Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 241442 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 942 AM EST Sat Feb 24 2018 .SYNOPSIS... A stationary boundary situated across our area will gradually lift northward as a warm front through Sunday. A cold front will then pass through from the west by Sunday evening, with high pressure building in to the region through early next week. && .NEAR TERM /THROUGH TONIGHT/...
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Visibility is gradually starting to improve across the area, but we remain solidly beneath a temperature inversion continuing to be fed by north/northeasterly winds. Dense Fog Advisory remains in effect through 11 AM, and some of the guidance hints we may need to extend it longer for portions of the area (especially near DC and waterways). Stay tuned. The boundary is on the doorstep, as evidenced by S/SW winds and higher temps/dewpoints at Waynesboro (KW13) and sites just south of the CWA in Louisa and Essex Counties in Virginia. Meanwhile, an area of showers has spread across the northern CWA this morning, dropping up to a half inch of rain. This is followed by a lull, but another area of showers will push in from the west late this morning and afternoon. I raised PoPs and QPF in the south based on trends. The forecast update overall leans heavily on the NAM/NAM Nest for temps/dewpoints, and on a combination of NAM and HRRR for everything else. The incoming rain and north winds should not allow the boundary to return northward much, if at all, today, and if the NAM is correct, it may even push a bit south. Highs today range from the upper 40s near the Mason-Dixon line (which could be optimistic) to the upper 60s in our far southern CWA (which could be pessimistic). I expect very little movement in temps tonight, with lows ranging from the mid 40s near the Mason-Dixon to the mid/upper 50s in the far southern CWA. Rain chances continue throughout the night, along with a likely resumption of low clouds and fog. Would be no surprise to need another Dense Fog Advisory.
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&& .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Come Sunday morning, a cold front will be moving eastward and nearing the western portions of our CWA. The aforementioned warm front will be lifting through and north of our area by Sunday morning, allowing much warmer temperatures to prevail. High temperatures will near 70 degrees for much of the area, with the cooler middle to upper 60s over the northern third of the CWA. Scattered shower activity Sunday morning will increase by midday as the front nears from the west. With the warmer temperatures and dewpoints creeping in to the lower 60s, along with frontogentical forcing, a slight chance of thunderstorms is possible as the front crosses the region by Sunday evening. Behind the front, clearing and drying conditions will win out Sunday night, with temperatures still above normal for late February, ranging from near 40 degrees to the north/west to around 50 degrees along our southern border. High pressure will build in from the west as we kick off the work week. The frontal boundary will linger along the Carolina coast on Monday, with the ECMWF indicating at least some rain chances for our southern zones. As such, will keep chance POPs for these areas on Monday, but the majority of our CWA will remain dry. Weak CAA behind the front on Monday will not prevent temperatures from reaching the upper 50s on Monday, with lows Monday night cooler in to the 30s area wide as CAA is a bit stronger and 850 temperatures reside at or just below 0C. This is in response to a low/mid level trough swinging through the region Monday evening. && .LONG TERM /TUESDAY THROUGH FRIDAY/... The middle of the week will be quiet as high pressure will be overhead. Tuesday will be the sunnier day; moisture will advance Wednesday due to return flow/warm advection as the center of the high will move into the western Atlantic. It remains to be seen if there will be any showers. GFS pointing that way more than ECMWF. Will back off PoPs for the period. Have greater confidence that the end of the week will end on the wet side, in spite of guidance timing/organizational differences. (ECMWF placing more emphasis on distinct southern stream shortwave/energy vs triple point from a well-defined northern stream system.) Either solution provides strong evidence of measurable precipitation, and neither scenario suggests there will be type issues until the back side (Friday night), when the qpf will be exiting. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
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LIFR/IFR have been the rule at all terminals, but conditions are slowly improving. Guidance remains pessimistic, especially at DCA/BWI for improvement, but I do see at least a brief window this afternoon before we likely fog back in tonight. Decent chances of rain at all terminals, especially this afternoon and tonight. Will be trying to refine those chances better for the 18z TAF package. A cold front will move through the terminals Sunday afternoon, with plentiful rain chances and IFR/LIFR VIS/CIGs continuing ahead of it. Southwesterly breezes at around 10 knots, gusting to near 20 knots during the day on Sunday, will shift out of the northwest Sunday night and fall to less than 10 knots in the wake of the frontal passage. Conditions should improve to MVFR/VFR Sunday night, and back to VFR through Monday night as high pressure moves in from the west. Exception to this on Monday would be CHO, where some residual shower activity is possible as the aforementioned boundary lingers well to our south. This could result in periods of sub VFR conditions there. Outlook for Tuesday and Wednesday... VFR conditions expected at this time due to high pressure.
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&& .MARINE...
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No advisories expected through tonight for our waters as winds will remain light as a stationary boundary to our south starts to lift northward as warm front. As a cold front approaches from the west on Sunday, winds will pick up and become gusty out of the south southwest Sunday morning, with SCA conditions looking possible through much of the day on Sunday until the front passes through our waters. The frontal passage may bring with it a gusty line of showers, with a slight chance of thunderstorms as well. High pressure will then build in to the region behind the front through Monday night, delivering lighter winds across our waters, and sub SCA conditions. Outlook for Monday through Wednesday... Winds will remain below Small Craft Advisory thresholds due to high pressure. Southerly flow will increase late Wed in the wake of the high.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Dense Fog Advisory until 11 AM EST this morning for DCZ001. MD...Dense Fog Advisory until 11 AM EST this morning for MDZ003>006- 011-013-014-016>018-503>508. VA...Dense Fog Advisory until 11 AM EST this morning for VAZ040- 051>054-501-502-505-506. WV...Dense Fog Advisory until 11 AM EST this morning for WVZ052-053. MARINE...None.
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&& $$ SYNOPSIS...BKF NEAR TERM...JE SHORT TERM...BKF LONG TERM...HTS AVIATION...JE/BKF/HTS MARINE...JE/BKF/HTS

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