Area Forecast Discussion

WHAT HAS CHANGED

A large area of moderate to heavy rain and embedded thunderstorms was moving west to east between I-70 and I-64 as of 8:30 PM. Near and south of US-50 is still unstable enough that, with increasing low-level wind this evening, severe weather (damaging wind gusts/brief tornado) remain possible. A Flood Watch remains in effect for north-central to northeast Maryland with continued heavy thunderstorms in the area.

KEY MESSAGES

- Severe weather risk will continue through this evening with the greatest risk across central VA, southern MD, and the Appalachian region. - Below average temperatures and mainly dry conditions are expected Thursday through the weekend.

DISCUSSION

KEY MESSAGE 1...Severe weather risk will continue through this evening with the greatest risk across central VA, southern MD, and the Appalachian region.

Widespread showers and thunderstorms continue this evening. The main near-surface-based instability is near and south of US-50. An MCV from earlier convection crossing Page County VA is likely advecting a narrow plume of SBCAPE around its eastern periphery, which will continue to support a gusty wind risk east over the next couple hours as it approaches near/south of the immediate DC metro area. To the north, warm/moist advection due to an increasing LLJ will continue between I-66 and I-70, focusing heavy thunderstorms in these areas.

Thanks to ongoing drought, most flooding to this point has been isolated. Still, urban areas or areas that have already received substantial rain recently will continue to be at risk of flooding until late evening.

Still a risk of severe wx Wed, but likely for areas south of the Capital Beltway and southern MD.

KEY MESSAGE 2...Below average temperatures and mainly dry conditions are expected Thursday through the weekend.

KEY MESSAGE 2 DESCRIPTION...

A sprawling area of high pressure will build to our north for Thursday and Friday, before shifting off the New England Coastline on Saturday. Northerly flow ahead of the approaching high will advect a much cooler and drier airmass into the region for Thursday. Well below normal temperatures and dewpoints are forecast for late July, with high temperatures only in the upper 70s/low 80s. Dewpoints in the 50s will also make it feel much less humid than is typical for July. Mostly sunny skies will accompany the well below normal temperatures/dewpoints. Temperatures will stay well below normal for Friday and Saturday within easterly flow. Both temperatures and dewpoints begin to rise again on Sunday as flow turns southerly, with highs in the mid to upper 80s and dewpoints in the 60s. The return of higher dewpoint air should lead to slightly greater chances for afternoon/evening showers and thunderstorms, but most locations will likely still stay dry.

Forecast confidence decreases substantially by Monday and Tuesday, but chances for showers and thunderstorms may further increase as multiple shortwave disturbances move through in northwesterly flow aloft. Depending on how the forecast details evolve, there could be a conditional threat for severe thunderstorms on either Monday or Tuesday.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Widespread showers and thunderstorms expected through mid evening with the risk of some severe wx. Thunderstorm risk Wed will likely stay south of the main terminals.

Prevailing VFR condtions are expected Thursday through Saturday. Winds will generally be out of the north on Thursday, east on Friday, and east to southeast on Sunday.

MARINE

Numerous to widespread showers and thunderstorms expected, some of which may require SMWs this afternoon and this evening and again Wednesday.

SCA conditions may linger in northerly flow into Thursday morning. Thereafter, sub-SCA level winds are expected Thursday afternoon through Saturday. Winds will generally be out of the north on Thursday, east on Friday, and east to southeast on Saturday.

TIDES/COASTAL FLOODING

Elevated southerly flow continues through this evening, pushing anomalies close to one foot. No tidal flooding is forecast at this time, but Annapolis and Havre de Grace will be close. Water levels will drop as a cold front passes Wednesday.

LWX WATCHES / WARNINGS / ADVISORIES

DC...None. MD...Flood Watch until 2 AM EDT Wednesday for MDZ004>006-008-011- 503>508. Small Craft Advisory until 8 AM EDT Wednesday for MDZ008. VA...Flood Watch until 2 AM EDT Wednesday for VAZ025-026-036-037- 503-504-508. WV...Flood Watch until 2 AM EDT Wednesday for WVZ505-506. MARINE...Flood Watch until 2 AM EDT Wednesday for ANZ530. Small Craft Advisory until 8 AM EDT Wednesday for ANZ530-531- 538-539. Small Craft Advisory until noon EDT Wednesday for ANZ532-533- 540>542. Small Craft Advisory until 6 PM EDT Wednesday for ANZ534-537- 543. Small Craft Advisory until 4 AM EDT Wednesday for ANZ535-536.