2:00 PM | An upcoming polar stratospheric warming event could prolong winter-like conditions in the central and eastern U.S.
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 central 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 over the North Pole that could prolong winter-like conditions across the central and eastern U.S. as we progress through March and into the month of April.
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. The entire process from the initial warming of the stratospheric at high latitudes to the cooling in the troposphere at middle latitudes can take several weeks to unfold. 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 central and eastern U.S. continuing well into the month of April. Indeed, the very latest NCEP Couple Forecast System (CFS) temperature anomaly forecast (below) for the month of April is colder-than-normal for much of the central and the eastern U.S.
1:15 PM | The coldest December/January/February in 35 years for the contiguous U.S. and March is off to an amazingly cold start
[Niagara Falls has frozen over for the second time this winter; courtesy Reuters]
December through February
The 3-month winter period of December, January and February was the coldest in the last 35 years across the contiguous United States as measured by the U.S. Historical Climatology Network (USHCN). In fact, according to USHCN data, this was the 10th coldest 3-month winter period ever for the contiguous U.S. going back to the late 1800’s (below). The last time nationwide temperatures averaged this low in the December, January and February time frame was during the winter of 1978-1979 which happened to immediately follow two other very cold winters of 1976-1977 and 1977-1978. The cold weather this winter season has been the most dramatic “relative-to-normal” across the Great Lakes and Upper Midwest with some impressive results. For example, Chicago registered its 3rd coldest winter ever in the 3-month time period of December through February with the most days ever having its low temperature at or below zero. Additionally, the ice cover extent on the Great Lakes is at a record high for the month of March and quite close to the all-time record high. Finally, a rarity has occurred at Niagara Falls where the water has frozen over for a second time this winter season (above).
The month of March has begun in much the same fashion as its three preceding months – namely, much colder-than-normal in much of the U.S. There have been numerous all-time record low temperatures set for the month of March that include the following:
-Atlantic City, New Jersey at 2 degrees (3/4)
-Dover, Delaware at 6 degrees (3/4)
-Charlottesville, Virginia at 1 degrees (3/4)
-Dulles Airport, Virginia at -1 degrees (3/4)
-Baltimore, Maryland at 4 degrees (3/4) [this broke the March record that held for the city of Baltimore since 1873]
-Kansas City, Missouri at -3 degrees (3/3)
In addition, the following locations have experienced their coldest days ever in the month of March (i.e., the coldest high temperature in March):
-Little Rock, Arkansas at 28 degrees (3/3)
-International Falls, Minnesota at -9 degrees (3/1)
-Erie, Pennsylvania at 9 degrees (3/3)
-Kansas City, Missouri at 5 degrees (3/2)
Finally, the snow cover across the Lower 48 states according to NOAA's National Ice Center is at ~54% which is the highest level in 10 years at this late date in the winter season.
Detailed description of the USHCN data
The United States Historical Climatology Network (USHCN) is a high-quality data set of daily and monthly records of basic meteorological variables from 1218 observing stations across the 48 contiguous United States. Daily data include observations of maximum and minimum temperature, precipitation amount, snowfall amount, and snow depth; monthly data consist of monthly-averaged maximum, minimum, and mean temperature and total monthly precipitation. Most of these stations are U.S. Cooperative Observing Network stations located generally in rural locations, while some are National Weather Service First-Order stations that are often located in more urbanized environments. The USHCN has been developed over the years at the National Oceanic and Atmospheric Administration's (NOAA) National Climatic Data Center (NCDC) to assist in the detection of regional climate change. Furthermore, it has been widely used in analyzing U.S. climate. The period of record varies for each station. USHCN stations were chosen using a number of criteria including length of record, percent of missing data, number of station moves and other station changes that may affect data homogeneity, and resulting network spatial coverage.
1:10 PM | More on the incredible cold winter weather including the latest on the Great Lakes record ice cover extent and some dazzling winter phenomena
[Ice caves at "Apostle Islands National Lakeshore" in northern Wisconsin (Lake Superior);
photo taken by Brian Peterson, Minneapolis Star Tribune, via Associated Press]
The incredible winter cold and the Great Lakes
This winter has brought some incredible cold to much of the eastern two-thirds of the nation, but nowhere has it been more persistent than in the Upper Midwest/Great Lakes region of the country. As a result, the Great Lakes have now reached a record ice cover extent for this late in the season at 90.5% (as of 3/2). The all-time record for ice cover on the Great Lakes is 95% set back in February 1979 (records date back to 1973). That all-time record is certainly within reach as brutal cold air continues to dominate the scene in the eastern two-thirds of the country which will very likely contribute to additional icing on the Great Lakes. In fact, the average 2-meter temperature this morning at 8AM (ET) across the continental U.S. was 19.9°F which may very well be the coldest reading ever this late in the season (source WeatherBell Analytics using NCEP RTMA 2.5 km analysis data).
