Area Forecast Discussion
Issued by NWS Sterling, VA

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423
FXUS61 KLWX 111528
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1028 AM EST Sun Feb 11 2018

.SYNOPSIS...
Low pressure will track along a slow moving front today, with
the front gradually moving south of the area by Monday. High
pressure will cross north of the area Tuesday. A warm front will
cross the region during midweek, followed by a cold front late
in the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
The forecast area remains in the transition zone between warm
air to the south (70 degrees in Wakefield) and cold air to the
north/northwest (35 in Pittsburgh, 39 in Martinsburg and York).
The axis of heaviest rainfall remain in the warm sector...
approximately just east of I-95.

A strong low level jet will continue to provide support for
heavier rain elements into early afternoon. Will gradually taper
PoPs off this afternoon as the jet departs, but still maintain
at least likely PoPs. Have received numerous storm totals in the
2-3 inch range, with multiple reports in excess of 3 inches.
Latest forecast suggests another inch of water within reason. The
Flood Watch remains in effect through this evening, with
multiple Warnings in effect.

As low pressure lifts northeast from Ohio this afternoon, there
may be some subsidence in its wake, allowing rain to become
more intermittent. Where the front ultimately ends up, it may be
drizzly to the north, with drier conditions to the south. There
will also likely be a sharp temperature gradient ranging from
the 40s northwest of the front to near 70 to the southeast. Have
let climatology lead the way, and blended in a little of the
newest ensemble guidance. Some weak instability will likely
develop in the warm sector and have thus added a slight chance
of thunder to the forecast early this afternoon as well.
Southerly winds will also become gusty south of the front, as
the surface based inversion inhibiting mixing of winds or
temperatures.

The upper level pattern will give the front a nudge to the
southeast tonight. With the weak instability still in place, a
convective fine line may develop along the I-95 corridor as the
front pushes southeast. Winds may gust 20-30 mph with the
frontal passage, possibly higher if the convection is organized.
This will bring an end to precipitation in most of the area,
although another wave could cause showers to linger in far
southern MD. An upper level front and jet streak may cause a mix
of precipitation along the Allegheny Front as temperatures
cool.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
There`s not particularly high confidence in the remainder of
the short term forecast as guidance still differs on the
position of the front to our south and the impact of any
disturbances crossing the area. On the whole, the trend has been
drier for the first half of the week, although some light
precipitation can`t be ruled out at times. High pressure will
build to the north by Tuesday, resulting in a cold air damming
wedge. Sometime Tuesday into Tuesday night would be the best
opportunity for some light overrunning precipitation, but there
isn`t a clear cut source of lift. Of course if this did occur,
some mixed precipitation could occur in mainly western portions
of the CWA.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
High pressure will slide east off the coast on Wednesday, with a
southerly flow returning. This will allow the front stalled to
our south to push back north, though it may be ill-defined by
the time it returns to the area. Expect temperatures to rise
back above normal, with 50s common. An isolated shower or some
drizzle could accompany the frontal passage, but overall
Wednesday looks relatively dry.

On Wednesday night, many models are depicting a weak upper level
disturbance crossing the area. With a comparatively milder and
more moist environment in place, this could result in some
showers during the night, potentially stretching into Thursday
morning.

Warmest day of the week looks like Thursday, as southwesterly
flow reaches peak strength just ahead of another cold front
approaching from the northwest. Highs should top the 60s in most
of the region. A few showers can`t be ruled out given increasing
forcing aloft, but overall it looks relatively dry.

Guidance is not in good agreement regarding Friday, with the GFS
plowing the front through the region before dawn thanks to a
much faster wave of low pressure moving northeast along it,
while the EC holds it up til afternoon due to the much slower
low pressure wave. If the slower EC solution is correct,
Thursdaynight would be drier and Friday would be rainier and
much milder, while the GFS solution depicts a wetter Thursday
night and drier Friday with cooler temperatures overall. Have
half- stepped for the time being.

