Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 181437 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 937 AM EST Sat Feb 18 2017 .SYNOPSIS... High pressure will linger over the area today. A weakening low crosses the area tonight. A backdoor cold front will enter the area Monday into Tuesday, followed by another disturbance Tuesday night into Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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High pressure can be found off the southeast US coast early this morning. 12Z IAD sounding shows a very sharp nocturnal inversion...and the rate at which that erodes is currently displayed in the wide range of area temperature observations. However, it shows that the mid 60s to lower 70s currently in the forecast will be easily attainable. Cirrus will continue to overspread the area, with perhaps some mid clouds late. Some guidance indicates sprinkles or very light showers currently in Kentucky may make it to the southwestern CWA late in the afternoon, so have added some low end POPs to that area after 4 PM.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/... An upper level disturbance along the Gulf Coast will head our way tonight. Aside from the vorticity, there`s not much else supporting this feature. The positively tilted trough weakens and is absorbed into the mean flow as it passes south of the forecast area. QPF is light (a few hundredths) and spotty. PoPs highest in the mountains, but no higher than chance there. Will be keeping the tip of southern Maryland dry. Many areas will see nothing more than a few sprinkles, as the low-levels don`t ever really saturate. Any lingering clouds Sunday morning will quickly mix out as a large scale ridge axis approaches Sunday. Concerns will shift to shortwave energy reinforcing the trough across New England. Temperatures at 850 mb won`t be as warm as today, and will only be dropping as a backdoor cold front slips toward us. It will be in the neighborhood by Monday. Am keeping Sunday high temperatures similar to today, as the low level airmass doesn`t really change up to that point. Monday will be significantly more challenging though-- both high and low temperatures. Believe that the core of the warm air in Virginia will remain. Have much less confidence in what will happen in northeast Maryland. Temperatures will drop; the questions are by how much, and where that dividing line will be. For the time being, am using the Potomac River Valley as the dividing line. && .LONG TERM /TUESDAY THROUGH FRIDAY/... A ridge axis (both surface and aloft) passes across the region on Tuesday, with return flow commencing and increasing humidity. A weak upper wave runs headlong into what`s left of the ridge on Tuesday night, and there could be enough moisture and instability to spark a few showers -- nothing major though. Surface warm front passes through around that time as well, ushering in another unseasonably warm stretch. Reaching 70 degrees is plausible on Wednesday, and even more likely on Thursday and Friday. Went above guidance throughout the warm stretch and my gut feeling is that I may still be a couple degrees too cool. However, there is some guidance showing more clouds and even isolated or scattered showers during the late week, so I am hedging a bit toward that potential. The next storm system arrives right at the end of the forecast period (next Friday night), but it currently appears we would be south of the triple point and thus the heavier rains would again miss us to the north; though it could bring temps at least closer to late-February expectations by next weekend. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... VFR for the valid TAF period, and for much of the weekend. An upper level disturbance will pass by tonight, but low levels remain dry enough that the impact will be increasing mid-high clouds this afternoon-evening, thickening to a 5000-6000 ft stratocumulus deck with perhaps a few sprinkles. MRB stands the best chance to have the ground moistened. By Sunday the disturbance will be east of the terminals, with Sunny skies returning. May have some 15-20 kt gusts Sunday afternoon. A backdoor cold front will drop south toward the area Monday. Its unclear at this point whether any marine stratus will advect inland. Currently the forecast indicates no. Would still keep that in the back of the mind as a worst-case possibility. VFR Tuesday with building thick high clouds. Lower clouds are possible for a time Tuesday night with possible MVFR or lower in scattered showers. Whether that happens or not, VFR returns on Wednesday. && .MARINE...
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Light winds on the water continue this morning, and that will be the case for much of the day. Any wind would come from the southwest today, but at speeds 10 kt or less. A disturbance will cross the area tonight, with perhaps a few sprinkles. The larger impact will come Sunday in the wake of this low, when northwest flow increases. Believe that the cold water/warm air combo will prevent winds from mixing across the marine area. For that reason, am not forecasting winds higher than 10-15 kt. The flow will turn north/northeast Monday as a backdoor cold front slips toward the waters. It remains unclear how far south this boundary will make it. Since area won`t be clear from this discontinuity, am once again keeping winds under Small Craft Advisory thresholds. Winds should remain below headline criteria Tuesday through Thursday. Some potential for southerly channeling on Friday which could lead to reaching advisory criteria in gusts.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...HTS NEAR TERM...ADS/HTS SHORT TERM...HTS LONG TERM...JE AVIATION...JE/HTS MARINE...ADS/JE/HTS

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