Mar 19, 2013: Winter Pattern Continues Into Spring

[notice]This is the second part of tonight’s post, which is separated into two parts. The first part is an overview of the March 18 snowstorm, and the second part is a forecast update.[/notice]

Forecast Highlights:

A snowstorm affected the area yesterday with more snow than expected, adding onto what is already a cold and snowy March compared to recent years. Temperatures today were warmer than those of yesterday, but still remained colder than average. Winter is still not over, however; a highly anomalous blocking pattern remains in place, with temperatures remaining mostly colder than average for the rest of the month, continuing past the first official day of spring, with the potential for more snow events in the region.


Tuesday, March 19 Observations:

Following the widespread snowfall on Monday night, temperatures warmed up enough early on Tuesday to result in a changeover to rain across the area as a secondary low pressure developed just south of the area, tracking into New England as it intensified. Cloudy skies with occasional showers, mixing with snow towards far interior NW NJ and SE NY, continued through the morning and early afternoon hours, with a squall observed in the evening producing thundersnow in Orange county and thunderstorms in Long Island.

Temperatures were initially chilly across the area, in the 30s for most, although following the end of the storm, skies cleared for parts of the area, resulting in temperatures quickly rising mostly in northern NJ into the mid to upper 40s, while the rest of the area saw highs in the upper 30s to low 40s.



Tonight – Friday: Cold Continues; Some Snow Thursday

As the radar image above shows, scattered precipitation continues across the region, including the area; earlier this evening, thunderstorms were reported in parts of Long Island with thundersnow in Orange County, NY. Clearing cloud cover is expected tonight, with mostly sunny skies for Wednesday with high temperatures peaking in the upper 30s to low 40s inland and the low to mid 40s across the rest of the area.

On Wednesday night, a coastal low pressure will develop well off the southeast US coast, quickly intensifying as it quickly tracks north, parallel to the coast but offshore, reaching Atlantic Canada by Thursday night where a major snowstorm is expected. While this coastal low will remain too far offshore to directly affect the region, a weaker low pressure is expected to develop closer to the Mid Atlantic coast, and has the potential to produce snow for coastal parts of the region on Wednesday night and Thursday. The GFS is currently the most bullish model with this feature, showing over 0.10″ of precipitation across most of the area, while the rest of the models are drier and further east, with only scattered snow showers in Long Island. I kept a chance of snow showers in the forecast east and south of NYC; at this time, little to no accumulations are expected, although the possibility is there for heavier precipitation out of the low pressure extending into the area further west than currently modeled, closer to what the GFS shows, in which case light accumulations could take place from NYC and further east.

With the weak low pressure off the coast, mostly cloudy skies are expected on Thursday, with high temperatures in the mid to upper 30s across the area, perhaps reaching 40 degrees near the immediate NYC area. More sunshine is expected for Friday with highs reaching the upper 30s inland and the low 40s across the rest of the area.

Weekend – Next Week: Slightly Warmer; Storm Potential Mon-Tues

By next weekend, the upper level low is expected to shift into Canada, with a weakening stale cool air mass left over the region, but with the pattern still not conductive for any sustained warm up in the pattern. A mainly dry and mostly to partly sunny weekend is expected with high temperatures slightly warmer, in the low to mid 40s across most of the area.

Meanwhile, the next storm is expected to develop in the central US, reaching the region by Monday and Tuesday. The latest models are in generally good agreement regarding the overall setup, with a low pressure initially tracking inland with a secondary low further east, either near or off the coast. The specifics are more uncertain, however, with many different scenarios depicted on the models regarding the track, either south or north of the area, and the intensity of the storm, with some models showing a heavy precipitation event while other show only light precipitation. These specifics are unlikely to be resolved for at least the next few days as the models continue to show different scenarios, although the probability of this storm affecting the area is increasing, with the potential for rain and/or snow on Monday and Tuesday. Stay tuned for more information on this storm.

Pattern Overview: Anomalous High Latitude Blocking

As has been mentioned with most of this month’s updates, there is a highly anomalous blocking pattern in place, with significant blocking extending into Greenland and up to the north pole. 500mb heights over Greenland and the pole are approaching 564dm, which as the 500mb geopotential height anomaly map to the left shows from the Pennsylvania State University e-Wall, is over 48dm above average, making this the most significant winter-time block over Greenland and Canada since 2010-11.

As can be expected with this blocking pattern, the Arctic Oscillation index is strongly negative and approaching record low levels, forecast by the GFS ensembles to drop to nearly -6, which can be seen to the left on the AO graph from the Climate Prediction Center (CPC). This blocking will remain a key player in the pattern for the rest of the month, as throughout this week and next weekend the blocking shifts from Greenland to northern and central Canada while an upper level low remains over the region, keeping temperatures colder than average. This pattern looks to last through at least early April, which will keep temperatures mostly colder than average with additional snow chances in the forecast. It should be noted, however, that just because the beginning of spring is cold does not mean the entire spring will end up this way; there have been years in the past where the pattern was initially cold but trended warm later on, such as April 2007 which started out cold but with a warm ending.

With yesterday’s storm included, Central Park so far has had 7.3 inches of snow this month, making it the 2nd snowiest March out of the last 17 years, only behind March 2009, and with a seasonal total of 26.1 inches to this point, which is slightly above average. Should Central Park receive an additional 1 inch of snow this month, it will be the snowiest March since 1996 with 13.2″. Looking at a historical perspective, since record keeping began in 1869, this March is the 30th snowiest on record.

As of March 18, Central Park is 1.4 degree colder than average for the month. With almost every day for the remainder of the month expected to end up colder than average, March will end up at least moderately colder than average, making it the second consecutive month with colder than average temperatures as February was 1.4 degree colder than average. The last time 2 consecutive months were colder than average was during December 2010 and January 2011, over 2 years ago. This month is in a significant contrast compared to last March, which was abnormally warm and dry; the average monthly temperature was 50.9 degrees, nearly 10 degrees warmer than this March and about 8.4 degrees warmer than average; temperatures reached the 70s a total of 8 times, peaking at 78 degrees on 3/22/12, while this March has yet to reach 60 degrees, the first March since 2001 to fail to reach this mark.

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