Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 270749 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 349 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 /UNTIL 6 PM THIS EVENING/... Clear skies and light winds have led to radiation cooling...with temperatures across the area nearing the dewpoint. Shallow low-level moisture (as sampled by the 00z IAD RAOB and forecast by model soundings) is trapped beneath low-level inversion. This has led to the gradually development of shallow fog (and some low clouds) east of the Blue Ridge over the past few hours. Fog is expected to spread in coverage and may become locally dense later this morning...and may necessitate a dense fog advisory if the areal extent becomes large enough. Any fog should quickly burn off shortly after sunrise as decent mixing commences and erodes low-level inversion. 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. && .SHORT TERM /6 PM THIS EVENING 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/... The backdoor cold front will still 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. && .AVIATION /08Z THURSDAY THROUGH MONDAY/... VFR expected to develop at most sites over the next several hours as fog continues to expand/develop across the area. Fog could become locally dense...with LIFR possible near sunrise. Fog should rapidly dissipate a few hours after sunrise as strong mixing develops. VFR then expected to remain through the remainder of the day. 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 very low at this point. 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. && .MARINE... Southerly flow will increase today...with low-end small craft wind gusts near 20 kts requiring a SCA (begins at noon for all waters). Winds will begin to diminish overnight...with SCA winds becoming less likely. 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 influence 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. && .TIDES/COASTAL FLOODING... Tidal anomalies remain elevated across the area. Coastal Flood Advisory continues for Straits Point...as it is currently exceeding minor flood stage. Added a Coastal Flood Advisory for this tide cycle at Annapolis as anomalies have been increasing and latest forecast places tidal levels a tenth or two above minor flood stage. SNAP-EX ensemble guidance also confirms this trend. Minor flooding is also possible this morning at DC...though confidence is still to low to act on it now...will continue to monitor. 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). && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory until 10 AM EDT this morning for MDZ014. Coastal Flood Advisory until 6 AM EDT early this morning for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory from noon today to 5 AM EDT Friday for ANZ531>534-537-539>541-543. Small Craft Advisory from noon today to midnight EDT tonight for ANZ530-535-536-538-542. && $$ SYNOPSIS...MSE NEAR TERM...MSE SHORT TERM...MSE LONG TERM...HTS AVIATION...MSE/HTS MARINE...MSE/HTS TIDES/COASTAL FLOODING...MSE

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