Most guidance has the cold front down close to the Gulf coast by
Saturday, placing us solidly in the cooler air mass. Highs
should be close to normal. The GFS has high pressure building
back across the region on Saturday, which would also mean a dry
day. However, the EC keeps the front close enough so that
another wave riding east along the it could cause the chance of
precip to linger. Given the cooler air mass, if this were to
occur a little wintry weather wouldn`t be out of the question.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
A mix of LIFR/IFR conditions persists across the area this
morning. There`s also a stalled front draped across the area,
which will be critical to whether any improvement occurs later
this morning into the afternoon after a wave of low pressure
lifts to the north. But, as the heavier rainfall elements exit,
have started to see hints of improvement.

South of the front, southerly winds may increase to 15-20 kt at
times this afternoon. Showers can`t be ruled out. TAFs offer a
slow improvement through IFR, but MVFR/VFR holding off until 20
UTC or later.

The front will push back to the southeast tonight. There may be
a thin line of showers (perhaps a rumble of thunder) that
develops that could bring gusty winds. Otherwise, northerly
gradient winds behind the front could gust 20-25 kt into early
Monday morning.

Drier weather and VFR conditions will return for Monday and
Monday night. Tuesday will most likely be dry too, but clouds
could lower late in the day and at night as flow becomes
onshore.

Sub-VFR conditions possible due to low cigs early Wednesday
while a warm front passes. Additional sub-VFR conditions
possible Wednesday night and early Thursday as a passing
disturbance could bring some showers.

&&

.MARINE...
Stronger southerly winds remain in the central Bay/lower tidal
Potomac, only slowly moving north. Guidance still suggests that
these winds will overspread the waters as the low-level
inversion breaks. Will maintain the Small Craft Advisory for all
waters this afternoon/evening. There is still a chance it won`t
materialize if the front does not lift north.

There may be a lull this evening, then winds will shift to the
north and increase as the front moves back south tonight. A line
of showers and thunderstorms may also bring locally higher
winds. The winds will gradually subside later Monday into
Tuesday as high pressure builds north of the region. Small
chance SCA lingers into Monday night on the Bay, but looks
increasingly unlikely with time into Tuesday.

Southerly channeling is likely to develop as a warm front
crosses the area Wednesday, with a risk of small craft advisory
conditions. As southwest flow increases on Thursday ahead of the
next cold front, advisories will again be possible.

&&

.HYDROLOGY...
The axis of heavier rain...with solid 2-3 inches and locally up
to 4 inches...has fallen across the southeastern section of the
forecast area-- from Charlottesville across southeastern DC
metro. Have numerous Flood Warnings in this stretch. Up to
another inch will within reason in this area.

Since the heavier rain stayed southeast of the river-based
forecast points, the most recent forecasts substantially lower.
Will be able to drop the River Flood Watch for Martinsburg.
However, Dawsonville still near the axis of rain with more to
come, and the river stage has reported a sharp rise in the last
couple of hours. Therefore will be holding onto the River Flood
Watch there.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flood Watch through this evening for DCZ001.
MD...Flood Watch through this evening for MDZ004>006-011-013-014-
     016-503>508.
VA...Flood Watch through this evening for VAZ025>031-036>040-
     050>056-501>508.
WV...Flood Watch through this evening for WVZ053-055-505-506.
MARINE...Small Craft Advisory until noon EST Monday for ANZ532-536-540.
     Small Craft Advisory until noon EST Monday for ANZ530-531-535-
     538-539.
     Small Craft Advisory until 6 PM EST Monday for ANZ533-534-537-
     541>543.

&&

$$
SYNOPSIS...RCM
NEAR TERM...HTS/ADS
SHORT TERM...ADS
LONG TERM...RCM
AVIATION...HTS/ADS/RCM
MARINE...HTS/ADS/RCM
HYDROLOGY...HTS



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