Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 191439 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1039 AM EDT Fri May 19 2017 .SYNOPSIS... A cold front will approach from the north today before passing through tonight. The boundary will stall out to the south later Saturday through Saturday night. The boundary will return north as a warm front into our area later Sunday and Sunday night before a stronger cold front passes through from the west Monday. High pressure will briefly return for Tuesday before another cold front impacts the area Wednesday. && .NEAR TERM /THROUGH TONIGHT/... Mainly clear skies are allowing temperatures to warm quickly into the 80s. Around 90 to lower 90s seems attainable for much of the area, so have made an upward adjustment to the MaxT forecast. It still looks like thunderstorm development mainly holds until after 2 PM for much of the area. Modified 12Z IAD sounding plus model forecasts indicate plenty of instability, so convective forecast as described below seems on track. However, feel like coverage may be pretty decent especially near and south of DC, so have bumped up POPs. There may be a some storms which drop south from PA as well which initiate along the actual cold front. Clusters and storm mergers will pose the severe threat, as deep layer shear is only forecast to be around 20 kt. Convective evolution (how long the threat for showers/storms linger) tonight remains highly uncertain, and is likely dependent on what happens over the Ohio Valley today. Previous discussion: High pressure will remain near Bermuda today while a cold front approaches from the north. An upper-level ridge will remain to our south today and weak shortwave energy will approach the area in the westerly flow aloft. More heat and humidity is expected today due to the flow around the Bermuda high. Max temps will be in the upper 80s to lower 90s for most areas. Did lean toward the warmer guidance due to a downsloping westerly flow that is anticipated. The heat and humidity will lead to an unstable atmosphere once again. A pressure trough is expected to develop overhead...likely acting as the lifting mechanism to trigger isolated to scattered thunderstorms this afternoon into this evening. Despite weak shear profiles...moderate to high instability is expected with over 2000 j/kg expected near and south of the pressure trough. Steep low-level lapse rates are expected and mid-level dry air in the soundings should lower Min Theta E values...causing an elevated threat for locally damaging wind gusts in stronger thunderstorms that do develop. Latest forecast soundings show a bulk of CAPE in the mid and upper-levels of the atmosphere which suggests large hail is possible in stronger thunderstorms as well. Best chance for strong to severe thunderstorms should be along and south of the pressure trough which will setup near Interstate 66 in northern Virginia into southern Maryland. Farther north...thunderstorms are possible as well but a downsloping westerly flow may limit coverage. The cold front will pass through our area tonight from north to south. A north to northeast flow behind the cold front will usher in noticeably cooler conditions by daybreak. Min temps will be in the 50s to lower 60s across most locations...perhaps middle 60s in central Virginia. Isolated to scattered thunderstorms should diminish in intensity this evening due to the loss of daytime heating. However...isolated to scattered coverage of showers may continue through tonight due to the cold front passing through. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... The boundary will move to the south Saturday before stalling out over North Carolina into the Appalachian Mountains Saturday night. An onshore flow behind the boundary will usher in much cooler conditions compared to recent days. Any leftover showers early Saturday morning should dissipate and much of the day will turn out dry. The one exception may be near the Allegheny Highlands. Thunderstorms are likely to develop in the warm sector west of the Appalachians. This activity may propagate into the mountains before dissipating over the cooler air. Max temps Saturday will be in the upper 60s to lower 70s for most locations...with middle 70s possible across central Virginia valleys of the Potomac Highlands. Current thinking is that there will be breaks of sunshine Saturday...but should low clouds from marine air develop quicker than anticipated then high temps will be several degrees cooler. More easterly winds and cool conditions are expected Saturday night through Sunday. The boundary will return north as a warm front Sunday. should remain mainly to our south and west across central Virginia into the Appalachian Mountains. Overrunning ahead of the warm front along with more marine air should cause more low clouds later Saturday night through Sunday. Max temps Sunday