10:45 AM | *April pattern change is likely to bring more sustained warmth to the Mid-Atlantic, rains to California and the Southwest US, and a sharp increase in severe weather to the Plains/Midwest/Ohio Valley*

[00Z GFS Ensemble run forecast maps of 500 millibar height anomalies for tomorrow (left) and April 7th (right); map courtesy Penn State e-Wall, NOAA]

Discussion

Overview The end of winter in the Mid-Atlantic region may be finally at hand. For several weeks now, the overall upper-level pattern has featured a ridge of high pressure along the west coast of North America and a trough of low pressure in the northeastern US. This pattern has brought about sustained colder-than-normal weather in the Mid-Atlantic region for the past few months, warmer and drier-than-normal weather for the western US, and it has helped to suppress severe weather activity in the Plains, Midwest and Ohio Valley. In fact, the tornado season is off to an historically low start as we approach the end of March (circled area on plot at bottom). All of this should begin to change over the next week or so as there will likely be a flip in the overall pattern with an upper-level trough forming in the western US and an upper-level ridge developing in the northeastern part of the country. Indeed, this upcoming upper-level pattern flip will have weather implications across the entire nation for much of the spring season.

Mid-Atlantic warm up The latest GFS Ensemble computer forecast model run for the 500 millibar height anomalies (above) shows a reversal of positions for the ridge and trough in the US between tomorrow and a week later on April 7th. Other computer forecast models generally agree with this potential pattern flip including the European (not shown) and Canadian (below). This change in positions of upper-level features will have serious implications on the overall flow of air across the entire US with an increasing southwesterly flow pumping milder air in the Mid-Atlantic region. This southwesterly flow of air should result in above-normal temperatures on a more sustained basis than recent weeks in the Mid-Atlantic region beginning in the next week or so. January, February and March were colder-than-normal months in the Mid-Atlantic region, but April and May have good chances to be above-normal.

[00Z Canadian model forecast map of 500 millibar height anomalies averaged out for the 7-10 day time period ending on April 7th; map courtesy Penn State e-Wall]

California drought The flip of upper-level systems from ridge-to-trough out in the western US is quite likely going to open the door for more rainfall in the Southwest US and this should include California which has been suffering through a long-term drought. The upcoming change in the overall weather pattern should allow for more eastward penetration of Pacific Ocean moisture into the western US and this should help alleviate the drought situation during much of the spring season. (San Francisco is suffering through its driest "January/February/March" time periods on record).

Threat for severe weather/tornadoes Finally, in addition to the warm up in the Mid-Atlantic region and the potential additional rainfall in the southwestern US, the increasing southwesterly flow of air in the eastern half of the nation will pump moist air northward from the Gulf of Mexico into the Plains, Midwest and Ohio Valley. This influx of moist air has been one important missing ingredient so far this spring in terms of the development of severe weather and tornadoes. Unfortunately, the addition of more humid and warmer air into the Plains, Midwest and Ohio Valley will likely result in a quick ramp up of severe weather and tornado activity that could take us from early-to-mid April into the month of May.

[Tornado season is off to an historically low start through the first three months of the year; courtesy NOAA]

12:30 PM | The Atlantic Ocean is showing signs of a possible significant long-term shift in temperatures from warm-to-cold

[Sea Surface Temperature Anomaly comparison chart between July 2014 and today; courtesy NOAA]

Overview

In addition to solar cycles, temperature cycles in the planet’s oceans play critical roles in climate and on the ever-changing distribution of global sea ice. Oceanic temperature cycles are often quite long-lasting and a warm or cold phase can persist for two or three decades at a time. The Atlantic Ocean experienced a cold phase from the early 1960’s to the mid 1990’s at which time it flipped to a warm phase and that has continued for the most part ever since. The current warm phase; however, is now showing signs of a possible long-term shift back to colder-than-normal sea surface temperatures (SST) and this could have serious implications on the climate and sea ice areal extent in the Northern Hemisphere.

Recent temperature trends in the Atlantic Ocean

Sea surface temperatures have dropped considerably during the past eight months or so in much of the northern Atlantic Ocean. The comparison chart above of SST anomalies between July 2014 (top portion) and today (bottom portion) shows a big drop in temperatures across much of the northern Atlantic Ocean. First, the rather limited colder-than-normal (blue) regions from July 2014 have increased noticeably in areal extent during the past eight months. Second, the well above normal waters (orange) of July 2014 that existed east of Greenland have cooled off dramatically during this time period to only slightly above normal (yellow) and there has even been a switch from well above normal (orange) to below normal (blue) in this area east of Greenland. Notice also that most of warm pockets of water off the Northeast US coastline in July 2014 (orange) have persisted for the most part during this general cool-down in the northern Atlantic Ocean during the past eight months; however, that may be about to change.

