8:16 AM

The day before Thanksgiving is always a very busy travel day throughout the United States and especially for the New York City and Philadelphia metropolitan areas.  This year will be no different and the weather conditions over the next few days will have a major roll in traveling conditions.  Throughout this forecast, I leaned heavily on the NAM, ECMWF, and Canadian guidance.  The GFS this morning is too over phased in my opinion and does not have a good handle on the current conditions.  Therefore I stayed away from that guidance and went with a blend of the other three.

Water Vapor-NOAA

This morning a ridge of high pressure is nosing down through the coastal plain, providing a cool, maritime air mass for the forecast area.  Winds are from the east ahead of an area of low pressure over the Great Lakes.  Meanwhile, a deep trough continues to dig over the Mississippi Valley, drawing moisture northward along the East coast.  The combination of these features will make for a cool and cloudy day for much of the northern Mid Atlantic along with a risk of a few isolated showers.

Cloudy conditions will continue through tonight and into tomorrow morning, however much of the region will remain dry with temperatures generally near normal.

Note the convection over Florida  and the eastern Gulf of Mexico.  That convection is due to a disturbance in the Sub Tropical Jet Stream that will interact with the Polar jet stream now diving into the Mississippi Valley.  Through tomorrow afternoon, a surface low pressure will develop off the North Carolina coast.  As the 500 MB trough intensifies and a closed low forms over the Tennessee River Valley, the coastal low will strengthen with increasing clouds over the northern Mid Atlantic through the day and into the early evening hours.

On Thanskgiving night into Friday morning, light to moderate rain will move into the New Jersey coast and the New York City metropolitan area including Long Island.  As the low intensifies, showers will break out throughout the Philadelphia metropolitan area as well.  Winds will also steadily increase from the northeast around 5 to 15 mph over the interior and 10 to 20 mph along the coast.

On Friday, the surface low will stall and meander around the New England coastal waters with a trough hanging back through the New Jersey coastal waters.  The 500 MB low will be right over New Jersey and eastern Pennsylvania through Friday, keeping the atmosphere highly unstable and thus supporting the threat of light to moderate precipitation with some heavier downpours at times through the day for the entire New York City and Philadelphia metropolitan areas.  Temperatures above 900 MB will be at or slightly below freezing and any strong lifting from passing mid level disturbances will support the mixing over or change over to snow and sleet at times.  However, I stress that no accumulation is expected for 99% of the area with the exception of locations above 1500 feet where some dusting or an inch of snow or sleet is possible in a passing snow shower.  Otherwise, the rain/snow mix will look nice but won’t amount to much.

I do want to address one out side risk here for northeastern Pennsylvania and northwestern New Jersey.  Temperatures for locations above 1200 feet will support the accumulation of snow and/or sleet in these locations and the lifting in these locations is forecasted to be rather strong.  While, overall I don’t expect much of a threat, I am going to keep an eye out for any mesoscale banding of precipitation in these area as accumulation can happen pretty quickly in higher elevation set ups.

High pressure will take hold for Saturday, Sunday, and Monday.  Temperatures will average near to slightly below normal for the weekend with highs struggling in the 40′s and lower in the 30′s.  However, as a strong cold front organizes over the Mississippi Valley with an impressive cold air mass for the Plains, a surge of warm air will allow temperatures to rebound significantly into the 50′s throughout the region.  Conditions will remain dry through the next three days.

A strong cold front will move through on Tuesday with wide spread rainfall for the region.  High temperatures will occur early in the day followed by temperatures crashing through the 40′s and 30′s in the afternoon.  I would not be surprised to see snow mixing in with the rain as the cold front passes and temperatures fall on Tuesday afternoon, especially over interior locations.

Extended Thoughts:

I really like the way the ECMWF is currently handling the pattern for early December.  There is a lot of teleconnections pointing to a negative EPO pattern developing, which is exactly what the ECMWF is illustrating for the start of December.  Now, the brunt of the cold air should be focused over the Plains, however that does not mean the Mid Atlantic remains warm.  The edge of the Polar air mass should build into the region, and that means one thing.  Storms!  I want to stress that the immediate coast is not in a favorable period, climatology speaking, for snowfall or even frozen precipitation.  The coastal waters are simply too warm and will have too strong of an influence to not have an impact on boundary layer conditions.  However, that doesn’t mean there won’t be areas in the northern Mid Atlantic that won’t get some frozen precipitation for the first few days of December.  I don’t want to go into details yet.  However, the point I want everyone to hone in on is this.  Being on the edge of the Polar air mass means that the thermal gradient is going to be very tight along the coast.  The Mid Atlantic will be on the divergent side of the trough and will be in a place highly favorable for storm development.  The start of December will not be a quiet one and surely will not be warm.

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