Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 200230 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 930 PM EST Thu Jan 19 2017 .SYNOPSIS... High pressure will move off to the east tonight. A warm front will move into the area Friday and stall out nearby through Saturday. Low pressure will affect the area Sunday and Monday. Weak high pressure may approach the area Tuesday before a cold front approaches Wednesday. && .NEAR TERM /THROUGH FRIDAY/...
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High pressure is currently located along the eastern seaboard. With thin spots in the cirrus, some locations have already fallen to their dewpoints (lower to mid 30s), necessitating lowering the low temperatures in some locations. However, with thicker clouds arriving from the southwest, the temperatures should hold steady or rise a few degrees the second half of the night. Any rainfall arriving in the southwest corner of the CWA late tonight will be very light. Warm advection, albeit rather weak, starts late tonight and continues on Friday as the closed low opens into a shortwave and lifts northeast across the region. With this overall being a weakening feature, rainfall is not expected to be particularly significant on Monday, generally less than a quarter of an inch. Rain may move through as several bands as suggested by short-range high resolution guidance, so went with "periods of rain" given potential for dry intervals. It`s possible the first band is little more than sprinkles, with the second band providing most of the measurable rainfall. The gap between any such bands remains uncertain. That all having been said, in general we expect rain to arrive across metro DC 8-10 AM, with light rain likely occurring at swearing in, and the rain should mostly depart 2-4 PM. However, the low clouds, fog and drizzle may persist given light southeast flow and no frontal passage expected. With the rain expected on Friday, expect highs to stay below guidance, mid to upper 40s generally.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... Friday night into Saturday night, the region is more or less between systems, but with the warm front stalling to our south and nothing to disturb the air mass, low clouds, mist, drizzle and perhaps fog look likely to remain over the region, at least through early Saturday. Some insolation may reach the ground enough to dissipate the drizzle, mist and fog on Saturday afternoon, but it will be short lived. Warm advection precip with the next complex low pressure system will start Saturday evening, with rain likely to overspread much of the region late Saturday night. With the clouds, mist, drizzle and patchy fog, lows on Friday night should not drop much, with readings generally in the 40s. Highs on Saturday will be dependent on the clouds thinning a bit with a break in the fog and drizzle. Assuming this occurs, we will likely rise into the low-mid 50s. Saturday night lows should stay in the 40s with the clouds and rain arriving late. && .LONG TERM /SUNDAY THROUGH THURSDAY/... A storm system will develop over the Mississippi Valley and deepen as it moves toward the east-northeast Sunday and Sunday night. This low will send energy into our region Sunday into Sunday night, bringing rain to the region. On Monday, the main low will move to the northeast a little further. The threat for rain continue Monday into early Monday evening. Temperatures will remain mild for January but cooler air quickly lines up on the doorstep of the region to move in Monday night. Some guidance suggests enough cold air may arrive before precipitation ends to allow a change to snow, particularly across higher elevations well northwest of I-95 corridor. Will need to watch this potential. Other concern Monday is for gusty winds, and some guidance is hinting at wind advisory gusts as the low passes just to the east. Will need to look at this harder in coming day or two. High pressure will usher in drier air Tuesday and Tuesday night. Clouds will break for sun, except for clouds and upslope rain and/or snow showers in the Potomac Highlands possible. An upper level trough of low pressure will sag from the Great Lakes region toward the mid-Atlantic Wednesday. A return of mild air from the south will evolve ahead of the trough with some sunshine giving way to some clouds. Temperatures will be seasonable with some sunshine Thursday. An upper level trough of low pressure will be just to the north but dry conditions should prevail. There is a chance of upslope rain or snow showers in the Potomac Highlands. && .AVIATION /03Z FRIDAY THROUGH TUESDAY/...
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VFR through late tonight with high pressure overhead sliding to the east. With the approaching warm front, clouds will thicken and lower, with intervals of rain starting early-mid morning Friday and continuing into the afternoon. Visibility and cigs will lower at least to MVFR. IFR is much more uncertain, with statistical guidance seeming to drop conditions too quickly given light rain. Have coded the lower cigs as "sct" to indicate the uncertainty. While rain ends in the afternoon, low clouds, mist and drizzle with a better chance of IFR conditions likely to persist through Friday night into Saturday morning before improving perhaps to MVFR Saturday afternoon and evening. Conditions may drop again late Saturday night as rain moves back in. Sub-VFR conditions Sunday through Monday with gusty winds developing Monday. Improving conditions Tuesday.
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&& .MARINE...
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Winds should remain below SCA through Saturday night, becoming southeasterly. Rain, drizzle and fog/mist likely to arrive Friday morning and persist through Saturday morning. Improved conditions seem likely Saturday afternoon and evening before rain returns late at night. Approaching system looks to bring increasing winds Sunday and Monday as it strengthens and moves northeast along the coast. SCA looks likely with gales possible Monday. Winds begin to diminish Tuesday.
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&& .TIDES/COASTAL FLOODING...
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Tidal anomalies have risen considerably today. However, tide levels look to remain below flood for this high tide cycle. With little impetus for draining the water, levels will need to be monitored over the weekend, although guidance does not suggest flooding at this time. With low pressure moving up the coast later Sunday into Monday, the set-up will be favorable for increasing water levels in the Bay.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...RCM LONG TERM...KLW AVIATION...ADS/RCM/KLW MARINE...ADS/RCM/KLW TIDES/COASTAL FLOODING...

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