Area Forecast Discussion
Issued by NWS Sterling, VA

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515 FXUS61 KLWX 161123 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 623 AM EST Fri Feb 16 2018 .SYNOPSIS... A cold front will drop through the area today. High pressure will briefly build north of the region tonight, then a wave of low pressure will pass to the south Saturday. High pressure will return briefly again on Sunday before another storm system approaches from the west early next week. && .NEAR TERM /THROUGH TONIGHT/... A subtle surface trough out ahead of the main cold front will drop south across the area bringing showers this morning. There will likely be a brief lull before another round of showers with the main cold front early this afternoon. Temperatures will start out mild (50s to around 60), but then fall through the day as cold air moves into the region on gusty NW winds (gusts to 30+ mph late this afternoon and early evening). A few brief downpours are possible. Tonight will be much colder with lows dropping below freezing just about everywhere. Gusty winds in the evening will make it feel several degrees cooler, but the wind will abate pretty quickly by late evening as high pressure builds over/just north of the area. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Surface low pressure will develop over the Tennessee Valley Saturday morning, driven by a digging upper trough flicking across the Great Lakes/Ohio Valley. Transient cold air in place will result in a myriad of precipitation types Saturday afternoon and evening. The latest guidance has trended a little cooler with a little more precipitation, with appreciable snow/sleet amounts of 2-4 inches seeming more likely along the western ridges and the Mason-Dixon line, tapering to an inch or less south of US 50/E of the Blue Ridge, and mainly rain for southern Maryland. Snow-to-liquid ratios are still expected to be below climatology even in areas of all snow due to most of the column where precipitation is developing being above -10 C, though upper jet dynamics could compensate for this somewhat especially over northern Maryland. A light glaze of freezing rain also appears likely for much of the area except northern MD (mainly snow) and extreme southern MD (plain rain). Slightly higher amounts around a tenth of an inch are possible over the central Shenandoah Valley where shallow cold air would be more easily trapped. Surface ridge axis/weak wedge orients itself more or less along I-95 so believe the metros could see a touch of freezing rain as well (usually along this ridge axis is where lower dew points/cooler air pools). Latest guidance is in very good agreement on timing, with little or no precipitation entering the CWA before noon. Clouds and precipitation quickly increase from SW to NE between 1 and 4pm, with the entire area seeing some sort of precipitation by 6 or 7pm. Conditions clear just as quickly around or shortly after midnight, perhaps lingering a few extra hours over the western slopes. Temperatures are expected fall to around freezing late Saturday night and early Sunday morning. Models are slightly deeper and stronger with the low pressure and resultant gradient in its wake, but still believe winds will be light enough to result in a little patchy ice formation/re-formation early Sunday morning. && .LONG TERM /MONDAY THROUGH THURSDAY/... As high pressure moves offshore Monday, isentropic lift from the return flow will permit showers to overspread the area. Am sticking with likely PoPs for the period. Temperatures should not be a problem-- we`ll start the day above freezing, and reach the lower- mid 50s. The forecast area be in the warm sector Tue-Wed. And with 850 mb temps around +12 or +13C, that could mean highs in the 70s again...without necessarily ample sunshine. Then the cold front drops southeast Wed night into Thu. There are position/timing uncertainties with the front, which will affect PoPs. Tuesday- Tuesday night should be holding onto a slight chance mainly Blue Ridge west. Wednesday is a little less certain, as the GFS is a bit faster with frontal progression. Believe the threat would increase late in the day (as opposed to the start). Temperatures will drop Thursday due to frontal passage. Again, there are timing/strength differences to resolve which will affect precipitation, but regardless of solution, highs should be in the 50s...not the 70s. Lows, in the mid 30s-lower 40s, will still be above climo too. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... LLWS at times early this morning with strong LLJ just ahead of a cold front. Patchy MVFR or even IFR in isolated heavier downpours, but overall minimal restrictions expected in occasional -SHRA this morning. Patchy MVFR possible along cold front early this afternoon. Winds will switch to NW late this morning into early afternoon, becoming gusty (to around 30 knots) by mid to late afternoon. VFR expected tonight. Winds go light late this evening. IFR likely Sat aft-eve with a wintry mix expected. Winds light NE becoming SE. VFR returns late Sat night as winds become NW. Flight restrictions likely Monday as a warm front and associated precip lifts across the terminals. Tuesday should be better in terms of a much lower chance of showers. Am not as certain with respect to cloud bases though. && .MARINE...
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Expecting a lull in SCA level gusts mid to late morning right as a cold front drops through. Winds will increase markedly out of the NW this afternoon and persist into this evening, with a few gusts to around 30 knots possible, especially in any heavier showers right along the cold front. Winds go light late tonight and remain light through Saturday. A few SCA gusts are possible late Saturday night into Sunday behind departing low pressure before going light again late Sunday into Sunday night as high pressure moves across. South winds will affect the waters Mon-Tue. Winds Monday will be less (approx 10 kt) due to poor mixing associated with anticipated rain. However, this forcing will be north of the waters by Tuesday. The biggest hindrance will be warm air over cold waters. For that reason, forecast winds will not exceed 15 kt.
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from noon today to 10 PM EST this evening for ANZ530-531-535-536-538>540. Small Craft Advisory from noon today to 3 AM EST Saturday for ANZ532-533-537-541-542. Small Craft Advisory from noon today to 6 AM EST Saturday for ANZ534-543.
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&& $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...HTS AVIATION...HTS/DHOF MARINE...HTS/DHOF is the U.S. government's official web portal to all federal, state and local government web resources and services.