Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 200106 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 906 PM EDT Wed Apr 19 2017 .SYNOPSIS... High pressure off of Atlantic Canada will move further away from the coast tonight and Thursday. A warm front will cross the area Thursday. A cold front will follow Friday afternoon. Weak high pressure builds over the area Friday night before another low pressure impacts the area Saturday night and Sunday. && .NEAR TERM /THROUGH THURSDAY/... Latest surface analysis indicates, in spite of southerly flow, forecast area still influenced by the southwestern periphery of cool high pressure. The warm front is back in the Ohio Valley. Thunderstorms have erupted along the cold front in the lee of the Great Lakes. Guidance in agreement with respect to weakening that line prior to its arrival in the Potomac Highlands toward dawn. Have therefore extended the period of dry weather past midnight before allowing PoPs (in the form of showers) to creep subtly upward again. It appears as though any late afternoon breaks in the overcast have filled back in again, or have been covered by higher cloud decks. We need a solid overcast to prevent radiational fog/low clouds to develop. Those breaks were in the Potomac Highlands, and since the mountains and its valleys will be nearer to the warm front, and any advection fog, believe those areas have the better chance to see any fog development. Am confining the threat to patchy coverage, due to somewhat low certainty. Any residual clouds and fog will break apart during the morning as the warm front crosses the area. Isentropic lift from the warm front, along with favorable moisture advection, increased CAPE, decent mid level lapse rates, and modest shear suggest that scattered to numerous showers and thunderstorms will develop coincident with daytime heating Thursday. Have kept likely PoPs west of the Blue Ridge; mountains will provide lift as well. Temperature forecasts on track this evening. Will have a relatively uniform mid-upper 50s for low temps tonight. Will touch up using latest LAMP shortly. On the warm side of the front tomorrow, a bubble of 12-13C suggest that highs should reach within a few degrees of 80F. && .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... Shra/tsra activity will wane with loss of heating Thu evening, but approaching cdfnt from the west could bring another round of late night and early morning showers Thu night and Fri. Still warm on Fri with sufficient moisture and instability Fri afternoon for another round of showers/t-storms mainly across the southern half of the fcst area. The 12Z GFS wants to push the front all the way into North Carolina, but the latest Euro keeps the front across srn VA keeping the risk of showers across srn zones. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... An area of low pressure will take shape over the south-central CONUS Saturday morning, then slowly push east just south of the area through the second half of the weekend. Models are in general agreement with this setup, which should result in a beneficial soaking rain across much of the area. A cool area of high pressure will persist to the north following the weekend system which should keep temperatures near to below normal for this time of year during the first half of next week. Periods of clouds or sprinkles will be possible in onshore flow as well. && .AVIATION /01Z THURSDAY THROUGH MONDAY/... A band of MVFR ceilings (OVC025) will be crossing the metros this evening. Behind that, clearing was evident on recent visibile satellite pictures. It will be interesting to see if those trends continue into the nighttime hours. RAP thinks so. However, the first nighttime images suggest that this area in the Potomac Highlands is starting to fill in again. LAMP has been backing off on poor flight conditions overnight. Have done likewise in the TAFs. It would take a solid MVFR cloud deck to prevent LIFR fog/low clouds from developing. Do not have a strong level of confidence, but am cautiously optimistic that a brief period of predawn MVFR will be the biggest concern at IAD/BWI, and maybe not even that for DCA. CHO, on the other hand, more likely to see some IFR. More recent guidance suggesting that scattered to perhaps even numerous showers and thunderstorms will be developing Thursday afternoon in daytime heating combined with lift from a warm front. This timeframe now entering the TAF window. Am taking no restrictions at this time due to spatial uncertainties. The associated cold front will cross the terminals on Friday. Again, showers and thunderstorms will be a possibility. Its unclear whether coverage will be as much as tomorrow. Sub-VFR likely over the weekend with rain. Winds will generally be out of the northeast around 10 knots or so. && .MARINE... South winds maintaining a fairly steady 10-15 kt this evening. Latest guidance suggests this will remain the case for much of the night, with a slow drop off toward dawn. Gradient flow should remain below Small Craft Advisory thresholds Thursday (from the south) and Friday (veering west behind a cold front). However, scattered showers and thunderstorms are anticipated both days, primarily in the afternoon and evening on Thursday, and during the daytime hours on Friday. A few of these storms could be on the stronger side, containing gusty winds. Special Marine Warnings may be required. Northeasterly winds of 10 to 15 knots are expected across the waters over the weekend as low pressure passes to the south. Gusts would exceed 20 knots for a time especially Saturday as the pressure gradient tightens between low pressure developing to the southwest and high pressure developing to the northeast. && .TIDES/COASTAL FLOODING... Observations this afternoon/early have been coming in lower than forecast...on the order of three-quarters of a foot lower at Strait`s Point. Therefore, the Coastal Flood Advisory for St. Mary`s County has been cancelled. Forecasts elsewhere seem to be doing ok, but have infused some of the latest guidance to fine- tune these projections. Water levels will continue to be elevated through Thursday. Departures will be running about a foot above normal. Some Action Stages will be threatened. Since levels rose within the past day, and the morning tide cycle will be astronomically higher, would not be surprised if an upward adjustment ultimately will be needed to the Thursday AM tide cycle. Corrections likley won`t come before a cold front on Friday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...LFR/HTS NEAR TERM...HTS SHORT TERM...LFR LONG TERM...DFH AVIATION...HTS/DFH MARINE...HTS/DFH TIDES/COASTAL FLOODING...HTS is the U.S. government's official web portal to all federal, state and local government web resources and services.