12:15 PM | Kind of a "polar vortex" look to next week's weather maps

[12Z Euro 500 mb height anomaly forecast map for next Tuesday, July 15th; map courtesy Weather Bell Analytics at weatherbell.com]

Discussion

Last summer, the hottest weather in the Mid-Atlantic region took place in the week between July 14th and July 20th with Philadelphia, for example, averaging an impressive 95 degrees for high temperatures over the 7-day period. The same time period this summer - which happens to be next week - looks like it may have a far different result and the upper level weather maps may actually resemble the “polar vortex” pattern made famous this past winter.

Today's European computer forecast model indicates there will be an anomalously strong upper level low situated over the central Great Lakes in much the same manner that occurred frequently during the past winter (see above; 12Z Euro forecast map courtesy Weather Bell Analytics at weatherbell.com). Also, in much the same manner as this past winter, the coldest air relative-to-normal is likely to occur over the Great Lakes and Upper Midwest, but cooler-than-normal air is likely to extend all the way to the Mid-Atlantic coastline. Normal high temperatures have now climbed to 87 degrees in Philly, 89 in DC, and 84 at Central Park, NY and these levels are more than likely not going to be reached for at least a portion of next week. The cold front that will usher in this cool air mass is likely to arrive in the Mid-Atlantic region in the Monday/Monday night time frame - perhaps associated with numerous showers and thunderstorms.

12:30 PM | Arthur caused a dramatic drop in ocean temperatures

[Sea surface temperature (SST) Anomalies before Arthur]

[Sea surface temperature (SST) Anomalies after Arthur]

Discussion

In their very essence, tropical storms are “heat machines” that are fueled by warm sea surface temperatures and they are one of mother nature’s most efficient methods of transporting heat from the tropics to the mid and high latitudes in her ceaseless attempt at balancing out the atmosphere. Often, there can be an immediate impact in this perpetual balancing act of nature with respect to sea surface temperatures in areas near the path of a tropical storm. In fact, Hurricane Arthur, which moved on Friday from the Outer Banks of North Carolina to off the Mid-Atlantic coastline, generated a dramatic drop in sea surface temperatures at the Jersey Shore. As the tropical storm moved over nearby ocean waters, it acted to churn up the water at the Jersey Shore so that colder water from lower levels in the ocean rose to the surface in a process caused “upwelling”. In the same manner that warm air rises and cold air sinks in the atmosphere, cold water will typically sink to lower levels in the ocean as it is more dense than warm water. The “upwelling” generated by Arthur helped to cause sea surface temperatures to plunge at the Jersey Shore from the lower 70’s on Thursday to the upper 50’s on Friday – they have since recovered some back to the lower 60’s. In fact, sea surface temperatures dropped significantly from “pre-to-post Arthur” off the southeast US coastline where Arthur first formed and hovered for a few days (see “before and after” SST maps above). While the change in sea surface temperatures is usually just a short-term phenomenon on the order of days and not weeks, it can act to suppress any new tropical activity until sea surface temperatures climb back to warmer-than-normal levels.

2:00 PM | Possible signs for another snowy winter

["Centrally-based" El Nino during the winters of 2002-2003 and 2009-2010]

Discussion

Winter is still a long ways away, but there are already some developments around the world that can give us a clue as to what kind of weather we can expect here in the Northeast U.S. during the upcoming winter of 2014-2015. One such development is now underway in the tropical Pacific Ocean as El Nino conditions (i.e., warmer-than-normal sea surface temperatures) are gradually forming just off the west coast of South America. There are two important factors with respect to an El Nino – magnitude and location - that can play significant roles in winter weather conditions here in the Northeast U.S.

