May 18, 2013: Cloudy, Stormy Week Ahead

Forecast Highlights:

An unexpected area of light-moderate rain affected most of the area this afternoon, with cooler than expected temperatures observed as a result. This is only the beginning of what will be a more active week, with partly to mostly cloudy skies and scattered showers and thunderstorms expected every day from Sunday through Friday, possibly severe towards the mid-late week, followed by potential clearing in time for Memorial Day Weekend.



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Tonight – Monday: Slowly Warming Up

A warm front will continue to slowly move northeast through the region, approaching the area, with mainly cloudy skies expected to continue through tonight and Sunday. Scattered showers are expected to develop again and affect the area at times through Sunday and Sunday night, with rainfall amounts generally under 1/4 inch. Due to the cloud cover, rain and onshore SE wind at 5-10 mph, temperatures will remain chilly, peaking in the low 60s from NYC and further east and the low to mid 60s in northern NJ and SE NY, with some locations especially east of NYC struggling to reach 60 degrees for a high. By Monday, the area will enter the warm sector, with mostly cloudy skies with light SW winds and highs peaking in the mid 70s from NYC and further north/west and the upper 60s to low 70s in Long Island and southern CT. Scattered thunderstorms are expected again, mainly north/west of NYC which are expected to remain non-severe.

Tuesday – Thursday: Frontal Boundary Nearby

Expected Set-Up: The general mid-upper level pattern in the first half of next week will consist of a nearly stationary mid-level low over the Midwest, with the surface low resulting in a risk for widespread severe weather in the central US region, and troughing in southeastern Canada extending into New England. Between these two features, ridging is expected in the eastern half of the US, with a WNW flow aloft over the Northeast US region. At the surface, a frontal boundary will extend from the Midwest low pressure into the region, with widespread thunderstorms likely along this boundary. The main areas of uncertainty in the outlook depend on the interaction of the two troughs aloft, as well as the timing and southern extent of the frontal boundary.

The frontal boundary will initially be north of the area on Tuesday, gradually dropping south, with a warm and unstable air mass covering the region as highs warm into the upper 70s and 80s away from the coast. With instability in place and somewhat favorable lapse rates and shear, the potential is there for thunderstorms, possibly strong or severe, near and north of the area as the boundary approaches, especially after Tuesday afternoon/evening. From there, the models diverge with the frontal boundary, ranging from earlier runs of the GFS which keep the boundary to the north with highs in the 80s, to the ECM which only cools eastern parts of the area, to the CMC which keeps the boundary nearby with highs in the 60s on Weds-Fri.

The GFS has displayed a bias of keeping the frontal boundary too far north in similar situations this spring, and I am siding towards the frontal boundary moving through most of the area on Tuesday night, ending up nearby on Wednesday with locations north-east of NYC cooling down into at least the mid 60s-low 70s for highs while locations west-south of NYC slightly cool down but remain in the 70s for highs. The boundary is then likely to shift back north on Thursday as a warm front, with the northern extent determining how warm temperatures end up and any risk of strong or severe thunderstorms with the next frontal passage likely towards the late week.

General Outlook: A brief surge of warmth is expected on Tuesday as a frontal boundary gradually shifts south, with highs likely warming into the upper 70s to low 80s in northern NJ and SE NY, mid to upper 70s in southern CT and the low to mid 70s in Long Island and coastal CT with partly sunny skies and a SW wind at 5-15 mph. The frontal boundary is expected to reach the area later overnight, with scattered thunderstorms, possibly strong or severe, likely especially after Tuesday afternoon/evening. There is some uncertainty regarding the placement of the frontal boundary on Wednesday and Thursday, although at this time the boundary is likely to reach the area, with highs on Wednesday cooling down into at least the mid 60s to low 70s east of NYC and the low to mid 70s from NYC and further west with continued scattered showers, before slightly warming up on Thursday with thunderstorms possible again. As previously mentioned, there is still some uncertainty regarding the timing of the frontal boundary, with the outlook subject to change over the next few days.

Memorial Day Weekend Preview: Cooler, Drier

The outlook for Memorial Day Weekend depends on the aforementioned frontal boundary, which at this time is likely to clear the area on Friday but with the timing still subject to change around, ranging from Thursday night to Saturday, although the model guidance is generally in agreement that a trough drops into the region by next weekend, which if verifies would result in drier conditions and more sunshine for the weekend into Memorial Day along with cooler than average temperatures. More information will be posted on the Memorial Day Weekend outlook as details become clearer.

2 thoughts on “May 18, 2013: Cloudy, Stormy Week Ahead

  1. Anonymous Reply

    Do you expect severe weather and flood potential for Tues-Thursday?

    • NYC Weather Reply

      Scattered storms are possible for Tues-Weds, although at this time the best risk for severe weather and flooding is likely to be west of the area. The highest probability of storms is on Thursday with the cold front, when there may be a risk of severe weather.

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