Longer-term trends in the Atlantic Ocean

On a longer time scale, there is supporting evidence from the National Oceanographic Data Center that something significant is indeed occurring in the Atlantic Ocean. Since around 2006/2007, there has been a definitive downward trend in “monthly heat content anomaly” in the top 700 meters of the northern Atlantic Ocean (below). The heat content in this part of the Atlantic Ocean ramped up rather sharply beginning around the middle 1990’s (first arrow) and seemingly peaked during 2006/2007 (second arrow).

[Monthly heat content anomaly trend in the top 700 meters of the northern Atlantic Ocean; courtesy National Oceanographic Data Center]

Atlantic Multidecadal Oscillation (AMO)

One way meteorologists can monitor sea surface temperature patterns in the North Atlantic Ocean is through an index value known as the Atlantic Multidecadal Oscillation (AMO). The monthly values for the AMO index are shown in the plot below for the period of 1856-2013 where positive values (reds) represent warmer-than-normal time periods and negative values (blues) indicate cold phases. Sea surface temperature cycles have tended to last for two or three decades at a time before phase changes take place. Since late last summer, the AMO index has actually dropped rather consistently from +0.355 to a now barely positive value of +0.016 (not shown in plot) providing supporting evidence that a cool-down is indeed occurring in recent months in the Atlantic Ocean.

[Atlantic Multidecadel Oscillation monthly index values where red indicates warm phases and blue cold phases; courtesy NOAA]

Model Forecast for Atlantic Ocean sea surface temperatures

One indication that this downward trend in Atlantic Ocean sea surface temperatures may continue comes from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). JAMSTEC’s global seasonal forecasting of sea surface temperature anomalies has a pretty good track record and its most recent long-range forecast (below) suggests there will be a fairly widespread area of colder-than-normal water in the northern Atlantic Ocean by the fall of 2015 (blue regions) and this includes the noticeable disappearance of those very warm pockets that are currently situated in the western Atlantic near the US coastline.

[JAMSTEC model forecast of sea surface temperature anomalies for fall season 2015; courtesy Japan Agency for Marine-Earth Science and Technology]

Ramifications on Northern Hemisphere Sea Ice Areal Extent

If the Atlantic Ocean is indeed slipping back into a colder-than-normal phase (i.e., negative AMO) then this would quite likely have a significant impact on Northern Hemisphere (NH) sea ice areal extent. The NH sea ice areal extent was generally at above-normal levels before the middle 1990’s (arrow in plot below) which is when the Atlantic Ocean temperature phase change took place from cold-to-warm. Once the warm phase of the Atlantic Ocean became established in the late 1990's, the NH sea ice areal extent trended sharply downward from positive levels into well below-normal territory. In recent years, there has been a jagged, but generally sideways trend in NH sea ice areal extent at those well below normal levels. However, if these recent signs of a possible long-term Atlantic Ocean temperature phase change from warm-to-cold are "real and sustained" (and sometimes there are false starts), then the NH sea ice areal extent will very likely return to above-normal levels in the not too distant future - just as it was during the last cold phase pre-mid 1990’s.

[Northern Hemisphere sea ice areal extent; courtesy University of Illinois "cryosphere", NOAA]

Ramifications on Climate

The impact on climate from a potential Atlantic Ocean temperature shift of warm-to-cold are less clear as the all-important Pacific Ocean would have to be factored into the equation. The temperature cycles in the Pacific Ocean are critically important to weather and climate across the US and elsewhere and will have to be monitored in the years to come. Certainly, the last cold phase of the Atlantic Ocean produced numerous cold winters in the Northeast US such as during the latter part of the 1960's and also in the 1970's. If the eastern Pacific Ocean remains warmer-than-normal for next winter season as it was for this year and the western Atlantic Ocean cools down as predicted, this could very well set the stage for another colder-than-normal winter in the Northeast US for 2015-2016.

Paul Dorian

Vencore, Inc.

