Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 180154 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 954 PM EDT Thu May 17 2018 .SYNOPSIS... A front will remain nearly stationary across the region through Saturday. Ample moisture, from a non-tropical low pressure system over the northeast Gulf of Mexico and Bermuda high pressure off of the Southeast U.S. Coast, will bring the potential for heavy rain and flooding through Saturday morning. A cold front will advance from the northwest late Sunday, before becoming stationary over the mid-Atlantic at the beginning of next week. && .NEAR TERM /THROUGH TONIGHT/... At this moment radar shows the main precipitation band towards the southeastern part of our CWA with with light to moderate rainfall coinciding with the boundary that has been over/near us recently. Scattered to isolated showers are also observed for the rest of the CWA. The high moisture environment was captured again by the 00Z balloon with PWATs at 1.64 inches. Instability is limited over our CWA, therefore as the precipitation moves north -as suggested by guidance- expecting mainly stratiform precipitation overnight, with periods of heavy rain possible. Flood Warnings are in effect for some areas over North- Central MD, Eastern WV and portions of Northern Virginia. Flood Watch continues in effect through Saturday morning. Previous forecast... The 12Z GFS has strong omega throughout the vertical overnight, associated with wind convergence throughout the vertical, and warm air advection at the low levels as winds turn easterly bringing in Atlantic moisture. The northern portion of the CWA will see a brief respite from moderate or heavy rain until midnight, then the areas of convection from later this evening will move into the northern half of the CWA overnight, and be heavy at times. && .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... Deep moisture is still present from the surface to the tropopause through Saturday morning, creating not much of a change, except that the 12Z GFS showing slow tapering beginning on Saturday into Saturday night. During this time we will see an increase in both the local area streams and mainstem rivers. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Lingering showers will continue on Sunday, as cold front drops through the area from the northwest. High temperatures will be in the low 80`s. The dry pattern will not last too long, as a low pressure system moving across the northeast will drag a cold front across the region late Sunday into early Monday. This front will move through the region by the end of Monday, but another low pressure system will pass by closer to the region. The exact location is a it uncertain at this point. However, it seems pretty clear that we will have another cold front pass by in this general time frame, bringing about another chance for rain. Based off of the latest model guidance, these amounts do not look significant, and are generally around a quarter inch of total precipitation. However, there will be a chance for thunderstorms both Monday and Tuesday, so heavier amounts may fall anywhere a thunderstorm sets up. Temperatures again will remain in the low 80`s throughout this timeframe. The details of Wednesday`s forecast are murky at this stage, as the aforementioned cold front looks to stall somewhere along or south of the area. Where this front stalls will determine if we things dry out a little. Right now, maintaining a chance of showers and isolated afternoon thunderstorms through Wednesday. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/... All terminals will continue to see MVFR to IFR conditions primarily due to low CIGS this evening, then becoming LIFR overnight. These conditions are expected to continue through at least mid-morning on Friday. Clearing skies Sunday afternoon, with another approaching cold front late Sunday into Monday. Low ceilings will be possible during this period. A chance for some scattered thunderstorms on Monday as well. && .MARINE... Increasing easterly flow on Friday will bring small craft conditions on Friday, predominantly in the northern portion of the Chesapeake Bay, but extending to all waters during midday Friday. Areas of fog will continue giving visibilities at or below one mile at times, particularly overnight tonight. Winds approaching SCA criteria will be possible on Sunday afternoon as a cold front is exiting the region. Calmer winds expected Monday, but the potential exists for some thunderstorms. && .HYDROLOGY... Conditions continue to worsen as this slow-evolving hydro event continues. A pretty widespread three inches or more of rain has fallen across the northern half of the forecast area already, with amounts that high more spotty to the south. (Parts of Culpeper and Fauquier Counties haven`t even seen an inch so far.) The evolution is so far going along similar to model projections, with light to moderate rain across the north, and more showery and heavy rain developing this afternoon to the south. This activity will pivot northward, and it appears will be followed by multiple additional rounds of precipitation. Confidence has increased in the rainfall prolonging into Friday night, and accordingly the Flood Watch was extended through that time. Also, the area has been expanded to include more counties. We still expect another 2 to 5 inches of rain between now and Saturday. Streams (including the mainstem Potomac) are already elevated or in flood, so flood potential is high. Forecasts now call for moderate flooding on the mainstem rivers downstream of Shepherdstown and minor flooding at several tributary locations. River Flood Warnings will be in effect for these locations based on a high confidence of at least minor flooding. It should be noted that depending on where heavy rain axes set up during the course of the event, there could be even more significant flooding than we are forecasting and interests near any river should heed warnings and keep themselves and their belongings away from the rivers. && .TIDES/COASTAL FLOODING... Fairly persistent onshore/up-the-bay winds increase the threat of minor tidal flooding for the more sensitive locations starting Saturday morning. Additionally, freshwater inundation will pose a threat to the DC/Alexandria tidal zone, and a Coastal Flood Watch is in effect from Friday through Sunday, with a focus really starting Friday afternoon/evening, for moderate flooding or greater. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flood Watch through Saturday morning for DCZ001. Coastal Flood Watch from Friday evening through Sunday morning for DCZ001. Coastal Flood Advisory until 8 PM EDT Friday for DCZ001. MD...Flood Watch through Saturday morning for MDZ003>006-011-013- 014-016>018-501>508. VA...Flood Watch through Saturday morning for VAZ025>031-036>040- 050>057-501>508. Coastal Flood Watch from Friday evening through Sunday morning for VAZ054. Coastal Flood Advisory until 8 PM EDT Friday for VAZ054. WV...Flood Watch through Saturday morning for WVZ050>053-055- 501>506. MARINE...Small Craft Advisory from 6 AM Friday to 3 AM EDT Saturday for ANZ530>533-538>541. Small Craft Advisory from 2 PM to 8 PM EDT Friday for ANZ534>537-542-543. && $$ SYNOPSIS...Lee NEAR TERM...Lee/IMR SHORT TERM...Lee LONG TERM...CJL AVIATION...Lee/IMR/CJL MARINE...Lee/CJL HYDROLOGY...JE TIDES/COASTAL FLOODING...JE

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