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.

10:00 AM | ****Unfolding weather pattern to bring significant snow and brutal cold to the Mid-Atlantic region during the next few weeks and perhaps right through the month of February****

Discussion

Overview Some winters in the Mid-Atlantic region never bring significant snow. Some winters never feature the brutal cold that winter can offer. It is quite likely that we’ll get to experience both significant snow and brutal cold here in the Mid-Atlantic region during the next few weeks and that type of pattern with snow and cold could actually last right through the month of February. There will be numerous snow threats along the way beginning with the one from this evening into Saturday and then a second from Sunday night into Monday. In addition, brutal cold air outbreaks are likely to make numerous visits here as well during the next couple of weeks with the first one to arrive by Tuesday of next week. In fact, low temperatures by Wednesday morning could be sub-zero in many parts of the Northeast US and single digits in the I-95 corridor region from DC-to-Philly-to-NYC.

An especially interesting and scary-looking time period will be as we end January and begin February. Not only are there signs for a major coastal storm around February 1st (Super Bowl Sunday) or so, but brutal cold is likely to follow during the first few days of February that could bring temperatures to near zero again in many places across the Northeast US (see forecast map below for 2-meter temperature anomalies in the 5-day period from 2/2 to 2/7).

[GFS 2-meter temperature anomaly forecast map for 5-day period of 2/2-2/7; map courtesy Weather Bell Analytics at weatherbell.com]

Temperatures and the connection to the 500 millibar height anomaly pattern One week ago we discussed the likelihood for strong ridging to develop along the west coast of North America as we progress into the latter part of January. Indeed, the forecast map (below) from yesterday’s 12Z GFS Ensemble run for February 1st continues to insist on strong ridging (oranges) along the west coasts of Canada and the US and this type of upper air pattern usually leads to multiple Arctic air mass incursions into the northern US from northern Canada. In fact, this forecast map 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 beginning of February.

[GFS Ensemble height anomaly forecast map for February 1st; courtesy Penn State eWall]

Short-term snow threats Tonight/Saturday Precipitation from an intensifying coastal storm should reach the DC metro region early this evening (5pm or so), the Philly region between 9pm – 11pm, and NYC between 1 and 3am. The main thumping of snow from this system will be on its front end before a changeover takes place to freezing rain and sleet and perhaps even to plain rain from the cities to the coast. This event is likely to end in the DC metro region by early tomorrow afternoon, late afternoon in Philly and early evening in NYC. Snow and ice accumulation estimates are as follows:

1-3 inches in the northern and western suburbs of the District; lesser amounts in DC and points south and east 3-6 inches in the Philly and NYC metro regions and in their northern and western suburbs; lesser amounts to the south and east

Sunday night/Monday Another system will drop southeastward across the Great Lakes region late Sunday in much the same manner as some of the recent “clippers”. This system, however, has more potential than those as it will intensify in the Mid-Atlantic region on Monday, tap into some coastal moisture, and will have a cold air mass in place as it arrives. As a result, snow is likely to be the dominate precipitation type with this system on Sunday night and Monday and anywhere from several inches of snow are possible from DC-to-Philly-to-NYC. There is quite likely to be a significant disruption with the Monday morning commute due to the snowfall.

Buckle up, it’s going to be a wild ride.

Video

httpv://youtu.be/clS8roYhJ8k

9:00 AM | NOAA upgrades its main global computer forecast model (GFS) and enhances its computing power

Discussion

Overview Effective 12Z Wednesday, January 14th, 2015, NOAA’s Environmental Modeling Center (EMC) began running an upgraded version of its main computer global forecast model called the Global Forecast System (GFS). The upgraded GFS model has greater horizontal resolution compared to its predecessor version and the higher resolution runs now extend farther out in time. In preparation for the upgraded computer forecast model, NOAA has recently upgraded its computing capability with each of its two supercomputers by more than tripling their capacity. Much of this effort of upgrading the computer forecast model and computing capability was accelerated after Hurricane Sandy struck the Northeast US in 2012 as the main NOAA forecast model produced some inferior results in its prediction of the storm track when compared to its equivalent European counterpart.

Global Forecast System (GFS) The upgraded GFS model now has increased horizontal resolution in the first segment of its forecast package from 27 to 13 km and this higher resolution now extends out to 10 days whereas previously the higher resolution only went out to 7.5 days. The second segment of the GFS forecast package has seen an increase in horizontal resolution from 55 to 35 km which extends out from 10 days to 16 days. Furthermore, the upgraded model has enhanced physics and a better handling of important parameters such as sea surface temperatures, sea ice concentration, and snow depth which are all utilized by the model.

