Area Forecast Discussion

WHAT HAS CHANGED

A line of showers has developed in the past hour or so from Frederick and Carroll Counties in Maryland southwest to Jefferson County West Virginia and northwestern Loudoun County in Virginia. While there is still a chance for additional showers and thunderstorms to develop around mid-afternoon further to the west and north, most of the stronger and more intense showers and thunderstorms should be late this afternoon through early evening. Small Craft Advisories remain in effect over the upper waters through early evening. Heat indices may briefly approach 95 degrees between 3pm and 6pm in a few spots.

KEY MESSAGES

- 1) A strong to severe thunderstorm threat glances northern portions of the region late this afternoon into this evening.

- 2) Early summer heat continues through the weekend.

- 3) Temperatures briefly drop early next week before increasing late in the week.

DISCUSSION

KEY MESSAGE 1...A strong to severe thunderstorm threat glances northern portions of the region late this afternoon into this evening.

A surface trough of low pressure and a little wind shear have allowed for an environment to support a line of showers across central Maryland, eastern Panhandle of West Virginia, and parts of northeastern Virginia during the middle part of this morning. No lightning with this activity...perhaps just a brief downpour or a wind gust over 35 mph. In the meantime, an upper trough of low pressure will continue to swing southeast across the Lower Great Lakes with a trailing cold front that is expected to move slowly into and through the region later today through early Sunday.

Dewpoint temperatures are climbing through the upper 50s and into the middle 60s. Currently, there is very little instability, but that could soon change as temperatures climb into the 80s to the lower 90s in places. The highest chance of strong to severe thunderstorms to develop and feed off of this instability and some wind shear will be along and north of I-66.

Mid-level (i.e. 700-500 hPa) flow increases to 30-40 knots this afternoon into this evening near the Mason-Dixon line as the upper trough to the northwest encroaches on ridging to the southeast. Low- and mid-level lapse rates steepen accordingly with the height falls aloft and robust diurnal heating. RAP forecast soundings indicate pre-convective lapse rates around 7 deg C/km, steepest in the mid-levels in the I-70 corridor and steepest in the low-level south of I-66/US-50. This is favorable for gusty to damaging downburst winds in any storms that manage to develop, with storm coverage and timing still somewhat in question given a lack of stronger ascent locally.

Most guidance indicates the highest chances of convection this evening. However, some members (i.e. WRF-ARW, RAP) have convection developing a few hours earlier. If convection develops a bit earlier (mid afternoon) near the Appalachians, it would move eastward during or just after peak heating. This would increase the damaging wind threat conditionally, though overall forcing will be rather weak. Forcing increases a bit this evening, but the loss of diurnal heating may cause storms and their wind threat to trend down, should the later timing pan out.

All in all, this result in a risk of a few strong to locally severe storms mainly in the I-68/I-70 corridor of northern and western MD, perhaps down into the eastern WV panhandle, far northern VA, and the Baltimore metro. Damaging wind gusts appear to be the main threat, though lapse rates/shear aloft lend to a conditional risk of some hail particularly in stronger, more isolated updrafts/marginal supercell structures, should they form and impact our area. The chance of rain drops off rather significantly further south (i.e. south of I-66/US-50) due to forcing being displaced to the north through peak heating.

A few showers may linger as the front nears the area from the north tonight, but the threat for severe weather or even thunder looks rather low beyond late evening, barring any greater-than- expected upstream organization.

For Sunday, the cold front may remain near enough to trigger a few showers and thunderstorms south of I-66/US-50. Moderate deep layer flow and steepening lapse rates will again lend to a risk of a few strong to damaging thunderstorm gusts, though lapse rates aloft won't be quite as steep so the hail threat is lower.

KEY MESSAGE 2...Early summer heat continues through the weekend.

Despite some high clouds and gradually falling heights with the approach of a cold front, well above normal temperatures will persist through the weekend as any appreciable cold advection lags and rain remains relatively spotty and brief.

High temperatures are expected to reach the upper 80s and lower to middle 90s across much of the region this afternoon, with 70s over the higher elevations. Dew points in the upper 50s to lower 60s should keep heat index values near the air temperature, but a few readings around 95 are possible mid to late afternoon.

The drier nature of the airmass allows temperatures to fall into the 60s to lower 70s tonight before rebounding into the upper 80s to lower 90s (70s mountains) on Sunday. Again, dew points in the upper 50s to lower 60s will hold heat index values close to the ambient air temperatures.

Low temperatures will likely fall into the 60s (50s mountains) Sunday night as a cold front presses further south.

KEY MESSAGE 3...Temperatures briefly drop early next week before increasing late in the week.

An upper-level trough and low-level onshore flow behind a surface cold front will bring cooler temperatures Monday into Tue (widespread 80s/cooler in mountains). Overnight lows will be in the 50s and 60s.

Upper-level ridging builds in Wednesday onward with surface temperatures increasing daily. Temperatures look to rebound to the 90s by Thursday into next weekend. Moisture returns with the increase in temperatures resulting in an uptick in humidity. Diurnally driven showers and thunderstorms look to impact the area daily Thursday onward, with perhaps a higher chance Friday with an upper-level disturbance approaching.

AVIATION /14Z SATURDAY THROUGH WEDNESDAY/

VFR conditions are likely at the TAF sites through this afternoon as high pressure gradually departs off the Atlantic coast. Winds will be generally SW, with a more S component east of a surface trough and a more W component to the west. Metro TAF sites such as KBWI/KMTN/KDCA would be most likely to see a bit more S component. Given decent vertical mixing, gusts up to 20 knots are possible, especially during the afternoon hours (slightly lower at KCHO). Shower and thunderstorm chances increase this evening, with KMRB to KMTN seeing the highest chance for restrictions. Further south, high-resolution models depict a weakening trend into the night. Although the consensus has TS probs peaking in the evening, a few members indicate CI a few hours earlier. Will monitor for possible AMDs. Any restrictions or gusty winds in/near storms should be brief (sub-hourly).

Winds shift to northwesterly behind the front midday Sunday with any convective chances mainly near KCHO. High pressure building to the north will yield mainly north to northeasterly winds into Sunday into Monday with continued VFR conditions. Some low CIGs can't be ruled out Monday night in onshore flow.

VFR is expected to prevail Tuesday into Wednesday with S winds possibly gusting 15-20 kts each afternoon.

MARINE

Generally S/SW flow is expected this afternoon, becoming more W/NW by late tonight. Although the highest likelihood of gusts to around 20 kts is mid to late afternoon, forecast soundings indicate potential for deep enough mixing to result in gustiness as early as late morning, with a modest increase in low-level flow heading into early evening ahead of an approaching cold front. Have therefore issued SCAs for the upper waters covering 11am to 8pm. A few gusty showers and thunderstorms may approach the waters late afternoon into this evening, with a few SMWs possible (again mainly to the north).

Winds turn NW Sunday, N Sunday night, then NE to E Monday. Gusts could again approach SCA levels briefly Sunday afternoon. More widespread gustiness is probable Monday in onshore flow. Winds shift to southerly Tuesday into Wednesday with additional SCAs possible.

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.