Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 260747 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 347 AM EDT Thu Apr 26 2018 .SYNOPSIS...
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High pressure will build overhead today. Low pressure will approach from the south tonight before passing through the area Friday. Another cold front will cross the region late Saturday. High pressure will build across the region Saturday night and hold through the middle of next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Low pressure continues to move off to the northeast this morning while high pressure approaches from the Ohio Valley. A northwest flow between these systems has ushered in drier conditions. The weak high upstream over the Ohio Valley will build over our area today. This will keep dry conditions in place along with sunshine. However, there will be a thin deck of high clouds well ahead of low pressure that will be tracking through the southeastern CONUS. The sunshine will allow for milder conditions today with max temps in the upper 60s to lower 70s across most locations. The high clouds will lower and thicken toward sunset as the low begins to turn northeast toward our area.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
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The low will track from the southeastern CONUS north and east into the the Mid-Atlantic tonight. Clouds will continue to lower and thicken this evening ahead of the approaching low. The low will tap into some southern stream moisture, and warm/moist air will overrun the relatively cooler air near the surface. The overrunning will also be supported by decent frontogenetical forcing both at the lower and mid-levels of the atmosphere due to the closed off upper-level low and the surface low. The decent forcing along with deeper moisture will cause rain to overspread the area quickly from southwest to northeast overnight. A soaking rain is expected late tonight into Friday morning. In fact, with the forcing and deep moisture (pwats around 1 to 1.25 east of the Blue Ridge is close to the 90th percentile for this time of year), a period of moderate to locally heavy rain is possible. Rainfall amounts for most areas are most likely to be around one-half inch to one inch, with lesser amounts west of Interstate 81, and perhaps a little more (1-1.5 inches) across portions of central Virginia where forcing is expected to be strongest. Most of the rain is expected to fall within a three to six hour period, and latest 3hr Flood Guidance is around 1.5 to 2 inches for most areas (1 to 1.5 in urban areas), so current thinking is that the flood threat is low at this time but not able to be completely ruled out. The low will move away from the area later Friday morning through Friday afternoon. An upper-level trough axis will remain overhead, so a couple showers are possible Friday afternoon but much of the time will likely be dry since most of the cold air aloft associated with the upper-level trough will remain to our north and west. A cold front will approach from the northwest Friday night, but our area will remain mostly dry ahead of the cold front. A light gradient combined with a saturated ground may cause areas of fog to develop overnight. Min temps will be in the 40s for most areas, but near 50 in Washington and Baltimore. A cold front along with along with a potent upper-level trough axis will pass through the area Saturday. A few showers are possible, especially late Saturday morning through early Saturday evening ahead of the cold front. Shower coverage is expected to be scattered so it will not be a washout, in fact most of the time should be dry for any one particular location. Having that been said, there is plenty of cold air aloft, so an isolated thunderstorm cannot be ruled out across eastern areas later Saturday afternoon. Latest nam bufkit shows around 100-200 j/kg of MLCAPE across these areas. Will continue with the slight chance for thunder in the Washington and Baltimore Metropolitan areas. Canadian high pressure will build overhead Saturday night bringing chilly conditions to the area, but it will be dry.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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High pressure will build into the region Sunday through Monday night. Cooler temperatures and dry conditions expected Sunday and Sunday night. Slightly milder temperatures anticipated for Monday and Monday night. As the high shifts to the East Coast then offshore Tuesday through Wednesday, dry conditions will continue through the period. Warm temperatures expected due to a developing and persistent return flow Tuesday through Wednesday.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/...
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High pressure will build over the terminals today. Northwest winds gusting around 15 to 20 knots this morning will gradually diminish this afternoon. Low pressure will approach the terminals tonight before passing through Friday. A period of moderate rain is expected late tonight into Friday morning. SubVFR cigs/vsbys are expected during this time and IFR conditions are possible. Low clouds may hang around into Friday afternoon before drier air slowly works its way in behind the departing low. A cold front will pass through the terminals Saturday. Scattered showers and perhaps even a rumble of thunder are possible late Saturday morning and afternoon. Northwest winds will increase Saturday night and Sunday behind the cold front with gusts around 20 knots likely. High pressure will build overhead Sunday night through Monday night.
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&& .MARINE...
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High pressure will build toward the waters today. A northwest flow will continue through this morning and a Small Craft Advisory is in effect. The winds will weaken this afternoon as the high settles overhead. Low pressure will approach the waters tonight before passing through Friday. The gradient should be weak enough for winds to remain below SCA criteria, but it will be close for middle portions of the Bay and the lower Tidal Potomac River. A period of moderate rain is expected well after midnight tonight through Friday morning as the low passes through. A cold front will pass through the waters late Saturday afternoon into Saturday evening. A few showers and perhaps even an isolated thunderstorm are possible ahead of the frontal passage. Winds aloft will be light, but a deep mixing layer is expected and if heavier showers or isolated thunder develop, an inverted V in the forecast soundings suggests that locally gusty winds cannot be ruled out in addition to the possible of small hail (from cold air aloft). High pressure will build in behind the cold front Saturday night through early next week. A Small Craft Advisory may be needed for portions of the waters Saturday night through Sunday night.
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&& .TIDES/COASTAL FLOODING...
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Anomalies will continue to fall today due to a northwest flow. Low pressure will approach from the south tonight before passing through Friday. An onshore flow is expected ahead of the low. This will cause anomalies to increase a bit but latest thinking is that the flow will be light so water levels should remain below minor flooding thresholds. However, it will be close for sensitive areas.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ530>543.
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&& $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...KLW AVIATION...BJL/KLW MARINE...BJL/KLW TIDES/COASTAL FLOODING...BJL

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