Area Forecast Discussion
Issued by NWS Sterling, VA

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043 FXUS61 KLWX 290808 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 408 AM EDT Mon May 29 2017 .SYNOPSIS...
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Upper level low pressure will drift east across the north side of the Great Lakes this week. In the meantime, weak high pressure will prevail over the Mid Atlantic outside of weak cold frontal passages Tuesday night and Wednesday night. Low pressure approaches from the west this weekend.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Early this morning, low pressure centers are located near the Great Lakes and east of the DelMarVa. A warm front links the two lows but curves south of the LWX CWA. Thus most of the area is locked in a stable marine layer of low clouds, fog and drizzle. Some observations have briefly hit 1/4 mile, so will have to keep an eye on dense fog advisory potential, but on the whole, this is more of a low stratus situation. A cold front trailing the low to our west will push eastward today, overtaking the stalled warm front. This front will act to scour out the low clouds through the morning hours, but perhaps taking until early afternoon in the Baltimore area. With the low offshore and high pressure remaining to the northeast, it is looking like the front will get hung up around the Chesapeake Bay this afternoon. A temperature (as well as cloud) gradient will be in place across northeast MD...and have thus shaved highs several degrees east of Baltimore. Elsewhere, highs should surge into the 80s. In addition, the frontal convergence zone may spark a few showers and perhaps a thunderstorm across the eastern quarter of the area. The front becomes more nebulous tonight, drifting to the south. Onshore flow will be re-introduced, bring a return of lower clouds, especially the northeastern quadrant of the area. Fog and reduced visibility will be possible, whether due to low clouds (east) or radiational cooling of the moist airmass (west). Low pressure will develop along the frontal zone to our south, which could bring a shower threat to the far southeast, but otherwise dry. Lows will be in the upper 50s to mid 60s.
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&& .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
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Upper level low pressure will continue to spin over Ontario Tuesday and Wednesday with the jet stream located just to our northwest. Surface features will continue to be less defined though, as weak fronts (more pressure troughs than huge airmass differences) waver across the region. The first will push southeast on Tuesday afternoon. With low stratus to start the day, it is uncertain how quickly and how far east it will erode. West of the marine air, modest instability will develop. Deep layer shear increases in magnitude with northward extent. Thus there is some risk of strong/gusty thunderstorms developing ahead of the front. SPC has placed northern parts of the area in a Marginal Risk. This front will lose its definition as it slips southward Tuesday night, ending the chance for showers. A final and slightly more well- defined front will push south Wednesday afternoon and evening. While low level convergence isn`t great, some scattered showers and storms will still develop along the front. The front will align better with diurnal heating to our west, so expect a decreasing trend for any storms that enter the area. Once again though, deep layer shear will be strong, so any organized storms could have a gusty wind threat. A Marginal Risk is in place for the northwestern half of the area. Highs both days will be in the 70s and 80s with lows in the 50s and 60s.
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&& .LONG TERM /THURSDAY THROUGH SUNDAY/...
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Canadian high pressure will dominate Thursday as a trough passes aloft. Warm air will begin returning northward on Friday, but by Saturday the next cold front is already dropping back south across the region as another trough rotates around the stalled upper low over southern Canada. That front looks to stall south of the region by Sunday as the upper low starts moving further east, with the front perhaps lifting back north as a warm front early next week. We will have a chance of showers and thunderstorms with the front dropping south across the area late Friday into Saturday. By Sunday, with the potential for the front to stall south of us, the weather is uncertain, but if high pressure builds in strongly enough from the north, it could be dry. Temperatures will mostly be cooler than normal, with the warmest day likely Friday.
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&& .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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IFR/LIFR conditions through daybreak with an onshore flow. How quickly cigs lift this morning will depend if the warm front is able to lift north, or if cold frontal passage will be required to mix out the clouds. Regardless, improved conditions are likely by early afternoon, except perhaps near MTN where the front may become hung up. A few showers may redevelop around Baltimore this afternoon, but not enough coverage to put in the TAF. The front will not have a lasting effect though, with onshore flow redeveloping tonight. Low MVFR or IFR likely in Baltimore area, and possible at the rest of the terminals except CHO. It`s uncertain how quickly these clouds will erode on Tuesday as well. Where they do, thunderstorms will be possible during the afternoon and evening as the next boundary drops southeast. It`s possible the low cloud pattern repeats Tuesday night as the front washes out. A more well-defined cold front will push through late Wednesday with scattered showers and thunderstorms possible. Mainly VFR later this week with high pressure returning. Thunderstorm chances are maximized late Friday into Staturday with a passing front.
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&& .MARINE...
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Overall light flow is expected on the waters through Wednesday. Expect variable wind directions as weak boundaries waffle across the area, though onshore flow may persist more often than not until a more defined cold front passes through Wednesday night. With the frontal passage after sunset, am not thinking strong winds will be found in its wake. Also can`t completely rule out a few thunderstorms each day, mainly focused in the afternoon and evening, although it`s uncertain how many reach the waters at this time. Sub-SCA winds expected later this week. Thunderstorm chances increase late Friday with an approaching front.
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&& .TIDES/COASTAL FLOODING...
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Water levels remain high and should continue above normal for the next few cycles due to the persistent onshore flow. Likely to continue reaching minor flood at many sites and advisories remain out into early this afternoon. Guidance also suggests advisories will need to get extended or reissued for the next cycles late tonight and tomorrow. After Tuesday, northerly flow may finally allow anomalies to start falling.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory until 2 PM EDT this afternoon for DCZ001. MD...Coastal Flood Advisory until 1 PM EDT this afternoon for MDZ014-018. Coastal Flood Advisory until 10 PM EDT this evening for MDZ017. VA...Coastal Flood Advisory until 2 PM EDT this afternoon for VAZ054. WV...None. MARINE...None.
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&& $$ SYNOPSIS...RCM NEAR TERM...ADS SHORT TERM...ADS LONG TERM...RCM AVIATION...ADS/RCM MARINE...ADS/RCM TIDES/COASTAL FLOODING...RCM

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