Computing power Modern numerical weather prediction uses the world’s most powerful computers and the associated software includes millions of lines of code. Some of the biggest computers in the world are used for weather and climate simulations. NOAA has upgraded its computing capability with each of its two supercomputers by more than tripling their capacity to at least 0.776 petaflops for a total capacity of 1.552 petaflops (a petaflop is a thousand trillion operations per second). The amount of data collected for operational numerical weather prediction is mind boggling. Petabytes of weather data are streaming to earth from dozens of weather satellites each day. In addition, hundreds of thousands of surface stations, roughly a thousand radiosondes, thousands of ships and buoys, thousands of aircraft, lightning detection networks, and other sensors are reporting each day adding up to hundreds of terabytes of information. All of this data is distributed around the world, quality controlled, and used to provide a physically consistent description of the three-dimensional atmosphere (info here provided courtesy Cliff Mass, University of Washington). Additional computing capacity and forecast model upgrades are scheduled by NOAA during the next few years.

9:00 AM | **Mid-winter update...cold pattern to continue and snow is sure to follow**

[00Z GFS Ensemble forecast map of 500 millibar height anomalies late this month; map courtesy Penn State eWall]

Discussion

Overview The first half of January has been colder-than-normal in much of the Mid-Atlantic region and indications are quite strong that the cold weather pattern will continue around here for the latter part of January and perhaps for much of February as well. In fact, there is reason to believe that the cold - relative to normal - could actually get much worse during the latter part of January and into early February compared to what we've experienced recently. So far this month, temperatures have averaged 3.5°F below normal at Philly International Airport in South Philadelphia, 3.7°F below normal in Central Park, New York, and 5.6°F below normal at Dulles Airport in Virginia. In addition, the colder-than-normal weather this month has extended across much of the eastern two/thirds of the nation and it is entirely possible that the nation as a whole ends up colder than last January which certainly was well below normal and made famous for its “polar vortex cold”. Snow, on the other hand, has been somewhat sporadic in the Mid-Atlantic region so far; however, sea surface temperature anomalies in both the Pacific and Atlantic Oceans - combined with the cold temperature outlook - suggest significant snowfall amounts are still possible as we progress through the second half of the winter season.

Temperatures and the connection to the 500 millibar height anomaly pattern There is reason to believe that a strong ridge will re-form along the west coast of North America as we progress into the latter part of January. Indeed, the forecast map (above) from last night's 00Z GFS Ensemble run for later this month features strong ridging (oranges) along the west coasts of Canada and the US and this type of pattern usually leads to multiple Arctic air mass incursions into the northern US from northern Canada. In fact, this forecast map suggests that the upper-level winds at 500 millibars – which tend to follow the height anomaly lines - could actually bring air from right around the North Pole into the northern US later this month.

Snowfall and the connection to sea surface temperature (SST) anomalies Snow has been rather sporadic so far this winter in the Mid-Atlantic region, but two far apart sea surface temperature patterns suggest significant snow is still on the table for the second half of the winter season. First, the latest SST anomaly map (below) shows some well-above normal sea surface temperatures near the US east coast (reds, oranges, yellows). Historically, this type of pattern with warmer-than-normal water off the US east coast has been linked with above-normal snowfall in the Northeast US during the second half of the winter season. Warmer-than-normal water just off the US east coast in the wintertime tends to help increase the temperature gradient along the coastline, decrease the stability of the lower atmosphere, and, in general, helps to intensify coastal storms.

In addition, the latest SST anomaly pattern in the central Pacific Ocean features an El Nino (warmer-than-normal; yellows, oranges) and this has often been correlated with above-normal snowfall during the latter half of winter seasons in this part of the country. As an example, the El Nino winter of 1982-1983, which featured little snowfall during the first half of winter, produced a blockbuster snowstorm in the Northeast US during February of that particular winter.

[Current sea surface temperature anomaly chart; courtesy NOAA]

1:45 PM | *Deep freeze grips the nation as we close out 2014...brutal cold coming to central and eastern US next week*

[7AM temperature map across the nation with average CONUS temperature of 14.3°F; map and data courtesy Ryan Maue of Weather Bell Analytics at "weatherbell.com"]

Discussion

It is certainly not too unusual to have cold air across parts of the nation at the end of the month of December; however, today’s nationwide deep freeze is indeed quite impressive. First of all, the average temperature this morning across the Lower 48 bottomed out at 14.3°F (at 7am ET). This is the lowest reading during the month of December since December 23, 1998 – some 16 years ago (data and temperature map above courtesy Ryan Maue of Weather Bell Analytics at "weatherbell.com"). In addition, about 85% of the nation was below freezing as of 7am (ET) this morning. The 12-hour 2-meter temperature anomaly forecast map from this morning’s GFS model run shows almost all “blues and purples” across the nation representing all areas predicted to experience below-normal temperatures (forecast map below courtesy “tropicaltidbits.com”). The current deep freeze extends virtually from coast-to-coast with the only exception being the Florida Peninsula where temperatures will remain above-normal (oranges/yellows).