9:30 AM | *Winter-like pattern to resume after "spring break"*

Discussion

Overview This break in our recent cold and stormy weather pattern will continue to provide us with glimpses of spring-like warmth over the next 7 days or so, but the signs for a return to a winter-like pattern in the Northeast US are rather convincing. There are numerous signals that point to a period of colder-than-normal weather in the Northeast US for the last third of March and the beginning third of April (roughly March 18-April 10) and it is quite likely to include more threats of snow. These signals that suggest there will be a return to a winter-like pattern are described below and involve such things as the Madden Julian Oscillation, stratospheric warming, 500 millibar height anomalies, and the Arctic Oscillation index.

Madden Julian Oscillation (MJO) The MJO is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is a large-scale coupling between atmospheric circulation and tropical deep convection. The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics. Furthermore, the MJO influences both precipitation and surface temperature patterns across the US. Specifically, one significant impact of the MJO in the U.S. during the northern hemisphere winter is an increase in the frequency and intensity of cold air outbreaks across the central and eastern US.

[European model forecast of the MJO index through March 23rd; courtesy NOAA]

MJO Phases Research has found that the location of the MJO, or phase, is linked with certain temperature and precipitation patterns around the world. The MJO phase diagram (above) illustrates the recent and forecasted progression of the MJO index through different phases which generally coincide with locations along the equator around the globe. When the index is within the center circle, the MJO is considered weak, meaning it is difficult to discern. Outside of this circle, the index is stronger and will usually move in a counter-clockwise direction as the MJO moves from west to east. The very latest European computer forecast model predicts an increasingly strong MJO index that will propagate through “phase 6” into “phase 7” on its way to “phase 8” over the next few weeks (follow green line in figure above in a counter-clockwise fashion). Phases 7 and 8 for the MJO index for this time of year (i.e., March/April/May) typically signal colder-than-normal temperatures in the northeastern U.S. (see circled “blue” regions below in "phases 7 and 8" temperature anomaly charts).

[US temperature anomalies for different phases of the MJO index during the period of March/April/May; courtesy NOAA]

Sudden Stratospheric Warming (SSW) Another way to monitor the potential for Arctic air outbreaks in the northeastern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events in the region of the North Pole have been found to set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from central Canada into the northeastern U.S. Indeed, there is reason to believe that there will be a significant stratospheric warming event during the next couple of weeks over the North Pole that could contribute to a return of our colder-than-normal temperature pattern in the northeastern U.S. as we progress through the latter part of March and into the month of April. The current stratospheric temperature pattern (at 10 millibars) and the 10-day forecast are shown below and they show an impressive area of stratospheric warming closing in on the North Pole later this month with the typical "polar-based" cold vortex getting displaced towards Asia.

[Temperature analysis at 10 millibars (stratosphere), current and 10-day forecast; courtesy NOAA]

500 millibar height anomalies The 500 millibar height anomaly forecast (below) for 10 days from now from the 06Z GFS Ensemble run indicates strong ridging (oranges) will re-develop in western Canada at the same time troughs of low pressure (blue) will form in the northeastern US and in the North Pacific region just to the south of Alaska. This type of upper air pattern usually leads to multiple Arctic air mass incursions into the northern US from northern Canada and, in fact, was in place for much of this winter season. This forecast map also suggests that the upper-level winds at 500 millibars – which tend to follow the height anomaly lines - could actually bring air directly from the North Pole into the northeastern US by the latter part of March. In addition, there are signs for “blocking” high pressure to form near Greenland at this same time period (oranges) and that tends to help keep cold air sustained in the northeastern US for a lengthy period of time.

[06Z GFS Ensemble 500 mb height anomaly forecast map for March 20th; map courtesy "tropicaltidbits.com", NOAA]

Arctic Oscillation index High-latitude blocking can be tracked by meteorologists through indices known as the Arctic Oscillation (AO) and its closely-related cousin called the North Atlantic Oscillation (NAO). When the AO is positive, for example, surface pressure is low in the polar region and this helps the mid-latitude jet stream to blow strongly and consistently from west-to-east keeping Arctic air locked up in the polar region. When the AO index is negative, there tends to be high pressure in the polar regions (i.e., “high-latitude blocking”), weaker zonal winds, and greater movement of polar air into the middle latitudes. The forecast of the AO index (red line on plot below) suggests there will be a plunge from the current strongly positive value to well into negative territory by later this month which supports the theory that cold air will be sustained in the northeastern US as an upper-level blocking pattern develops.

[Observed (black) and forecast (red) of the Arctic Oscillation index; courtesy NOAA]

Paul Dorian Vencore, Inc.