[CFS model forecast for a "centrally-based" El Nino during the upcoming winter season]

A super strong El Nino would tend to lead to a warm winter in much of the U.S., but I do not expect that to happen despite some model forecasts to the contrary (for more on this: http://thesiweather.com/2014/04/22/1100-am-el-nino-on-the-way-but-odds-are-against-a-super-one/). Meanwhile, a weak-to-moderate El Nino in the tropical Pacific Ocean has been associated with cold and snowy conditions in the Northeast U.S., but it tends to depend on the given location of the particular El Nino. Specifically, weak-to-moderate El Nino’s that are “centrally-based” in the central Pacific Ocean as compared with the warmest sea surface temperatures relative-to-normal being right near the west coast of South America (i.e., “eastern-based") have historically led to snowy winters in the Northeast U.S. For instance, the winters of 2002-2003 and 2009-2010 featured “centrally-based” El Nino conditions and both winters were very snowy in the Northeast U.S. Indeed, there are some longer-range computer forecast models suggesting that this El Nino ultimately becomes “centrally-based” in the Pacific Ocean by the fall and continuing into the winter season. In fact, the similarities between the forecasted sea surface temperature anomalies for the upcoming winter (middle map) and the actual anomalies during the snowy winters of 2002-2003 and 2009-2010 (top map) are rather amazing. Not only are they both featuring "centrally-based" El Ninos in the Pacific Ocean, but both have a noticeable warm sea surface temperature anomaly tucked in near the Alaska coastline and a colder-than-normal region just to its southwest.

We’ll continue to monitor both the magnitude and location of the unfolding El Nino in the Pacific Ocean over the next few months here at thesiweather.com. Currently, it can be said that there are signs that point to a “centrally-based”, weak-to-moderate strength El Nino which could very well result in another snow-filled winter around here.

Video

httpv://youtu.be/Z7qmkpIovHA

1:00 PM | The Atlantic Basin hurricane season has now officially begun and there is an impressive and very fortunate on-going streak

Discussion

The Atlantic Basin hurricane season officially started yesterday, June 1st, and there are actually some signs of tropical activity over the Gulf of Mexico region beginning as early as late this week. The U.S. has been extremely fortunate in recent years to not have a single major hurricane strike (i.e. category 3, 4 or 5). In fact, the last major hurricane to hit the U.S. occurred in October 2005 when Wilma hit southwest Florida as a category 3 storm in that very active tropical season. There have been category 1 and 2 hurricane strikes in the U.S. in recent years including Humberto (Texas), Ike (Texas), Gustav (Louisiana), Dolly (Texas), Irene (North Carolina) and Isaac (Louisiana), but none of those reached the status of 3, 4 or 5 after landfall. [Sandy was not technically a hurricane at its New Jersey landfall (post-tropical cyclone) and, if it were, it would have been categorized as a 1].

Such a streak, or “drought”, in U.S. major hurricane strikes is unprecedented going back to 1900. As of the start of this hurricane season, the span will be 3,142 days since the last U.S. major hurricane landfall. The previous longest span is about 2½ years shorter! During an average tropical season in the Atlantic Basin (using 1981-2010 as a baseline), there are 12 named storms and, of those, 6-7 become hurricanes, and 2 become major hurricanes (category 3-5). By the way, just as a point of comparison, in 1954 the U.S. was hit by 3 major hurricanes in less than 10 weeks.

For more on the 2014 Tropical Outlook: http://thesiweather.com/2014-tropical-and-mid-atlantic-summer-outlooks/

Days between major hurricane landfalls in the U.S., 1900-2013. (Credit: Roger Pielke, Jr., University of Colorado)

12:00 PM | Solar cycle 24 continues to rank as one of the weakest on record; the "solar maximum" phase of solar cycle 24 may have been reached and it is not impressive

[Current solar image with very few sunspots]

Discussion

Overview It appears that the solar maximum phase for solar cycle 24 may have been reached and it is not very impressive. In fact, this solar cycle continues to rank among the weakest on record which continues the recent trend for increasingly weaker cycles. 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 (plot below; courtesy Doug Biesecker of NOAA/SWPC). For this reason, many solar researchers are calling this current solar maximum a “mini-max”. Solar cycle 24 began after an unusually deep solar minimum that lasted from 2007 to 2009. In fact, in 2008 and 2009, there were almost no sunspots, a very unusual situation that had not happened for almost a century.