[12Z GFS 2-meter temperature anomaly forecast map for early tonight; map courtesy "tropicaltidbits.com"]

In addition to the widespread cold which has even extended into southern California, a rare snow is occurring today across the Southwest US at such locations as Lake Havasu, Arizona, Las Vegas, Nevada, and the southern California San Bernardino mountains. Today’s cold and snow is being fueled by intense upper-level low pressure over the Desert Southwest and a very strong Arctic high pressure system over the Northern Plains and Rockies.

[Southern California (Temecula) vineyard in the deep freeze this morning]

Looking ahead to next week, after a brief spike in temperatures on Sunday across the east coast, colder air will again make a move from Canada into the northern US anchored by more strong high pressure. This time the cold air outbreak is likely to spare the western US, but by the second half of next week, the central and eastern US could be in the deep freeze once again and this time Florida may not be so lucky. Next week’s outbreak could very well cause a freeze problem way down the Florida Peninsula and places in the Upper Midwest like Chicago, Illinois could experience sub-zero temperatures.

1:00 PM | Global sea ice surges to above-normal levels seen rarely in recent years

[Yellow line represents southern hemisphere sea ice areal extent throughout 2014; courtesy University of Illinois "cryosphere"; NOAA/NCEP]

Discussion

Overall Summary Southern Hemisphere sea ice areal extent continues to run at or near all-time record high levels in the satellite data era (circled area above) for this particular time of year in records going back to 1979. The northern hemisphere sea ice areal extent continues to run at below normal levels; however, it is well above levels seen on this particular date during the past several years. Overall, the global sea ice areal extent has surged into “above-normal” territory (below) that has been seen rarely during the past several years.

[red line represents global sea ice areal extent relative-to-normal (zero line); courtesy University of Illinois "cryosphere"; NOAA/NCEP]

Southern Hemisphere Sea Ice The rather remarkable period of consistently higher-than-normal sea ice areal extent in this part of the world actually began a few years ago. Back in 2011, the southern hemisphere sea ice areal extent was still at below-normal levels, but it has surged in recent years to nearly 1.5 million square kilometers above the 1979-2008 mean (data courtesy University of Illinois "cryosphere" web site with data originating from NOAA/NCEP Snow and Ice Data Center).

Northern Hemisphere Sea Ice The northern hemisphere sea ice areal extent is still below normal (circled area below) relative to all years going back to 1979 although it is well above the lowest point set during 2012 and noticeably above levels seen earlier this year. The northern hemisphere sea ice areal extent is currently about 0.5 million square kilometers below normal using the base period of 1979-2008 for comparison (data courtesy University of Illinois "cryosphere" web site with data originating from NOAA/NCEP Snow and Ice Data Center).

The northern hemisphere sea ice areal extent has generally trended lower since the mid 1990’s reaching primarily below-normal levels after the turn of the century. In the past several years, however, there has been a leveling off of that downward trend in terms of sea ice areal extent at those below-normal levels. In the time period before the mid 1990’s, the sea ice areal extent was generally above-normal dating back to 1979. The directional shift in the sea ice areal extent trendline that developed during the mid 1990’s in the northern hemisphere correlates quite well with a northern Atlantic Ocean sea surface temperature cycle that is tracked by meteorologists through an index called the Atlantic Multidecadal Oscillation (AMO). Indeed, Atlantic Ocean sea surface temperature anomalies play a critical role in the overall northern hemisphere sea ice areal extent. The AMO index flipped in phase during the mid 1990’s from negative (cold) to positive (warm) and the sea ice areal extent trendline changed direction at precisely that point in time. Once the northern Atlantic sea surface temperatures flip back to cooler-than-normal levels – perhaps in the next few years or so - the northern hemisphere sea ice areal extent should return to the normal or above-normal levels seen prior to the mid 1990’s.

[Yellow line represents northern hemisphere sea ice areal extent throughout 2014; courtesy University of Illinois "cryosphere"; NOAA/NCEP]

11:45 AM | An unfolding stratospheric warming event provides supporting evidence for cold weather ahead

[Polar view of current stratospheric (10-millibars) temperature pattern and the 10-day forecast using the GFS model; courtesy NOAA]

Discussion

One of the ways to monitor the potential for Arctic air outbreaks in the northern 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 northern Canada into the central and eastern U.S. Indeed, there appears to be a significant stratospheric warming event in the offing over the next 10 days or so (above) centered near the North Pole that provides supporting evidence for cold weather conditions ahead in the central and eastern U.S.

During the winter months in the lower polar stratosphere, temperatures are typically lower than minus 70° Celsius (purple area above). 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° in just a few days. This sets off a reversal in the west-to-east winds and the collapse of the polar vortex. 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. The entire process from the initial warming of the stratospheric at high latitudes to the cooling in the troposphere at middle latitudes can take weeks to unfold.

