Area Forecast Discussion
Issued by NWS Sterling, VA

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943 FXUS61 KLWX 181405 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1005 AM EDT Mon Jul 18 2016 .SYNOPSIS... A surface trough will remain over the region today and a cold front will approach from the north tonight. The cold front will slowly pass through the area Tuesday through Tuesday night. High pressure will build overhead for Wednesday and Thursday. High pressure over the Atlantic will control the weather pattern for late in the week before a cold front approaches the area early next week. && .NEAR TERM /THROUGH TONIGHT/...
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A line of showers and thunderstorms extends from Central NY southwestward to the Ohio River Valley this morning. There has been redevelopment along the front edge of this line as it moves eastward. Across the Mid-Atlantic region...temps are climbing into the 80s with dewpts in the low 70s leading to a muggy/humid morning. The line of showers and thunderstorms will continue to move eastward this morning and move over the mountains around noon. Marginal instability exists right downwind of the convection with a lull across most of the Mid-Atlantic region this morning. Thunderstorms are possible across the Potomac Highlands early this afternoon. Uncertainity increases further east as dewpts are in question. Model guidance has a large spread with dewpts ranging from the 50s to the upper 60s. Most guidance has activity diminishing as it moves into the Blue Ridge region and Northern VA with scattered showers and embedded thunderstorms. However if dewpts stay elevated thunderstorm coverage should be greater. Behind intial line of showers and thunderstorms... redevelopment is possible across WV and may move into the region late this afternoon and into the evening. Best chance will be across the SW...VA Highlands...Shenandoah Valley and central foothills. Prev discussion... Subtropical ridge will remain to our south today while low pressure in the subtropical jet tracks through eastern Canada. The cold front associated with the low will track through the Great Lakes into Pennsylvania today. A southwest flow ahead of the cold front will allow for hot and humid conditions. Max temps will top off in the lower to middle 90s across most locations. Some areas may even reach the upper 90s if convection holds off this afternoon. Dewpoints across most areas should be in the mid to upper 60s...which will cause heat indices to reach the upper 90s to around 100...especially across central Virginia into the Washington and Baltimore metropolitan areas. A few showers and thunderstorms are possible...mainly this afternoon and evening. Guidance diverges a bit on the exact details...but an MCS over the Great Lakes and Ohio Valley early this morning should dissipate as it moves toward our area since it will be running in to more stable air. However...a few showers and thunderstorms are likely to develop on the outlow of this activity this afternoon before building to the south and east into this evening. A surface trough may also enhance convection a bit near the Washington and Baltimore Metropolitan areas into central Virginia. Most guidance has MLCAPE values around 1000 to 1500 J/KG with the higher amounts near and east of the surface trough across the Washington and Baltimore Metropolitan areas into southern Maryland and central Virginia. However...shear profiles remain marginal at best and the stronger shear profiles will be across northern Maryland and eastern West Virginia. There is a threat for isolated severe thunderstorms with locally damaging wind gusts being the primary threat...but as of now the threat of widespread severe is low since the shear and instability will be displaced. Will have to pay attention to latest forecast because confidence is low. Timing issues of when convection would develop on the outflow of the MCS will impact the coverage of showers and thunderstorms this afternoon and evening. The cold front will drop south approaching the Mason-Dixon Line by early Tuesday morning. A few more showers and thunderstorms are possible ahead of the boundary tonight...but coverage should remain isolated to widely scattered due to the loss of daytime heating. Shear profiles will strengthen overnight...and if convection over Pennsylvania becomes organized there may be an isolated threat for severe thunderstorms across northern Maryland this evening. However...confidence is very low. Warm and quite humid conditions will persist ahead of the front tonight. Min temps will 60s in the mountains to the middle 70s in Washington and Baltimore.
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&& .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... The cold front will slowly drop through the area Tuesday. In fact...it may stall out over central Virginia and southern Maryland for much of the afternoon. Tuesday will turn out to be not as hot with temps closer to climo...maxing out in the upper 80s to around 90 for most locations. A few showers and thunderstorms are possible into the evening hours...especially across central Virginia into southern Maryland where dewpoints will be higher closer to or just ahead of the cold front. The cold front will continue to pass through Tuesday night. A northerly flow behind the front will make Tuesday night quite comfortable with dewpoints dropping into the 50s and lower 60s. High pressure will build overhead for Wednesday through Wednesday night...bringing dry and seasonably warm conditions along with lower humidity. && .LONG TERM /THURSDAY THROUGH SUNDAY/... The return and amplification of heat and humidity will be the main story for the end of the week and into the weekend. Thursday will be the "coolest" and least humid of the period with high pressure directly overhead and mostly sunny skies. Looking at highs generally in the 80s to around 90F. Temperatures will ramp up further Friday and Saturday under strong upper ridge extending from the Central US. Could be a few isolated showers or thunderstorms both Friday and Saturday that pop up in any instability or any MCS`s moving along the periphery of the ridge. Likely looking at widespread 90s to locally near 100F possible. When coupled with increasing humidity, heat headlines may be necessary. Frontal system approaches from the northwest on Sunday, and this will increase chances for some scattered showers/storms. However, heat and humidity continue with widespread 90s for highs. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... VFR conditions are expected most of the time through through this evening. A few showers and thunderstorms are possible this afternoon and evening...especially between 18 and 22z. Locally gusty winds are possible along with brief subvfr conditions. Patchy fog is possible overnight into early Tuesday. An isolated shower or thunderstorm cannot be ruled out ahead of the cold front but most places will be dry. The cold front will pass through Tuesday into Tuesday night and winds will shift to the north behind the boundary. A few showers and thunderstorms are possible near KCHO and perhaps toward KIAD and KDCA Tuesday into Tuesday evening. High pressure will bring VFR conditions overnight Tuesday through Wednesday night. Generally VFR expected from Thursday and through the weekend. A few isolated showers/thunderstorms are possible which may bring a brief period of sub-IFR conditions. Patchy fog also possible during overnights, but coverage not likely to be widespread. && .MARINE... A surface trough will remain near the waters through tonight. A south to southwest flow is expected during this time. A small craft advisory is in effect for this afternoon and evening and it will be marginal. Left the SCA in effect due to a deep mixing layer expected. A few showers and thunderstorms are possible this afternoon and evening. Locally gusty winds are possible in any thunderstorms that develop. A cold front will pass through the waters Tuesday into Tuesday night. A few showers and thunderstorms are possible ahead of the boundary...especially across the middle portion of the Bay into the Potomac River. Locally gusty winds are possible in any thunderstorms that do develop. High pressure will build overhead Wednesday into Wednesday night. A pressure surge behind the cold front may cause winds to gust near SCA criteria overnight Tuesday into Wednesday morning. Mostly sub-SCA conditions expected for the end of the week and weekend, however brief intervals of gusts up to around 20 knots possible each night. && .TIDES/COASTAL FLOODING... Tidal anomalies are near normal early this morning. A southerly flow will cause anomalies to increase a bit today...but the flow should be just west of south. Therefore...minor flooding is not anticipated at this time. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 9 PM EDT this evening for ANZ530>543.
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&& $$ SYNOPSIS...BJL NEAR TERM...BJL/HAS SHORT TERM...BJL LONG TERM...MM AVIATION...BJL/MM/HAS MARINE...BJL/MM/HAS TIDES/COASTAL FLOODING...LWX

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