3/21 All Zones AM Update: Arctic blast midweek, unsettled beyond

Some cloud cover and few scattered showers remain over the region this morning, associated with a very weak cold front and shortwave passing through. Subsidence will follow later this morning (sinking, drier air), with more clearing and sunshine for this afternoon. Temperatures will remain at seasonable levels today with light northwest winds and deep mixing in the atmosphere. High temperatures are anticipated to be in the mid-upper 50s over much of the region today. Some locations could reach around 60 degrees.

However, a stronger cold front associated with an upper-level trough arrives very late tonight and early tomorrow morning. Temperatures early tonight will fall into the upper 30s and lower 40s. Behind the front, temperatures will fall into the 20s and lower 30s by daybreak tomorrow morning. A tight pressure gradient will also lead to blustery conditions with winds gusting  between 30mph and 40mph during the day on Wednesday.

Wednesday night be the coldest night of this arctic blast as the upper-level trough moves over Northeast. By daybreak on Thursday morning, temperatures will be in the teens over the Interior and lower to middle 20s near the coast. High pressure over region will support more clear skies and light winds. Temperatures may slightly warmer by Thursday afternoon, in the upper 30s to lower 40s. Thursday night may be very cold again, especially over the suburbs and Pine Barrens, with mostly clear skies and calm winds leading to more radiational cooling.

GFS model showing low temperatures Wednesday night/Thursday morning

GFS model showing low temperatures Wednesday night/Thursday morning

High pressure will then gradually shift off the Mid-Atlantic coast with a return light southerly flow beginning on Friday. Mid-level heights and temperatures will also be rising, with the trough lifting out of the Northeast. This will help surface temperatures warm up on Friday. Awarm front approaching the region will cause more clouds and possibly some scattered showers, especially during the afternoon and evening hours.

Uncertainty with temperatures and precipitation remains high for this weekend. Some residual Atlantic blocking will result in stronger confluence underneath an upper-level low over Southeast Canada. This will cause surface high pressure to the north to push a backdoor cold front south over the Northeast this weekend. Temperatures will largely depend on the position of this frontal boundary and there might be a large difference in temperatures from north to south over the region.

Meanwhile, a fast and active southern branch of the pacific jet will cause a cut-off low to form over Central Plains and track into the Ohio Valley this weekend. But as this storm system runs into more confluence over the Great Lakes and Northeast, the cut-off low will weaken and a secondary surface low will likely form near Mid-Atlantic coast. How intense and far north the secondary low gets will also determine the northern extent of more significant precipitation later in the weekend into early next week.

Overnight ECMWF model showing more wintry precipitation over the Interior Northeast by Sunday evening

Overnight ECMWF model showing more wintry precipitation over the Interior Northeast by Monday morning

Overall, the synoptic setup will lead to more unsettled weather with more clouds and some precipitation at times, with a strong thermal gradient or baroclinic zone somewhere over the Northeast or Northern Mid-Atlantic. What form this precipitation takes will depend on how far south another cold airmass sinks into our area. We currently favor mainly rain from about I-80, New York City and Long Island southward, due to late March climatology, and the flow still being somewhat progressive with mid-level heights above normal. But chances for more frozen or wintry precipitation will increase with distance further north. Stay tuned for zone forecast updates this week on this storm system!