textproduct: Wakefield

WHAT HAS CHANGED

Marginal Severe risk has been expanded south to the north-side of the Richmond metro and through the Northern Neck and southeast MD for this afternoon and evening.

KEY MESSAGES

1) Isolated to scattered showers and thunderstorms are expected through this evening, primarily from the Piedmont through central VA and the Eastern Shore. Damaging wind gusts are possible with any thunderstorms, with the greatest threat from the north-side of the Richmond metro and through the Northern Neck and southeast MD for this afternoon and evening.

2) Heat and humidity will persist through the remainder of the weekend and into the middle of next week. A low chance of afternoon and evening showers and thunderstorms continues Sunday, mainly dry Monday, with diurnal chances of showers/thunderstorms returning Tuesday through Thursday.

DISCUSSION

As of 320 PM EDT Saturday...

KEY MESSAGE 1...Isolated to scattered showers and thunderstorms are expected through this evening, primarily from the Piedmont through central VA and the Eastern Shore. Damaging wind gusts are possible with any thunderstorms, with the greatest threat from the north-side of the Richmond metro and through the Northern Neck and southeast MD for this afternoon and evening.

GOES water vapor channels depict SW flow aloft this afternoon between and upper trough over the Great Lakes and a subtropical ridge offshore. There is a shortwave trough over the upper Ohio Valley embedded within the flow and scattered convection has developed well in advance of this feature along the higher terrain of the Appalachians and in vicinity of a lee side trough. Meanwhile, surface high pressure is centered well offshore of the Southeast coast. Hot and humid with temperatures in the lower 90s and dewpoints in the lower to mid 70s. This results in heat indices in the upper 90s to lower 100s.

The aforementioned shortwave trough is expected to push E across the northern Mid-Atlantic through this afternoon and evening. Effective bulk shear is around 25-30kt, and should remain similar through early this evening. MLCAPE has reached 2000-2500J/kg per SPC mesoanalysis, and may evening increase to 2500-3000J/kg over the next 2-3 hours. DCAPE is about 800-1000J/kg and is expected to remain steady through peak heating, with steep low-level lapse rates ~8C/km, and max 2-6km AGL lapse rates are even ~7C/km per the latest analysis. Given sufficient shear and instability, initial cellular convection has the potential to organize into line segments, with the main point of uncertainty through the afternoon and evening being how widespread the convection becomes. The main threat will be damaging wind gusts with 800-1000J/kg of DCAPE and steep low- level lapse rates, but some large hail is possible in any initial cellular convection. SPC expanded the marginal risk area southward this morning to basically I-64 from the Louisa/Fluvanna line to the north- side of the RIC metro, then eastward to about Wallops Island. A slight risk has been added immediately N of the local area. Much of the area is in a marginal ERO. However, showers/tstms should have enough motion to limit the excessive rainfall threat.

KEY MESSAGE 2...Heat and humidity will persist through the remainder of the weekend and into the middle of next week. A low chance of afternoon and evening showers and thunderstorms continues Sunday, mainly dry Monday, with diurnal chances of showers/thunderstorms returning Tuesday through Thursday.

The subtropical ridge offshore will continue to prevail Sunday as the Great Lakes trough shifts to the Northeast. Deterministic models and ensembles continue to hint at a more zonal pattern across the area by early next week, with the strongest flow remaining north of the area. Meanwhile, at the surface high pressure continues to thrive off the coast continuing to usher in heat and moisture. Overall a rather typical stagnant late summer pattern is expected with temperatures on the warm-side of seasonal averages. Dewpoints will struggle to mix out through the day and remain in the low to locally mid 70s during peak heating. The mix of heat and humidity will allow heat indices to be in the upper 90s and low 100s, but locally ~105F from the northern shore of the Albemarle Sound though southside Hampton Roads.

A subtle boundary settles into the area later tonight and linger Sunday. This along with the sea-breeze could trigger additional showers/tstms Sunday. High-res models continue to struggle with the extent and location of convection, but the 12z/08 HREF suggests at least slight chc to low chc PoPs for much of the area. Locally heavy rain will be the primary threat. Mainly dry Monday with generally deep layer westerly flow. Potentially more unsettled energetic WNW flow Tuesday through Thursday (highest PoPs N) as a stronger trough dives SE toward New England. This trough may eventually push a stronger cold front through the region later in the week or early next weekend, but confidence in this remains low at this time.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

As of 725 PM EDT Saturday...

VFR conditions are favored to prevail through tonight and into Sunday. The coverage of showers/storms has generally been lower this evening and don't expect much additional development. Mid- level cloud cover will generally be SCT in nature tonight with 5-10 kt SW winds. A weak boundary settles into the region Sunday, and this along with the sea- breeze could trigger a few showers/tstms (15-25% chc) Sunday aftn. The wind will generally be W/WSW 8-12 kt Sunday afternoon.

Outlook: Primarily VFR Sunday night through Thursday. Mainly dry Monday, with a 20-40% chc of aftn/evening showers/tstms Tuesday/Wednesday, and ~20% Thursday.

MARINE

As of 230 PM EDT Saturday...

Key Message:

- Sub-SCA marine conditions are expected to prevail through the weekend and into early next week.

This afternoon, Bermuda high pressure remains centered offshore, keeping S-SW flow across the local waters. Latest observations show S-SW winds averaging 10 to 15 knots with gusts to 20 knots. Seas are running around 2 to 3 feet nearshore and 3 to 4 feet offshore. Waves in the Chesapeake Bay are running around 1 to 2 feet. Winds shift to the S or SSE later this afternoon and evening due to diurnal seabreeze influences. Southerly winds increase to 15 to 20 knots over the coastal waters later this evening into the first part of the night. Gusts ~25 knots will be possible across the offshore coastal waters during this timeframe. The pressure gradient relaxes slightly Sunday into early next week with a S to SW wind averaging ~10 knots which will allow for more of a true sea- breeze with the wind shifting to SE near the coast during the afternoon/evening hours. Seas may briefly increase to around 3 to 4 feet nearshore and 4 to 5 feet offshore later this evening, before averaging 2 to 3 feet for the remainder of the forecast period. A weakening cold front or at least a prefrontal trough eventually looks to push south, getting closer to the local area by mid to late week. Winds may periodically shift to the W-NW and increase, but any elevated winds would probably be short- lived.

AKQ WATCHES/WARNINGS/ADVISORIES

MD...None. NC...None. VA...None. MARINE...None.