Area Forecast Discussion
Issued by NWS Sterling, VA

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649 FXUS61 KLWX 121917 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 217 PM EST Fri Jan 12 2018 .SYNOPSIS... Low pressure and its associated cold front will pass through the area tonight. Canadian high pressure will build overhead Saturday through Monday. Low pressure and a reinforcing cold front may impact the area Tuesday into Wednesday. && .NEAR TERM /THROUGH TONIGHT/... A southerly flow will has continue to usher in unusually warm and moist air through this evening. Max temps this afternoon will be well into the 60s for most areas and a few locations may even reach 70 degrees...especially across northern and central Virginia into portions of eastern West Virginia. For locations along the western shore of the Bay and the shore of the Tidal Potomac River...areas of dense fog are possible through this evening. This is because of warm and moist air over the colder waters. Will continue to monitor visibilities through this evening...but confidence is too low for a Dense Fog Advisory at this time since it appears to be localized. Also...incoming rain may help keep vsbys up just a bit for most areas. Having that been said...vsbys around or below a quarter mile are possible especially right by the water. The warm and moist air will cause plenty of clouds along with some showers. Showers will increase in coverage late this afternoon due to the strong southerly steering flow causing convection upstream over southern Virginia/North Carolina to move into the area. Latest mesoanalysis does show limited instability over the area. In fact...some of that instability has become surface based. Therefore...an isolated thunderstorm or gusty shower cannot be ruled out late this afternoon into this evening. The best chance for that will be across northern and central Virginia where instability is highest. A strong cold front located over the Ohio Valley and western Pennsylvania this afternoon will move into Potomac Highlands late this evening...before passing through the rest of the area overnight. An upper-level low in the southern stream will also pass through our area during this time...and this will cause surface low pressure to develop along the cold front. The developing low will cause a swath of rain/isolated thunder to accompany the frontal passage. A stronger low-level jet just a few thousand feet from the surface suggests that heavier showers or isolated thunder may contain gusty winds. However...instability will be limited and also elevated for most areas during the frontal passage. Therefore...the threat for severe storms remains low. Strong northwest winds will develop right behind the frontal passage and it will usher in much colder air. Temperature drops around 15 to 25 degrees are likely within just a few hours of the frontal passage. The rush of cold air may cause rain to end as wintry precipitation. There is still low confidence for most areas on how much moisture will be left by the time that cold air arrives to change the ptype to a wintry mix. The one exception is along/west of the Allegheny Front. Rain will change to a wintry mix late this evening before changing to snow overnight. A Winter Weather Advisory is in effect for these areas. For other locations...rain may change to a light wintry mix toward morning. Despite low confidence in accumulating wintry precipitation...it is important to note that temperatures rapidly falling below freezing will cause any wet surfaces to freeze quickly. Therefore...icy conditions are possible toward Saturday morning especially from the Washington and Baltimore Metropolitan areas north and westward. The Winter Weather Advisory may need to be expanded farther south and east from where it is now should confidence increase for wintry precipitation tonight. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... The cold front will move away from the area Saturday morning while Canadian high pressure approaches from the north and west. A strong northwest wind will usher in much colder conditions with temps falling into the 20s to lower 30s for most areas. Temps in the single digits and teens are expected along the ridge tops. A longwave upper-level trough will sharpen a bit overhead Saturday as a northern stream system digs into the Great Lakes. This system will cause the upper-level jet to strengthen overhead and the trough axis should remain to our west. Our area will be in the left-exit region of the upper- level jet and this may provide just enough lift for light wintry precipitation Saturday morning. Again...confidence is too low at this time but the Winter Weather Advisory may need to be extended in area to account for this. Regardless...a rapid freeze is possible Saturday morning and this may result in icy spots on paved surfaces. Highest confidence is from the Washington/Baltimore Metro areas north and westward. By Saturday afternoon...there should be enough dry