Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 121503 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1003 AM EST Fri Jan 12 2018 .SYNOPSIS... Low pressure developing in the Tennessee Valley today will pass near the area tonight, dragging a cold front eastward early Saturday morning. High pressure of Canadian origin will build across the region through early next week. && .NEAR TERM /THROUGH TONIGHT/...
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A southerly flow will continue to usher in unusually warm and moist air through this afternoon. Max temps will be well into the 60s for most areas and a few locations may even reach 70 degrees...especially across central Virginia. The warm and moist air will cause plenty of clouds along with some showers. The most widespread rain will be along/east of Interstate 95 through early this afternoon due to the low-level jet. However...the low-level jet will depart and there will be breaks in the precipitation across these areas this afternoon. Elsewhere...there will be frequent breaks in precipitation but deep moisture and shortwave energy will result in showers from time to time. A strong cold front will approach the region this evening before passing through overnight. Low pressure develop and as it moves northeast along the cold front...passing through our area. A strengthening low-level jet is expected ahead of the frontal passage and this will cause widespread showers with the frontal passage. Elevated instability is still progged to be in place ahead of the frontal passage...so an isolated thunderstorm cannot be ruled out. Do think that the threat for severe is low since instability will be limited and elevated. However...with the strong low-level jet in place there can be gusty showers that develop along the cold front late this evening into the overnight. Guidance maintaining its signal that low-level cold air will come rushing east behind this system. Its a question of whether the precip will exit before the cold air arrives. Through sunrise, believe that risk will remain west of the Blue Ridge, although possibly along the Mason-Dixon line too. Thermally, the column would support a change to a mix (sleet or perhaps freezing rain) if there were still precip present. Am not completely certain what type will dominate, and there is still time to figure it out. Do have a likely wintry mix in the aforementioned areas, and precip changing to snow in the mtns (with a couple of inches accum possible).
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... The precip type challenges that dominate northern/western sections tonight spread east to the rest of the area Saturday morning. At the same time, precip will be exiting to the northeast. The ECMWF remains about the slowest solution with regard to the qpf and thus its the coldest solution. Am leaning upon NAM/NAMnest/HiResW-ARW, which is a bit faster than the Euro wrt qpf timing, but not nearly as fast as the GFS. This means that PoPs drop off to about 20-30%, but with the same type thoughts (possibly a wintry mix before ending, but low confidence). Even in the mountains, precip shuts off during Saturday morning as the Great Lakes (especially Erie) is frozen. Additional snow accumulation will be minimal. For the rest of the weekend, the story will be the cold. Saturday will be windy too (20-30 mph), so wind chills will be an issue. Am not straying far from blended temperature guidance. That suggests that temperatures will fall most of the day Saturday, to a low in the teens Saturday night. Sunday nights lows will be similar. Temperatures likely won`t get above freezing Sunday. The lowest wind chills likely will be Saturday night, as the gradient relaxes as high pressure nears on Sunday. && .LONG TERM /MONDAY THROUGH THURSDAY/... Arctic high pressure will begin to retreat Monday as an Alberta clipper tracks across the midwest Mon into the eastern Great Lks by mid next week. Clipper will be generally weakening as it moves east into an area of strong high pressure over Atlantic Canada but it may still be able to bring a period of very light snow (less than 0.1 inches liquid equivalent) Tue-Tue night as it crosses the area. A new surge of modified Arctic air will follow behind it Wed. A moderating trend in temperatures is set to begin next weekend Jan 20- 21 as east coast trough lifts out and heights rise. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/...
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MVFR to IFR conditions are expected through this afternoon. There will be showers around...especially across the eastern terminals through midday. LIFR conditions are possible right along the Bay for KMTN. A cold front/low pressure system will track across the terminals tonight, providing a better opportunity for showers with brief LIFR conditions and perhaps with gusty winds. After that, winds will veer northwest, and increase to 15-25 kt. Timing will be predawn. There is a slight chance that cold air will arrive before all precip departs. If this occurs, then rain would change over to a wintry mix before ending. Low confidence on specifics, but worth mentioning. The weekend and beginning of next week will feature VFR conditions and northwest winds. The strongest winds will be Saturday into Saturday evening.
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&& .MARINE...
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A southerly flow will continue through this afternoon. A Small Craft Advisory remains in effect...but it will be marginal due to the relatively cooler waters. The gradient will increase tonight...so a Small Craft Advisory remains in effect for the waters. The advisory extends through Saturday into Saturday evening...when mixing profiles much more favorable behind a cold front. This likely will need to be continued into Saturday night, and perhaps Sunday. A Gale Warning may be needed for portions of the waters overnight through Saturday morning from a pressure surge behind the cold front. It remains possible to get brief gales just behind the cold front early Saturday morning. Will continue to include this potential in the synopsis due to continued uncertainties.
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&& .HYDROLOGY... Warm conditions are expected again today-tonight, along with periods of rain. Break up of ice may cause localized issues, but confidence is currently too low and threat too isolated to include in the HWO at the moment, but we will continue to monitor. Around one to one and a half inches of rainfall is anticipated through tonight. While precipitable water is well above climatology, suggesting that flooding is possible, relatively quick movement of individual elements and dry antecedent conditions should preclude a widespread hydrologic issue. Limited rainfall up to this point has helped. Even though soils are warming, absorption will still be limited, thus substantial ponding is possible, but not enough issues are expected to need a Flood Watch at this time. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 10 PM EST Saturday for ANZ531>534- 537-539>541-543. Small Craft Advisory until 6 PM EST this evening for ANZ530- 535-536-538-542. Small Craft Advisory from 1 AM to 10 PM EST Saturday for ANZ530-535-536-538-542.
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&& $$ SYNOPSIS...BJL/HTS NEAR TERM...BJL/HTS SHORT TERM...HTS LONG TERM...LFR AVIATION...BJL/LFR/HTS MARINE...BJL/LFR/HTS HYDROLOGY...ADS/HTS

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