April 28, 2013: Clouds, Showers Briefly Return Mon-Tues

Forecast Highlights:

A noticeably slower pattern takes hold over the next week, setting up for a more complex forecast with several interesting features in the pattern, including a persistent east/NE flow that keeps the area initially cloudy and chilly with some showers, a blocking pattern later in the week where temperatures remain seasonable near NYC with the warmth displaced well to the north over Canada, and a late week cold front that stalls in the central US where another late season snowstorm is possible.



Forecast Overview: More Complex Pattern Ahead

Last weekend, a strong 1040mb high pressure moved through the region, slowly extending towards Atlantic Canada and the North Atlantic Ocean throughout the week. Unlike previous high pressures that went through the region earlier in the month, when the pattern was for somewhat mild weeks with a stormier ending and a cooler weekend, this one stalled south of Greenland in a region dominated by lower pressures for the majority of the month, which ended up blocking the flow with a strong cutoff low present over the central Atlantic ocean over the course of the last week. This cutoff low will shift west throughout this week, merging with another weak cutoff low and stalling south of Atlantic Canada through this week and next weekend while a broad high pressure stretches from the northern Atlantic ocean to the Northeast US, once again blocking the flow for the next 7-10 days with some interesting features in the weather pattern expected as a result.

Monday – Wednesday: Cloudy, Chilly, Some Showers

Originally, the outlook for this week was for the high pressure to stretch far south enough to block a weak Ohio Valley low pressure from tracking into the region and producing showers, although the high pressure is no longer expected to expand this far south, allowing for the associated showers to slowly expand northeast, reaching the area on Monday with mainly cloudy skies, cooler temperatures and an east flow expected. With the high pressure to the north and east, the showers will be prevented from advancing further northeast and will be forced south later on Tuesday into early Wednesday as the high pressure slides back south.

The models differ regarding how much rain falls and how long the rain lasts, with the GFS only showing scattered showers, the 12z ECM shows up to 1/2 inch in some areas, with the NAM showing over 1 inch near NYC. The NAM’s wet solution is considered as an outlier taking its typical wet bias into consideration. Otherwise, scattered showers are expected with amounts generally ranging from 1/4 to 1/2 inch from NYC and further west, locally higher towards western NJ. The showers will struggle to advance east of NYC where less than 1/4″ is expected, with parts of LI and CT potentially staying entirely dry. The showers are generally expected to fall on Monday and Tuesday, possibly into Wednesday morning.

Regarding temperatures, the cloud cover, showers and east flow complicate the forecast more than originally thought; the GFS, which keeps the area in the north end of the showers, has highs mostly in the upper 50s to mid 60s, while the ECM and NAM keep highs in the mid 50s, with some spots failing to break out of the low 50s, which by this time of the year is nearly 15 degrees below the average high. I sided at this time with a compromise between the two, with highs in the mid 50s to near 60 across most of the area for Mon/Tue and low to mid 60s in southern CT, with low-mid 60s north of NYC on Wednesday and mid 50s-low 60s to the south, including Long Island. The temperature outlook is subject to minor changes, with more information to be posted over the next few days. For more localized forecasts, refer to the 5-day forecast page.

Thursday – Next Weekend: Drier, Slightly Warmer

Several developments in the pattern are expected to occur during the mid-late week. A cold front will move through east central Canada, cutting off the ridging aloft. Around the same time, the majority of the models show a mid level low becoming cut off near Greenland and drifting SW all the way to Cape Cod’s latitude, or on some models even as far south as North Carolina’s latitude while remaining off the coast. Between these two features, blocking aloft will become stalled over the region with the surface high pressure persisting while suppressing the early week rain to the south, resulting in a sunnier and drier forecast for the late Wednesday-Friday time frame. A cold front will slowly approach from the central US, but with blocking over the region, a cutoff low will develop over the central US and the Midwest, where another late season snowstorm is possible around the same locations that have had an abnormally snowy April. The blocking will delay the storm, extending dry conditions through at least Saturday or Sunday.

An east flow will continue on Thursday, with partly cloudy skies expected and highs peaking in the mid to upper 60s north and west of NYC, including southern CT, with upper 50s to mid 60s in Long Island. Similar conditions are likely to continue on Friday, approaching 70 degrees north of NYC. Due to the placement of the high pressure and ridging aloft, however, the warmest temperatures will remain well north of the area; widespread 70s are expected across northern New England into south/eastern Canada, with the possibility of 80 degrees in Canada north of New England.

By next weekend, uncertainty increases regarding exactly where the Ohio Valley low becomes cut off and how far south the high pressure shifts; some models such as the DGEX keep the area chilly with highs in the 50s to low 60s, while the GFS and ECM, in a scenario somewhat similar to last year’s March significant warm spell, shift the warm south, warming the area into the mid 60s to low 70s with the warmest temperatures in New England as the central US cutoff low slowly drifts east. At this time, I am expecting mostly sunny skies with highs in the mid 60s to near 70 degrees on Saturday away from the coast, slightly cooling down on Sunday as the cutoff low approaches with rain likely holding off until at least late Sunday or early next week, although this part of the outlook is still subject to change. Stay tuned for more information on the late week and next weekend’s outlook.

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