Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 270753 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 353 AM EDT Thu Jul 27 2017 .SYNOPSIS... An upper-level disturbance will pass through the area this afternoon into this evening. Low pressure will intensify as it moves into the area Friday. The low will slowly pass through the area Friday night into Saturday. The low will stall out near the Mid-Atlantic coast through Monday. High pressure is expected to build overhead during the middle portion of next week. && .NEAR TERM /THROUGH TONIGHT/... High pressure has moved off the Mid-Atlantic and New England coast early this morning. An onshore flow has developed...and some low clouds are expected...especially for locations along and east of the Blue Ridge and Catoctin Mountains. Patchy fog is also expected in sheltered valleys and rural areas. The high will remain offshore throughout this afternoon and a southerly flow will usher in a return of seasonably warm and more humid conditions. more humid conditions. An upper-level disturbance will pass through the area this afternoon into this evening and a surface trough will also pass through the area during this time. Isolated to scattered showers and thunderstorms are expected during this time. Shear profiles will strengthen a bit in response to the shortwave energy moving through and this may cause a few thunderstorms to become severe. However...widespread severe weather is not expected since the forcing will be weak and shear profiles will still be marginal. Locally damaging wind gusts are the primary threat. Max temps will be in the mid and upper 80s across most locations. The shortwave energy and surface trough will move off to the east tonight while a stronger upper-level low digs through the Great Lakes. Our area will remain in between these systems...so any showers will likely remain isolated to scattered. Min temps will range from the 60s in most areas west of the Blue Ridge Mountains to the lower and middle 70s near Washington and Baltimore. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... Closed off upper-level low will track through the Ohio Valley Friday and into our area Friday night. This system will interact with a cold front to our north...causing surface low pressure to develop along the boundary. The upper-level low and surface low will strengthen as it moves into our area Friday night. Coastal low pressure is expected to develop near the Delmarva Peninsula Friday night and latest guidance causes this system to remain nearly stationary around this area through Saturday night. This is because the closed upper-level low cuts off from the jetstream. This makes sense because of a split flow in the jetstream and the upper-level ridge that is building over the Rockies. As the low approaches the area Friday...showers will become more widespread along with some thunderstorms. Deep moisture in place suggests that showers and thunderstorms will be locally heavy. The widespread showers and thunderstorms are expected to continue through Friday night as the low pressure strengthens overhead. Strongly considered a Flash Flood Watch with this cycle...but held off for now. The reasons for that are that there is still some uncertainty as to exactly where the heaviest rainfall is going to setup. The heaviest rain is likely to be just north of the surface low and near the 850mb warm front...where the most frontogenetical forcing is expected. Latest guidance seems to paint this area across northern and central Maryland...northern Virginia...eastern West Virginia and toward the Washington and Baltimore metropolitan areas. However...there is some uncertainty with some guidance showing that band a bit farther south toward central Virginia and other guidance showing it closer to the Mason-Dixon line. Most likely rainfall amounts for Friday through Friday night are around 1 to 3 inches...again with the best chances of the highest amounts being across the areas mentioned. However...locally higher amounts are possible due to the convective nature of the precipitation. The low is expected to stall out near the Delmarva Peninsula Saturday through Saturday night. A gusty north to northeast flow is likely along with more rain. Unusually cool conditions are expected due to the cloud cover and rain. Rain may also be locally heavy with the upper-level low overhead. This may produce additional flooding concerns. The 00z guidance is the first to consistently show this solution so confidence is low. However...the trend over the past couple days has been slower and farther south/west with this system for Saturday through Saturday night. Therefore...the forecast has been adjusted to account for this. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... Guidance has slowed considerably with the departure of upper-level low late in the weekend as it remains cutoff from the main belt of westerlies to its north. Thus, unsettled weather may now remain into Sunday and perhaps even Monday as the upper-level low only gradually pulls away from the area. Precipitation potential during this time will be tied to position of the low -- the closer it remains to our area, the greater the potential. Low would become quasi-vertically stacked, which would generally limit precipitation intensity. Though, antecedently we may not be able to handle much extra rainfall after previous days rainfall. Low will eventually move east of the area with drying conditions behind it. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... MVFR cigs from low clouds are expected across the eastern terminals this morning. Patchy fog is possible elsewhere. Cigs should improve later today. Isolated to scattered thunderstorms are expected this afternoon and evening. A couple storms may be severe...with damaging winds the primary threat. Low pressure will approach the terminals Friday before moving overhead and intensifying Friday night. Widespread showers and thunderstorms are expected to develop and rain may be locally heavy. A few storms may also be severe. More rain is likely Saturday and Saturday night along with Subvfr cigs and vsbys as the low stalls out near the Delmarva Peninsula. Gusty north to northeast winds are also possible during this time. Rainfall may linger Sunday (and perhaps into Monday) as low pressure meanders near the area. Improving conditions expected once low moves further out to sea by the middle of the week. && .MARINE... High pressure will remain offshore through tonight and an onshore flow is expected. Will continue with the SCA for middle portions of the Bay and the lower Tidal Potomac River through tonight...but it will be marginal. Low pressure will approach the waters Friday before strengthening overhead Friday night. The low will likely stall out near the Delmarva Peninsula Saturday through Saturday night. Gusty winds are possible with the low nearby...especially Friday night through Saturday night. Rainfall and gusty winds AOA Small Craft Advisory criteria may remain Sunday into Monday as low pressure remains nearby. && .HYDROLOGY... Heavy rain is becoming increasingly likely later Friday through Saturday night. There is still some uncertainty as to where the heaviest rain is to setup...but based on the latest guidance the most likely scenario is across northern Virginia...northern and central Maryland...eastern West Virginia and toward the Washington and Baltimore Metropolitan area. Most likely rainfall amounts during this time will be around 3 to 5 inches. This may cause minor flooding...particularly across the Potomac River Basin. Latest MMEF shows minor flooding possible along portions of the Potomac River Basin later Saturday into Sunday. Confidence remains low at this time due to some uncertainty as to exactly where the heaviest rain will setup. There is a chance that the heavier rain band could be a bit farther north or even a bit farther south...and this would have a significant impact on river levels this weekend. && .TIDES/COASTAL FLOODING... Persistent southerly flow has allowed tidal anomalies to increase to over a foot with Coastal Flood Advisories currently in effect for this morning at Straits and Annapolis and then again this evening at Straits Point. Advisories may also be needed at Baltimore and/or DC for this mornings high tide cycle...though, will continue to monitor, as confidence is too low at the moment. Low pressure is expected to impact the area Friday into the weekend, bringing the potential for elevated tidal anamolies with a risk of coastal flooding. Confidence is timing and areal extent of the threat is currently low due to uncertainty on the track/timing of the low. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory until noon EDT today for MDZ014. Coastal Flood Advisory until 9 PM EDT this evening for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory until 6 AM EDT early this morning for ANZ531-536-539. Small Craft Advisory until 2 AM EDT Friday for ANZ532>534-537- 540-541-543. && $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...MSE AVIATION...BJL/MSE MARINE...BJL/MSE HYDROLOGY...BJL TIDES/COASTAL FLOODING...MSE

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