Nov 22, 2011: Heavy Rain Tonight, 2+ Inches

As a relatively weak yet very moist low pressure developed in the southern US and moved towards the Ohio Valley, today brought cloudy skies across the area with occasional showers. The storm’s heavier rains spread into the area this afternoon, and places from NYC and further west are currently observing moderate to locally heavy rain. With the cloud cover and rain, today brought colder temperatures across the area, which ended up colder than expected in NW NJ and interior SE NY, where highs only peaked in the mid to upper 30s, although precipitation type was rain due to above freezing temperatures aloft. Temperatures in the immediate NYC area ranged from the lower to mid 40s in the northernmost suburbs to the upper 40s to lower 50s in NYC, and Long Island/S CT saw temperatures in the upper 40s to lower 50s as well.

The low pressure will continue to move east, bringing as much as 2 to 3 inches of rain in parts of the area, which will result in some flooding as well as placing 2011 as the second wettest year on record for NYC. A strong ridge and high pressure will bring in sunshine just in time for Thanksgiving, and although chilly temperatures are still expected, temperatures will warm up by the weekend, with temperatures passing the 60 degree mark once again. Although this will not be a brief warm spell, the warmth is expected to end around Tuesday of next week as another storm approaches, bringing another risk of heavy rain to the region.

Tonight and Tomorrow: Heavy Rain

Storm Set-Up: The low pressure is currently located near central Ohio. The storm has been moving to the northwest throughout the day, but as it approaches a high pressure to its north, the low pressure will take more of an eastern track, taking it through central Pennsylvania and central New Jersey tonight and tomorrow. The storm originated in the southern US, pushing the warmth along with it further north, and as the storm moves into the Northeast US, it will run into the cold air, producing an axis of heavy snow from the Adirondacks in northern New York to southern/central Maine. The high pressure to the north will keep the cold air stuck in place, resulting in cold temperatures throughout the storm as far south as Sussex county in NJ and Orange county in NY, where temperatures will stay in the upper 30s to lower 40s until tomorrow morning before slightly rising. There is no strong cold air mass associated with this storm, however, and the storm will only pull in a weak and very short lasting trough into the region as it moves out before a more significant ridge builds in, which will be discussed in more details in the next section.

Scenario Forecast: This storm is associated with a lot of moisture, as can be seen with the latest radar posted above showing widespread moderate rain covering the region. As the overnight hours continue, the steady moderate to heavy rain, mixing with snow in northern New England, will spread further northeast, while the NYC area will be in the southern edge of the heavier rain. With more instability further south under the warm sector from eastern Virginia into central NJ, some thunder is possible overnight near NYC, but is not expected north of NYC. To the left, I posted a graphic representing the potential future radar towards 2 AM, when the low pressure will be near the PA/OH border, showing the heavy rain from NYC into central New England, thunderstorms in the southern Mid Atlantic, and developing heavy snow in New England. This map does not account for areas of sleet and freezing rain that will fall between the rain and snow zones in central New England and near central NY.

The heavy rain is expected to continue through about 8 AM on Wednesday morning, when the intensity of the rain will weaken. Occasional showers will continue through the afternoon hours, with cloudy skies and an isolated shower possible for the evening hours. Temperatures overnight will rise across the entire area, peaking in the morning hours in the mid to upper 40s inland, lower to mid 50s in the rest of SE NY excluding Orange county, mid 50s to lower 60s in NE NJ and NYC, lower to mid 50s in southern CT, and upper 50s in Long Island. As the low pressure slides just to the south of NYC during the early afternoon hours, temperatures will drop into the mid 40s to lower 50s across the area by the afternoon hours.

