Area Forecast Discussion
Issued by NWS Sterling, VA

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541 FXUS61 KLWX 170127 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 927 PM EDT Wed Aug 16 2017 .SYNOPSIS... High pressure over the region will slowly move east off the coast tonight. A warm front will move northward Thursday into Thursday night followed by a cold front that should pass through the region Friday night. Weak high pressure will build into the area over the weekend. && .NEAR TERM /THROUGH THURSDAY/...
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Weak stalled front remains stretched west to east across the middle of the forecast area. Isolated showers have been forming in this zone, although a general dissipating trend is expected to continue as nocturnal stability increases. Sharp inversion around 14kft (below the freezing level) on the 00Z IAD sounding explains why no lightning has been observed. Overnight, warm advection will develop aloft, resulting in an increase in mid-level clouds, but perhaps not until late. Thus questions remain about low clouds and fog. The NAM has definitely be the most pessimistic model in terms of both, while the GFS only suggests some clouds between 5-7kft. Considered SREF and NARRE probs (which are low for significantly reduced visibility) and thus kept the flavor of the forecast similar, with patchy fog in the typical prone locations. Dew points suggest lows will bottom out in the mid 60s to low 70s. May need to adjust up a little, especially in areas of thicker clouds and to the south of the stalled boundary. Boundary will slowly move northward back across the area during the day Thursday. PW`s will surge significantly, with values likely to be above 2 inches by late afternoon. A weak shortwave and vort max will also cross the region. However, cloud cover will be abundant, limiting CAPE availability somewhat. The result in soundings is a skinny CAPE profile, more suggestive of a heavy rain/flash flooding threat than a severe threat, but the latter can`t be completely ruled out given around 25 knots of shear. The best overlap of shear and instability will be in the northwestern quadrant of the area...with the highest chances of storms in general near and west of the Blue Ridge. With the clouds, temps should end up a little cooler than Wednesday, but still a very warm and sticky day.
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&& .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
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After the trough/max passes and insolation wanes, expect precipitation to wind down during the evening, but with a very moist environment, a few showers may linger. With the clouds and high humidity, lows will stay very high, with 70s being widespread. Any areas that see a little clearing might see some patchy fog. Depending on the location of forcing/boundaries and available instability aloft, there could be a continuation of thunderstorms with locally heavy rain late in the night into Friday morning due to an approaching low level jet. Current model projections suggest this would be over the northeastern quadrant of the area. Friday the next cold front approaches. With more opportunity for sun early in the day and cooling aloft, CAPE should be more plentiful, with shear increasing to around 30 knots. This should result in more of a severe threat versus flash flood threat, though with PW`s still over an inch and a half, any cells that train could still pose a flood risk. With more sun, highs should approach if not reach 90 in the warmer spots. Front passes across the region at night, with chance of precip rapidly declining. However, warmth and moisture at low levels will be slower to exit, with lows likely in the 70s near I-95, cooler in the mountains.
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&& .LONG TERM /SATURDAY THROUGH WEDNESDAY/... A cold front will be stalling to our southeast as a weak high pressure builds over our area on Saturday. Upper level energy could bring some showers Saturday night, with dryness returning on Sunday. High pressure moves offshore as the front slowly pushes away from us and stall over the Carolinas Sunday into Monday. Southerly flow will advect humidity into our area with increasing temperatures into Monday. A pressure trough could enhance afternoon showers and thunderstorms Monday into Tuesday. A cold front will approach from the west on Wednesday with additional showers and thunderstorms possible. High temperatures will be in the mid to upper 80s... some 90s and 70s at higher elevations. && .AVIATION /01Z THURSDAY THROUGH MONDAY/...
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It looks like any remaining showers near a stalled front will avoid the terminals. There certainly are a range of solutions regarding cigs/vsby overnight. NAM has been the most pessimistic with moisture overrunning with widespread IFR or lower, while the GFS only suggest clouds between 5-7kft. Given low IFR probs in the NARRE and SREF, have kept flavor of previous forecast with MVFR at IAD, MRB, and CHO. One caveat to the forecast: if low clouds do develop, they may take some time to lift Thursday morning, until a warm front can lift to the north. Main concern Thursday and Friday will be thunderstorms, mostly during the afternoon and early evening hours, which could result in brief intervals of IFR cigs and vis, along with gusty winds. VFR conditions expected Saturday into Monday, with short periods of sub-VFR conditions Saturday night and maybe Monday afternoon due to possible showers and thunderstorms.
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&& .MARINE...
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Sub-SCA at present, with variable wind directions due to a stalled front as well as isolated showers which have produced outflow boundaries. Southerly winds should develop Thursday morning. An approaching cold front later Thursday into Friday looks likely to bring southerly channeling to the bay and lower Potomac. SCA raised starting late Thursday and likely will need to be extended through Friday. Other marine concern will be thunderstorms each of the next two days, with gusty winds a concern. Mainly dry conditions expected Saturday into Monday, with periods of showers and thunderstorms possibly developing Saturday night and Monday afternoon. Wind gusts are expected to stay below the small craft advisory threshold.
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&& .TIDES/COASTAL FLOODING...
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Anomalies are around a half foot tonight, which will keep water levels below flood stage. A return of southerly winds will bring rising anomalies back to the region on Thursday, with potential minor flooding at sensitive sites by Thursday afternoon/evening and continuing into Friday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 4 PM Thursday to 6 AM EDT Friday for ANZ533-534-537-541-543. && $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...RCM LONG TERM...IMR AVIATION...ADS/IMR/RCM MARINE...ADS/IMR/RCM TIDES/COASTAL FLOODING...ADS/RCM

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