May 13, 2011: Week Of Rain Coming Up

NOTE: This post was updated on Saturday morning, 5/14, but there are no changes from yesterday’s outlook.

The 5-Day Forecast has been updated for the immediate NYC area only at this time.

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After a long week of mostly to partly sunny skies and high temperatures in the 60s and 70s across most of the area, today finally brought an increase in cloud cover as the high pressure and offshore storm keeping the region dry began to leave the Eastern US. Due to the storm and high pressure, a storm in the central US was unable to advance much further east, but with these obstacles out of the way, the storm is going to slowly make its advance into the region before stalling as well, bringing consistent clouds, showers and thunderstorms that will continue through almost all of the upcoming week.

Tomorrow’s Outlook:

As the storm continues to slowly move towards the Northeast, cloudy skies are expected tomorrow again, though tomorrow will be the last dry day until at least next weekend. An isolated shower is possible, though any shower will be brief with most of the shower activity staying well west of NYC. Temperatures, however, will be colder than they have been this week, reaching the mid to upper 60s from NYC and further west, and the upper 50s to mid 60s for Long Island and southern Connecticut. A few lower 70s are expected near NYC and further west.

Sunday – Monday: Rain, Some Storms, Mild

Sunday: The low pressure in the Ohio Valley will slowly move into Pennsylvania on Sunday, which will bring warmer temperatures into the area as well as a risk of thunderstorms. Mainly cloudy skies are expected with high temperatures in the upper 60s to lower 70s from NYC and further west and the upper 50s and 60s east of NYC. Showers and thunderstorms are expected, especially towards the evening and early overnight hours, however it will not rain during the entire day, as some breaks are expected in between the rain. Some convection is expected, and I mentioned a risk of thunderstorms in the 5-Day Forecast, but with a lack of supportive parameters, no severe thunderstorms will take place near the area.

Monday: Monday will bring a similar scenario to Sunday, but with less of a shower/thunderstorm risk as the low pressure pushes itself south of the area. There is some slight uncertainty with temperatures, though it is a possibility that places from NYC and further west once again reach the lower to mid 70s as the GGEM and GFS models have been suggesting. These models, however, have been lowering their forecast temperatures with every run, and while chilly temperature are not expected, I went with slightly lower temperatures, expecting upper 60s to lower 70s once again.

Tuesday And Wednesday: Storm Brings Heavy Rain, Wind Potential

Tuesday: With the low pressure drifting south, ending up near North Carolina, occasional showers and some thunderstorms will continue. An ESE wind is expected, with high temperatures colder than Sunday and Monday, reaching the mid to upper 60s from NYC and further west and the upper 50s to mid 60s east of NYC. It is possible that temperatures may end up warmer if there is less rain than expected, or slightly colder if there is more rain than expected.

Tuesday Night – Wednesday: During Tuesday, a high pressure is expected to stick around Nova Scotia and northern Maine, staying there through Wednesday. With this high pressure blocking the low pressure in the Carolinas from moving north, it will begin to move NW, likely moving through North Carolina and SW Virginia. With a relatively tight pressure gradient in place, a breezy ESE wind is expected, and the storm is expected to pull in a lot of moisture, resulting in the potential for heavy rainfall in parts of the region.

The location where the heaviest rainfall will take place is still slightly uncertain, with yesterday’s models targeting the NYC/NJ area for some of the heavier rainfall amounts. The GFS is the driest model at this time, but even then, it still shows a general 1.5 to 3 inches of rain in the area. The other models have been showing at least 2 to 4 inches of rain, and I am siding with the wetter models at this time, expecting a general 2 to 4 inches in the places that see the heavy rain, locally as high as 5 inches.

There has been a trend recently to shift the high pressure and the storm slightly SW, with the GFS yesterday taking it through VA/DC and bringing heavy rain into the area, with its 06z run shifting south and bringing the heavier rain further south as well. The 00z GGEM also shifted south with its latest run, keeping most of the rain south of NYC. Despite this trend, at this time, I am keeping the potential for heavy rain in the forecast, as the area is still expected to be in the moderate to heavy rain zone, with the potential for 1 to as much as 3-4 inches of rain across most of the area which may result in some flooding. The probability for a moderate to heavy rain event is relatively high at this time, but as the area is currently expected to be close to the northern edge of the heavy rain zone, any south trend could reduce the expected rainfall amounts. More information will be posted on this with the next full update.

Late Week And Beyond:

Beyond Wednesday, the models handle the storm differently, however it is likely that scattered showers and potentially thunderstorms will continue during Thursday and Friday. Temperatures will warm up once again, likely reaching the lower 70s from NYC and further west, with breaks in the cloud cover expected as well. As this is in the longer range, there is still uncertainty and details may change, though according to some models such as the GFS, there is a risk of scattered showers through next weekend as well. Regardless of the exact scenario, much warmer temperatures will not be quick to return into the area, and it may not be until the last full week of May that NYC reaches the 80s for the first time this month.

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