Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 191826 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 226 PM EDT Tue Jun 19 2018 .SYNOPSIS... A cold front dropping south out of Pennsylvania will stall over the Mid-Atlantic during the middle part of the week. A wave of low pressure will move along this front Wednesday into Wednesday night. The front will sag southward toward the North Carolina border Friday as high pressure briefly builds in from the Ohio Valley, then the front will return northward over the weekend. && .NEAR TERM /THROUGH TONIGHT/... Current analysis shows a weak cold front dropping south into northern Maryland this afternoon. A pressure trough is located over southern Maryland into central Virginia. A northwest flow behind the pressure trough has caused enough downsloping and sunshine for more hot and humid conditions. The heat and humidity will continue to cause heat indices around or slightly above 100 degrees across the Virginia Piedmont into central Virginia, until convection arrives later this afternoon. The heat and humidity has led to an unstable atmosphere, and latest mesoanalysis shows around 1500-2000 j/kg of mlcape ahead of the cold front across eastern West Virginia into the Washington Metropolitan area and southern Maryland (ahead of the approaching cold front). The instability has combined with forcing from the boundary to trigger scattered to numerous showers and thunderstorms. This convection will gradually propagate south throughout the rest of this afternoon/early evening. Shear profiles are marginal, but with the higher amounts of instability, inverted V soundings, and mid-level dry air there is a threat for locally damaging wind gusts in storms along/ahead of the boundary into early this evening. The best chance for stronger storms will be south and west of the Potomac River in Virginia into southern Maryland. Veering low-level flow and enhanced low-level shear along the boundary would suggest an isolated tornado is possible, particularly over central Virginia where instability should be greatest (probability would be higher with any boundary/river breeze intersections). There is also a heavy rain/flood threat, but it appears too isolated for a watch at this point (see Hydrology section for more details). The boundary will stall out over the Potomac Highlands into central Virginia overnight. A few showers are possible along the boundary, but much of the time may end up dry due to the loss of daytime heating. Patchy fog is expected along and ahead of the boundary as well. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... Low pressure will track along the boundary, passing through our area later Wednesday into Wednesday night. Widespread showers and thunderstorms are likely, especially later Wednesday afternoon into Wednesday evening. There is an isolated severe threat for locally damaging wind gusts and large hail. Also, moisture convergence from the low and the instability will cause thunderstorms to produce heavy rain. Localized flash flooding is possible as well. More details are in the hydro section below. The boundary will drop to the south overnight Wednesday through Thursday night. A few showers are possible near the Potomac Highlands into the central Shenandoah Valley, but most other locations should be dry as cooler and more stable air moves in. && .LONG TERM /FRIDAY THROUGH MONDAY/... On Friday attention will turn to an upper-level low located over the central Mississippi Valley. At the surface, an area of low pressure will develop ahead of the upper-level low and move into the lower Ohio River Valley. Meanwhile, high pressure will depart off the New England coast. We`ll be located in stable air to the north of a frontal boundary, but rain chances can`t be ruled out by late afternoon as warm advection commences aloft. Chances for rain will increase Friday Night as deeper warm air advection and associated overrunning precipitation overspreads the area. By Saturday morning, the upper-level low will shear out into more of an open wave as it tracks toward the Lower Great Lakes. The surface low is also forecast to track toward the lower Great Lakes. As this low moves off to the northeast, it will move a surface warm front northward through the area Saturday morning. As a result, Saturday will be noticeably warmer and more humid than previous days. Model guidance suggests that areas to the south of the warm front will destabilize by Saturday afternoon, with MLCAPE values reaching around or slightly above 1000 J/kg. This instability coupled with seasonably strong flow aloft (0-6 km shear values around 40 kt), could lead to some stronger storms Saturday afternoon. The area of low pressure will continue to track off to the northeast toward the Canadian Maritimes on Sunday. The cold front associated with this system will progress through the region sometime during the Sunday afternoon through Monday time period, but differences between individual models and their respective ensembles are substantial. Rain chances can`t be ruled out on Sunday or Monday, but the general trend with time will be for rain chances to lessen and drier, cooler air to work into the region in the wake of the cold front. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... Scattered to numerous thunderstorms are expected along and ahead of a cold front through early this evening. The best chance for storms in DCA and IAD will be through 20z, and for KCHO 20-22z. Farther north, the strongest storms have ended. Storms will be capable of producing locally damaging wind gusts and IFR/subIFR cigs/vsbys via torrential rain. The boundary will stall to the south of the terminals overnight into Wednesday, before low pressure rides along the boundary later Wednesday and Wednesday evening. Some storms may be strong to locally severe and IFR/subIFR conditions are possible in heavy rain. The boundary will stall to the south of the terminals overnight Wednesday through Thursday night. Showers and thunderstorms will be the main concern Friday into Saturday as a warm front slowly lifts northward. Sub-VFR clouds may also occur before the front lifts north, particularly Friday night. && .MARINE... An SCA is in effect for the waters this afternoon. Otherwise, winds should stay largely below SCA levels outside of any convection through Thursday. Scattered to numerous thunderstorms this afternoon will be capable of producing gusty winds over the middle portion of the Bay and the Tidal Potomac River. Another round of strong to locally severe storms are possible later Wednesday afternoon and evening. The main concern in the long term will be chances for thunderstorms on Saturday. Otherwise, there may be a chance for low- end SCA winds on Friday/Friday Night in easterly flow to the north of a warm front. && .HYDROLOGY... High PWATs (around 2 inches) are expected this afternoon ahead of a cold front dropping southward out of Pennsylvania. Despite warm cloud layers in excess of 12 kft and steering flow somewhat parallel to the forcing boundary, low-level NW flow and forward storm motion should prevent a more widespread threat of flooding. The forward motion of the front forcing the convection may accelerate some as it is modulated by residual cold pools from overnight convection, which would help move storms along. We also have not had much appreciable rain in about a week. Therefore, there is no watch at this time. However, some training is still possible and given the conditions listed above. A second, more widespread and possibly persistent round of showers and a few heavy thunderstorms are likely Wednesday into Wednesday night as low pressure developing over the Ohio Valley moves eastward over the Mid-Atlantic along the stalled front. High moisture content, wetter antecedent conditions and additional training potential would result in a higher flood potential, and may ultimately require a watch. && .CLIMATE... Above normal temperatures are forecast today. While record highs are unlikely, record warm lows are still possible. Below is a list of daily record warm temperatures. Washington DC area (DCA) Date Record High Record Warm Low June 19 99 (1994) 75 (2011, 1994, 1981, 1978) Temperature records for the Washington DC area have been kept at Ronald Reagan Washington National Airport since 1945. Additional temperature records observed downtown date back to 1872. Baltimore MD area (BWI) Date Record High Record Warm Low June 19 99 (1994) 74 (1905) Temperature records for the Baltimore MD area have been kept at Baltimore-Washington International Thurgood Marshall Airport since 1950. Additional temperature records observed downtown date back to 1872. Dulles VA area (IAD) Date Record High Record Warm Low June 19 98 (1994) 71 (2014) Temperature records for the Dulles VA area have been kept at Washington Dulles International Airport since 1960. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 4 PM EDT this afternoon for ANZ530- 531-538-539. Small Craft Advisory until 6 PM EDT this evening for ANZ532>537-540>543. && $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...KJP AVIATION...BJL/KJP MARINE...BJL/KJP HYDROLOGY...DFH CLIMATE...LWX is the U.S. government's official web portal to all federal, state and local government web resources and services.