Area Forecast Discussion
Issued by NWS Sterling, VA

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086 FXUS61 KLWX 240130 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 930 PM EDT Sun Apr 23 2017 .SYNOPSIS... High pressure near Long Island will slide out to sea tonight. Low pressure in Georgia will very slowly move eastward, reaching the South Carolina coast on Monday, the North Carolina coast Tuesday, and moving east of Mid-Atlantic on Wednesday. A cold front will reach the region late Thursday or early Friday. && .NEAR TERM /THROUGH MONDAY/... Latest surface analysis depicts low pressure over Georgia, with a stationary front extending eastward towards Wilmington, NC and a cold front extending south into the Gulf of Mexico. To the north, high pressure is centered near the Jersey Shore and over the southern Great Lakes, with another center in the central Plains. Aloft, we have a closed low over western Tennessee, with a shearing shortwave over southern New England and another ridge over the southern Plains. This closed upper low will be moving at a snails pace over the next few days, and its associated surface features will do likewise. Overall, this is acting like a mid-winter storm, except that its too warm for any snow. Other than the lack of snow, the cool northeast wind, minor coastal flooding and long period of precipitation also reflect its rather winter-like nature. Another element of similarity will be the excruciatingly slow progress of the precipitation into our area. As the low pushes east tonight, the confluence over our region will only very slowly give way tonight and Monday, and the drier air in place, fed into the region by the elongated high pressure ridge (classic banana ridge often present on the northwest side of winter storms) will continue to feed dry air into it. This trend has continued into the evening, with the very low clouds still confined to far southern Virginia. Light rain has made it up to Staunton and Charlottesville, but it`s northward progress is somewhat uncertain overnight. Even though there will be better chances for a steady rain as Monday progresses, model QPF suggests it may be of little substance north/east of the Potomac River. As the best forcing stays south, significant heavy rain is generally not expected across the region, so no flood watches have been issued, though if any were needed, it would be in central VA near the Blue Ridge where the upslope flow and upper level forcing is maximized. Lows tonight will be in the 40s with highs generally in the 50s on Monday. Have nudged highs down in the southwest portion of the CWA where steadier rains are expected. As a proxy, daytime temperatures in southwest VA today were in the mid 40s to around 50. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... The slow moving low will only reach Cape Hatteras on Tuesday and won`t even make its closest approach to the region until Tuesday night, though at that point it should be starting to weaken with diminishing precip across our region. Bottom line is that we`ll continue to see periods of rain through Tuesday evening, which should then diminish later Tuesday night. Northeast winds will continue and temperatures will stay in the upper 40s to mid 50s. A period of heavier rain is possible late Monday night into Tuesday as the low lifts northward and forcing increases, by which time the dry air feeding into the system from the north should be mostly eliminated, but again, no flood watches, as uncertainty about placement and timing remains rather significant. From Monday night onward, the setup also becomes more favorable for low clouds and drizzle when it`s not raining. Have also nudged down high temperatures Tuesday in central/northern portions of the area where models are likely too fast in dislodging the rain-cooled CAD wedge. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... A cut off low moves from the Mid-Atlantic coast into the NE, and away from the coast on Wednesday. Some isolated to scattered showers possible, mainly to the eastern half of our CWA on Wednesday. High pressure builds Wednesday night into Thursday before a cold front approaches from the west. This front is expected to stall over or near our CWA Friday and into Sunday keeping warm and humid conditions over our region, allowing for showers and maybe thunderstorms at times. Temperatures will be above normal during this period, with highs reaching the 70s and 80s, lows in the 50s and 60s. && .AVIATION /01Z MONDAY THROUGH FRIDAY/... VFR now, but deteriorating slowly tonight and Monday as low pressure moves slowly toward the region with rain, a northeast wind, and lowering CIGs and VIS. For 00Z TAFS, made some significant adjustments to slow down timing of MVFR/IFR cigs. NAM is the fastest model to deteriorate, so have incorporated its solution as "SCT" groups, but with dry air feed from the NE, think it will take well into Monday for lower cigs to reach the metros/MRB. While rain chances increase, it will likely be light through much of Monday and unlikely to cause vsby issues (except at CHO). IFR does become likely by Monday night and Tuesday as the low continues slowly moving northeast up the coast. Period of heavier rain also appears likely late Monday night into Tuesday morning. LLWS could also be an issue during this time. Some improvement likely to start Tuesday night as the low starts heading into the Atlantic. Sub-VFR possible for the terminals early on Wednesday before VFR conditions return on Wednesday night as low pressure moves away. VFR conditions continue into Thursday, and sub- VFR conditions possible Friday with a lingering front nearby. && .MARINE... Most of the waters are still well below SCA conditions this evening, although there is an uptick in winds near the mouth of the Potomac. Any stronger winds will likely remained confined to this region overnight. Expect widespread SCA by Monday afternoon as a slow moving low moves its way up the eastern seaboard. Right now not expecting gales, but could be close in the middle bay. Lowest visibilities with low clouds/fog/moderate rain will be Monday night into early Tuesday. Winds may start diminishing by Tuesday night. Isolated to scattered showers possible early on Wednesday before dry conditions return Wednesday night as low pressure moves away. Dry conditions continue into Thursday, and showers and maybe thunderstorms possible on Friday with a lingering front nearby. Wind gusts should remain below the small craft threshold. && .TIDES/COASTAL FLOODING... Have issued a Coastal Flood Advisory for St. Marys again for the overnight high tide, as even a slightly lower trend would still produce minor flooding. Guidance suggests Annapolis will also need to be watched overnight, although anomaly trends this evening currently suggest it would fall short. A persistent onshore flow will continue over the waters through Tuesday. Elevated water levels are expected to continue. The onshore flow will strengthen a bit for Monday through Tuesday. Minor flooding is possible near times of high tide. The flow should gradually turn north Tuesday night into Wednesday as the low moves away from the area. Water levels should decrease during this time. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory until 3 AM EDT Monday for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory from 6 AM Monday to 6 AM EDT Tuesday for ANZ530>532-535-536-538>542. Small Craft Advisory until 6 AM EDT Tuesday for ANZ533-534-537- 543. && $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...ADS/RCM LONG TERM...IMR AVIATION...ADS/IMR/RCM MARINE...ADS/IMR/RCM TIDES/COASTAL FLOODING...ADS/RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.