["Red line" represents solar cycle 24 showing far fewer sunspots during its maximum phase compared to most cycles going back to 1755]

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, 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 prolonged period of time 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 an 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 couple of 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 these 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.

Video

httpv://youtu.be/K98k1NaN__c

1:00 PM | First signs of potential tropical activity in the Atlantic Basin

[Latest European model forecast of MJO index; "green line" represents the model forecasted daily movement of the MJO index through phases 2 and 3 during the period from May 29th to June 12th]

Discussion

The official start to the Atlantic Basin hurricane season is this Sunday, June 1st, and there are some signs that there could be some early tropical activity to monitor. The Madden-Julian Oscillation (MJO) index is raising the possibility of tropical troubles in the Atlantic Basin during the early-to-middle part of June and this could ultimately have an impact on the east coast. The MJO index tracks a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days. The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperatures around the global tropics and subtropics. Research has found that the location or “phase” of the MJO is linked with certain temperature and precipitation patterns around the world. The very latest MJO forecast by the European computer forecast model (see plot above) propagates the MJO index into phases 2 and 3 during the early-to-middle part of June and studies have shown that these two phases tend to be correlated with increased tropical activity in the Atlantic Basin.

Indeed, the last few GFS computer forecast model runs have been hinting at some possible development late next week in the western Caribbean Sea or southern Gulf of Mexico region (see forecast map below) and this potential tropical disturbance could ride up the eastern seaboard. In fact, the first named tropical system last year, Tropical Storm Andrea, formed during the early part of June in a period in which the MJO index passed through phases 2 and 3. That system developed near the Yucatan Peninsula region of Mexico, tracked across northern Florida, and then moved right up the eastern seaboard producing more than three and a half inches of rain in Philly on June 7th and 8th.

[GFS computer model forecast from yesterday for late next week showing a disturbance in the southwestern Gulf of Mexico with plenty of moisture; forecast map courtesy Weather Bell Analytics at weatherbell.com]

11:15 AM | Amazing cloud formation in supercell thunderstorm

Discussion

A supercell is defined as a thunderstorm that is characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. One such supercell thunderstorm in northeast Wyoming on Sunday produced some spectacular cloud formations and little in the way of precipitation which is a key in the great viewing. These “low precipitation” supercells are more common in the Plains or northern Rockies where dry air often interacts more easily during their formation as compared to locations on the east side of the Mississippi River. In fact, due to the little rainfall produced from this type of “low precipitation” supercell, the storm’s rotation is viewed more easily. Normally, precipitation obstructs the view of the cloud structure that develops with the normal supercell rotation. A two-minute video (below) of the developing supercell is available on YouTube (picture and video courtesy of the storm chasers team called “Basehunters Chasers”).

Video

httpv://www.youtube.com/watch?v=VoO89cqDgJU

10:30 AM | 2014 Tropical and Mid-Atlantic Summer Outlooks

httpv://youtu.be/-ZYdt0lt3HU

Discussion

Tropical Outlook Summary The overall numbers may be down this year in terms of the number of Atlantic Basin tropical storms, but the sea surface temperature pattern in the western Atlantic and Gulf of Mexico makes the U.S. quite vulnerable to some late season tropical hits. The two major factors involved with this year’s tropical outlook include a developing El Nino in the equatorial Pacific Ocean and the current sea surface temperature pattern across the Atlantic Ocean which features an area of colder-than-normal waters off of the west coast of Africa, but warmer-than-normal temperatures near the U.S. east and Gulf of Mexico coastlines. As a result, there are likely to be fewer-than-normal “African-wave” type tropical systems this season that travel long distances across the tropical Atlantic Ocean and perhaps more in the way of “home-grown” type systems that develop much closer to the U.S. Typically, the “African-wave” type of tropical storm dominates the first half of the season (June/July/early August) and the “home-grown” type of tropical storm plays more of an important role during the second half of the season (late Aug/Sept/Oct).

