Area Forecast Discussion

WHAT HAS CHANGED

Added another Heat Advisory for the same area on Friday. Additionally, added some new details/observations regarding today's severe weather threat. Made a few changes to the Friday severe weather threat as well, just to bring in some model uncertainty.

KEY MESSAGES

- 1) Hot and humid conditions persist through Friday with a daily threat for strong to severe thunderstorms.

- 2) Shower and thunderstorm chances return late Sunday into next week as a cold front moves through the region.

DISCUSSION

KEY MESSAGE 1...Hot and humid conditions persist through Friday with a daily threat for strong to severe thunderstorms.

Today presents a complex convective forecast, with a high end potential for severity, but a very low confidence in if/how many storms develop. Starting at the surface this afternoon, it seems like many things are in favor of storms today. Temperatures are currently well into the 90s, with dew points rising into the 70s which is yielding MLCAPE values in excess of 2000 J/kg. Even more impressive is the pooling of 3000+ J/kg of MLCAPE pooled along the Chesapeake Bay. There is a surface trough located along the Blue Ridge roughly, with winds out of the SW to the east, and more westerly to the west of the boundary.

Per the latest ACARS sounding data, westerly flow is prevalent at the surface and in the mid-levels, which could inhibit convection for much of the afternoon. Thinking that development on terrain circulations is less likely due to this fact, but some development is slightly more likely along the Bay Breeze. However, the area to watch closely is the convection developing along a remnant MCV from this morning over the Ohio Valley. This activity will eventually push into the region later this afternoon/evening, likely becoming the primary threat for severe weather today. As storms are already developing, timing has come a bit more into focus, and is trending a bit later, and several hours later compared to what we saw last evening. What this convection does over the Allegheny Front is largely going to depend on the mode it crosses in. If it is still not well developed, it may struggle to maintain itself in the mid-level westerlies. However, if this activity develops into an established squall line, then it should be able to maintain itself as it moves into an even more unstable airmass over our region. As the wave of energy moves into the much more robust environment near/east of I-95, storms may become more vigorous again, possibly congealing into small clusters with a damaging wind and sporadic large hail threat. One concerning thing is just how much DCAPE is available for very strong downdrafts today (around 1000-1200 J/kg). This is substantial, and has me concerned for a few significant gusts if things do manage to organize into bowing structures especially. Still, this threat is not guaranteed, which is why POPs are not higher for this event. However, given the potential higher end nature should it materialize, keep a very close eye on the forecast this evening.

Thunderstorm activity should wind down late this evening, though with the residual warm and humid airmass in place a pop up shower or even a bit of thunder can't be ruled out overnight.

Outside of the convective potential today, the other big issue is going to be the heat, as heat indices are into the 100s for most. Near and east of I-95, slightly more humid air to the east of the trough and closer to the Chesapeake Bay has heat indices near/above 105, peaking mid afternoon. It will be hot regardless, so make sure to stay hydrated and avoid strenuous activity during the heat of the day.

Heat and humidity lingers Friday, and unless there is a lot of morning cloud cover it looks to be a bit hotter. There is the potential for a bit more mixing down of lower dew points given lingering westerly flow, but heat index values likely approach 100 to 105 again in the afternoon. For now, have highlighted the same areas in a Heat Advisory for tomorrow as was highlighted today. Could see an expansion west being possible, but given the potential for lower dew points compared to today, this was a good first stab at it, and will allow further shifts to make a final call on the westward extent. However, as always, just because an advisory may not be in effect for all areas, it will be very hot/humid either way, so all should take proper precautions once again.

Stronger forcing associated with an upper trough glancing by to the northwest and an approaching cold front lends to higher certainty of more numerous thunderstorms Friday afternoon and evening. Flow aloft will tick up slightly, though still modest and out of the W to WNW. There is some concern with the amount of spread amongst the model guidance as to where the best coverage is. Many of the hi-res models thus far keep convection suppressed to the south, likely due to that westerly flow aloft potentially stifling convection. However, with abundant instability in place once again, am thinking that storms may once again develop as they move east of the I-81/Blue Ridge corridor as they run into the most favorable airmass. Given the amount of instability in place once again, thunderstorms may produce some rather gusty winds. Severe/damaging wind gusts are plausible, especially with the added forcing that should help force storms into bands. Isolated large hail is possible with the most intense updrafts, and again could see the potential for some significant wind gusts.

Thunderstorm activity should wind down by the evening hours.

Following the Friday night cold frontal passage, high pressure will briefly pivot over the Mid-Atlantic for Saturday. This results in notably lower humidity and temperatures several degrees cooler (relatively speaking) as well.

More sun than clouds amid light northwest breezes are expected. Temperatures falling into the 60s Saturday night with dew points around 60 will feel like a stark contrast to the more oppressive humidity in the preceding couple of days/nights.

