Area Forecast Discussion
Issued by NWS Sterling, VA

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752 FXUS61 KLWX 211840 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 240 PM EDT Sat Apr 21 2018 .SYNOPSIS...
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High pressure over the Great Lakes will slowly move east into the Northeast U.S. by Sunday night. This high will move offshore Monday. A low pressure system will bring unsettled weather to the region Monday night through the middle of the week.
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&& .NEAR TERM /THROUGH SUNDAY/...
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Latest surface analysis depicts high pressure centered over the Great Lakes, with a second center, slightly weaker, located near Norfolk. This high will dominate our weather into Monday, providing mostly dry and slightly cooler than average conditions, though temperatures will be slowly moderating towards normal. Tonight, while the high will be overhead, a shortwave will be advancing toward the region from the northwest. This shortwave should bring a band of clouds with it as it passes, and while they will at midlevels, it will likely be enough to prevent temps from getting too close from freezing in the growing season region. Thus, no frost or freeze headlines. Temps will get chilly, with colder spots in the 30s, but along and east of I-81 it should generally be no lower than the upper 30s. Clouds will remain through much of Sunday, and in fact lower a bit. As the axis of the shortwave itself passes the region late in the day, there could even be a stray sprinkle or light shower. Guidance depicting this potential concentrate it near the Appalachians of eastern West Virginia and western Virginia, though one or two models do show a little light precip near southern MD. For the time being have the forecast dry since odds remain low, but if this trend continues, may have to introduce an isolated shower/sprinkle into the forecast. While clouds will dominate on Sunday, there should be enough insolation, coupled with the higher starting point, to bring temps back to the 60s for mores spots once again.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
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Odds of precip diminish Sunday night as a weak ridge moves overhead. Clouds should also diminish, but there will likely still be enough, combined with the continued moderation of the air mass, to keep temps above frost/freeze thresholds, and in fact, it should mostly stay in the 40s. The ridge aloft pushes northeast past us on Monday as the closed low moves across the lower Mississippi Valley. At the surface, the high will move off the coast. This will allow clouds to return, and in fact guidance is now edging precip into west- central VA by day`s end. However, there will still be enough insolation to boost temps into the 60s for most of the region. As the low slowly pushes northeastward toward us Monday night, the chance of precip will continue to increase from southwest to northeast as the night progresses. Temps should be warm enough for plain rain even at the higher elevations, though some spots could edge into the upper 30s. Otherwise, temps will mostly stay in the 40s with the clouds preventing any radiational cooling.
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/...
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Early on Tuesday, we will be dealing with a couple different features that will act together to bring precipitation to the region. First, the surface low moving out of the southeast and developing off the North Carolina coast. The second is a cut-off upper low that will move out of the southeast into the southern Ohio Valley. Recent model runs are relatively consistent with one another, and begin to merge this cut-off upper low with the main axis of the jet stream. As this occurs, the resulting shortwave moves into our area from the west Tuesday evening. The combination of lift provided by the shortwave and the surface low moving up the eastern seaboard should bring a period of moderate rainfall Tuesday night, and continuing into Wednesday as light to moderate rainfall. Temperatures on Tuesday and Wednesday will remain slightly below average thanks to this unsettled pattern. Things should taper off to lingering showers on Wednesday evening, as the low pressure system lifts to the north. However, unsettled weather will continue in the form of light showers on Thursday, as a relatively strong shortwave moves south of the area. A weak surface low begins to form off the NC/VA coastline, in association with this shortwave, late Thursday. This low should quickly be taken to sea and have little effect on the area. Temperatures on Thursday will be very similar to Tuesday and Wednesday. With so many disturbances moving through the area, it is going to be hard to get really warm. A few lingering showers very early on Friday should taper off, and skies should clear early in the day. A more potent low pressure system will be moving along the US/Canada border through the day Friday, but the cold front associated with this system will affect us sometime during the day on Friday. Model spread is great with exactly when the front will pass through at this time, but a period of precipitation is possible as this moves through. The timing of this front will be a big determining factor when it comes to temperatures on Friday afternoon, but for now the latest thinking is that the front will move through early to mid-afternoon, allowing for temperatures to at least get closer to average for this time of year. The development of this low pressure system beyond Friday introduces great uncertainty to the forecast. The GFS brings precipitation to our northern areas as the low pressure system strengthens in the northeast, while the Euro brings practically nothing to the area on Saturday. Some ensembles do support the possibility for some rain on Saturday, but the key is that it does look to be light at this point if we do see any. Temperatures look to return to below average.
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&& .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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VFR thru Monday all terminals as high pressure dominates. Winds mostly light and variable, though becoming easterly Monday. Cigs and vis may diminish in rain later Monday night, especially at CHO. Least likely terminal to be impacted through Monday night is BWI. A very unsettled pattern will develop on Tuesday, and last through Thursday, as several systems move across the region. Low clouds will certainly be on the table throughout the long-term period, so expect sub-VFR conditions throughout much of this period.
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&& .MARINE...
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Winds light through Monday morning, generally 5-10 knots on the waters. Southeasterly flow increases Monday as high pressure slides off the coast and low pressure approaches from the southwest. SCA level gusts may begin late Monday or Monday night as the system approaches. The primary concern in the long term will be out ahead of a developing surface low on Tuesday along the NC coast, which will move north into the area. Windy conditions can be expected on the waters, bringing about the possibility for SCA. Wind should be calmer by Wednesday.
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&& .TIDES/COASTAL FLOODING...
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Persistent onshore flow will develop Monday and continue Tuesday with an approaching storm system. This will likely result in increased tidal anomalies, and possibly some coastal flooding, by the middle part of the upcoming work week.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...CJL AVIATION...RCM/CJL MARINE...RCM/CJL TIDES/COASTAL FLOODING...RCM

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