In a typical Atlantic Basin tropical season, there are about 12 named storms with 6 or 7 reaching hurricane status and only 2 or 3 actually reaching major status (i.e., category 3, 4 or 5). This year there may be more on the order of 8-10 named storms with 3-5 reaching hurricane status, but despite these expected slightly below normal overall numbers, the U.S. could actually see more tropical activity than usual due to the sea surface temperature pattern in the western Atlantic and Gulf of Mexico. One final note, fortunately, the US mainland has not been struck by a "major" hurricane (i.e. category 3 or higher) since Hurricane Wilma in October 2005 and this is one of the longest stretches ever recorded without a "major" hurricane hit. In fact, when the 2014 official hurricane season begins on June 1st, it will have been 3,142 days since the last category 3+ storm made landfall in the US and this shatters the record for longest stretch between intense hurricanes in the US going back to 1900.

El Nino in the tropical Pacific Ocean What goes on in the tropical Pacific Ocean does indeed have an effect on the tropical Atlantic Ocean. El Nino, which refers to warmer-than-normal waters in the central and eastern tropical Pacific Ocean, affects global weather patterns and tends to produce faster-than-usual high-altitude winds over the tropical Atlantic Ocean. This increase in the upper atmospheric winds over the tropical Atlantic Ocean is an inhibiting factor for tropical storm formation in the Atlantic Basin and tends to rip apart growing storms. Currently, there are numerous signs for the development of an El Nino this summer in the tropical Pacific Ocean and this should inhibit storm formation in the tropical Atlantic Ocean.

Atlantic Ocean sea surface temperature pattern The main breeding grounds for Atlantic Ocean tropical systems are in the region between the west coast of Africa to the Caribbean Sea and the Gulf of Mexico. Above normal sea surface temperatures in this region generally help to intensify tropical waves that come off of the west coast of Africa and move westward in the trade winds. This year, however, there is a pocket of colder-than-normal sea surface temperatures off of the west coast of Africa and this should inhibit the formation of tropical storms in that part of the tropical Atlantic Ocean. On the other hand, there are warmer-than-normal waters just off the U.S. east coast and across much of the Gulf of Mexico and this should aid in the development of storms in nearby locations such as the Caribbean Sea, Gulf of Mexico or just off the Southeast U.S. coastline.

Mid-Atlantic Summer Outlook In general, I believe there is little chance for a hot and dry summer in the Mid-Atlantic region, but rather somewhat near normal in terms of precipitation with a slight leaning towards the cool side of normal when it comes to temperatures. The developing El Nino in the tropical Pacific Ocean will play a role in our summertime weather pattern and there are two other factors that should turn out to be meaningful. First, the far higher-than-normal Great Lakes ice cover extent this past winter and spring is generally a useful indicator for normal to cooler-than-normal temperatures in the Mid-Atlantic region (not necessarily because of the ice cover itself, but because of the continuing overall weather pattern which produced the anomalous ice cover in the first place). Second, soil moisture content is rather high around here thanks in part to the very snowy winter and also to the plentiful spring rains that we have experienced. High soil moisture content tends to significantly reduce the chances for summertime drought and excessive heat.

11:50 AM | El Nino likely on the way, but odds are against a "super" one

[computer model forecasts courtesy International Research Institute at Columbia University]

Discussion

Summary El Nino, which refers to warmer-than-normal waters in the central and eastern tropical Pacific Ocean, can have an important effect on global weather patterns; especially, if it develops into what is commonly referred to as a “super” El Nino. In fact, we have had “super” El Nino’s in the tropical Pacific Ocean in the not too distant past (e.g., 1997-1998, 1982-1983) which caused global temperatures to spike to well above normal levels for a sustained period of time. Indeed, there are numerous signs that an El Nino is likely to develop this summer in the tropical Pacific Ocean and continue into the fall season and it is likely to have an impact on the upcoming tropical season in the Atlantic Basin. Many are suggesting that this potential El Nino will evolve into a “super” El Nino; however, I believe the odds are against that and I provide some of the reasoning in this discussion and also in the video (below).

