Area Forecast Discussion
Issued by NWS Sterling, VA

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848 FXUS61 KLWX 221835 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 235 PM EDT SUN MAY 22 2016 .SYNOPSIS...
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Low pressure will develop near Cape Hatteras tonight and head northeast Monday and Tuesday. High pressure will slide southeastward from the Ohio Valley on Monday into the southeast on Tuesday and then move off the coast Wednesday, where it will remain for the rest of the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As guidance progged, an area of rain has dropped south from Pennsylvania into most of the forecast area today. One can see the pivoting taking place on animated radar loops, as the 500 mb low is positioned nearly overhead. On visible satellite pictures cumulus clouds are developing on the western edge of the cloud mass, which includes the Appalachians. Presently thunderstorms are forming in West Virginia, but near term guidance suggests its still possible for the mountains to get in on this. Have transitioned database over to steadier rain, evolving to occasional showers before days end. In addition, have preserved the slight chance of thunderstorms for the western zones. RAP/HRRR guidance suggests that there will be a decreasing trend in radar coverage toward and especially after sunset. NAM/GFS holds onto precipitation near the center of the low. That could be drizzle. Regardless, there`s still a decent likelihood that measurable precipitation may occur between midnight and dawn. So PoPs will reflect an overall downward trend, but will remain quite high in central Virginia and southern Maryland through the nighttime hours.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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The upper low will slip offshore Monday and begin its trek up the coastline. Another coastal surface low will develop, and the system will become vertically stacked. The forecast area will be situated in a similar placement to where West Virginia is today. There will be minimal instability (via CAPE) and better lapse rates (6 C/km) between 850-500 mb. Anticipate redevelopment of showers during diurnal heating, and cannot rule out thunder either. Since mean layer RH will be a bit thinner, there will be the potential for brief glimpses of sunshine, which will drive the heating necessary for thunderstorms to form. MOS has been real consistent suggesting that high temperatures will approach 70F Monday, so will continue with that forecast. Have a better chance that the nocturnal minimum will result in showers ending overnight. Believe there still will be enough influence from the upper low that skies will remain mostly cloudy. In turn, that will influence low temperatures, keeping them a bit warmer once again. Ridging will begin to build across the area on Tuesday. Won`t completely rule out a morning shower or afternoon thunderstorm along the coastline. Instability looks a little better for convective development compared to Monday. However, heights will be much higher. Only the coast, nearer to the departing upper low, will vertical velocity be enough to take advantage of this lift. Skies will be partly sunny east of the Blue Ridge, due to mean layer moisture. To the west, subsidence from the building ridge suggests skies will be mostly sunny. The higher heights will also manifest in the temperature forecast, which will feature a substantial rise...to the mid to upper 70s.
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&& .LONG TERM /WEDNESDAY THROUGH SUNDAY/...
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While temperatures climb into the 80s Wednesday, dewpoints remaining in the M50s, on downsloping westerly flow, will keep it from becoming too oppressive. Noticeable changes arrive late Wednesday into Thursday, as surface high offshore to our SE promotes strong moisture advection. By Thursday afternoon, dewpoints forecast to rise at least 10F, with M/U 60s dewpoints commonplace through the weekend. This will help to increase afternoon instability into at least the moderate range, though lack of lifting mechanism and nearby ridge axis aloft, will likely keep thunderstorm activity diurnally driven and CI confined to the higher terrain out west. Furthermore, lack of meaningful steering flow could prevent thunderstorms from moving too far off the higher terrain, but could also lead to a few hydro issues (especially with increasing PWATs). Places devoid of afternoon thunderstorm activity will remain hot and humid, with afternoon highs at least 5-10F above normal. Increased moisture will also keep nighttime lows elevated, with 60s across most areas (near 70 in urban areas).
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&& .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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Flight restrictions prevail across the terminals. They have been variable between MVFR and IFR, depending upon intensity of rain and positioning of lower clouds. Will be in a similar pattern through the night. Areal coverage of rain should become less, especially MTN/BWI/MRB, but that could lead to drizzle and fog instead. Suppose that LIFR possible, but have low confidence in when/where. The upper low will begin its departure Monday, which should give ceilings at least an opportunity to rise. Have VFR in TAFs by afternoon. Confidence quite low in the details of its progression, but believe we ultimately will get there...perhaps by afternoon. In the heating that will result from the lifting clouds, there will be the chance for diurnal showers, and maybe even a thunderstorm. Renewed restrictions would result, which would be more localized and of a shorter time duration. Trends will continue to improve Monday night. Higher confidence VFR by Tuesday as high pressure builds. Slim chance of a shower along the Chesapeake Bay shoreline, which will be near the periphery of the exiting low. This includes BWI/MTN. VFR conditions Wednesday, with high pressure remaining in control. While chances at any one location are low, a few thunderstorms are possible Thursday and Friday, with locally sub-VFR in and around storms, otherwise VFR expected.
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&& .MARINE...
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Mixing will be quite poor through Monday, as low pressure remains nearly overhead. Winds will below any critical thresholds. Flow will be northerly tonight, but then become northwest in the wake of the low. While winds remain light through the end of the workweek, there is a low-end chance for an isolated thunderstorm Thursday and/or Friday afternoon.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...HTS NEAR TERM...HTS SHORT TERM...HTS LONG TERM...MSE AVIATION...HTS/MSE MARINE...HTS/MSE

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