will range from the upper 60s in northern and eastern Maryland to the lower and middle 70s across most other locations. A cold front will move into the Ohio VAlley Sunday night. Increasing southerly flow will allow for deeper moisture to move into the area. Shortwave energy ahead of the boundary along with a strengthening low-level jet will likely cause showers to move into the area from west to east. Min temps will hold in the 60s across most areas due to the southerly flow. Also included the slight chance for thunderstorms due to some elevated instability in the forecast soundings. && .LONG TERM /MONDAY THROUGH THURSDAY/... Showers will linger into Monday as the cold front crosses the area. A few thunderstorms will be possible too based on CAPE/shear profiles, but significant instability likely won`t be able to be generated due to saturation. Anticipate there will be a quiet stretch from Monday night through Tuesday night as high pressure crosses the northeast and Mid Atlantic. That said, the return of warm advection on Tuesday suggests that clouds will be making an early resurgence, so the first part of the week won`t be all that sunny. A deeply amplified northern stream trough axis will be pivoting across the CONUS Wednesday into Thursday. The local impact will be for moisture to be drawn off the Gulf of Mexico ahead of a cold front Wednesday. While PoPs will reside in the database both Wednesday and Thursday, the best chance (and higher PoPs) will be on Wednesday. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... VFR conditions are expected through tonight for most of the time. However...scattered to perhaps numerous thunderstorms are expected this afternoon and evening. Some thunderstorms will be capable of producing locally damaging wind gusts and large hail. Confidence in severe thunderstorms is low but the best chance appears to be near KIAD..KDCA and KCHO closer to a pressure trough. A cold front will pass through the terminals later tonight and northeast winds are expected behind the boundary. Onshore flow will continue Saturday through Sunday. The marine air will increase the chances for low clouds...especially later Saturday night and Sunday. More low clouds are likely Sunday night along with increasing chances for showers ahead of a cold front. Flight restrictions will be likely Monday as a cold front crosses the terminals. Its too soon to detail when/where/how low, but an improving trend should take place during the afternoon/evening hours. VFR likely will prevail Monday night into Tuesday. && .MARINE... Bermuda high will continue to influence the weather pattern through today. A southwest flow will continue through this morning. A pressure trough will develop across northern Virginia into southern Maryland later this morning through this afternoon. Southwest winds are expected south of the trough...but winds will shift to the west or even slightly north of west across the Northern Chesapeake Bay and upper Tidal Potomac River north of the trough. Across these areas...winds will approach SCA criteria later this morning through this afternoon but average winds through the mixed layer indicate gusts should be limited. A cold front will pass through the waters later tonight. The pressure surge with the cold front will cause north to northeast winds behind the boundary to gust around 20 knots. A Small Craft Advisory is in effect for the waters later tonight into Saturday morning. An onshore flow will continue through Sunday before turning southerly Sunday night ahead of an approaching cold front. A Small Craft Advisory may be needed for portions of the waters Sunday night. Behind a cold front Monday will be the best chance to see Small Craft gusts in the extended part of the forecast. High pressure builds Monday night and Tuesday. && .CLIMATE... BWI broke a record high minimum temperature for Thursday...May 18th. The high minimum temperature was 70 degrees which broke the old record of 68 degrees in 1900. More unseasonably hot conditions are expected today. Record heat is not anticipated for BWI or DCA today...but it is possible for KIAD. Here is a list of record high temperatures and record high minimum temperatures for today...May 19th. Friday (05/19) Record Warm Lows: DCA: 72 in 2015 BWI: 75 in 1877 IAD: 66 in 2015 Friday (05/19) Record Highs: DCA: 96 in 1997 BWI: 98 in 1962 IAD: 92 in 1997 && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 11 PM this evening to 8 AM EDT Saturday for ANZ530-531-538-539. Small Craft Advisory from 1 AM to noon EDT Saturday for ANZ532>534-537-540>543. Small Craft Advisory from 1 AM to 8 AM EDT Saturday for ANZ535- 536. && $$ SYNOPSIS...BJL NEAR TERM...BJL/ADS SHORT TERM...BJL LONG TERM...HTS AVIATION...BJL/ADS/HTS MARINE...BJL/ADS/HTS CLIMATE...BJL is the U.S. government's official web portal to all federal, state and local government web resources and services.