Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 281354 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 954 AM EDT Wed Sep 28 2016 .SYNOPSIS...
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An upper level low will impact the area through Thursday night... bringing the potential for heavy rainfall to the Mid Atlantic. This will gradually move away from the area this weekend.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Complex weather situation is unfolding across the area...as strong upper-level low is dropping southward into the Ohio River Valley. At the surface, frontal boundary is across southern VA...where it will remain through much of today. Very challenging forecast over the next couple of days. It`s interesting to see how much the atmosphere moistened up in the 12 hours betweeen the 00Z and 12Z IAD sounding. This increase in moisture has led to some very heavy ranifall across Lower Southern Maryland - radar estimates of over 4" in 3 hours has led to the issuance of a flood warning for Calvert and St. Marys Counties. Training storms could lead to some flooding issues later today. Another factor with the sounding/VWP is the wind field - low level winds are from the east...turning abruptly to the southwest above 6000 feet. Turning in the atmosphere has led SPC to put the CWA east of the Blue Ridge at a 5% tornado risk. Limiting factor would be cloud cover..but given we had to issue severe warnings at 550 am this morning implies that there is energy in the atmosphere.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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Heaviest rain expected later tonight into Friday morning as strong upslope flow (up to 50 kts out of SE) and high precipitable water transport set up over the area (axis of heaviest rainfall expected over our area). Flash Flood watch in good shape...as repetitive bouts of moderate-to-heavy rain could eventually saturate the dry soils and lead to runoff concerns. Storm total QPF still greater than 3 inches over a large area...with the potential for higher totals in localized areas. Despite the weak instability on Thursday...strong wind field remains...so could also be a few stronger storms with wind/hail threats. General consensus is for the bulk of the heaviest rainfall to begin to wind down during the day Friday as low pressure system begins to weaken as it becomes vertically stacked.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/... Upper low will remain across the Ohio Valley this weekend. Hence, we will need to keep a chance of showers in the forecast for at least Saturday. However, that chance will be less than the end of the week as the low will begin to lift toward the Great Lakes and eventually the St. Lawrence Valley. That should permit a drying trend, making Sunday the better of the two weekend days. High temperatures will be near normal for the start of October. Lows, on the other hand, will be about 5 to 10 degrees above normal, due to the humidity from the influx of moisture. High pressure should be back for early next week. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... Upper-level low will impact the area today through Friday...bringing periods of IFR (and perhaps lower) to all the terminals. Heaviest rain expected late tonight into Friday. Isolated thunderstorms are also possible this afternoon and Thursday afternoon. Could see a gradual return to VFR later Friday afternoon. Gradual improvement through the weekend, with windows for flight restrictions becoming more scarce. VFR should generally prevail by Sunday. && .MARINE...
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SCA in effect for the lower half of the Chesapeake Bay and lower Tidal Potomac now...and for all waters starting later this evening through 12z Friday...as gradient winds increase with strengthening system. Gradient should begin to relax by later Friday...with SCA winds less likely. Upper low will slowly lift and fill this weekend. No flags anticipated at this time.
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&& .TIDES/COASTAL FLOODING... Current forecast keeps tidal sites below flood stage at least through most of Thursday. Believe guidance is not capturing strength of easterly flow component...so this will need to be monitored to see if coastal flooding concerns arise sooner. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch from late tonight through Friday morning for DCZ001. MD...Flash Flood Watch from late tonight through Friday morning for MDZ003>006-011-013-014-016>018-502>508. VA...Flash Flood Watch from late tonight through Friday morning for VAZ027>031-038>040-050>057-501-502-505>507. WV...Flash Flood Watch from late tonight through Friday morning for WVZ050>053-055-504. MARINE...Small Craft Advisory from 6 PM this evening to 8 AM EDT Friday for ANZ530>532-535-536-538>541. Small Craft Advisory until 8 AM EDT Friday for ANZ533-534-537- 542-543.
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&& $$ UPDATE...WOODY! PREVIOUS...MSE/HTS

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