Area Forecast Discussion

WHAT HAS CHANGED

A Severe Thunderstorm Watch is in effect for western Maryland, the eastern West Virginia Panhandle, and a small portion of northern Virginia. This watch is in effect until 8pm this evening. Strong to severe thunderstorms could bring damaging winds and large hail to these areas.

KEY MESSAGES

- 1) A Severe Thunderstorm Watch is in effect until 8pm this evening for western Maryland, eastern West Virginia Panhandle, and parts of northern Virginia.

- 2) Dry and warm late Sunday and into Monday.

- 3) Hot and humid conditions for the second half of the week with daily thunderstorm chances.

DISCUSSION

KEY MESSAGE 1...A Severe Thunderstorm Watch is in effect until 8pm this evening for western Maryland, eastern West Virginia Panhandle, and parts of northern Virginia.

A mid-level trough of low pressure, moving out of the eastern Ohio Valley, will ignite showers and thunderstorms across our northern third of our region, mainly along and north of I-66. A Severe Thunderstorm Watch is in effect until 8pm this evening for western and central Maryland, northeastern West Virginia including the eastern Panhandle, and far northern Virginia. A fast- moving squall line of heavy showers and severe thunderstorms are currently racing east- southeastward across southwest Pennsylvania and northern West Virginia. Frequent lightning, wind gusts over 55 mph, and large hail are occurring in this activity. A tornado can't be ruled out within this squall line. This activity will reach Garrett County in Maryland between 315pm and 330pm and should continue onward toward the east- southeast over the next hour or two. The added heat, temperatures reaching upper 80s to middle 90s, and humidity with dewpoint temperatures in the lower to middle 60s will aid in this convection continuing on strong. CAPE is 1000-1500 J/kg and MLCAPE is around 500 J/kg so instability isn't overly high. Wind shear is 20-30 kts. Much of the convection will impact western and central Maryland and most of northeastern West Virginia, including the eastern Panhandle. Parts of northern Virginia could also feel the impact of damaging winds and hail between 4pm and 7pm. Additional strong to severe thunderstorms could develop ahead of the squall line or near the periphery and some could even make it close to the northwestern suburbs of Baltimore City. Low confidence on this. The main threats will be damaging wind gusts over 60 mph, large hail, downpours, and lightning. A cold front at the surface will follow this evening's convection later this evening and overnight. It looks as if the front will slow its forward movement on the south and southwest flank of it. Thus, a few lingering showers and a thunderstorm could exist anywhere in our northern half of the region into the 1am to 2am timeframe. Some isolated convection could linger or refire during the Sunday morning or midday hours due to this possible stalled front.

KEY MESSAGE 2...Dry and warm late Sunday and into Monday.

A front will slide to the south and southwest, stalling as it moves. Limited shower activity expected. As a matter of fact, most places north and east of the Virginia Piedmont will be dry and remaining warm. Highs in the middle 80s to near 90 on Sunday. Low temperatures Sunday night will likely fall into the 60s as a cold front presses further south.

Monday will be dry with high pressure building into the region. Highs in the lower to middle 80s.

KEY MESSAGE 3...Hot and humid conditions for the second half of the week with daily thunderstorm chances.

After the relatively lower temperatures Monday and Tuesday, heat will begin to build Wednesday as the surface high moves off the coast. Ensemble temperature plots suggest Thursday through Saturday will be the hottest days with temperatures potentially well into the 90s. The heat index may rise above 100 degrees in some areas as dew points will be higher these days compared to some of our recent hot days.

The higher theta-e airmass will arrive on Wednesday as well. Details are still a little uncertain with how strong mid/upper level ridging will be, and when exactly shortwaves may approach from the northwest. In the end, there is some shower and thunderstorm potential each day Wednesday through Saturday, even if it is ultimately terrain-driven and limited in coverage. There may be some severe weather risk given the heat and humidity, although it may be isolated due to relatively weak shear (assuming a stronger trough does not unfold). It's possible a cold front and stronger jet streak could approach at some point next weekend.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

VFR conditions are likely at the TAF sites of CHO and maybe IAD and DCA. This would depend on the convection this afternoon not making it these sites. Otherwise, MRB could encounter a period up to an hour of IFR or LIFR should the squall line in southwest PA make its way to the terminal or additional convection develops between 4pm and 7pm. BWI and MTN could get impacted between 6pm and 9pm but confidence is low. Winds southwest to west 5 to 10 knots, except brief gusts 30 to 40 knots in any strong thunderstorms near MRB and perhaps BWI and MTN. Winds becoming northwest 5 to 10 knots Sunday. Any isolated heavy showers around IAD, DCA or CHO on Sunday could bring brief MVFR conditions and gusts over 30 knots.

VFR is expected to prevail Monday into Tuesday.

Thunderstorms are possible during the afternoon and evenings of Wednesday and Thursday. Winds likely remain 10 kt or less out of the south or west.

MARINE

SCAs remain in effect for the waters through 8pm. Any convection that reaches the northern and central Chesapeake Bay late this afternoon and evening could bring about SMWs or MWSs depending on the duration. Winds could gust over 35 knots in strong thunderstorms. MWSs may be warranted in the central and southern Chesapeake Bay and much of the Potomac waters on Sunday should any convection linger near the surface front.

As high pressure moves off the coast, southerly flow will prevail Monday through Thursday. Wednesday may have the highest chance for winds near advisory criteria. However, channeling events could bring marginal advisory conditions each evening/night as well.

LWX WATCHES / WARNINGS / ADVISORIES

DC...None. MD...Small Craft Advisory until 8 PM EDT this evening for MDZ008. VA...None. WV...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ530- 531-535-538.