10:00 AM | *The cold is getting old, but it just won't fold*

[NOAA CFSv2 45-day temperature anomaly forecast; map courtesy Weather Bell Analytics at weatherbell.com, NOAA]

Discussion

Not only will February turn out to be one of the coldest February’s ever in much of the Northeast US and Upper Midwest, but it will likely turn out to be one of the coldest months ever in many locations. For example, Bridgeport, CT, Islip, NY and Cleveland, OH are all on pace for their coldest month ever; JFK and LaGuardia Airports in New York City are likely to join with Boston, MA and end up in the top two or three for their coldest month ever recorded.

In the I-95 corridor region of DC, Philly and New York City, temperatures have averaged around 10 degrees below normal through February 24th, and there is certainly no reason to believe these numbers will change much over the remainder of the month which will continue to average well-below normal. Philly is currently on pace for its third coldest February ever and likely will end up in the top ten for coldest months ever. All indications are that this colder-than-normal weather pattern in the Northeast US will continue right through March and perhaps even into April despite some breaks along the way.

A recent temperature anomaly forecast by NOAA’s CFSv2 longer-range model shows an impressive colder-than-normal temperature pattern right through early April across the eastern two-thirds of the nation (above; map courtesy Weather Bell Analytics, NOAA). There is reason to believe this forecast is on to something by looking at the latest GFS forecasted 500 millibar height anomalies during the next few weeks. As an example, the 500 millibar height anomaly forecast for late next week (below; map courtesy "tropicaltidbits.com", NOAA) depicts the similar upper-level pattern that has brought us cold weather during the month of February – namely a strong western North American ridge of high pressure (oranges) and a deep trough in the Northeast US (blues). This combination allows for the transport of Arctic air from northern Canada into the northern US on a consistent basis.

[06Z GFS 500 millibar height anomaly forecast for March 6th; map courtesy "tropicaltidbits.com, NOAA]

The one saving grace is that “normal” temperatures are climbing pretty rapidly as we approach the spring equinox. For example, the normal high temperature in Philly on February 23rd is 46 degrees, but by the time we reach March 23rd it climbs to 55 degrees. In other words, 10 degrees below normal a month from now won’t have quite the same sting that it has right now.

11:00 AM | *The sun has gone quiet again during the weakest solar cycle in more than a century*

latest_solar_image

[Latest solar image with little sunspot activity; courtesy "spaceweather.com"]

Discussion

Overview

The main driver of all weather and climate, the entity which occupies 99.86% of all of the mass in our solar system, the great ball of fire in the sky – has gone quiet again during what is likely to be the weakest sunspot cycle in more than a century. For the past 5 days, solar activity has been very low and one measure of solar activity – its X-ray output – has basically flatlined in recent days (plot below courtesy NOAA/Space Weather Prediction Center). Not since cycle 14 peaked in February 1906 has there been a solar cycle with fewer sunspots. We are currently more than six years into Solar Cycle 24 and today the sun is virtually spotless despite the fact that we are still in what is considered to be its solar maximum phase. Solar cycle 24 began after an unusually deep solar minimum that lasted from 2007 to 2009 which included more spotless days on the sun compared to any minimum in almost a century.

x-ray output flatlined

[The flatlining of solar X-ray output in recent days; courtesy NOAA/SWPC]

Solar maximum

The sun goes through a natural solar cycle approximately every 11 years. The cycle is marked by the increase and decrease of sunspots which are visible dark regions on the sun’s surface and cooler than surroundings. The greatest number of sunspots in any given solar cycle is designated as the “solar maximum" and the lowest number is referred to as the “solar minimum” phase. There was an uptick in the number of sunspots in April 2014 which produced a second peak during solar cycle 24 and it is looking increasingly likely that this will be considered the solar maximum point for this particular cycle (figure below courtesy NASA). Many solar cycles are double peaked; however, this is the first one in which the second peak in sunspot number was larger than the first peak which occurred in February 2012. Going back to 1755, there have been only a few solar cycles in the previous 23 that have had a lower number of sunspots during its maximum phase.

solar_cycle_24

[Sunspot numbers for solar cycles 23 and 24 (current) with second peak; courtesy NASA]

Consequences of a weak solar cycle

First, the weak solar cycle has resulted in rather benign “space weather” in recent times with generally weaker-than-normal geomagnetic storms. By all Earth-based measures of geomagnetic and geoeffective solar activity, this cycle has been extremely quiet. However, while a weak solar cycle does suggest strong solar storms will occur less often than during stronger and more active cycles, it does not rule them out entirely. In fact, the famous Carrington Event of 1859 occurred during a weak solar cycle (#10) [http://thesiweather.com/2014/09/02/300-pm-the-carrington-event-of-1859-a-solar-superstorm-that-took-places-155-years-ago/]. In addition, there is some evidence that most large events such as strong solar flares and significant geomagnetic storms tend to occur in the declining phase of the solar cycle. In other words, there is still a chance for significant solar activity in the months and years ahead.

