Area Forecast Discussion
Issued by NWS Sterling, VA

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657 FXUS61 KLWX 241439 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 1039 AM EDT Tue Apr 24 2018 .SYNOPSIS... Low pressure to our south will impact the weather today. The low 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 /THROUGH TONIGHT/... Cutoff upper-level low pressure over TN this morning will catch up to the sfc low tonight with upper system weakening and phasing with northern stream trough dropping from the eastern Great Lks. Steady rain expected this afternoon through midnight due to decent onshore flow and warm air aloft overrunning cooler at the sfc. Still expecting a a good soaking rain with totals between 1-1.5 inches south of I-66 and 2+ on the eastern slopes of the Blue Ridge with lesser amounts over northeast MD. Previous afd... A deep moisture fetch from the Gulf Stream is advecting into this system. The warm and moist air from the Gulf Stream will overrun surface cooler air resulting in a soaking rain for the entire area. The tricky part will be the timing of the rainfall arrival. Rain is expected to fill in early this morning across the Potomac Highlands into central Virginia. A soaking rain is expected for the morning rush across these areas. This band of rain will gradually spread north and east...most likely making it into the Washington Metropolitan area late this morning into early this afternoon, then towards Baltimore by late this afternoon. The heaviest rain is expected across central Virginia into the Potomac Highlands where it will be raining all day. Temp/dewpoint depressions early this morning remain pretty large so evaporative cooling is expected once rain begins. Therefore, ran a non-diurnal trend for the hourly temperatures and it will turn out to be quite chilly for this time of year with temps holding in the 40s across the mountains to the 50s across most other locations. A few areas in northeastern Maryland will approach 60 where it will take longer for the rain to move in. The cutoff low will pass by to the south this evening before getting picked up by the jetstream and eventually moving up the Mid-Atlantic Coast overnight. Our area will remain on the cool side of the boundary, and with more overrunning in place this will allow for a soaking rain for much of the night. Rainfall rates will subside a bit as the dry slot works its way in aloft. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... The surface low will continue to move up the Mid-Atlantic Coast Wednesday while the upper-level low associated with this system passes by to our south. A cold front will also pass through the area Wednesday evening. Unsettled conditions are expected with a few showers likely Wednesday due to the low passing by combined with the approaching cold front. However, it does not appear that Wednesday will be a washout since there will be breaks in the precipitation. There may even be some breaks of sunshine, but cloud should hang around for much of the day. Wednesday will turn out a bit milder ahead of the cold front with max temps in the 60s for most areas. Drier air will move in behind the cold front Wednesday night due to a northwest flow. High pressure will return for Thursday, bringing some sunshine and dry conditions along with seasonable temperatures. Low pressure may impact the area overnight Thursday with rain possible, but confidence remains low at this time and large divergence among guidance as to how strong the low will be and how far north it will track. && .LONG TERM /FRIDAY THROUGH MONDAY/... An area of low pressure should move across our southeastern zones Friday and bring a chance for rain showers to the region. This chance of rain showers could extend back as far as I-81. There remains some uncertainty on the track of the low and the strength of the low between the models. For now, we will keep the chance of rain showers. Temperatures should remain near to slightly below average, and could vary greatly depending on the track of the low. A weak cold front Saturday will move across our region before midday. A few rain showers cannot be ruled out along and ahead of this front. Temperatures will be close to average for this time in April. High pressure builds in on Sunday and Monday. Dry conditions and warmer temperatures will ensue each day. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... Rain will overspread the terminals from southwest to northeast this morning into this afternoon. Along with the rain will be lowering cigs/vsbys. Timing is a bit uncertain, but it looks like MVFR conditions will overspread the terminals late this morning into this afternoon and IFR conditions will overspread the terminals tonight. However, conditions are expected to deteriorate a little quicker for KCHO where IFR conditions are possible this afternoon. Also, periods of IFR vsbys are possible in bands of moderate rain, especially this afternoon. Low clouds along with rain and drizzle will hang around for tonight with IFR conditions expected. Conditions will gradually improve Wednesday as low pressure passes by to our east, but there will be a few showers around. Drier air and a northwest flow are expected Wednesday night through Thursday with VFR conditions. Low pressure may impact the terminals overnight Thursday but confidence remains low. Sub-VFR conditions possible Friday with a nearby low pressure system. VFR conditions at all terminals Friday night through Saturday night. Winds southwest around 5 knots Friday and Friday night. Winds becoming northwest 5 to 10 knots Saturday and Saturday night. && .MARINE... An easterly flow will gradually strengthen this morning as low pressure approaches from the south. The low will move up the Mid-Atlantic Coast late tonight and pass by to our east Wednesday. The gradient will subside during this time. A northwest flow is expected on the back side of the low Wednesday night into Thursday. A Small Craft Advisory is in effect for the waters today into this evening, and for the Bay and lower Tidal Potomac River into late tonight. Will continue with a lull in the winds for Wednesday as the low passes by to the east, then a Small Craft Advisory will likely be needed Wednesday night and Thursday for portions of the waters. Low pressure may impact the waters later Thursday night but confidence remains low at this time. No marine hazards expected Friday through Saturday night. Winds southwest 5 to 10 knots Friday and Friday night. Winds becoming northwest 10 knots Saturday and Saturday night. && .HYDROLOGY... Widespread rainfall amounts from 1 to 2 inches are forecast through Wednesday, with locally higher amounts likely along the central Blue Ridge and adjacent foothills to the east. Due to relatively dry antecedent conditions and extended time period over which the rain falls, widespread hydrological concerns are not anticipated. However, with increasing southeast flow, localized amounts of up to 3" are possible, again mainly near the higher elevations of the Blue Ridge and near-by highlands which could lead to some minor hydrology issues. && .TIDES/COASTAL FLOODING... An easterly flow will strengthen today as low pressure approaches from the south. The easterly flow will continue tonight before becoming light Wednesday and turning northwest Wednesday night. Tidal anomalies will continue to increase today. There is a better chance for minor tidal flooding with the high tide cycle tonight into Wednesday morning mainly at Straits Point, despite it being the lower of the two astronomical norms. Water levels should subside later Wednesday night with the northwest flow. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 4 AM EDT Wednesday for ANZ530-531- 538>540-542. Small Craft Advisory until 4 AM EDT Wednesday for ANZ532>534- 537-541-543. Small Craft Advisory until 10 PM EDT this evening for ANZ535- 536. && $$ SYNOPSIS...BJL NEAR TERM...LFR SHORT TERM...BJL LONG TERM...KLW AVIATION...BJL/KLW MARINE...BJL/KLW HYDROLOGY...BJL TIDES/COASTAL FLOODING...LFR

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