May 31, 2011: Hot, Stormy Start To Summer

Note: As tonight’s update focuses on tomorrow’s severe weather risk, the 5-Day Forecast page was not updated. An afternoon update will be posted tomorrow, as well as storm updates in the late afternoon if needed.
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After warm temperatures yesterday across the area, a weak back door cold front moved in from the north, but the impact of this front was underestimated. Despite a sea breeze in the afternoon lowering temperatures, high temperatures still reached the lower to upper 80s were still observed in Long Island/southern Connecticut, upper 80s in NYC, and upper 80s to lower 90s in northern NJ and SE NY. Yesterday, all of the models were in agreement showing colder temperatures, with an ESE wind and a back door cold front to the west supporting this, but only this morning the models suddenly changed to show warmer temperatures across the area.

The back door cold front will move east and out of the area tonight as a warm front, with hot temperatures once again tomorrow. With the main cold front moving towards the area, however, thunderstorms will be a risk tomorrow, and it is possible that tomorrow may bring the area’s first significant severe weather outbreak of the year.

Tomorrow’s Outlook:

Mainly cloudy skies along with some fog are expected in the morning across the area. With the warm front moving through, some showers and thunderstorms are very possible in parts of the NE, potentially including parts of the area, and even another MCS similar to what we saw yesterday morning cannot be ruled out, but most of the storms would stay to our north should there be any activity tomorrow morning.

With the warm front to the east of the area, a southwest wind is expected, which combined with sunshine in the late morning into the early afternoon hours, will allow temperatures to warm up into the upper 80s to lower 90s from NYC and further west. Temperatures in Long Island and southern Connecticut will range from the mid 70s to mid 80s. With dew points potentially reaching the lower 70s, heat index will reach the lower to potentially mid 90s from NYC and further west.

There is a clear risk of severe weather tomorrow in the region, but how big the risk ends up is uncertain. Most of the models continue to show impressive parameters, with CAPE up to 4000, LI up to -10, and lapse rates up to 7. Bulk shear up to 35-40 knots is expected, which is enough for at least an enhanced 30% risk of severe weather. The higher bulk shear values are to the north of the area, and I placed the area in the southern end of the 30% risk, which extends into New England.

Most models show a risk of severe weather, but the NAM model is the main outlier, consistently showing scenarios where severe weather fails to take place, ranging from storms to the south of the area on its 18z run to morning storms sticking around the area, preventing any formation of storms in New England in its 00z run. Interestingly, it shows very little activity in New England, where the higher risk of severe weather is expected to end up. Even the SREF ensemble mean shows a more supportive set up for severe weather. At this time, I am using the other models, but morning storms in the Northeast and potentially in the area cannot be ruled out, which would limit instability and temperature rise, therefore reducing the severe weather risk.
Thunderstorms are expected to begin forming in the central Northeast around 1-4 PM tomorrow and move towards the east southeast, reaching the area by the late afternoon. Most models place the best risk of precipitation from northern NJ into New England, where I placed a 30% risk, but some thunderstorms are expected south of there as well. Strong wind gusts are the main risk with tomorrow’s storms with small-moderate hail also possible, and the risk of a tornado, especially into New England, cannot be ruled out. It is possible that tomorrow ends up drier than expected, with less widespread thunderstorms, which may also reduce the risk of severe weather. For now, I am sticking with a 30% risk area, but should the main model guidance verify, I may upgrade the risk to an enhanced 30% risk. Stay tuned for an update on the storms tomorrow afternoon, along with storm updates in the late afternoon.

Longer Range: Tomorrow night’s update will focus more on the longer range, discussing the cool down for Thursday and Friday bringing high temperatures into the mid 70s to lower 80s across the area, followed by slightly warmer temperatures for the weekend which may come along with a rain chance before cooling down again early next week, as we enter a pattern where the biggest heat remains to the south and west of the area with more seasonable temperatures in the area.

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