Area Forecast Discussion
Issued by NWS Sterling, VA

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076 FXUS61 KLWX 271348 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 948 AM EDT Thu Apr 27 2017 .SYNOPSIS... A weak cold front crosses the area from the west tonight. A backdoor front will slide into the area Saturday evening into Sunday before lifting north as a warm front early Monday. A stronger cold front will move through the region from the west Monday night. && .NEAR TERM /THIS AFTERNOON/...
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Atmosphere is in a rather stagnant pattern. Without a wind to mix out low level moisture has manifested itself as low cloud and fog. On visible satellite this can be seen to be eroding along the edges. This will continue during the morning. By afternoon the sun should reappear. Remainder of today will feature warm weather as ridging slides across the area and moderately strong southerly flow develops. Southerly wind gusts up to 25 mph by afternoon with highs in the L/M80s and dewpoints rising into the L/M60s.
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&& .SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/... Cold front will be approaching the area from the west this evening and will weaken as it crosses the area. Thunderstorms likely west of the Blue Ridge...as time of arrival near or shortly after sunset will allow some instability to remain. While forecast soundings suggest instability will be rather limited across this area (due to relatively poor mid-level lapse rates)...strong shear could allow for some organization into convective clusters...which could briefly become marginally severe with isolated wind damage. SPC has outlined this area in a marginal risk because of this. Any storms will be moving into a progressively hostile environment as they progress east of the Blue Ridge...since instability rapidly wanes across this area. Thus, expect any storms to rapidly become elevated and weaken as they approach the metros...with a rather precipitous decline in lightning activity. Warm weather continues Friday...with afternoon highs similar (if not a couple of degrees cooler) to today. Though, cold front will have shunted best moisture to the south...so dewpoints will be slightly lower. Warm nose near 800 mb will keep the majority of the area moderate-to-strongly capped which will suppress convection across much of the area. One exception could be across across our far southern counties closer to where better moisture resides...with an isolated shower/thunderstorm possible. Currently believe any activity would be just to our south...so left forecast dry for now. Saturday appears to be the warmest day of the period...with highs nearing 90F and dewpoints peaking near 70F. This will create moderate instability by early afternoon (MLCAPE 1000-2000 J/kg). Backdoor front may slide into the area during the afternoon...which may focus thunderstorm development. Still some uncertainty with regards to the southward spatiotemporal evolution of front...and thus whether we would be able to take advantage of the primed environment. Best chances for thunderstorm activity would reside along/near front with strong thunderstorms possible given strong instability and shear (EBS > 40 kts). Any thunderstorms would become increasingly elevated overnight with a gradual weakening trend. SPC has introduced a Day 3 Marginal Risk across the northern half of the forecast area. Spatial location and category look very good considering aforementioned discussion of parameters and uncertainty in frontal position. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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The backdoor cold front will be lurking across the forecast area at the start of the extended forecast period (Sunday). Present indications are that northeast/north-central Maryland will be most affected. Exact frontal location/movement will be key, as temperatures north/east of the boundary will be in the 70s (at best), while it`ll be approaching 90 south/west of the front. Confidence improving a little vs. yesterday, but still not all that high. Will seek to strike a balance in the forecast. Will need the warmer air mass to generate enough instability for thunderstorms. The front will retreat north Sunday night, but without much fuel for storms. Mid levels will actually be on the dry side. Surface dewpoints, on the other hand, will be high for late April. Most locations will have lows in the 60s. The forecast area will be in the warm sector before the arrival of a fairly well-defined cold front Monday night. The PM hours have the best chance of seeing thunderstorms. It remains to be seen how strong those storms will be, as temperatures won`t fully recover from the effects wedge, and thus instability values/lapse rates will be on the lower side. Behind the front, surface high pressure will reside under zonal flow. That would support scattered clouds and perhaps a few late-day sprinkles/showers.
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&& .AVIATION /14Z THURSDAY THROUGH MONDAY/...
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With the exception of DCA ceilings/vsby have been oscillating/doing a slow improvement but are still either IFR or MVFR. This afternoon VFR is expected to return. A few showers/thunderstorms are possible this evening into the overnight. The best locations for thunderstorms would be at KMRB...with less of a chance as activity spreads east of the Blue Ridge. Could be some fog around again tonight...though confidence is low. VFR expected Friday. Shower and thunderstorm chances then return Saturday as frontal boundary slides into the area. Some strong storms are possible depending on where the boundary sets up (best chance across northern half of the area). The backdoor cold front will be lurking across the terminals on Sunday, especially in the morning. There is a potential for flight restrictions with east winds to the north of the front. Frontal position remains a challenge to forecast. BWI/MTN stands a better chance at being impacted than CHO though. The front will return north by Sunday night. There could be some showers/storms Sunday PM, but confidence and areal coverage both limited. A much better defined cold front will arrive from the west Monday PM. Scattered to numerous thunderstorms appear to be a good bet at this time, with brief flight restrictions possible within storms.
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&& .MARINE...
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SCA begins at noon for all waters. Winds will begin to diminish overnight. Relatively light winds expected over the waters Friday into Saturday. Shower and thunderstorms are possible Saturday afternoon as a front slides into the area...some storms could be strong. The waters will be influenced by a backdoor cold front on Sunday. Mixing likely will be poor, and the gradient won`t be all that strong either. The front will retreat by Sunday night, permitting stronger southerly flow on Monday. A better chance at Small Craft caliber gusts will develop ahead of a cold front. But, mixing may not be ideal at the water/air temperature interface.
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&& .TIDES/COASTAL FLOODING...
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Tidal anomalies remain elevated across the area. Coastal Flood Advisory continues for Straits Point...as it is currently exceeding minor flood stage. Southerly flow today will keep tidal anamolies elevated. Appears most sites will remain below minor flooding stage for todays high tide cycle (it is the lower of the two) but will then threaten minor flooding tonight (especially the sensitive sites).
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory until 1 PM EDT this afternoon for DCZ001. MD...None. VA...Coastal Flood Advisory until 1 PM EDT this afternoon for VAZ054. WV...None. MARINE...Small Craft Advisory until 5 AM EDT Friday for ANZ531>534-537- 539>541-543. Small Craft Advisory until midnight EDT tonight for ANZ530-535- 536-538-542.
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&& $$ .UPDATE...WOODY! .PREVIOUS...MSE/HTS

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