Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 091457 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 957 AM EST Fri Dec 9 2016 .SYNOPSIS... High pressure builds east from the Great Plains through Saturday before moving off the Mid-Atlantic coast Sunday morning. A cold front crosses the area Monday followed by a reinforcing cold front Wednesday. High pressure builds back into the area for the latter half of next week. && .NEAR TERM /THROUGH TONIGHT/... Morning surface analysis shows 1040+ mb high pressure extending east of the continental divide. 12Z IAD 850 mb temperature of -11C indicates today`s chilly forecast is on track. CAA mountain wave stratocumulus is generally west of the Blue Ridge, but may expand a little through the day. The 36 MPH wind gust at IAD at 9 AM should be the upper end of what`s expected today, with gusts only gradually subsiding this evening. Observations from the Highlands show less than an inch of snow through 7 AM, so sub- advisory amounts over the 30 hour period seem reasonable. Previous valid discussion follows: Cold and blustery today...with tight pressure gradient remaining as large area of high pressure over the central CONUS gradually builds towards the area. Even with lots of sun...most places remain AOB 40F the entire day. Gusty winds up to 25-30 mph will create winds chills in the teens through midday...even by afternoon wind chills remain AOB 30F. Westerly component through a moist layer will continue to promote off and on snow showers across upslope areas through tonight. Generally less than 3 inches expected through 7 AM Saturday before any activity begins to wane. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... High pressure builds overhead Saturday through early Sunday...thereby relaxing the pressure gradient. Perhaps a few degrees colder than Friday as CAA continues...with many places failing to get above 40F despite sunshine. However, diminishing winds by afternoon will offer some relief. High pressure moves offshore Sunday with a southerly flow developing on its backside. This should help to nudge temperatures up a few degrees. Though, increasing cloud cover ahead of next system will temper any appreciable warming. Precipitation may move into NW/N portions of area Sunday...thermal profiles during this time would suggest mainly snow initially...with a transition to a snow/rain mix by afternoon. Any wintry precipitation will be fighting warm air advection aloft (with warm nose developing near 850 hPa) and at surface Sunday into Monday morning. Eventually most places will transition to all rain...but there could be a brief period of freezing rain near the Mason-Dixon line early Monday morning. Timing of precip over DC/Balt metros suggests activity should remain of the liquid variety. && .LONG TERM /MONDAY THROUGH THURSDAY/... Storm system passing well to the north will bring warmer air north on Monday, allowing temps to rise into the 50s in many areas and causing precipitation to fall in the form of rain. High temps may be underdone in fact, but will wait til we get a little closer to push us above model guidance averages. Right now the rain looks like it will be mostly in the morning, with potential for a mostly dry, breezy but relatively mild afternoon. High pressure pushes overhead Monday night with cold advection bringing temps back down into the 30s. Tuesday will start out dry with high pressure overhead, however some guidance develops a wave of low pressure to the south which may brush the region later in the day. Disagreement grows as we head into Wednesday, with some guidance showing a stronger and warmer system coming in from the southwest, while others show only a weak disturbance and rapid cold advection already underway. Bottom line is there is a chance of precipitation and it could be snow, but it would seem slightly more likely that it either is mostly rain for I-95, or very little falls whatsoever. Still a long way out and a lot can change. Either way, very strong cold advection takes place behind that system so that we are probably seeing well below normal temps by Thursday. Temps may in fact struggle to rise above freezing while lows will be well below freezing. Northwest flow will mean that the only precip will likely be upslope flow on the Allegany Front. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... VFR continues through much of Sunday for most terminals. Cigs FL040-060 most likely at MRB today. By Sunday, MRB could have sub-VFR in rain/snow showers, but confidence in precipitation coverage is low. Sub-VFR then possible at all sites late Sunday into Monday as precipitation (possibly a mix) spreads across much of the area. Strong NW winds with gusts up to 20-30 kts continue through this evening before moderating back below 15 kts overnight. Sub VFR with potential IFR Monday as storm system with rain showers moves across the area. Conditions improve back to VFR Tuesday for a time before next system potentially arrives midweek. && .MARINE... Solid SCA conditions continue through much of tonight. Based on strong gradient, did not think gusts would immediately subside with sunset, so extended SCA headlines across all waters through 9 PM. SCA then will taper off for the upper tidal Potomac...and then the upper Chesapeake and inlets at midnight. Headlines clear rest of waters at 6 AM Saturday as pressure gradient relaxes and winds fall under SCA criteria with high pressure building overhead. Will peruse latest data to see if headline needs to be extended into Saturday morning. Marginal SCA conditions could develop Sunday with southerly flow ahead of approaching cold front. SCA may continue Monday as the front passes. They may diminish briefly Tuesday under a very transitory bubble of high pressure. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 AM EST Saturday for ANZ531>534- 537-539>541-543. Small Craft Advisory until midnight EST tonight for ANZ530-536- 538-542. Small Craft Advisory until 9 PM EST this evening for ANZ535. && $$ SYNOPSIS...MSE NEAR TERM...ADS/MSE SHORT TERM...MSE LONG TERM...RCM AVIATION...ADS/MSE/RCM MARINE...ADS/MSE/RCM

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