Area Forecast Discussion
Issued by NWS Sterling, VA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KLWX 280840
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
440 AM EDT Wed Sep 28 2016
An upper level low will impact the area Wednesday through Thursday
night...bringing the potential for heavy rainfall across portions of
the area. The upper level low will gradually move away from the area
Friday through the weekend.
.NEAR TERM /THROUGH TONIGHT/...
Complex weather situation is beginning to unfold across the
area...as strong upper-level low is starting to drop southward into
the Ohio River Valley. At the surface, frontal boundary is across
central VA...where it will remain through much of today. Overrunning
showers have already been developing this morning across the VA
Piedmont and are now pushing into parts of northern VA. Expect this
trend to continue through the morning and into the afternoon...with
a northward shift in precipitation chances through the day. While
instability remains rather anemic, strengthening wind field could
lead to some stronger storms across southern portions of the
area...with wind/hail the primary threats in the strongest storms.
SPC has much of central VA and the VA Piedmont in a marginal risk
for SVR...which seems very reasonable considering the setup.
Training storms could lead to some flooding issues...but dry
antecedent conditions should prevent most hydro issues at least for
.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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). Spatiotemporal characteristics of the 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.
.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 /09Z 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
Gradient winds should remain below SCA criteria through much of the
day today...though SCA gusts are possible in showers/thunderstorms.
Small craft advisory was issued starting later this evening through
12z Friday...as gradient winds strengthen with strengthening system.
Gradient should begin to relax by later Friday...with SCA winds less
Upper low will slowly lift and fill this weekend. No flags
anticipated at this time.
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.
-- Changed Discussion --DC...Flash Flood Watch from late tonight through Friday morning for
MD...Flash Flood Watch from late tonight through Friday morning for
VA...Flash Flood Watch from late tonight through Friday morning for
WV...Flash Flood Watch from late tonight through Friday morning for
MARINE...Small Craft Advisory from 6 PM this evening to 8 AM EDT Friday
-- End Changed Discussion --