Weekly Overview: Transitional weather, interior winter storm possible late week

It was dreary today with lots of cloud cover and spotty light rain and drizzle in some areas. High pressure is now moving out into the Atlantic, while low pressure from the Midwest tracks into Southeast Canada.  This will allow for a warm front to lift further northward through the Northeast states this evening, with winds turning more southwest overnight. Cloud cover will remain in place most of the night with some light rain or drizzle still possible.

Temperatures will gradually rise through the 40s into the 50s later tonight and Tuesday, especially closer to the coast.  These temperatures will be mild for late December. A cold front moves through the Northeast states with more showers possible Tuesday morning and early afternoon.  A mid-level shortwave and upper-level jet streak drifting through may aid in more lift, but moisture appears somewhat limited with this front based on this evenings guidance. So heavy rain is not anticipated.

Skies will clear later Tuesday afternoon, as high pressure begins building in behind this mentioned cold front. Temperatures will fall back into the 40s tomorrow afternoon, with a colder airmass beginning to move into the region. Another mid-level shortwave will move across the region Tuesday night. Despite how vigorous this shortwave appears, models are keeping most areas dry with limited low-level moisture and a subsidence inversion between 700-850mb that puts a cap on more showers developing. Behind this, more subsidence (sinking air) will result in mostly sunny skies on Wednesday. A colder air mass will be moving in, with northwest downslope winds. High temperatures should reach the middle 40s which is a few degrees above normal.

18znamwed

NAM model showing strong shortwave energy moving through region on Wednesday

Our interest turns towards the next storm system that will likely impact the Northeast and Mid-Atlantic region on Thursday. Model guidance suggests a low pressure will track north of the Great Lakes, with a frontal boundary moving through parts of the Northeast and Mid Atlantic on Thursday. This is associated with an enlarging upper-level trough digging and amplifying over the Ohio Valley and Mid-Atlantic region. A 500mb shortwave energy with PVA (positive vorticity advection) rounding the base of trough results in enough rising motion for a secondary low to from along the frontal boundary near the Northern Mid-Atlantic or Southern New England coast. The airmass prior to this system is cold enough that some snow or wintry mix for the Interior locations is possible on Thursday, especially if the secondary low deepens and takes a track far enough south.

However, there are some substantial model differences between the most reliable models, the GFS and ECMWF, on the strength of this secondary low —  that would result in either more or less frozen precipitation.  The ECMWF model has been most bullish on a secondary low bombing out and strengthening near New England, producing more significant snowfall over parts of Interior New England and Eastern NY. Other models such as the GFS show somewhat weaker and more progressive low pressure with lighter precipitation. Considering the lack of more high-latitude blocking and a strong West Coast ridge, we are a little skeptical of storm being as intense as the ECMWF suggests it will be. But recent trends on much of the model guidance suggest careful monitoring of the system.

The latest 18z NAM showing strong 500mb shortwave and PVA along with strong 300mb jet streak moving trough causing the secondary low rapidly intensify just south of Long Island.

The latest NAM model showing strong 500mb shortwave and PVA along with strong 300mb jet streak moving trough causing the secondary low rapidly intensify just south of Long Island.

For the major cities, along the I-95 corridor and coastal plain, a primary low tracking over the Great Lakes and high pressure shifting east into Canadian Maritimes will probably result in enough warm-air surging in the lower and middle levels of atmosphere for predominately rain from this system. Some wrap around snow showers are possible towards the end of the storm as colder air rushes in behind secondary low which will be deepening over New England. But we don’t anticipate any accumulating snow for the I-95 corridor or coastal plain.

This storm will finally exit the region on Friday. A colder, drier airmass will be settling in over Northeast with temperatures closer to normal. It will be blustery Friday, with daytime highs likely upper 30s or around 40.  Overnight lows could be in the 20s over much of the region Friday night. Over the weekend, another low associated with an upper-level trough moving over the St. Lawrence River Valley will drag a frontal boundary through the Northeast US — with showers anticipated at some point during the weekend over Northeast. There is some uncertainty with regards to timing and amount of rainfall, but it doesn’t look a washout at this time. A southerly flow ahead of this front may cause temperatures to moderate by Saturday night or Sunday, so it may be just a chilly with temperatures in the 30s if you’re heading home from New Year’s Eve celebrations Saturday night.  Stay tuned for more updates this week on Thursday’s storm and the weather for New Year’s Weekend.

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