GFS model showing some high latitude blocking (above normal height anomalies) to start the month of May. Past data offers interesting clues as to what this might mean for our summer weather.

Does blocking in May hint at summer weather?

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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Despite deary start, sun returns by midweek

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Dreary conditions, which took control of the areas weather on Monday into early Tuesday, will give way to the return to sun and seasonable temperatures by later on Tuesday. After a cloudy and potentially drizzly start to the day, high pressure to the north and east will begin to strengthen and nudge towards the area as an atmospheric disturbance aloft weakens. As this occurs, clearing is expected to push into the area from the east. Areas that see the sun on Tuesday will be able to warm up into the 60’s despite a light east/southeasterly wind.

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Visible satellite imagery from the afternoon of Monday, April 29 2013 showing clouds overspreading the area. Many areas were observing drizzle and light rain in response to a weak disturbance moving near the area.

Clouds, showers to begin the new week

Plenty of sun, and warm temperatures, dominated the areas weather late last week into this past weekend. But late Sunday, mid and high clouds took over owing to an approaching disturbance to the west. This disturbance is over the area today — nestled between a tight mid level atmospheric flow and bringing clouds and showers with it. The unsettled weather will also bring cooler temperatures — topping out in the upper 50’s to lower 60’s. The air will feel cooler and damp, however, with winds off the cooler ocean waters.

The showery and cloudy conditions are expected to persist through the early week, but will become less numerous on Tuesday and taper off completely by Wednesday. By mid week, the sun should make a return. Winds will remain out of the east, so the air will still have a bit of a chill to it, but the sun and temperatures in the 60’s will be pleasant compared to the damp and dreary conditions that the week began with.

Today: Cloudy with showers likely. Highs in the upper 50’s to lower 60’s. Southeast winds around 15 miles per hour. Chance of precipitation 80 percent.

Tonight: Cloudy with showers likely. Lows in the upper 40’s to lower 50’s. Southeast winds around 15 miles per hour. Chance of precipitation 60 percent.

Tuesday: Mostly cloudy with a chance of showers. Highs in the upper 50’s to lower 60’s. Southeast winds around 15 miles per hour. Chance of precipitation 40 percent.

Wednesday: Partly cloudy. Highs in the mid 60’s. Southeast winds around 10 miles per hour.

GFS model showing height anomalies at 500mb next week. Notice the anomalous cutoff low over the Southeast States, and the strong blocking to the north of New England. Image courtesy Weatherbell/Ryan Maue.

Blocking episode may create forecast headache

Blocking, both over Eastern Canada and in the high latitudes, more often than not means a forecast headache for our forecast area. We’ve seen it on many occasions with winter storms — as the strong blocking highs create anomalous weather patterns. We saw it with Hurricane Sandy, and the Nor’Easter that followed. And it seems that we may see it again, as we head towards the first week of May.

Forecast models are coming into agreement on the potential for an anomalous blocking pattern to develop over Eastern Canada and the Northwest Atlantic towards Newfoundland. With a shortwave trough predicted to be over Central Canada at that time, the trough may be forced underneath the block into the Ohio Valley and the Mid-Atlantic/East Coast. This would create the opportunity for a “cutoff low” and possibly even a coastal storm — with widespread below normal temperature anomalies wherever the cutoff low and upper level trough tracks.

At this range, the question becomes: How strong will the blocking be, and where will it develop? The positioning of the block, as well as its strength, will have a great deal of impact in determining where the cutoff low will eventually track. This will, in turn, have major impacts on the sensible weather forecast for our area. Needless to say, the potential is there for a prolonged period of showers and unsettled weather — as well as a coastal storm — if the cutoff tracks near our area. If it remains farther south, we may be spared the worst effects.

Forecast models are still struggling with the positioning of the block and positioning of the cutoff low. The Euro (or ECMWF), for example, has the cutoff low staying well to our south while the GFS and Canadian models bring it closer to our area with a prolonged period of rain in the Mid-Atlantic states.

In situations like this one, forecasters can utilize ensembles to see the variance amongst forecast models. In this case, we can check out the GFS Ensembles — which show the average height anomalies at 500mb (mid levels of the atmoshpere) on the left, and the “spaghetti” plot of each ensemble on the right. It is easy to see that most ensembles agree on the strong block developing over Newfoundland and Eastern Canada, but the exact positioning of the upper level trough over the Central US is less certain.

Forecasters will certainly be monitoring the models over the next several days, so be sure to stay tuned for updates on the potential unsettled pattern as we head towards next week. For now, enjoy the pleasant weather through the weekend!