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Potential for Late Season Freeze

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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:

Last3mTDeptUS

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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.

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Pleasant pattern will come to an end next week

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[alert type=”info”]Don’t Forget! The Eta Aquarid meteor shower peaks Saturday Night through Sunday Morning. Good viewing conditions are expected with clear skies. Up to 30 meteors per hour possible. Let us know if you see any![/alert]

Blocking patterns often produce anomalous weather somewhere in the United States — and frequently, it occurs in the Eastern 1/3 of the United States. Not this time. The blocking pattern, which became anomalous late this past week, drove a cutoff low into the Central United States that produced an amazing late-season snowfall across parts of the United States. The unsettled weather, however, has remained over the Central United States thus far and has not made much progress to the east — as the block has actually collapsed overhead and high pressure in association with it has settled over the Northeast US.

This will change next week. Until then, however, a few more days of pleasant weather can be expected through the weekend and into early next week. The game changer will occur once the decaying cutoff low, once responsible for historic late season snow over the Central US, drifts up the Eastern seaboard. Forecast models are in good agreement that it will bring increased chances of showers and thunderstorms as well as cooler temperatures. In addition, a slightly cooler and more unsettled pattern looks likely through the medium to long term forecast period.

For a day-by-day breakdown of the forecast, check out our Forecast Brief. If you’re looking to get more details and technical reasoning, check out our Technical Forecast Discussion. Have a great Saturday night!

Blocking sends disturbance south from Greenland

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High latitude blocking, which we discussed earlier this week as an important predictor in summer temperature patterns around our area, is at it again. A large and anomalous blocking ridge, building north from Canada towards the higher latitudes into Newfoundland and Southeastern Greenland, is forcing a disturbance and upper level low to shift southward. The disturbance will track southward today — and can be seen on water vapor and visible satellite imagery early this afternoon. The feature won’t directly impact our area, nor will it cause any widespread unsettled weather on the east coast, but it could reinforce east/southeasterly winds as it passes later this week.

The track of the disturbance is the most unusual. To see an upper level low or disturbance track southward from Greenland all the way to a point just a few hundred miles off of the US East Coast is exceedingly rare. However, one glance at the blocking pattern aloft tells you the story — the disturbance had nowhere else to go, being forced into a small window between the trough over the Atlantic and the blocking ridge to the west over Canada. (Click read more for more imagery).

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Does blocking in May hint at summer weather?

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Seasonal forecasting is one of the more tedious and intricate tasks for meteorologists. Not only is it difficult to predict by nature (no pun intended) — but forecast model accuracy skill is greatly reduced at that range. Often, meteorologists find themselves looking back towards past events, and analogs, for help with predicting the months ahead. In our case, interest was piqued when we noted what seemed to be higher than normal frequency of high latitude blocking this year. Hurricane Sandy, the nor’easter which immediately followed, and many events this winter featured patterns that were driven by blocking patterns over Canada, Greenland, and the higher latitudes towards the pole. The blocking pattern was not overly anomalous at least on a per-year basis, but it certainly seemed to be more frequent than the calendar year which preceded it.

As we look forward to May, forecast models are in agreement on the continuation of higher than normal height anomalies at 500mb over Central and Eastern Canada as well as farther north towards Greenland. The continuation of blocking in the higher latitudes, relative to our location, can offer some interesting insights into the summer temperature forecast as we move forward. Is there a common theme amongst the historically warm and historically cool summers in the NYC Area — and can we relate it to Springtime blocking patterns? The answer, may surprisingly to some, is yes.  For the sake of example, take the summer of 2009 and the summer of 2010. Both of these years featured wildly different patterns, with 2009 remaining very cool throughout the summer while 2010 was warm. In 2009, there was very little blocking observed from March through May. 2010, on the other hand, featured periodic blocking in the high latitudes from February through May. Interested yet?

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