Reasoning against a "super" El Nino Several dynamical computer forecast models tend to develop an El Nino during the summer months in the tropical Pacific Ocean, but then project it to level out or even weaken as we approach the late fall and upcoming winter season. One model in particular that I track from Japan called the JAMSTEC has a pretty good track record with respect to El Nino forecasting and while it does predict an El Nino to develop during the summer months and continues it through the fall season, it then tends to weaken it heading into the upcoming winter season. Also, if one looks at the recent history of the “multivariate El Nino Southern Oscillation (ENSO) index”, it is generally the case that “super” El Nino’s tend to follow relatively warm periods with weak El Nino conditions in the tropical Pacific Ocean whereas weaker El Nino’s tend to follow relatively cold periods (i.e., La Nina conditions). The last few years have in fact been dominated by La Nina (cold) conditions in the tropical Pacific Ocean which is supporting evidence that a “super” El Nino is less likely to form. Finally, another important index that meteorologists track with regard to the state of the tropical Pacific Ocean is called the Southern Oscillation Index (SOI). This index gives an indication of the development and intensity of El Nino or La Nina events in the tropical Pacific Ocean and it is calculated using the pressure differences between Tahiti and Darwin. If there are negative values of the SOI then this can be indicative of upcoming El Nino episodes. A “long-lasting and sharply negative” SOI value can be a useful predictor of a very strong or “super” El Nino. Currently, the SOI values that we are observing are nowhere near the values experienced during or just preceding the “super” El Nino years of 1982-1983 and 1997-1998.

Potential winter implications One final note, the difference between a weak and “super” El Nino can be huge when it comes to impact on winter weather in the Northeast US. An ongoing “super” El Nino next winter would increase the odds of a warm winter in the Northeast US whereas a weaker El Nino - especially one based in the central tropical Pacific Ocean - can still allow for a cold and snowy winter (more on that outlook at a later time).

Video

httpv://youtu.be/_s1LUjWAPBY

2:20 PM | 2014 tornadoes near historic lows across the US thanks in large part to the persistent cold air outbreaks

Discussion

While the middle of this week will likely bring the threat for tornadoes into the central part of the country, there is nothing but good news so far this year on the tornado front and a large part of the thanks can go to the persistent cold air outbreaks that we have experienced during the spring in much of the eastern half of the nation with the primary focus across the Great Lakes and Upper Midwest. In fact, the number of tornadoes reported to NOAA’s Storm Prediction Center (SPC) through April 20th is 109 (above) and this is one of the slowest starts in years to the tornado season - perhaps even as long as a century - which continues a downward trend that began a few years ago. The average number of tornadoes for this time of year is 413 based on the period from 2005-2013. The yearly nationwide tornado totals for the last two years (943 in 2013, 1119 in 2012) were the lowest annual amounts of any of the past ten years according to NOAA’s SPC. One other favorable bit of news on the tornado front is the fact that we have not had an EF-3 or stronger tornado in the last 152 days which is the 4th longest span in the past 60 years without that type of strong tornado (below).

The overriding reason for the low number of tornadoes so far this year - as well as for much of last spring - has to do with the fact that the persistent cold weather pattern in much of the central and eastern U.S. has greatly reduced the chance for severe weather by inhibiting moisture-laden warm air from the Gulf of Mexico to flow northward into the southern U.S. This is an important, and generally necessary, ingredient for severe weather and the generation of tornadoes. Indeed, this same persistent cold weather pattern has contributed to the record high ice cover extent across the Great Lakes – still continuing at this late date – and more cold air masses are in sight for this region of the country through at least next week and the beginning of May.