2:15 PM (Thursday) | ***Torrential downpours, strong-to-severe thunderstorms, 50-60 mph wind gusts and isolated tornadoes tonight as powerful "pattern-changing" cold front arrives***
Paul Dorian
One of the ingredients that can lead to damaging surface-level wind gusts of 50-60 mph is a very strong upper-level jet streak as depicted here by the 06Z GFS for 2AM early Friday morning; map courtesy NOAA, tropicaltidbits.com
Overview
All the ingredients are coming together for a wild weather event tonight in the Mid-Atlantic region and northeastern US. Vigorous energy in the upper part of the atmosphere associated with a powerful jet streak will combine with a low-level jet, very warm and humid air, and an intensifying surface low pressure system to generate a squall line along a slow-moving cold frontal boundary zone. This squall line should arrive in the Mid-Atlantic’s I-95 corridor region of DC-to-Philly-to-NYC between 8 and 11 PM with torrential downpours, strong-to-severe thunderstorms, 50-60 mph wind gusts and perhaps isolated tornadoes.
Much colder air will arrive late tonight riding in on strong NW winds and a secondary push of colder air will arrive on Sunday leading to the first widespread freeze from late Sunday night into early Monday morning in the I-95 corridor. In fact, the overall weather pattern will tend to change dramatically with tonight’s powerful cold frontal passage with colder-than-normal conditions likely in the Mid-Atlantic/NE US for much of the month of November following a warmer-than-normal October.
One of the ingredients that can lead to damaging surface-level wind gusts and even isolated tornadoes is a powerful low-level jet streak as depicted here by the 06Z GFS for 8PM later this evening; map courtesy NOAA, tropicaltidbits.com
Details
Deep upper-level low pressure will push into the eastern Great Lakes this evening and take on a “negatively-tilted” orientation to its trough axis (i.e., oriented NW-to-SE). This will allow for strong upward motion in the Mid-Atlantic region and northeastern US which will aid in the development of a north-to-south oriented long squall line that will extend all the way from central NY-to-central PA-western Virginia early tonight. Giving additional support to the formation of strong-to-severe thunderstorms will be very strong winds in the upper atmosphere (e.g., 250 millibars or 35,000 feet) and in the lower atmosphere (e.g., 850 millibars or ~ 5000 feet) as well. The turbulent downdrafts produced in the squall line thunderstorms will help to “mix” some of these strong winds from aloft to ground level which could result in 50-60 mph wind gusts. In addition, while the primary threat from these thunderstorms will be of “straight-line” winds, given the very strong low-level jet, there could be isolated tornadoes as well within the squall line region.
NOAA’s Storm Prediction Center has much of the Mid-Atlantic region in an “enhanced” risk for severe weather through tonight; map courtesy NOAA
Given the fact that there are still many leaves remaining on the trees as the month of October winds down, these kind of wind gusts can lead to power outages in the Mid-Atlantic region and NE US - both before and after the cold frontal passage. Ahead of the front, winds will be quite strong from a S-SW direction and then they’ll shift to W-NW following the passage of the surface cold front and continue at strong levels into Friday morning. The timetable for the arrival of the squall line in the DC-to-Philly-to-NYC corridor is roughly 8-11 PM as depicted (below) by some high-resolution computer forecast models (e.g., NAM (3-km)).
High-resolution models such as the 3-km version of the NAM depict a squall line over or just to the west of Route I-95 by late in the evening; map courtesy NOAA, tropicaltidbits.com
It’ll turn much cooler late tonight with temperatures dropping well down into the 40’s in most spots and highs on Friday and Saturday will be confined to the 50’s in the Mid-Atlantic region as compared to the 70’s today. Excellent radiational cooling conditions will take place tomorrow night (i.e., clear skies, diminishing winds) and low temperatures by early Saturday morning could be near the freezing mark in many suburban locations along the I-95 corridor for the first time this season. An even cooler air mass will arrive on Sunday and this will likely lead to a widespread freezing event late Sunday night/early Monday morning as, once again, there will be excellent radiational cooling conditions as high pressure moves directly overhead in the overnight hours.
A vigorous wave of energy in the upper atmosphere over the eastern Great Lakes this evening will aid in the formation of an intensifying surface low pressure system and the squall line that is expected to form later today along a surface cold frontal boundary; map courtesy NOAA, tropicaltidibits.com
One final note, this is the weekend in which we turn back the clocks by an hour.
Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com
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