air and subsidence from the approaching high for just dry and cold/blustery conditions. Canadian high pressure will continue to settle over the region Saturday night through Sunday night...bringing dry and cold conditions. The combination of cold air and blustery northwest winds will cause wind chills to drop into the single digits and lower teens for most areas. Wind Chills below zero are expected along the ridge tops of the Blue Ridge and Allegheny Highlands Saturday night into Sunday morning. A Wind Chill Advisory may be needed. && .LONG TERM /MONDAY THROUGH FRIDAY/... Surface high pressure will be located over eastern Canada on Monday as broad upper level troughing encompasses most areas east of the Rockies. After a cold start, highs will struggle to reach freezing across northern parts of the area. Ahead of the main trough axis, a clipper low will be moving through the Great Lakes Monday night into Tuesday. There remains some timing uncertainty with the trailing cold front and how much moisture will be associated -- although overall it should be light. Temperatures will be cold enough to support snow. Besides the snow potential, the front will reinforce the cold weather. After a slight warm up on Tuesday, high temperatures may struggle to reach freezing once again on Wednesday. The mid/upper level trough will sharpen and cross the area Wednesday night. At a minimum, some upslope snow showers will be possible, but the trough will also likely interact with the baroclinic zone to our southeast and induce cyclogenesis. Will have to keep an eye on this period to see how close the low is to the coast. High pressure will begin building from the west late Thursday into Friday as heights rise aloft. Temperatures will be moderating toward climatology. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... Southerly winds will continue to usher in unusually warm and moist conditions through this evening. The warm and moist air over the Bay has resulted in low clouds and fog for KBWI and KMTN. IFR/LIFR conditions are expected through this evening. IFR/LIFR conditions are possible for KDCA as the surface wind comes off the Potomac River. Elsewhere...MVFR to IFR conditions are expected through this afternoon. Showers will become more widespread late this afternoon and evening as a cold front approaches. The cold front will pass through overnight. A strong wind shift to the northwest winds gusts around 25 to 35 knots are expected behind the cold front. The wind shift is most likely between 03 and 05z for KMRB and 06-09z for the other terminals. Rain may end as a light wintry mix Saturday morning. Any wintry precipitation amounts appear that they will be light...but a wet ground may quickly freeze toward Saturday morning with temperatures rapidly falling below freezing. Gusty northwest winds through Saturday evening will diminish a little overnight Saturday through Sunday. VFR conditions are expected Saturday afternoon through Sunday night. No issues expected Monday with high pressure. A period of snow is possible Tuesday as a clipper system crosses the area. There are still plenty of details that need to be refined with this system. && .MARINE... A southerly flow will continue through this evening. A Small Craft Advisory is in effect for the waters...but it will be marginal due to relatively cooler waters. A wind shift to the northwest is expected behind the cold front...generally between 07 and 11z and winds will be gusty. A Gale Warning is in effect for the waters overnight through Saturday morning. Winds will diminish a little for Saturday afternoon and Saturday night...but a Small Craft Advisory remains in effect for the waters during this time. High pressure will continue to build over the waters Sunday through Sunday night. A Small Craft Advisory may be needed for portions of the waters during this time. High pressure will be north of the region on Monday resulting in light southerly winds. A cold front will cross the waters on Tuesday, and Small Craft Advisory conditions will be possible in its wake. If the front moves far enough away, winds should lessen by Wednesday. && .HYDROLOGY... Rain is expected late this afternoon into tonight. 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 0.75-1.50 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... DC...None. MD...Winter Weather Advisory from 10 PM this evening to 10 AM EST Saturday for MDZ501. VA...Winter Weather Advisory from 10 PM this evening to 10 AM EST Saturday for VAZ503. WV...Winter Weather Advisory from 10 PM this evening to 10 AM EST Saturday for WVZ501-503-505. MARINE...Small Craft Advisory until midnight EST tonight for ANZ530>543. Small Craft Advisory from noon Saturday to 6 AM EST Sunday for ANZ530>543. Gale Warning from midnight tonight to noon EST Saturday for ANZ530>543. && $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...ADS AVIATION...BJL/ADS MARINE...BJL/ADS HYDROLOGY...ADS/HTS/BJL

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