Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 161346 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 946 AM EDT Tue Jul 16 2019 .SYNOPSIS... A warm front will lift northeastward across the area this morning. The remnants of Barry will then affect the region late Wednesday into Thursday. High pressure will return for Friday and into the weekend. && .NEAR TERM /UNTIL 9 PM THIS EVENING/... A warm front remains positioned across the northern reaches of the forecast area this morning. Its subtle to find, but based on dewpoints in the lower-mid 70s DC south vs dewpoints in the upper 50s/lower 60s in central Pennsylvania and upstate New York there definitely is a boundary in place. This front will lift northeastward and through the region by this afternoon with moist southwest flow increasing out ahead of the remnant trough of Barry. There will be mid/high clouds streaming overhead through much of the day, but despite this, still expecting high temperatures to reach into the 90s for the majority of the region. This combined with dew points reaching 68-73F will provide heat index values in the low to mid 90s west of the Blue Ridge to the upper 90s and even some low 100s east of the Blue Ridge. Do expect to keep heat index values below advisory threshold though (105F east of Blue Ridge). This increasing warmth/humidity will promote the development of 1000-2000 J/KG of MLCAPE across the region this afternoon. Based on the upper end of the forecast temperature/dewpoint range was able to generate over 2000 J/kg MLCAPE (and over 3000 J/kg of SBCAPE) on a modified 12Z LWX RAOB. Despite lack of strong forcing and upper level ridging, some weak energy in the flow aloft combined with terrain circulations should be enough to spark scattered shower/thunderstorm development in the early to mid afternoon over the Allegheny/Potomac Highlands. There should also be enough westerly flow aloft for some propagation eastward towards the I-95 corridor later this afternoon. The extent of eastward propagation will depend on how organized any convection can become, the more organized the further east they should be able to go. Differences currently exist in hi-res guidance as to the evolution. Given ample CAPE and 0-6KM shear of roughly 20 knots, some isolated severe activity is possible, especially with any organization. && .SHORT TERM /9 PM THIS EVENING THROUGH THURSDAY NIGHT/... Any shower/thunderstorm activity will wane this evening and a mostly dry overnight is expected. Lows will range through the 70s. The remnant trough that was once Barry will then move into the Ohio Valley on Wednesday, with plenty of heat and humidity out ahead of it. Expecting highs well into the 90s with dew points in the low 70s. Heat index values by the afternoon likely reach into the 100- 105F range and heat advisories will likely be necessary for portions of the region. Additionally, as the trough approaches, shower/thunderstorm chances will increase during the afternoon and evening, and likely linger overnight. Given stronger forcing and slightly stronger wind field, some thunderstorms may become strong to locally severe, in addition to having the ability to produce very heavy rain. Lows from 70-80F Wednesday night. The trough will be overhead through at least the first half of Thursday, and should pull eastward by late in the day. Scattered shower/thunderstorm development is expected again, and will likely happen sooner in the day than either today or Wednesday. Therefore, given more cloud cover and earlier chances for precipitation, temperatures will likely be several degrees "cooler" than Wednesday, although dew points may tick up a degree or two. Highs Thursday will generally be from 87-92F, with heat index values in the upper 90s to low 100s. Lows Thursday night from 70-80F. && .LONG TERM /FRIDAY THROUGH MONDAY/... Dangerous heat is becoming increasingly likely Friday through this weekend. 850/925 hPa temps of 25/30 C respectively are expected Friday through the weekend. Full mixing would result in surface temperatures exceeding 100 degrees F. Given uncertainties in just how much we mix, not willing to go quite that high this far out in the forecast, but the potential is there for multiple days near or exceeding 100. This combined with high humidity will likely result in dangerous heat, with heat index values of 105 to 115 well within the realm of possibility. Given the fact that high heat and humidity will likely occur for multiple days in a row, this compounds the danger of excessive heat. Please visit www.weather.gov/safety/heat for tips to beat the heat. A rather strong trough and associated surface cold front approaches early next week, and when it collides with the heat and humidity in place, it seems possible that strong thunderstorms may result. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... Predominantly VFR conditions are expected through Thursday night. Each afternoon and evening will see chances for scattered showers and thunderstorms, and therefore brief reductions to visibilities/ceilings and some localized gusty winds. Any locations that see rain during the day/evening may also see some patchy fog the following night. Mainly VFR Fri-Sat, but very hot and humid with a pop up shower or thunderstorm possible. && .MARINE... Sub-SCA conditions are expected through tonight. Southerly winds less than 15 knots are expected today and tonight. Some scattered showers/thunderstorms with localized gusty winds are possible late this afternoon and evening, and Special Marine Warnings may become necessary. Southerly winds will then increase Wednesday and Wednesday night and SCA conditions are possible. In addition, chances will continue for scattered showers and thunderstorms. Largely sub-SCA conditions return Thursday, although chances persist for showers/thunderstorms. Sub-SCA likely Friday-Saturday, but very hot and humid. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...MM NEAR TERM...MM/HTS SHORT TERM...MM LONG TERM...DHOF AVIATION...MM/DHOF MARINE...MM/DHOF

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