Second, it is pretty well understood that solar activity has a direct impact on temperatures at very high altitudes in a part of the Earth’s atmosphere called the thermosphere. This is the biggest layer of the Earth’s atmosphere which lies directly above the mesosphere and below the exosphere. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation and are highly dependent on solar activity.

Finally, if history is a guide, it is safe to say that weak solar activity for a very prolonged period of time (several decades) can have a negative impact on global temperatures in the troposphere which is the bottom-most layer of Earth’s atmosphere - and where we all live. There have been two notable historical periods with decades-long episodes of low solar activity. The first period is known as the “Maunder Minimum”, named after the solar astronomer Edward Maunder, and it lasted from around 1645 to 1715. The second one is referred to as the “Dalton Minimum”, named for the English meteorologist John Dalton, and it lasted from about 1790 to 1830. Both of these historical periods coincided with below-normal global temperatures in an era now referred to by many as the “Little Ice Age”. In addition, research studies in just the past couple of decades have found a complicated relationship between solar activity, cosmic rays, and clouds on Earth. This research suggests that in times of low solar activity where solar winds are typically weak; more cosmic rays reach the Earth’s atmosphere which, in turn, has been found to lead to an increase in certain types of clouds that can act to cool the Earth.

Outlook

The increasingly likely outcome for another historically weak solar cycle continues the recent downward trend in sunspot cycle strength that began over twenty years ago during solar cycle 22. If this trend continues for the next few cycles, then there would likely be more talk of another “grand minimum” for the sun. Some solar scientists are already predicting that the next solar cycle, #25, will be even weaker than this current one. However, it is just too early for high confidence in those predictions since some solar scientists believe that the best predictor of future solar cycle strength involves activity at the sun’s poles during a solar minimum and the next solar minimum is still likely several years away.

Paul Dorian Vencore, Inc.

8:30 AM | ****Incredible looking overall long-term pattern: afternoon/night accumulating snow with heavy squalls, damaging winds late tonight/early Sunday with possible power outages, painful cold on Sunday, multiple additional snow threats and brutal cold air outbreaks****

[Early morning NEXRAD radar image with snow over the Great Lakes/Ohio Valley and headed our way; image courtesy University of Wisconsin]

Discussion

The next 24-36 hours will bring us accumulating snow with heavy squalls, high impact winds, painful cold and dangerous sub-zero wind chills as a result of tremendous intensification of a storm system off the Northeast US coastline. This storm is currently located over southeastern Ontario and it will drop rapidly today towards the I-95 corridor region. Once this system reaches the coastal waters it will deepen explosively by some 36 millibars in a 24-hour period. The combination of the relatively warm waters of the western Atlantic Ocean, an extremely vigorous upper-level wave of energy, and a powerful upper-level jet streak will help cause this rapid intensification during the next 24 hours or so. This type of rapid and significant intensification will lead to a very sharp pressure gradient across the Northeast US which, in turn, will lead to extreme wind gusts on the order of 55 mph or so in the period between late tonight and Sunday morning (approximately 3 am to noon). Unfortunately, power outages are quite possible with this type of wind from late tonight into early which is the same time the coldest air of the season comes pouring into the region. Winds will slowly decrease later Sunday afternoon and evening.

As far as the snow is concerned, while there can be some snow seen this morning, the majority of the snow showers and possible heavier snow squalls should occur this afternoon and continue this evening. The snow part of this event will really consist of two parts: one part associated with the Arctic front from this afternoon into the late evening hours and then the second part associated with the developing storm late tonight and early Sunday. Some of the snow squalls this afternoon and evening can be quite intense with brief heavy snowfall, gusty winds, very poor visibility ("white out" conditions) and quick accumulations. Late tonight and early tomorrow will pose another chance for additional snowfall from eastern PA northeast across New Jersey and New York City as the storm deepens rapidly off the coastline. In fact, New York City could end up with its heaviest snowfall during this latter portion of the storm when an “inverted trough” extends to the northwest of the intensifying surface low pressure center. One final note on the snow, it is possible that some "thunder snow" occurs during the next 24 hour in portions of the I-95 corridor given this tremendous upper-level suport and upward motion in the atmosphere.

