Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 250153 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 953 PM EDT Tue Apr 24 2018 .SYNOPSIS... Low pressure over South Carolina will slowly move up the Mid- Atlantic coast tonight before passing by to our east Wednesday. A cold front will pass through Wednesday evening and high pressure will return for Thursday. Low pressure may impact the area overnight Thursday into Friday before a cold front passes through Saturday. High pressure will return by the end of the weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... As low pressure lifts northeast toward the forecast area, the moist conveyor will be lifting north into Pennsylvania and New Jersey. East/onshore flow remains, with overrunning/isentropic lift present. Thus, the rain has transitioned into a light rain/drizzle scenario, with low clouds and areas of fog. As the center of the low approaches, believe that the upglide will decrease somewhat-- enough to gradually decrease PoPs from cat to likely...and for central Virginia/Potomac Highlands, to a high end chance by dawn. With such a setup, temperatures won`t be falling far tonight...with upper 40s west and lower 50s east by dawn. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/... Steady rain will end by 12Z everywhere as onshore flow weakens and low pressure passes just to the east. Not much in the way of forcing available to generate precip, but can`t completely ruled out a few showers as weak instability develops with passage of upper level trof axis. Otherwise, most cloudy skies with some breaks in the clouds possible over the VA piedmont in the afternoon. Very weak high pressure builds Wed night and Thu. A second area of low pressure will move across the area Thu night with increasing chances of showers. && .LONG TERM /FRIDAY THROUGH TUESDAY/... On Friday, a upper level 500mb trough will be situated over the Midwestern US. A 500mb shortwave will move through the system and be over our region on Friday. The associated sfc low pressure system will shift into the region from the southwest. The exact location of this sfc low is still uncertain as the 12Z GFS keeps the low further north and west in northern Maryland and PA. The 00z Euro has the low approaching and moving east north east just to our south. The difference between solutions is that the GFS tries to keep the majority of the precipitation to the north and west of our forecast area while the Euro tries to take the low further southward. The 12Z Euro run may bring the low further north like the GFS increasing the the chances for higher precip amounts. As of now, this seems like a fairly weak frontal passage that will bring light precipitation. The front associated with this sfc low will likely move out of our forecast area by early Saturday morning. Temperatures will rise Friday up into 60s with some areas reaching the 70s due to warm air advection from the south to southwest. A isolated thunderstorm wouldn`t be out of the question on Friday. Behind the weak frontal passage, a strong high pressure will build into the area from the Upper Midwest and the Great Lakes region. The high will remain over the region through early next week leading to clear skies and temperatures close to average for this time of the year with temperatures in the upper 60s to low 70s. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... The synoptically forced precipitation has lifted north of the terminals. With residual moisture, lift, and onshore flow... low clouds and fog has developed in the light rain and drizzle. Terminals are bouncing between MVFR and IFR, but believe that IFR and even LIFR will prevail overnight. Aside from CHO, am forecasting no lower than IFR at this time. If it were to go lower, believe that cigs would be the culprit. Gradual improvement expected Wed as the low moves northeast of the terminals, but it will be a slow process. If we`re lucky, then VFR will return mid-late afternoon. Otherwise it will wait until early evening. No flight restrictions anticipated Thu. The first part of the day may be breezy (from the northwest) in the pressure gradient in the wake of the departing low, but gusts should not significantly exceed 20 kt. Clouds will increase on Friday as a low pressures system moves in from our south. Brief lowering of cloud bases and visibility will be possible if showers and isolated thunderstorms move through the region. Sub-VFR conditions will be possible on Friday. Clearing skies along with light winds are expected over the weekend leading to VFR conditions. && .MARINE... East flow across the waters this evening in advance of low pressure. With low clouds and fog, mixing not ideal. Have cancelled the Small Craft Advisory for the Potomac above Indian Head. Believe an impulse will be supporting a burst of better wind travelling up the Bay tonight, so will be keeping the rest for now. Light winds expected on Wed, then strengthening somewhat Wed night behind departing low. Marginal SCA conditions possible, but confidence low attm for any SCA. A low pressure approaching from out south on Friday will lead to a southerly flow and increasing clouds. The southerly flow will likely warrant the need for small craft advisory on Friday as winds flow up the chesapeake bay. Over the weekend, a strong high pressure will be present leading to clear skies and lighter winds. SCA unlikely Saturday and Sunday as of now. && .TIDES/COASTAL FLOODING...
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Water levels running about a foot above astronomical normals this evening, with most of that increase coming during the daytime/evening hours Tuesday. The departures are a pinch higher than that for the tidal Potomac below the 301 Bridge. Based on these trends and current projections, Piney Point will brush the minor threshold tonight. With continued south/east flow into tomorrow, departures will be steady or continue to rise slightly. Therefore, anticipate a repeat tomorrow morning. Therefore, have issued a Coastal Flood Advisory for St Marys Co for the next two tide cycles. Although water will be running high, it is unclear whether any other sites will be affected. If there isnt enough northwest wind to push water out on Wed (which is a real possibility), then Annapolis may briefly touch minor flood too. Also, SW DC forecast to come just inches away for the next few cycles. Will be monitoring those sites, but not enough confidence to issue anything at this time.
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DC...None. MD...Coastal Flood Advisory until 2 PM EDT Wednesday for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory until 4 AM EDT Wednesday for ANZ530>534- 537>543. Small Craft Advisory until midnight EDT tonight for ANZ536.
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&& $$ SYNOPSIS...LFR NEAR TERM...HTS SHORT TERM...LFR LONG TERM...JMG AVIATION...HTS/JMG MARINE...HTS/LFR/JMG TIDES/COASTAL FLOODING...HTS is the U.S. government's official web portal to all federal, state and local government web resources and services.