Area Forecast Discussion
Issued by NWS Sterling, VA

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565 FXUS61 KLWX 101521 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1021 AM EST Sat Dec 10 2016 .SYNOPSIS... High pressure over the center of the continent builds east today before moving off the Mid-Atlantic coast Sunday morning. A warm front develops over the area and lifts north Sunday before a cold front crosses Monday. A reinforcing cold front crosses Wednesday before cold high pressure builds back into the area. && .NEAR TERM /THROUGH TONIGHT/... Northwest flow continues across the Mid-Atlantic as high pressure builds eastward from the Ohio and Tennessee Valleys. There will still be enough of a gradient for gusty winds of 15-25 mph at times, particularly the northern part of the area. While accumulating snow is likely ending across the Allegheny Front, traffic cams show flurries continuing to fall, so have added that mention into the early afternoon. Max temps will range from the 20s at higher elevations to the 30s elsewhere...and perhaps a few 40s south/east. Even though the high will be moving overhead tonight, mid-high level clouds will increase from the west late tonight as low pressure moves into the Central Plains. Warm air advection will begin across the Ohio Valley and light snow showers are possible across the northern Allegheny Front and Potomac Highlands late tonight. Little to no accumulation is expected through Sunday morning. Calm conditions will result in another cool night with temperatures dropping into the teens/20s tonight...with the thickness of cloud cover potentially impacting the outcome. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Moisture advection will spread across the Mid-Atlantic region Sunday. Snow showers are expected to spread eastward but hug the MD/PA border as the area of best mid-level convergence shifts northward into PA. High pressure will be off the Mid-Atlantic coast by Sunday afternoon and southerly flow will increase across the region. Temperatures will still be on the cool side due to cloud cover across much of the region especially across the north. An area of lower pressure will move northward along the SE U.S. coast Sunday afternoon associated with Atlantic moisture. Rain showers are possible across the Tidewater region and Southern MD into Sunday night and amts will be light. Low pressure will move into the Great Lakes region Sunday night and warm air advection aloft will continue... 850mb temperatures will cross 0 deg and keep rising into Monday morning. Sfc temperatures are expected to drop into the low to mid 30s across the region with warmer temperatures along and east of I-95. This will make for multiple p-types across the region Sunday night into Monday morning. Guidance shows sfc temperatures rising overnight into Mon morning and after analysis of boundary layer winds it may actually occur as SW winds will be 10-20kts. This can be a flaw in model guidance as they can overestimate sfc warm air advection. A wintry mix is expected across the region as precipitation moves into the west Sunday night. Snow will likely change to sleet and then freezing rain for places below freezing. As precipitation moves eastward by Monday morning...SN/IP will change to rain. Temperatures will climb into the 50s Monday and rain is definitely expected. Overall...a brief period of ice accumulation is possible mainly across the Allegheny Front and Potomac Highlands Sunday night into Monday. Confidence is low at this time to issue any advisories. Decent gulf moisture associated with the advancing cold front Monday will result in 0.25-0.5 of rainfall through Monday. Clearing will occur by Monday night with snow showers ending along the Allegheny Front. High pressure will move overhead Monday night. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Guidance is in somewhat better agreement through the long term period of the forecast than it was last night. We start out on Tuesday with a weak area of high pressure over the region which looks to keep the weather dry with near normal temperatures. It looks like Tuesday night stays mostly dry ahead of an approaching cold front. On Wednesday however, latest guidance is notably in better agreement regarding an approaching shortwave and vort max helping to initiate cyclogenesis along the front as it crosses the area, which could lead to a period of mixed precipitation or snow. Still a long way out but we continue to watch this potential closely. By Thursday, this system is long gone and we end up with arctic high pressure and temps struggling to reach the freezing point even in the warm spots. The arctic high settles right overhead on Friday with continued very cold conditions. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... VFR conditions expected through tonight. NW wind gusts to 20 kt will be possible (except CHO) today. -SN may move into MRB late tonight into Sun morning and may result in vsby restrictions however confidence is low. -SN will move out of MRB by Sunday afternoon and VFR conditions expected Sunday at all terminals. Sub-vfr conditions expected Sunday night into Monday as moisture advection occurs. -SN/IP/FZRA is possible at MRB during this time. -SN/IP possible at other terminals however activity should become -RA early Monday. RA expected at all terminals Monday. VFR conditions expected Monday night- Tuesday. Sub VFR conditions possible Wednesday as a storm may try to bring rain or snow to the region again just ahead of the next arctic surge. && .MARINE... Enough pressure gradient remains across northern portions of the area for SCA conditions to continue today. Expanded the SCA one tier of zones south, including the Potomac near DC. Even though the SCA was expired for the remainder of the waters, winds will be on the threshold, so an SCA may need to be reissued based on obs. Lighter winds expected with high pressure Saturday night into early Sunday. Increasing southerly flow ahead of the next storm system looks to warrant another SCA by Sunday evening. SCA level winds expected Monday...dropping off Tuesday before possibly redeveloping Wednesday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ530>533-535-538>541. && $$ SYNOPSIS...RCM NEAR TERM...ADS/HSK SHORT TERM...HSK LONG TERM...RCM AVIATION...ADS/HSK/RCM MARINE...ADS/HSK/RCM

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