Accumulations by tomorrow morning are likely to be in the 1-3 inch range across Philly, a coating to 2 inches in the DC metro region, and 2-5 inches in and around NYC metro region. However, we'll have to watch for a potential surprise late tonight and early Sunday for additional snowfall in the Philly metro region and especially in the New York City metro region which could add to these preliminary accumulation estimates. Much of the snow that does accumulate will be whipped around by powerful winds late tonight and on Sunday with considerable blowing and drifting possible in some areas. Temperatures are likely to drop to single digits by early tomorrow throughout the I-95 corridor, then struggle to rise at all during the day, and then plunge to possible record-breaking territory by early Monday morning in the area of 0 degrees or even slightly below.

Another system could throw significant accumulating snow our way late Tuesday into early Wednesday, but that storm track is still somewhat uncertain. However, what is quite certain is that there will be more bitter cold air for the second half of next week and beyond and there will be multiple additional snow threats as an unbelievable long-term pattern is shaping up for the Mid-Atlantic region.

1:00 PM | ***Friday to Wednesday - the worst weather that winter has to offer***

[12Z GFS forecast map for next Tuesday early afternoon (blue=snow); map courtesy "tropicaltidbits.com", NOAA]

Discussion

Overview Get ready; it’s going to be a wild ride. The stretch from Friday to Wednesday will feature some of the worst weather that winter can offer in this part of the country. All of the following are on the table for the 6-day period from tomorrow through next Wednesday:

- Painful cold with 0 degrees within reach (Sunday AM, Monday AM) - Weekend accumulating snow (late Saturday, Saturday night) - Very strong winds with potentially damaging wind gusts (Sunday) - Dangerous sub-zero wind chills (Friday and especially on Sunday) - Significant snow threat (late Tuesday into early Wednesday)

Here are the details: Arctic air arrives today in the Mid-Atlantic region accompanied by snow showers and increasing winds. Those winds will remain strong tonight and temperatures will plunge in the overnight hours into the single digits in many suburban locations by early Friday. The winds remain strong through much of the day on Friday and this will produce sub-zero wind chills at times before the wind finally relaxes noticeably early tomorrow night.

On Saturday, another clipper system and Arctic front will be plowing towards the I-95 corridor from the Great Lakes region. This system has more potential in terms of snowfall in our area compared with today’s frontal system. In fact, snowfall on the order of a coating to two inches is possible late Saturday and Saturday night in the DC metro region, 1-3 inches in Philly, and 2-5 inches in NYC associated with this next Arctic blast. Then, Sunday promises to be the coldest day yet with painful cold, dangerous sub-zero wind chills, and potentially damaging wind gusts past 50 mph. The last thing we need on Sunday is for scattered power outages to develop, but that possibility cannot be ruled out with the most powerful winds likely late Saturday night and early Sunday.

After a bitter cold start on Monday - near 0 degrees in many portions of the I-95 corridor - moisture will begin streaming towards us from the south-central US. The combination of very cold ground temperatures, an entrenched Arctic air mass in the Northeast US and copious amounts of “southern” moisture does not bode well for the Mid-Atlantic region. Indeed, this could turn into a significant snow event for the Mid-Atlantic region from late Tuesday into early Wednesday, but since the storm track cannot be pinned down quite yet, it is just too early to make this call. And yes, this mid-week storm will be followed by more brutal cold.

Buckle up…the next 6 days will be quite interesting.

[12Z GFS forecast map for late Tuesday night (blue=snow); map courtesy "tropicaltidbits.com", NOAA]

Video

httpv://youtu.be/7vLfp462510

1:30 PM | **Back-to-back Arctic invasions to produce amazing cold around here by early Sunday**

Discussion

Overview The normal high temperature for today at Philly’s International Airport is 43 degrees and we are going to end the day several degrees below that mark, but this cold is nothing compared to the type of cold that is coming. Back-to-back Arctic invasions will send temperatures plummeting in the Mid-Atlantic region over the next few days and the second blast is likely to feature the coldest air of the season so far in much of the area from Maine to Florida. The last time temperatures dropped to less than 10 degrees in Philly during the month of February was in 1996 (6 degrees on 2/6/96). There is little doubt that Philly will see temperatures that low on Sunday morning (and again on Monday morning). The last time sub-zero readings occurred in Philly during the month of February was in 1979 (-2 degrees on 2/11/79) and this is not out of the question for the Sunday/Monday Arctic blast.

