Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 170826 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 326 AM EST Fri Feb 17 2017 .SYNOPSIS... High pressure builds overhead through late morning. A weak system passes nearby Saturday night into Sunday morning. A backdoor cold front will be near the region on Monday followed by the return of high pressure. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Chilly to start today with nearly the entire CWA AOB 30F near daybreak due to strong radiational cooling of a dry airmass. High pressure has built overhead...so, winds are relatively light and wind chills aren`t too far below temperatures. This morning is the last through this forecast period with lows below freezing. Transition day today as high pressure crosses the area and slides just to the east with southwesterly return flow developing by afternoon. Onset of warm air advection has trended a bit earlier from last night...which will mean slightly warmer forecast highs across central VA (where U50s/L60s are likely). Elsewhere little change to forecast highs. Near 50 F for DC/Balt...cooler (L40s) across northeast Maryland where warm advection arrives later in the day. && .SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY NIGHT/... Warmup continues Saturday...with highs nearly 20 F above normal. M/U60s in most places...except right along the Chesapeake and Tidal Potomac where U50s/L60s will be more common. While we start the day sunny, clouds increase during the afternoon as a weakening disturbance approaches. This disturbance will bring a chance for some light precipitation to the area Saturday night into early Sunday morning (mainly south of Washington DC). Any precipitation expected to clear the area near daybreak Sunday leaving the rest of the day dry with increasing sunshine. Highs again well above normal (in the M/U60s). && .LONG TERM /MONDAY THROUGH THURSDAY/... A sharp ridge of high pressure over the area Monday will lead to warm, tranquil conditions. While the same ridge will be the influencing feature Tuesday, the result may be a little bit different. Current guidance suggests that the ridge axis will be atop the area, resulting in onshore flow. Hence, the forecast will have more clouds and cooler temperatures. The timing and strength of this will need to be watched. Potentially, this could result in a marine airmass advecting inland which would result in a much different forecast than previously advertised. Points east of the Blue Ridge would be most vulnerable. By Tuesday night, northern stream shortwave energy will be flattening the flow aloft (both mid and upper levels...850 mb and above). Heights looking a bit flatter than yesterday, which is allowing both GFS and especially the ECMWF to bring a surface front and associated precipitation east of the Appalachians Tuesday night into Wednesday. Any precipitation would be light (a few hundredths inch of rain). And, thermal profiles warm enough for this to be definitively rain. The subsequent front would be left behind across the area on Thursday. It remains to be seen how strong it would actually be. Consequently, forecasts will have mid range clouds and low end (20%) PoPs. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... VFR conditions expected to prevail through Saturday as high pressure remains in control of the weather. Weak disturbance moves through the area Saturday night into Sunday morning...with a chance for light showers. Light rainfall rates should keep flight restrictions rather marginal/brief...with perhaps a brief period of sub-VFR (best chance at KCHO). VFR returns after daybreak Sunday. VFR conditions should continue into Monday. Tuesday could be a little questionable, as there is a hint of onshore flow. What is yet to be determined is whether that would bring flight restrictions inland due to low clouds/fog. && .MARINE... Winds have fallen off rapidly overnight as high pressure continues to build overhead. Thus, expired the SCA headline a little early. Relatively light winds expected through Sunday...with the typical increases/decreases during the day/night due to diurnal mixing trends. Winds should primarily be 10 kt or less Monday and Tuesday. In terms of direction... northwest on Monday, but becoming onshore Tuesday as a ridge axis passes overhead. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...MSE NEAR TERM...MSE SHORT TERM...MSE LONG TERM...HTS AVIATION...MSE/HTS MARINE...MSE/HTS

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