Potential for Late Season Freeze

One of the coldest springs in recent history nationwide continues to roll forward with the potential for one more shot of winter across the Mid-west and Northeast. Since February 1st, we have seen a dramatic reversal in temperature anomalies from the blowtorch Dec-Jan period to the very cold Feb-Mar-April of 2013. The only mild areas have been northeastern New England and the SW US – all other locations have been feeling the chill. The epicenter of this cold has been the Mid-west and northern Plains, where many stations experienced over a 20 degree decrease in mean temperature from March 2012 to March 2013:

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Chicago  IL:    -27.7º

2012: 64.0º

2013: 36.3º

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Des Moines IA: -26.0º

2012: 64.4º

2013: 38.4º

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Indianapolis IN: -25.3º

2012: 66.5º

2013: 41.2º

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La Crosse WI: -27.5º

2012: 59.4º

2013: 31.9º

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Marquette MI: -24.3º

2012: 50.6º

2013: 26.3º

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Milwaukee WI: -26.3º

2012: 59.1º

2013: 32.8º

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Minneapolis MN: -25.2º

2012: 56.3º

2013: 31.1º

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St. Louis MO: -22.1º

2012: 70.4º

2013: 48.3º

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The month of May began with similar magnitude of cold in the same areas that have been targeted over recent months. Brutal, recording breaking cold encompassed the Plains, with the historic event of snow flakes flying as far south as Arkansas. The coastal Northeast has been near normal with a northeasterly flow off the Atlantic, but surface high pressure made the airmass a beautiful, dry, sunny one for us. The interior Northeast and OH Valley have been much warmer the past week, though that will change for a time next week.

Models are in consensus on a deep short wave trough that will amplify into the Eastern US early next week (Sunday 12th-Monday 13th-Tuesday 14th), bringing with it widespread sub 0c 850mb temperatures to latitudes as far south as 37N. In addition, unsettled weather is expected to grip the area beginning this week and continuing into this weekend. The following is the 18z GFS ensemble mean 500mb height anomalies valid for late Sunday night of the upcoming weekend. Notice the dark blue color over the Northeast – that is indicative of very low heights, and thus cold air, since the atmosphere contracts with chill and expands in heat.

Not quite convinced based on the NCEP/GFS model data? The 12z ECMWF for Monday morning has the chill as well. In the bottom right image in the link attached, we can see the black line representing the 0c isotherm at 850mb, is all the way south to Virginia. High temperatures may struggle into the low 60s on Monday with favorable conditions for radiational cooling Sun, Mon and Tuesday nights in the Northeast. Given surface high pressure nearby, most areas away from the immediate cities should see frost and freeze conditions. Some guidance actually depicts overnight lows in the 20s across the interior Northeast and Mid-west, which could be damaging to any vegetation that has already been planted. NYC itself will not fall below 32F due to the urban heat island effect, but they will see temperatures well below normal for a couple days.

There is support for this cold shot from the tropical Pacific too. The Madden Julian Oscillation (MJO ) will be propagating through phases 3-4 over the next 5-7 days. Phases 3 and especially 4 are historically chilly ones in the Eastern US  (phase 4 is the image on the far bottom left of the link).

Given the model support, pattern progression, tropical forcing, and cold air available, we are likely to see a continuation of the spring 2013 pattern through next week with a final shot of winter. Overnight freezes are very possible for most in suburbia across the Northeast.

Beyond the 13th/14th, the pattern should turn warmer with a +NAO/AO in place and a MJO progressing into phase 5-6. The potential exists for our first 90F readings in the May 15th-25th period, with a warmer than normal airmass, but that is a topic for another post. May will probably finish colder than normal in the Mid-west, Plains, and milder than normal in the Northeast due to a warmer second half of the month. Before that, get ready for some jacket weather early next week.