Thursday night/Friday Arctic blast A clipper system and its associated Arctic frontal system will arrive in the Mid-Atlantic region later tomorrow with scattered snow showers - perhaps even a heavier burst of snow - along with strengthening winds and then it’ll turn brutally cold behind it late tomorrow night and Friday. Temperatures early Friday could bottom out in the upper single digits in some suburban locations along I-95 and they will struggle to reach the 20’s on Friday along with much lower wind chill values.

Sunday/Monday Arctic blast Before that late work week Arctic air mass even has a chance to modify, another clipper system and its associated Arctic front will be plowing towards the Mid-Atlantic region. This system will be accompanied by snow for the I-95 corridor along with strengthening winds from later Saturday into early Sunday and it will usher in absolute frigid air for Sunday and Monday. The snow could turn out to be rather significant in portions of the Northeast US; especially, from New York City north and east. Sunday is liable to turn out to be the coldest day of the winter so far in much of the region between Maine and Florida. Low temperatures early Sunday morning and again early Monday morning could plunge to 0 degrees in many parts of the Mid-Atlantic region. High temperatures on Sunday will be confined to the teens in the I-95 corridor and with winds gusting up to 40 mph or so, there will be dangerously low (sub-zero) wind chill values.

The 12Z GFS 96-hour forecast map for low temperatures on Sunday morning (above) show below zero readings just to the west of the NYC metro region, near zero in the Philly suburbs, and low-to-mid single digits near DC (map courtesy Weather Bell Analytics). The 12Z GFS 96-hour 2-meter temperature anomaly forecast map for Sunday (below) shows the entire eastern half of the nation in a deep freeze and some sections (in purple) are more than thirty degrees below normal for this time of year (map courtesy Weather Bell Analytics).

12:00 PM | **Stormy and cold pattern developing for the next few weeks with multiple Arctic air outbreaks and numerous storm threats**

Discussion

Overview The pattern setting up for the next few weeks looks quite active and cold with multiple Arctic air outbreaks and storm threats. The southern branch of the jet stream will become more active and this will push Pacific Ocean moisture into the southern US which can move across the nation into the eastern US. After today’s precipitation event, there will be a couple of tranquil days in the Mid-Atlantic region with moderately cold conditions, but then the next clipper type of low pressure system and its associated Arctic blast will be ready to pounce on the area by Thursday night. Before that late week Arctic air mass even has a chance to modify, another clipper system and associated Arctic invasion will be plunging towards the Mid-Atlantic region by Saturday night. Beyond that, it appears yet another storm threat is likely around Tuesday or so of next week and this one will feature lots of moisture from the southern states. That storm next week could ultimately turn into a significant winter storm for the Mid-Atlantic region, but the storm track is, of course, still uncertain this far in advance.

This afternoon and tonight Colder air continues to trickle into the I-95 corridor as our Monday progresses and this will lead to mainly frozen precipitation in the Philly and NYC metro regions for the rest of this event. In Philly, the intermittent precipitation should fall as sleet and/or snow this afternoon with a change to all snow likely tonight. Accumulations by later tonight can be on the order of a coating to an inch with slick spots on untreated surfaces. In the NYC metro region, any wintry mix early this afternoon is likely to become all snow by later this afternoon and accumulations of snow and ice by later tonight should be on the order of 1-2 inches with slick spots on untreated surfaces. In DC, any plain rain that falls this afternoon is likely to turn into a wintry mix of freezing rain and sleet tonight, perhaps even ending briefly as all snow late tonight. Slick spots are likely on untreated surfaces in the overnight hours and there can be a coating of snow and/or ice.

Thursday night clipper and powerful Arctic blast By late Thursday, the next clipper and powerful Arctic front will blast into the Mid-Atlantic region with strengthening winds and numerous snow showers which could even include a heavier snow squall and/or a period of steadier snow. The upper-level pattern at 500 millibars for Thursday night is quite impressive with a deep trough of low pressure situated right on top of the Mid-Atlantic region (below). This will allow for rapid development of the clipper system once it reaches the warmer waters of the western Atlantic and this could lead to an all-out blizzard for – you guessed it – portions of New England. Temperatures by early Friday will bottom out in the single digits in many suburban locations along the I-95 corridor.

