Area Forecast Discussion
Issued by NWS Sterling, VA
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Issued by NWS Sterling, VA
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-- Discussion containing changed information from previous version are highlighted. --083 FXUS61 KLWX 211919 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 319 PM EDT Fri Jun 21 2024 .SYNOPSIS...-- Changed Discussion --Heat and humidity continue to increase through the weekend as high pressure sits overhead. Mostly dry conditions are expected with isolated showers and thunderstorms mainly focused over the mountains. A cold front will work across the region Sunday into Monday bringing a reprieve in the high heat and humidity. High pressure briefly builds back over the area Tuesday before an additional cold front sweeps through the region Wednesday and Thursday next week.-- End Changed Discussion --&& .NEAR TERM /THROUGH TONIGHT/...-- Changed Discussion --A persistent subtropical ridge aloft remains over the Southern Plains eastward into the Mid-Atlantic and southeastern U.S. This sprawling 594-597 dm anticyclone has maintained an area of above average temperatures across the local area. Early afternoon temperatures have risen into the upper 80s to low 90s, locally approaching 95 degrees in a few spots. Humidity levels have come down a bit as the deepening boundary layer continues to mix in some lower dew point air from above 850-mb. The resultant heat indices continue to sit in the 90s. Recent GOES-16 visible satellite imagery show much of the cumulus fields have concentrated over the mountains, particularly the Allegheny Front. Any convective attempts have been squashed by the subsidence aloft. However, some activity could track down from south-central Pennsylvania this afternoon. Some modest radar returns are dropping down toward the point where I-68/I-70 intersect in western Washington County. Lightning activity continues to diminish as this activity pushes southeastward toward the Mason-Dixon Line. Have maintained thunderstorm chances through dusk before the threat wanes. A toasty evening and night looms ahead with forecast low temperatures in the upper 60s to mid 70s. Some patchy fog may develop across the Allegheny mountain valleys.-- End Changed Discussion --&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...-- Changed Discussion --Oppressive heat continues into the weekend with widespread mid/upper 90s likely, locally to near 100 degrees. Although the mid/upper ridge begins to buckle which allows heights to decrease, the lower portion of the atmosphere will begins to heat up. From this afternoon (Friday) into the weekend, 850-mb temperatures are expected to go up by around 2 to 4 C. This net increase in low-level temperatures will bring daily conditions into the more hazardous levels. Heat indices rise to around 105 degrees on Saturday, locally near the century mark over areas west of the Blue Ridge. Consequently, Heat Advisories have been issued area a vast majority of the area from 11 AM until 8 PM. Although 110 degree heat indices are not completely out of the question, model soundings show some vertical mixing which could carry humidity levels down a bit during peak heating. However, there are a broad range of potential solutions in how much dry air ends up mixing in. Regarding convective threats, the guidance shows some isolated activity firing off the mountains which slowly pushes toward U.S. 15. Expect this all to decay by sundown which again leads to a warm and muggy night across the area. Heights are expected to further lower as the approaching trough and cold front begin to show their influences over the region. Heat-related products will likely be needed again on Sunday late morning into the early evening. The combination of upper 90s temperatures and upper 60s/low 70s dew points will support heat indices into the 103 to 107 degree range. One question that looms is how much cloud cover works its way eastward toward the Mid-Atlantic region. Depending on how extensive these clouds are, it could slightly hinder reaching the upper 90s threshold. Convection should be much more prevalent, particularly during the second half of the day and into Sunday. This could bring both a severe and flash flooding threat. For the latter, while the area has moved into a D0 to D1 drought, precipitable water values of around 2 inches should support that threat for localized flooding issues. The mentioned cold front begins to enter the Alleghenies late Sunday night before pushing into the I-95 corridor by daybreak. This will allow for a continued threat for showers and thunderstorms during the overnight hours. This is accompanied by lows in the low/mid 70s, locally to around 80 degrees in the urban hubs.-- End Changed Discussion --&& .LONG TERM /MONDAY THROUGH FRIDAY/...-- Changed Discussion --Overall it`s a variable and increasingly unsettled pattern throughout the extended period. A cold front will continue east of the area Monday with a brief reprieve in the heat and humidity Tuesday as high pressure settles overhead. A stronger cold front midweek will bring an additional threat for strong to severe thunderstorms to the region. Current 12z guidance continues to hint at a front along or just east of the Blue Ridge Monday morning into Monday midday. A few thunderstorms may accompany the boundary as it passes through. With peak heating Monday afternoon into the early evening, additional showers and thunderstorms are possible with the greatest concern for severe weather east of I-95 and down into southern MD/northern neck of VA. Primary threats with thunderstorms Monday looks to be wind. All convective activity looks to quickly wane Monday evening into Monday night as dry air filters back into the region. This is in association with a brief area of high pressure building back into the region. As for temperatures, Monday expect highs to run slightly cooler due to residual cloud cover as the front passes through. Highs