Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 130136 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
936 PM EDT Tue Sep 12 2017

High pressure over New York will move offshore today. The
remnants of Irma will track into the Tennessee Valley tonight,
the Ohio Valley Wednesday and into Pennsylvania Thursday. High
pressure will return for the weekend.



Models still insist bands of showers will lift northeast
overnight, but radar is currently quiet. Have kept POPs all
below 45%. Experimental GOES-16 data shows low clouds/stratus
layer covering much of VA attm with a strong subsidence
inversion noted on the low-level water vapor band over cntrl VA.
Believe model guidance is overdone. Stratus appears as the most
likely outcome overnight.


Believe Wednesday will be predominantly dry as a weak induced
ridge over the region will allow region to see some sunshine.
Highs around 80.

Irma`s last gasps will finally be felt in the Mid Atlantic late
Wednesday night and Thursday as the low gets picked up and
transported into the northeastern US. PoPs begin to increase
after midnight in the western part of the forecast area...
increasing to likely across the northern 2/3 of the region by
Thursday afternoon. Highest QPF totals will likely be along the
Masin Dixon Line - around 1/2 inch. Precip chances will decrease
Thursday night. Highs in the upper 70s/around 80 Thursday.


Upper level low/weak surface reflection (remnants of Irma) will
cross the region Friday. Favoring the slightly slower timing of the
SREF out of respect for resilient ridging over the northwest
Atlantic which has repeatedly verified stronger than most numerical
guidance over the past couple weeks, which would slow the forward
progress of the trough. This means lots of clouds and scattered
showers will remain possible through the day Friday.

A weak trough axis and some leftover mid-level vorticity will remain
over the area Saturday leading to a partly to mostly cloudy day.
High pressure, northerly winds and negative vorticity advection
should lead to more sun on Sunday.

Models diverge precipitously by early next week. Jose (provided it
holds together through then) will likely be meandering somewhere off
the southeast coast (likely still several hundred miles from land).
Long range guidance has exhibited significant run-to-run
variability, with the 12z GFS/GEFS shifting well offshore. The CMC
follows this path through 7 days as well, as does the 00z ECMWF/most
of the EC ensembles. However, the UKMET and its ensembles take the
system westward through the Bahamas which is notable because the
UKMET was, to my knowledge, the first global model to correctly
predict Irma`s track into northern Cuba thanks to the aforementioned
tenacious NW Atlantic ridge. So bottom line the future track is
highly uncertain and bears watching.

Aside from Jose, models also differ on timing/strength of incoming
shortwave trough and attendant surface cold front, but siding again
with slower guidance and indicating increased shower chances Tuesday
of next week.


VFR this evening. Low clouds look to develop before sunrise
along with some possible drizzle and fog putting airports into
IFR range, but improving quickly during Wednesday morning.
Showers/lower ceilings possible again Thursday.

Lingering moisture/scattered showers may lead to brief
restrictions Friday. Otherwise mainly VFR into the first half of the
weekend. N flow AOB 10 kts.


Winds expected to remain below SCA values through Thursday.

With troughing over/near the waters and a weak pressure
gradient, anticipate winds below SCA criteria Friday into the




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