Currently, Lake Ontario has the least ice cover of the five lakes at 38.2%, but that is more than three times its normal, and it has increased dramatically in recent days. Ontario has a reputation for never freezing as it quite deep so it retains heat longer than the other four lakes. Also, the Niagara River feeds water into Lake Ontario from Lake Erie, providing agitation which keeps the water's surface from freezing. Here are the latest ice cover percentages for all five lakes: Erie 95.4%, Superior 95.0%, Huron 94.5%, Michigan 90.1% and Ontario at 38.2%.
Dazzling winter weather phenomena
One of the positive by-products of this bitter cold winter has been the fact that the extensive ice cover has allowed hikers to visit “ice caves” accessible by foot for the first times in many years. For example, tens of thousands of people in northern Wisconsin have taken advantage of this year’s deep freeze to hike across Lake Superior to visit the caves at the western end of the “Apostle Islands National Lakeshore” (see above). These caves were carved out of sandstone by the waves from Lake Superior, the largest by volume and surface area of all five Great Lakes. In addition, an amazing picture from near Mackinac Island, Michigan shows this "frozen" wave on Lake Huron (below).
Philly's chance at history
Locally, the Philadelphia region has a chance to make some history late tonight with respect to the bitter cold Arctic air that is currently gripping the region. The record low for tomorrow’s date at Philly Int’l Airport is 7°F and the all-time low temperature for the month of March is 4°F – both within the realm of possibility. Many suburban locations to the north and west of the city will drop down to near 0 degrees by early tomorrow, but milder air will return by the end of the week with highs rebounding to the 40’s.
It looks like our colder-than-normal weather pattern of recent months in the Mid-Atlantic region will continue on average right through the month of March. One signal that suggests cold weather will indeed continue in March in the eastern U.S. comes from a tropical disturbance known as the Madden Julian Oscillation (MJO). In addition, support for a colder-than-normal month of March comes from a NOAA seasonal climate forecast model called the Coupled Forecast System version 2 (CFSv2).
Background information on the 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 over the U.S. during the northern hemisphere winter is an increase in the frequency and intensity of cold air outbreaks across the eastern US.
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 illustrates the 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 model MJO index forecast (below) propagates the MJO from phase 8 in early March to phases 1, 2 and 3. All of these particular phases of the MJO (i.e., 8, 1, 2 and 3) typically result in a colder-than-normal temperature pattern for the eastern US (see “temperature composites” figure centered on February-March-April).
The latest monthly forecast from NOAA’s Coupled Forecast Model (v2) strongly suggests the colder-than-normal weather pattern of recent months will continue (see below) throughout the eastern two-thirds of the nation. This model is updated on a daily basis and it has done a very good job in its temperature forecasts when reasonably close to event time as we are now with March only a few days away. As has been the case throughout the winter months, the core of the coldest air “relative to normal” will likely be centered from the Northern Plains to the Upper Midwest according to the model forecast, but an incredibly large area across much of Canada and the U.S. will be below-normal for the month of March.
How about snow
As far as snow is concerned, the combination of the expected colder-than-normal temperatures along with a newly activated southern branch of the jet stream (e.g., California storms) will quite likely produce above-normal snowfall for the Mid-Atlantic region during the month of March. Philadelphia has actually had 3 straight months (December, January and February) featuring monthly snowfall totals in the top ten for the given month using records dating all the way back to the 1880’s. The 3 consecutive months of top ten monthly snowfall amounts has not happened in Philly since January-February-March of 1978. There has never been a winter with 4 consecutive months ending up in the top ten for that given month so if it happens in March - certainly a possibility given the overall pattern - it would be a first for Philadelphia. As far as seasonal totals are concerned, Philly is in 3rd place on their all-time list some 20 inches shy of the seasonal record set in the winter of 2009-2010 with 58.4 inches so far this season - this record of 78.7" is also within reach given the current overall pattern.