[500 millibar height anomaly forecast map for late Thursday night with deep upper-level trough over the Mid-Atlantic; map courtesy "tropicaltidbits.com"]

Saturday night clipper and powerful Arctic blast By late Saturday, another clipper system will be dropping southeastward from the Great Lakes region into the Mid-Atlantic and this system could generate numerous snow showers in the I-95 corridor late Saturday into early Sunday – perhaps even a period of steadier snow. Once again, a deep upper-level trough will develop in the Mid-Atlantic region (below) in very similar fashion to the Thursday night Arctic blast. As with the Thursday night event, this type of upper-level pattern may allow for rapid development of the clipper system once it reaches the warmer waters of the western Atlantic. This clipper will usher in another brutally cold Arctic air mass for the latter part of the weekend and the early part of next week. In fact, it may turn out that the temperatures early Sunday morning are at the lowest levels seen so far this winter in much of the Northeast US.

[500 millibar height anomaly forecast map for Sunday morning with deep upper-level trough over the Mid-Atlantic; map courtesy "tropicaltidbits.com"]

10:00 AM | Stratospheric warming and the MJO: two far apart signals that suggest colder-than-normal weather in the northeastern US continues right through February and perhaps well into March

Discussion

Overview January 2015 was colder-than-normal in the northeastern US and it looks like that type of pattern may continue through February and perhaps well into March as well. Two separate signals that are far apart on the planet suggest cold weather could indeed continue in the northeastern U.S. for the foreseeable future. The first signal is coming from a tropical disturbance known as the Madden Julian Oscillation (MJO) and the second signal is coming from the highest part of the atmosphere over the North Pole.

Madden Julian Oscillation (MJO) The MJO is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is a large-scale coupling between atmospheric circulation and tropical deep convection. The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics. Furthermore, the MJO influences both precipitation and surface temperature patterns across the US. Specifically, one significant impact of the MJO in the U.S. during the northern hemisphere winter is an increase in the frequency and intensity of cold air outbreaks across the central and eastern US.

MJO Phases Research has found that the location of the MJO, or phase, is linked with certain temperature and precipitation patterns around the world. The MJO phase diagram (above) illustrates the recent and forecasted progression of the MJO index through different phases which generally coincide with locations along the equator around the globe. When the index is within the center circle, the MJO is considered weak, meaning it is difficult to discern. Outside of this circle, the index is stronger and will usually move in a counter-clockwise direction as the MJO moves from west to east. The very latest NOAA GFS Ensemble model forecast of the MJO index propagates it from its current "phase 6" location into "phase 8" as we progress towards mid-February (follow green line in figure above in a counter-clockwise fashion). Phase 8 for the MJO index this time of year (January/February/March) typically signals colder-than-normal temperatures in the eastern U.S. (see circled area below in "phase 8" temperature anomaly chart).

Sudden Stratospheric Warming (SSW) Another way to monitor the potential for Arctic air outbreaks in the northeastern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events in the region of the North Pole have been found to set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from central Canada into the northeastern U.S. Indeed, there appears to be a significant stratospheric warming event in progress right now over the North Pole that could continue our colder-than-normal temperature pattern in the northeastern U.S. as we progress through the month of February and perhaps even into the month of March. The current stratospheric temperature pattern and the 5-day forecast are shown below and they show stratospheric warming at the North Pole during the next five days as the typical "polar-based" cold vortex is displaced towards Asia.

SSW Consequences During the winter months in the lower polar stratosphere, temperatures on average are below minus 70 degrees Celsius. The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex. The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes. This dominant structure is sometimes disrupted in some winters or even reversed. Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50 degrees in just a few days. This sets off a reversal in the west-to-east winds and the collapse of the polar vortex. In recent SSW events, the polar vortex has split into two pieces and that opened the floodgates for Arctic air to move southward. In response to the stratospheric warming at the high latitudes, the troposphere in turn cools down dramatically and this cold air displacement is then transported from the tropospheric high latitudes to the tropospheric middle latitudes. This doesn’t mean that each and every day following an SSW event will be below normal as that will not be the case. However, it does suggest that, based on historical similarities, we could be looking at an overall below-normal temperature pattern in the northeastern U.S. through February and perhaps even well into March.