KEY MESSAGE 2...Shower and thunderstorm chances return late Sunday into next week as a cold front moves through the region.

A deep upper-level trough will form over the Great Lakes towards the end of the weekend, resulting in a cold front that will track through the East Coast between late Sunday and going into Monday. There are a couple of ML models that show a severe risk for this front, bringing back severe chances after a brief pause on Saturday.

Once this cold front moves through, we could begin to see a brief dip in temperatures, with highs for Monday currently ranging largely in the 80s. Although the low pressure system associated with this front will push north by late Monday, another area of low pressure could build towards the south and push north on Tuesday, bringing additional chances for rain and showers. By this point, severe chances become much more uncertain as instability looks modest, but shear will increase. Overall, expect a repeating pattern of frontal boundaries tracking through the region bringing several opportunities for precipitation.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Prevailing VFR conditions are expected through Friday. Thunderstorm coverage is uncertain this afternoon and evening, but any storm could contain very gusty winds or even some hail. Storms look to fire later this afternoon relative to yesterday. The 18Z TAFs honed in on a smaller convective window, though still couldn't waver from the PROB30s, as confidence is just too low at this time.

There is a higher chance of strong to severe thunderstorms Friday afternoon and evening as a cold front approaches. Added that threat in as a PROB30 for now. There is still some uncertainty amongst model guidance for now, but still thinking the chances are higher tomorrow. Some of the high-res model guidance just disagrees, so will have to see how the evening/overnight guidance changes.

Winds will be fairly light out of the west or southwest both days outside of any storms.

VFR conditions are predominantly expected through the weekend, with some reduced conditions possible beginning Sunday evening associated with potential showers and thunderstorms. Periodically reduced conditions associated with a low pressure system could continue through early next week. Gusts between 15-20 knots are possible Sunday afternoon/evening before falling again Monday, with southwest winds shifting easterly by Monday evening.

MARINE

Generally light west to southwest winds are expected through Friday, becoming northwest Friday night through Saturday. There is a potential of at least brief near SCA conditions behind a cold front Friday night into Saturday morning. Ahead of this front, potential showers and thunderstorms bring a risk of lightning and strong winds to the waters each afternoon/evening. Both afternoons may need a few SMWs, with 50+ knot winds possible in any storms.

SCAs will likely be needed Sunday evening with gusts up to 20 knots possible, but should go back below SCA thresholds by Monday morning. Southwest winds shift northerly Monday afternoon before shifting southeast.

CLIMATE

Temperatures are expected to soar to as much as 15 to 20 degrees above average through Friday. This approaches or exceeds daily record values at some sites.

Below is a list of record daily high and warm low temperatures for June 11th and 12th (Thursday and Friday).

================================================================= June 11th Record Daily Highs and Warm Lows ================================================================= Site Record High Record Warm Low DCA 101 F (set in 1911) 74 F (set in 2020 +6 others) IAD 95 F (set in 2016) 72 F (set in 2005) BWI 99 F (set in 1911) 75 F (set in 1914) DMH 100 F (set in 1984) 79 F (set in 1984) NAK 96 F (set in 2000) 79 F (set in 1973) HGR 97 F (set in 1911) 73 F (set in 2005) MRB 101 F (set in 1911) 71 F (set in 1927 and 1922) CHO 99 F (set in 1911) 76 F (set in 2020)

================================================================= June 12th Record Daily Highs and Warm Lows ================================================================= Site Record High Record Warm Low DCA 95 F (set in 2017 +6 others) 77 F (set in 2015) IAD 96 F (set in 1986) 72 F (set in 1986) BWI 96 F (set in 1986 and 1914) 77 F (set in 1947) DMH 96 F (set in 2025 +2 others) 77 F (set in 2017 and 2015) NAK 98 F (set in 1914) 77 F (set in 2000 and 1973) HGR 92 F (set in 2017 +5 others) 71 F (set in 2010 +5 others) MRB 97 F (set in 1938 and 1933) 73 F (set in 1914) CHO 98 F (set in 1914) 72 F (set in 1947 +2 others)

LWX WATCHES / WARNINGS / ADVISORIES

DC...Heat Advisory until 8 PM EDT this evening for DCZ001. Heat Advisory from 11 AM to 8 PM EDT Friday for DCZ001. MD...Heat Advisory until 8 PM EDT this evening for MDZ008-011-013- 014-016>018-508. Heat Advisory from 11 AM to 8 PM EDT Friday for MDZ008-011-013- 014-016>018-508. VA...Heat Advisory until 8 PM EDT this evening for VAZ054-057. Heat Advisory from 11 AM to 8 PM EDT Friday for VAZ054-057. WV...None. MARINE...Heat Advisory until 8 PM EDT this evening for ANZ530. Heat Advisory from 11 AM to 8 PM EDT Friday for ANZ530.