Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 140734
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
334 AM EDT Thu Sep 14 2017

.SYNOPSIS...
High pressure will remain offshore through the end of the week. The
remnants of Irma will scoot toward the northeast and across
Pennsylvania today then to southern New England tomorrow. A second
area of high pressure will build in the region over the weekend.
&&

.NEAR TERM /THROUGH TONIGHT/...
Water vapor imagery shows a circulation of the remnants of Irma that
are moving across southern Ohio toward Pennsylvania. Radar shows a
batch of rain showers, generally light in intensity, moving
northeastward from near Charlottesville and Louisa in central
Virginia toward northeastern Virginia and adjacent Washington D.C.
These showers are front-running showers well ahead of the remnants
of Irma and could bring a few hundredths of an inch over the areas
that they move over through daybreak. Both last night and early this
morning, computer models have done a good job at indicating these
showers would be around. The HRRR, NAM and GFS indicated these
showers both times. Other shower activity is moving east across
western Pennsylvania and West Virginia. Individual moderate
showers within this activity is moving toward the northeast.
Apart from the rain showers, some locations in northeast
Maryland are dry but with dewpoint temperatures in the middle to
upper 60s and have clear skies. These are prone to some dense
fog through 6am, particularly Harford and Baltimore Counties.
There is currently a dense fog advisory out for these counties,
where we could see reduced visibilities of one-quarter of a mile
or less through 6am. Use caution and leave space between you
and the vehicle in front of you.

Once the dense fog burns off in northeast Maryland by mid-morning
and any rain showers in northeast or eastern Virginia push off to
the northeast and east, we could see a break in any rain activity
the remainder of this morning. As the remnants of Irma slide east-
northeast across southern Pennsylvania later this morning into the
afternoon, additional rain showers and perhaps a thunderstorm or two
could develop due to some daytime heating and low level moisture. It
looks like the best chance for any thunderstorms would be between
1pm and 5pm this afternoon and between the Mason-Dixon line and I-66.
High temperatues this afternoon with be close to average with highs
in the upper 70s to near 80.

Although we can`t rule out a lingering shower or two this evening
and overnight, the center of the remnants of Irma should scoot to
our east to allow for NVA to evolve to our west. We left a slight
chance of rain showers in the forecast for tonight in much of the
area. Also, some patchy fog could redevelop tonight due to residual
moisture from recent rainfall and places that do clear out from
cloud cover. Lows tonight will be in the lower to middle 60s with
middle 50s out west.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Some upper level energy could still rotate around the remnants of
Irma on Friday. Although the remnants will be well to our northeast,
any energy that drops southward into our region could spawn a couple
of rain showers. Showers look to be isolated, but possible. Highs
will be near 80.

Conditions will dry out Friday night with lows in the lower 60s.

High pressure will gradually build in from the northwest Saturday
and Saturday night. A nearby trough of low pressure over portions of
the Delmarva Peninsula could spawn an afternoon shower in parts of
southern Maryland. A stray shower is also possible over the Potomac
Highlands. For the most part, high pressure should dominate with dry
conditions, but we could still have a renegade shower around. Highs
in the lower to middle 80s Saturday with lows in the middle 60s
Saturday night.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Surface high pressure will be atop the forecast area for much of the
extended forecast period. However, the ridging aloft will be
situated west of the Appalachians. Hence, heights are rather low and
low-level flow is onshore, an artifact of Jose in the western
Atlantic. As such, decent CAPE will be able to generate each
afternoon, especially Sunday and Monday. Therefore, the forecast
will feature isolated/scattered showers and thunderstorms. Since
these will be instability-driven, anticipate a rapid demise upon
sunset. Heights will be higher, and PoPs will be lower by Tuesday-
Wednesday.

Speaking of Jose, through the forecast period, much of the forecast
guidance has consolidated in an offshore solution. However, there
are still ensemble members...from both EPS and GEFS...that want to
split from the back and make a hard left turn. Thus, it is way too
early to state anythign conclusively.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
VFR conditions through daybreak, except for LIFR to perhaps VLIFR
conditions due to dense fog near MTN and BWI terminals. Visibility
could be reduced to one-quarter mile or less between the current
time through 6am. Winds light and variable to perhaps calm near MTN
and BWI.

VFR conditions at all terminals today. Any moderate showers or a
brief thunderstorm this afternoon near MRB, IAD, and DCA could
reduce conditions to MVFR or IFR for a brief period. Winds generally
southwest 5 to 10 knots shifting to the west at 5 to 10 knots today.

VFR conditions tonight. There could be some patchy fog. Winds
becoming northwest less than 5 knots.

VFR conditions Friday and Friday night at all terminals. There could
be a passing shower Friday afternoon. Winds northwest 5 knots
Friday, becoming light and variable Friday night.

Early morning low clouds and fog and late day widely scattered
showers and thunderstorms will be possbile Sunday and Monday. Flight
restrictions will be possible...the greater confidence of such will
be during the morning push.

&&

.MARINE...
No marine hazards expected today through Friday night. Winds
shifting southwest to northwest around 10 knots today into tonight.
Winds northwest 5 to 10 knots Friday, then light and variable Friday
night.

Winds Sunday-Monday will be north/northeast. At this time, the
gradient does not look strong enough to support winds much above
10kt.

&&

.TIDES/COASTAL FLOODING...
Water levels are running a good 3/4 to 1 ft above astronomical
normals, which is also about a half foot above ESTOFS/ETSS
guidance. Numerous caution stages have been reached... in
addition to minor flood at Annapolis and the cusp of minor flood
at SW DC. Maine Ave is cresting now; no additional Advisories
anticipated overnight.

The daytime tide cycle will be the lower astronomically. In
addition, guidance sugests that water levels will ease slightly.
This combonation will preclude the need for any Advisories during
the day. Hopefully these trends carry over into Thursday night as
projected.
&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Dense Fog Advisory until 6 AM EDT early this morning for
     MDZ006-011-507-508.
VA...None.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...KLW
NEAR TERM...KLW
SHORT TERM...KLW
LONG TERM...HTS
AVIATION...KLW/HTS
MARINE...KLW/HTS
TIDES/COASTAL FLOODING...HTS



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