Area Forecast Discussion
Issued by NWS Sterling, VA

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781 FXUS61 KLWX 301358 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 958 AM EDT Sun Apr 30 2017 .SYNOPSIS... A front which is draped across the area will lift north as a warm front later today. A stronger cold front move east of the Mississippi River tonight and approach the Appalachians by Monday morning. This front will cross the region Monday evening. High pressure will bring cooler and drier weather during the middle of the week. && .NEAR TERM /THROUGH TONIGHT/... At 9 AM surface front in form of wind shift is located from near Franklin WV to Warrenton to Chesapeake Beach. Low clouds are farther north than that line. While it appears the southern edge is trying to scatter into cumulus, am a little concerned erosion may be on the slow side since winds north of the boundary are verifying with a more northeasterly component than originally progged. While this could have some effect on the temperature gradient, the already-warm start means portions of the Baltimore area are already nearing their highs despite the clouds. Temperatures are on their way to the 80s south of the boundary. There is still a bit of uncertainty in the details of how convection will transpire today. In the bigger picture, though, it will be safe to say that north of the boundary surface-based instability will be nil. To the south, SBCAPE will be in the neighborhood of 1500-2500 J/kg with more than adequate deep shear (30-35 kt bulk 0-6km layer). The struggle once again is finding the trigger. Believe that terrain and frontal convergence will be that mechanism, suggesting that the Potomac Highlands will be the favored location. 12Z RNK sounding has very dry mid-level air which is expected to advect northward, and all regional soundings show multiple stable layers. Thus am continuing the previous thinking that coverage will be scattered at best. If a stronger updraft can develop, gusty winds will be a threat. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... Instability quickly dissipates with the loss of diurnal heating, and have removed PoPs for the nighttime hours. Area will be north of the warm front, in an air mass with 60s dewpoints, so patchy fog development will once again be a concern. Otherwise, it should be a quiet night. Lows won`t be far removed from dewpoints. On Monday, a well defined cold front will be approaching the area. There are still some timing differences amongst guidance. This bigger disparity, though, is whether thunderstorms will develop along the front or along a pre-frontal trough. The GFS is the quickest solution, keying on pre-frontal convection in a diurnally favored timeslot. If this happens, then strong to severe thunderstorms will be possible. Other solutions suggest thunderstorms will be nearer the front, and timed more in the evening. The challenge will be how much instability will generated; more than ample shear is present. PoPs will be likely to categorical nonetheless, with emphasis on the later afternoon and evening hours. Cooler and drier air will overspread the area Monday night (overnight) into Tuesday morning. We will have a glancing blow from the upper trough axis during the day Tuesday, which will sustain gusty winds much of the day. But since the air will be drier and flow will be downsloping, don`t believe that clouds will be numerous. Temperatures will be closer to normal during the day, with nighttime low in the 40s and 50s again. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... High pressure will build into the region Wednesday. Cooler conditions expected mid-week as northerly winds usher in cooler and dryer conditions. High temps in the U60s/L70s expected Wed. A storm system will be coming together across the Mississippi Valley Wed night. Warm air advection will begin across the region Wed night-Thursday and clouds and showers are expected. More showers are expected to move into the region Thu night- Friday as the system becomes more dynamic. The upper level low becomes a closed low Thu night and sfc low pressure will continue to deepen across the Ohio Valley. This system may bring thunderstorms capable of heavy rain as the wind field strengthens and dewpts rise rapidly Friday. Timing is uncertain and it is unknown at this time if conditions will be hazardous. Low pressure will likely move overhead Fri-night- Saturday. Clouds and showers will likely keep temperatures below normal mid-late week. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... A backdoor cold front has slipped south of the metro terminals but not to CHO, providing a complicated flight category regime. MVFR seems to be the rule on the north side of the boundary. Have rather low confidence at how quickly the restrictions will erode, especially since winds are verifying from a more northeasterly direction. However, satellite shows the southern edge of the clouds trying to erode. Expect BWI/MTN to be affected the longest. Scattered thunderstorms should develop south of the front, but direct impact at an airfield too limited to forecast at this juncture...am thinking the highest chance will be near MRB. Anything that does develop will dissipate with sunset tonight. The air will be humid enough for more patchy fog development overnight into early Monday morning. Will be keeping forecasts at MVFR. Lower results certainly possible. A well defined cold front will approach the terminals Monday, and cross the region late Monday into Monday night. Numerous thunderstorms anticipated along/ahead of the front, with brief aob IFR along with locally gusty winds. The front will clear the area late Monday night. VFR expected through Wednesday. Rain will spread east Wednesday night into Thursday. && .MARINE... A backdoor front has slipped to around Chesapeake Beach, and further southward progression may be limited before gradually returning northward. Flow will be northeast/east to its north, and southeast/south to its south and shouldn`t be much more than 10 kt sustained, so Small Craft Advisories are not anticipated. This front will return to the north this evening. Gradient flow (southerly) will increase overnight into Monday ahead of a stronger cold front approaching from the west. A Small Craft Advisory goes in effect after midnight tonight for southern waters, and then for all waters on Monday. The front itself will impact the marine area Monday night, although some thunderstorms may develop ahead of the front Monday afternoon. These thunderstorms may contain gusty winds. Behind the front flow turns westerly. Mixed profiles appear deep, and expect Small Craft conditions to extend into Tuesday. A storm system may impact the waters late in the work week. Showers and thunderstorms are possible as well as gusty winds that may produce SCA conditions. && .TIDES/COASTAL FLOODING... Water levels running very close to astronomical normals this morning. Anticipate that will remain the case through the next two tide cycles. ESTOFS does suggest that water levels will rise in the northern Bay, but believe that is a result of a model bias and am neglecting that solution. After that, uncertainty grows as model solutions become more divergent. Am providing a forecast of water levels below caution stage Monday. South flow could sustain an increase. Some solutions suggest these elevated levels could linger into Tuesday, as winds will turn gusty but will be from the west...and thus won`t prompt a blowout. Low pressure will come up the coast Thursday, which likely will increase water levels from wherever they stand up to that point. && .CLIMATE... Record highs were set at DCA and IAD yesterday, and record warm lows were set at all three airports (DCA, IAD, and BWI). The record warm low at DCA was also a record for the month of April. Records highs are less likely today as a backdoor front drops down. However, the record warm lows are a possibility. Here are the record highs/warm lows for Sunday: Washington DC... High 92 (in 1942); Warm low 67 (in 1983). BWI Airport... High 92 (in 1910); Warm low 63 (in 1983). Dulles Airport.. High 86 (in 2007); Warm low 64 (in 1983). && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 8 AM to 6 PM EDT Monday for ANZ530>532-535-536-538>540-542. Small Craft Advisory from midnight tonight to 6 PM EDT Monday for ANZ534-537-543. Small Craft Advisory from 4 AM to 6 PM EDT Monday for ANZ533- 541. && $$ SYNOPSIS...HTS NEAR TERM...ADS/HTS SHORT TERM...HTS LONG TERM...HSK AVIATION...ADS/HTS MARINE...ADS/HSK/HTS TIDES/COASTAL FLOODING...HTS CLIMATE...HTS

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