TL;DR

The Storm Prediction Center has issued a Marginal Risk for the coastal Mid-Atlantic today, covering parts of eastern Pennsylvania, New Jersey, Maryland, and Delaware. Thunderstorms developing this afternoon could produce isolated severe hail, damaging winds, and perhaps a tornado. Activity is expected between 11 a.m. and 1 p.m. EST, then move offshore by evening.

The severe weather focus shifts to the East Coast today as a weak surface low tracks across the Mid-Atlantic states. While yesterday brought 15 hail reports and 6 wind reports to the Midwest and Ohio Valley—including quarter-sized hail in Indiana and roof damage near Riley—today's threat zone moves to Philadelphia, the New Jersey coast, and the Delmarva Peninsula.

Where is the highest risk today?

The NOAA Storm Prediction Center's Day 1 outlook, issued at 06:48 a.m. CST Thursday, places a Marginal Risk across the coastal Mid-Atlantic. The risk area includes:

  • Eastern Pennsylvania
  • New Jersey (especially central and southern counties)
  • Maryland (eastern shore and central counties)
  • Delaware
  • Coastal Virginia (northern counties)

The northern extent of the severe risk will likely remain constrained by a warm front, meaning Long Island and southern New England should stay below severe thresholds. The most appreciable severe risk will tend to remain offshore and south of those areas.

What's driving today's storms?

A mid-level shortwave trough currently over the Ohio Valley is moving eastward across the Mid-Atlantic today. According to the SPC, a related weak surface low is expected to develop eastward across Maryland, Delaware, and southern New Jersey this afternoon.

An effective warm front will extend from the surface low across eastern Pennsylvania into New Jersey. Thunderstorms may initiate along or near this boundary by late morning—around 11 a.m. to 1 p.m. EST.

Low-level moisture ahead of these features should remain modest over land, with surface dewpoints generally in the low to mid 50s. Filtered daytime heating will allow weak instability to develop, but the overall environment will be marginal for severe weather.

Deep-layer wind shear of around 25 to 40 knots will support organized storm structures. Cold mid-level temperatures—around -20 to -22°C at 500 millibars—and modestly steepened lapse rates aloft will favor hail production in any stronger cells that develop.

What hazards are expected?

The SPC outlook identifies three primary hazards:

Severe hail

Given the cold air aloft and steep lapse rates, isolated severe hail is the primary threat. Any stronger cells that develop could produce hail larger than one inch in diameter. The hail threat will be highest where storms can tap into the most unstable air parcels near the warm front.

Damaging winds

Isolated damaging winds are also possible as convection spreads from eastern Pennsylvania into New Jersey and vicinity before moving offshore. While low-level winds and related shear should remain fairly modest, downdrafts from stronger cells could produce gusts capable of tree and power line damage.

Tornado

A somewhat more favorable zone for a tornado may exist if a persistent supercell can interact with or cross the surface warm front this afternoon across eastern Pennsylvania into New Jersey. Low-level shear will remain marginal, but the warm front provides a focused zone of enhanced helicity. Any tornado would most likely be brief and weak, but the threat is non-zero.

When will the storms hit?

Thunderstorm initiation is expected by 16:00 to 18:00 UTC (11 a.m. to 1 p.m. EST) along the warm front across eastern Pennsylvania and New Jersey. Storms will spread eastward through the afternoon, moving offshore by early evening.

The severe threat window will be relatively brief—roughly four to six hours—before storms push into the Atlantic and the warm front lifts north of the instability axis.

Cities to watch

Metro areas in or near the Marginal Risk zone include:

  • Philadelphia, PA — near the western edge of the risk area; storms may initiate nearby
  • Trenton, NJ — along the warm front; enhanced tornado potential if a supercell crosses the boundary
  • Atlantic City, NJ — coastal location; hail and wind threat as storms move offshore
  • Wilmington, DE — central to the risk area; all three hazards possible
  • Baltimore, MD — southern edge of the risk zone; isolated severe hail most likely
  • Dover, DE — Delmarva Peninsula; hail and wind threat this afternoon

Residents in these areas should monitor radar this afternoon and have a way to receive warnings if storms intensify.

What about tomorrow?

The SPC Day 2 outlook shows no severe weather risk for Friday, November 13. High pressure will build across the eastern United States, bringing quiet weather to most of the country.

By the weekend, attention may shift back to the southern Plains, where the Day 3 outlook mentions potential storm development across Texas and the Southern Plains. Check back tomorrow for updates on any emerging threats.

What should you do?

A Marginal Risk means severe weather is possible but not expected to be widespread. If you're in the coastal Mid-Atlantic today:

  • Monitor radar between 11 a.m. and 6 p.m. EST.
  • Have a way to receive warnings. Wireless Emergency Alerts for severe thunderstorm and tornado warnings will break through Do Not Disturb on most phones. Third-party weather apps are silenced by Do Not Disturb, though Wireless Emergency Alerts for tornado warnings still break through on most phones.
  • Move indoors if storms approach. Hail can damage vehicles and injure anyone caught outside.
  • Stay away from windows if a severe thunderstorm warning is issued for your location.
  • If a tornado warning is issued, go to the lowest floor of a sturdy building, away from windows. Interior rooms like bathrooms and closets offer the best protection.

The severe threat today is low-end, but any thunderstorm can produce dangerous lightning. When thunder roars, go indoors.

VORTEX is a free web app at vortexintel.app that monitors severe weather nationwide. Pro ($4.99/month) places phone calls to your phone when a tornado or flash flood warning is issued for a location you care about — calls can ring through Do Not Disturb once you enable Emergency Bypass for the number, unlike most third-party app notifications.