will range from the upper 80s and low to mid 90s east of I-81 with upper 70s and mid 80s further west. Overnight lows will fall back into the mid to upper 60s with upper 50s over the mountains. Surface high pressure briefly builds back over the area Tuesday giving us a brief reprieve in humidity as drier air settles in. Dewpoints will fall back into the mid to upper 50s and low 60s with light west to northwest breezes. The drier air combined with a downslope component to the wind will allow high temperatures to surge back into the low to mid 90s especially east of the Allegheny Front. Overnight lows Tuesday night will fall back into the upper 60s and low 70s. Unfortunately, high pressure pushes offshore Wednesday allowing for increased southwesterly return flow to funnel back in. This will allow temperatures to surge back into the mid to upper 90s with heat index values up and over the 100 degree mark east of the Blue Ridge Wednesday afternoon. Some uncertainty remains in regards to temperatures due to the progression of the next shortwave trough and incumbent cold front from the Ohio River Valley set to approach the region. 12z synoptic/ensemble guidance suggest fairly spotty convection Wednesday afternoon and evening with more widespread thunderstorm activity developing across the Ohio River Valley and advancing east toward the region Wednesday night. The front and upper level trough slow a bit into Thursday leading to continued thunderstorm chances especially in areas east of the Blue Ridge. With that said, extra cloud cover and a front overhead will lead to cooler temperatures in the mid to upper 80s. Drier weather returns Friday and Saturday as high pressure builds back over the region.-- End Changed Discussion --&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...-- Changed Discussion --VFR conditions are expected through early Saturday across the area terminals. While dry today, a few showers and thunderstorms could impact the western terminals by Saturday afternoon. Any restrictions would be brief in nature. Sunday brings the main threat for showers and thunderstorms which would lend itself to sub-VFR conditions at times. This would especially be the case the second half of Sunday into the night. Some of these storms could be severe in nature. Prevailing southerly winds eventually shift to southwesterlies on Sunday. Any wind shifts with the cold front likely hold of until early Monday. Prevailing VFR conditions are expected Monday outside of a few thunderstorms along and east of the corridor terminals late Monday morning into Monday afternoon. Convection will be spotty in nature and mainly tied to the cold front passing through. High pressure returns to the area Monday night into Tuesday bringing continued VFR conditions. Additional showers and thunderstorms are likely Wednesday into Thursday as another cold front passes through the area.-- End Changed Discussion --&& .MARINE...-- Changed Discussion --Aside from the middle/upper tidal Potomac, all marine locations are in Small Craft Advisories until 2 AM tonight. Southerly channeling will raise gusts up to 20 knots at times through the duration of the advisories. Additional southerly channeling effects are possible Saturday evening/night, and again on Sunday as the influences of convection begin to impact the waters. Special Marine Warnings are possible for any of that convection, with the threat persisting through the overnight hours on Sunday. Small Craft Advisories may be needed for portions of the waters Monday into Monday night as a cold front passes through. South to southwesterly flow will lead to channeling over the open waters before switching to the west and northwest Monday night into Tuesday as high pressure builds in. A few thunderstorms may develop along the front Monday afternoon especially over the middle and lower waters. The primary threat with thunderstorms will be strong winds. Sub-SCA level winds return with high pressure Tuesday into Wednesday.-- End Changed Discussion --&& .CLIMATE...-- Changed Discussion --Very hot temperatures are expected through Sunday. Several records could be in jeopardy, as seen below. Below is a list of record high temperatures for Jun 21st, 22nd, and the 23rd and the year the record was set, and the current forecast high temperatures for those days. RERs are only issued for DCA, IAD, BWI, and MRB, but other sites are shown for reference. Friday Jun 21st Climate Site Record High Forecast High Washington-National (DCA) 99F (2012) 94F Washington-Dulles (IAD) 98F (1988) 95F Baltimore (BWI) 100F (2012+) 94F Martinsburg (MRB) 102F (1931) 93F Charlottesville (CHO) 99F (1933) 93F Annapolis (NAK) 100F (1988) 89F Hagerstown (HGR) 97F (1923) 93F Saturday Jun 22nd Climate Site Record High Forecast High Washington-National (DCA) 101F (1988) 99F Washington-Dulles (IAD) 99F (1988) 99F Baltimore (BWI) 100F (1988) 99F Martinsburg (MRB) 102F (1933) 98F Charlottesville (CHO) 101F (1933) 97F Annapolis (NAK) 101F (1988) 94F Hagerstown (HGR) 100F (1988) 98F Sunday Jun 23nd Climate Site Record High Forecast High Washington-National (DCA) 98F (1988) 98F Washington-Dulles (IAD) 96F (1988) 97F Baltimore (BWI) 97F (2010) 98F Martinsburg (MRB) 100F (1934) 94F Charlottesville (CHO) 100F (1894) 96F Annapolis (NAK) 98F (1988) 94F Hagerstown (HGR) 98F (1988) 95F-- End Changed Discussion --&& .LWX WATCHES/WARNINGS/ADVISORIES...-- Changed Discussion --DC...Heat Advisory from 11 AM to 8 PM EDT Saturday for DCZ001. MD...Heat Advisory from 11 AM to 8 PM EDT Saturday for MDZ003>006- 008-011-013-014-016>018-501>508. VA...Heat Advisory from 11 AM to 8 PM EDT Saturday for VAZ026>031- 037>040-050-051-053>057-501-502-505-506-526-527. WV...Heat Advisory from 11 AM to 8 PM EDT Saturday for WVZ050>053- 055-502>504. MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ530>534- 537>543.-- End Changed Discussion --&& $$ SYNOPSIS...EST NEAR TERM...BRO SHORT TERM...BRO LONG TERM...EST AVIATION...BRO/EST MARINE...BRO/EST CLIMATE...LWX