Total Rain Forecast: By the time that the storm ends, at least 1.5 to 2.5 inches of rain are expected in Long Island/S CT with the highest amounts towards southern CT, and between 2 and 3 inches of rain from NYC and further west. These amounts will be capable of producing flooding in some areas, especially west of NYC. In addition, with at least 2-3 inches of rain expected in NYC, this storm will make 2011 the second wettest year on record for Central Park, with nearly 70 inches of rain since January. The wettest year on record was in 1983 with 80.56″; although 10 inches of precipitation are not expected to fall in December to surpass this record, especially if a wet pattern sets up for December, this year could get very close to the record.

Thanksgiving Weekend: Sunny, Dry, Warmer

With the low pressure moving out of the region, skies will clear just in time for Thanksgiving. A high pressure will move into the region on Thursday, at the same time that a weak and very short lasting trough moves into the region. This will result in mostly sunny skies and chilly temperatures, peaking in the upper 40s to lower 50s inland and the lower to mid 50s across the rest of the area. Cold temperatures are expected for Thursday morning, dropping into the mid 20s to lower 30s across northern NJ, SE NY and southern CT, lower to mid 30s in Long Island, and mid 30s in NYC.

With the high pressure slowly moving through the region, a much stronger ridge will build into the eastern half of the US, providing the region with southwest winds and warming temperatures. As a result, sunny skies will continue through Friday and mostly sunny skies on Saturday with warming temperatures, reaching the upper 50s from NYC and further west and the mid to upper 50s east of NYC on Friday. Well above average temperatures are expected for Saturday, reaching the upper 50s to lower 60s across most of the area, and temperatures may get near the 65 degree mark in parts of NE NJ.

Sunday will bring mostly cloudy skies as a low pressure system approaches from the west, with similar high temperatures, in the upper 50s to lower 60s across the area. Although no rain is expected across the area on Sunday, this low pressure will begin what may be a crazier week of weather across the eastern half of the US.

Next Week: Stormy Beginning, Cold End?

The graphic above showing the warm spell for the weekend shows that a low pressure will be moving through Wisconson with its cold front moving east as well. As this system approaches the eastern US, the trough will become neutrally and then negatively tilted as it begins to cut off from the main flow. Exactly where this storm cuts off is still uncertain and will result in timing uncertainty for the rain next week, but at this time, it is likely that the upper level low cuts off over the southeastern US, which would keep mild temperatures in place across the area through Monday while the Southeast sees a cold rain, potentially falling as snow in the higher elevations. The cutoff low is not expected to stick for a long period of time in the Southeast, and once it starts to drift north back towards the main flow, moderate to heavy rain will move into the region. The timing could range any time from Monday afternoon to Wednesday afternoon, but at this time, I am going along with the model consensus supporting Monday night through Tuesday for the rain to affect the region. There is still uncertainty with the timing, but the scenario does support the potential for heavy rain to affect parts of the region regardless of the timing. Stay tuned for more information on the outlook for early-mid next week.

For the longer range, after this storm, some models such as the long range GFS attempt to set up a significantly colder pattern, with no ridge in the southeast. Looking at the overall pattern, there are some changes that will allow more cold air into the central and eastern US for the very end of November into the first week of December, including transient ridging near the Davis Strait west of Greenland, the consistently positive EPO dropping towards neutral, hints of ridging near the west coast, and the displacement of the strong polar vortex near Alaska. These factors will likely play a role in a somewhat colder start to December, but none of these changes appear to be permanent yet as the Pacific pattern, unfavorable for sustained cold, fails to flip to a pattern favorable for East cold, with no sustained west based -NAO block setting up. With the factors above considered, as well as the MJO which continues to move through unfavorable phases for cold in the East, a somewhat colder start to December is expected across the region with average or slightly below average instead of above average temperatures possible, including the second half of next week, although it is likely that the mostly mild pattern with the occasional transient cold spells continues beyond early December through the middle of the month, potentially even longer than that. Even though the more sustained cold and snow will eventually get to the East, it’s going to take time for such a pattern to unfold, and it may not be until late December or even January that any sustained winter conditions affect the East, although transient periods of cold and some snow will take place until then. A more detailed write up on the longer range will be included with the winter outlook, which will be posted by the end of the month.

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