The plot (below) shows the stratospheric (10-millibar) temperature pattern for 2013 and 2014 in the high latitude region of 90°N to 65°N. Some stratospheric warming events in recent decades have, in fact, been followed by widespread very cold air outbreaks across southern Canada and the US some two to three weeks after the initial upper atmosphere warming. For example, such an event occurred during December 1984 and this led to an extremely cold January 1985 in much of the central and eastern US. Interestingly, last winter season featured a sharp spike in stratospheric temperatures at just about this same time of year (i.e., late December, left circled region below) and the following couple of months were indeed much colder-than-normal in the central and eastern US. This year saw a sharp, but temporary spike in stratospheric temperatures during the latter part of November and perhaps that atmospheric event contributed to the cold and snow of Thanksgiving week in this part of the country.

A detailed video discussion on “Stratospheric Warming” can be found on the “Meteorology 101” page of the weather web site.

[Stratospheric (10-millibars) temperature plot for 2013 and 2014 in high latitude region of 90N to 65N; courtesy NOAA]

11:30 AM | *Buckle up, it could be a wild ride*

Discussion

Overview This work week will be rather uneventful weather-wise with no big storms and nothing more than a warm frontal passage on Tuesday and a cold frontal passage on Wednesday. The upcoming weekend, however, will bring the threat for a coastal storm to the Mid-Atlantic region and there are numerous signs in the atmosphere and oceans around the world that this potential weekend event may be the beginning of an overall pattern change to “stormy and cold” – not only for the remainder of December, but perhaps right into the New Year as well.

500 mb pattern The overall wind flow in the upper part of the atmosphere around 500 millibars (~18,000 feet) during the next couple of days will be rather zonal (west-to-east) in the northeastern US and this will prevent any significant cold air outbreaks from Canada. Over the next 7-10 days, however, there will be a change to the upper-level wind flow with troughiness developing over the Aleutian Islands (blue area on forecast map below) and high pressure ridging forming along the west coast of the US and British Columbia (red area on forecast map below) [an increasingly +PNA and -EPO pattern]. This type of upper level pattern will increase the chances for Arctic air outbreaks to be able to drop from northern Canada into the northern US as we progress through the rest of December and into January.

[00Z GFS-Ensemble 500 mb height anomaly forecast for Christmas Eve (12/24); map courtesy "tropicaltidbits.com"]

High-latitude blocking In addition, upper-level high pressure ridging is likely to develop over eastern Canada, Greenland and the western Atlantic (circled area above) in the next 7-10 days and this generally leads to a “blocking” pattern in the higher-latitudes which often favors colder and stormier weather in the Northeast US. The change to a “blocking” type of pattern can be tracked by the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) indices which - when negative - usually suggest some “blocking” in the atmosphere at higher latitudes is underway. Indeed, the NAO and AO are forecasted to drop sharply in the next 7-10 days or so into negative territory (red, circled areas on forecast maps below).

[North Atlantic Oscillation index and Arctic Oscillation index forecasts; courtesy NOAA]

An active southern jet Elsewhere, the sea surface temperature (SST) anomaly pattern in the Pacific Ocean currently features colder-than-normal water (blue area) to the north of Hawaii and warmer-than-normal water (yellow, orange area) near Hawaii and this combined with the unfolding weak El Nino (circled area) in the central tropical Pacific Ocean will likely generate an active southern jet stream cutting from west-to-east across the Pacific Ocean (indicated by arrow) and right into the southern US. This type of pattern will likely keep multiple storms coming into California over the next several days and these disturbances will tend to move west-to-east across the country riding along the southern jet stream.

[Current sea surface temperature anomalies; courtesy NOAA]

Multiple storm threats There is the potential for storm development this weekend along the east coast from a system that will hit California on Tuesday, then head to Texas by mid-week, and then pull out of the southern states late in the week and make a move to the northeast. Computer forecast models are not in agreement with this potential threat – par for the course when several days out from any potential event – but the changes in the overall weather pattern as described above suggest that the threat is real. The air mass in place ahead of this potential weekend storm will be only marginally cold enough for snow in the Mid-Atlantic region so odds would probably favor rain at the coast and snow in interior higher-elevation locations with the I-95 corridor in the usual precipitation-type "battle zone" area. This storm may be the first of many over the next couple of weeks. A second storm is possible around the middle of next week and then another one a few days after that.

High-latitude “blocking”, a weak and centrally-based El Nino, and an active southern jet were all major factors in the 2014-2015 Vencore Weather Winter Outlook which favored a colder and snowier winter than normal: http://vencoreweather.com/2014-2015-winter-outlook/

Buckle up, it could be a wild ride.