Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 210749 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 349 AM EDT Sun May 21 2017 .SYNOPSIS...
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A quasi-stationary front will remain across the Carolinas today before lifting north into the area as a warm front late today. A cold front will move through the area from the west Monday. High pressure will briefly return Monday night before low pressure develops over the Carolinas Tuesday and moves northeast off the DelMarVa into Wednesday. Upper-level low pressure will then linger over the area into Friday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Today`s weather will be quite similar to yesterday...as persistent easterly onshore flow...resulting from high pressure to our NE off the NY/NJ coast and low pressure over western Great Lakes...keeps marine layer entrenched over the area. As of 3 AM...quasi-stationary front has settled over far southern North Carolina...it will remain there through much of today...with our area remaining in the cool sector. Low clouds will remain plentiful through the day hindering insolation and keeping temperatures near 70 F. It is also expected to remain dry through the day...except for far SW portions of the CWA where a late day shower could approach. Front will return northward as a warm front overnight. This will act to increase rain chances from SW to NE as WAA lift develops in vicinity of frontal boundary. Prefrontal showers chances continue overnight.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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Shower chances continue Monday morning as cold front approaches from the west. Front will struggle to move quickly through the area...as it remains well displaced from parent low over northern Canada. By afternoon...enough instability could reside ahead of the front for a few thunderstorms to develop...though, airmass recovery from morning rain showers casts doubt on how far west/north this would occur. Best chance for thunderstorm development across central VA into S MD. Weak shear/thermo space suggests disorganized general thunderstorms would be the primary mode. Respite from rain chances late Monday into Tuesday morning as high pressure briefly builds into the area. Latest guidance develops low pressure along frontal boundary to our south Tuesday and moves it up to near the DelMarVa coast Tuesday night. This will act to bring rain chances back to the area as we remain in the western quadrant of the lows track. Tight gradient suggests western areas might not see much rainfall...with best chances along/east of I-95. Current QPF probs suggest 0.5/1/1.5 inches at the 25/50/75th percentiles with the two systems through 8 AM Wednesday...with highest totals along/east of I-95.
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&& .LONG TERM /WEDNESDAY THROUGH SATURDAY/...
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The main synoptic feature dictating the forecast in the extended periods remains the deep trough axis pivoting across the Great Lakes Tuesday and tilting negatively across New England by Friday. There is model agreement that there will be a cutoff low somewhere in the base of the trough Thursday; where and how deep understandably still to be determined. Regardless, the period will be wet/unsettled. Precipitation will be dictated by location of vorticity maxima and enhanced by diurnal heating. PoPs will be likely or close to it. However, there will be dry periods. Too much moisture within the column to hope for much sun, although the upper layer does dry out by Thursday. Regarding thunder, ECMWF suggests there will be mean layer instability both Wednesday and Thursday PM, while GFS is focusing on only Thursday. Don`t believe these details can be resolved at this timeframe, so will preserve chance of thunderstorms both days. No good reason right now to deviate temps much from model blend...which is climo. The trough axis will move northeast of the area Friday, providing the area with northwest flow on the back side of the deepening surface cyclone. That setup suggests that precipitation chances will be decreasing as drier air infiltrates the area. Cold advection will initially keep temperatures down. The warm-up begin Saturday with the return of warm advection.
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&& .AVIATION /08Z SUNDAY THROUGH THURSDAY/...
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Cold front remains to the south today over the Carolinas...with persistent easterly flow around high pressure offshore to our northeast keeping low clouds across the area. VFR cloud decks should prevail...with bases 3.5-4.5 kft...though brief periods of MVFR ceilings are possible. Warm front lifts northward into the area late today and a cold front will cross the area Monday. These features will allow rain to develop with periods IFR (to perhaps locally LIFR) developing tonight due to low ceilings. Improvement is expected later Monday as cold front pushes east of the area. Coastal low may create more aviation restrictions late Tuesday. The forecast Wed-Thu will be unsettled, with numerous opportunities for flight restrictions. However, showers (and perhaps thunderstorms) will be hit-or-miss albeit numerous. The afternoon and evening hours should be preferred for showers/storms, but it is too soon to be detailed with forecast evolution.
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&& .MARINE...
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Westerly winds gusting 10-15 kts will prevail through much of today...before winds turn southerly later today as warm front begins to lift back northward. Small Craft Advisory was issued to account for low-end SCA criteria wind gusts starting late this afternoon through tonight. Wind gusts may remain elevated Monday...though confidence is currently low. Lighter winds expected late Monday into Tuesday as high pressure briefly builds over the area. SCA wind gusts could return late Tuesday depending on the track of the coastal low...best chances across southern portion of the Bay...which is closer to the current forecasted track. The waters will be under the influence of an upper level storm system crossing the area Wednesday and Thursday. Numerous showers/perhaps thunderstorms will result. Gradient flow likely won`t be that strong, although a coastal storm could develop late Thursday. At this stage will keep winds under any Advisory threshold.
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&& .TIDES/COASTAL FLOODING...
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Easterly onshore has allowed tidal anomalies to rise rather quickly. A Coastal Flood Advisory was issued for St. Marys county as observational trends coupled with most recent guidance suggested that Straits Point would exceed minor flood stage during this mornings tide cycle. Annapolis may also approach minor tidal flooding later this morning...though probability is too low at this time...and will continue to monitor. Anamolies increase further today...with minor tidal flooding likely at Straits (Coastal Flood Advisory extends through tonights cycle), Annapolis, and DC. Other sites may also approach minor flooding tonight. Evacuation from the estuary appears unlikely through at least Monday (and perhaps longer)...which will keep tidal levels elevated with continued minor flooding possible at the most sensitive sites near times of high tide.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Coastal Flood Advisory from 8 AM this morning to 3 AM EDT Monday for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory from 5 PM this afternoon to 5 AM EDT Monday for ANZ530>533-538>542. Small Craft Advisory from 8 PM this evening to 8 AM EDT Monday for ANZ534-537-543. Small Craft Advisory from 8 PM this evening to 5 AM EDT Monday for ANZ535-536.
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&& $$ 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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