Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 171547
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1047 AM EST Sat Feb 17 2018

.SYNOPSIS...
Low pressure will track northeastward along a stalled frontal
boundary well to our south today and tonight. High pressure will
build in from the west on Sunday, migrating offshore by Monday.
Low pressure will move northeastward toward the Great Lakes
Monday, lifting a warm front northward through our region. A
cold front is poised to approach the region during the latter
half of the work week.

&&

.NEAR TERM /THROUGH TONIGHT/...
High and mid-level clouds streaming over our area from the
southwest this morning, but ample sunshine has greeted much of
the region this afternoon. Temperatures in the middle to upper
30s for most this morning, but with the clouds encroaching in
response to an approaching mid-level trough and surface low
pressure over the Tennessee River Valley, we won`t see them
warm too much more, topping out near 40 in the warmest locales.

Skies turn mostly cloudy for all by this afternoon and moisture
begins to increase from southwest to northeast over the region.
Despite temperatures 5 to 10 degrees above freezing today, do
think as precipitation spreads across the region, wet bulbing
will occur with dewpoint depressions on the order of 10-15F this
morning. Precipitation will overspread the area early this
afternoon to our southwest, reaching the metro areas mid to late
afternoon. The precip may start as rain before sufficient
cooling occurs, especially along and south of I-66, then a
transition to snow/sleet. Expect snow at the onset across our
northern zones with colder temperatures at the surface and aloft
there.

Precipitation picks up in earnest by this evening as favorable
lift moves closer to our region with a potent shortwave nearby
and a the right entrance region of the jet moving overhead. Low
pressure will quickly develop off the VA/MD coast where a strong
baroclinic zone resides, then race off to the northeast.
Precipitation will end for most by midnight. Temperatures will
fall to near freezing in the metro areas, and below freezing to
the north and west overnight. This will allow for potential icy
spots early Sunday morning, so use caution if out traveling.

Thinking remains that the northern third of our CWA remains
mostly snow through the event, with Southern MD seeing
snow/sleet mix changing to plain rain the quickest. Metro areas
in between will reside near the snow/sleet/rain transition line,
and where that resides will obviously impact accumulations and
ptype. Will keep advisories as are for now, with no significant
changes made to the forecast package from earlier this morning.
We will digest the remaining 12z guidance to determine where
adjustments, if any, need to be made. As always, visit our
winter weather page for the latest on accumulations and timing
at weather.gov/lwx/winter.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
High pressure quickly moves in behind the exiting low pressure
area to the northeast, resulting in a dry and clear conditions.
After a cold and perhaps icy start to the morning, temperatures
will rebound nicely, topping out in the mid 40s to lower 50s
region wide. High pressure shifts offshore Sunday night, keeping
a light return flow over the region and temperatures above
normal.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Warm front slides to our north on Monday in return flow as high
moves well to our east. PoPs for rain likely in our west...
lesser to the east. Temps in the 50s.

Another surge of much above normal warmth is expected for the middle
of next week, followed by increasing rain chances by the end of the
week.

An anomalously strong upper level ridge will be centered off of the
southeast United States coastline Tuesday morning with deep
southwest flow entrenched across the region. This will lead to dry
and very warm conditions on Tuesday with temperatures likely on the
order of 25 degrees above climatological normals for mid-late
February. High temperatures are currently forecast to reach the 70s
area-wide, with lows Tuesday night holding in the 50s.

A cold front will then be on the approach by Wednesday, crossing in
the late Wednesday - Thursday time period, although timing
differences exist between model solutions, with 17/00z ECMWF slower,
and 17/00z GFS quicker with frontal passage. Will favor the slower
ECMWF solution as the less progressive ECMWF usually fares better in
these types of situations. This would keep Wednesday quite warm and
relatively dry, with increasing chances for rain Wednesday night and
Thursday. Temperatures will trend downward after frontal passage.

The boundary will then remain nearby for later Thursday and Friday
with strong southwest flow continuing aloft as upper ridge remains
intact over the southeastern US. This will lead to additional rain
chances into Friday, and temperatures considerably cooler than mid-
week but still likely near or above normal.

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
IFR and possibly LIFR CIGS/VIS at times late this afternoon
into tonight as wintry precipitation moves in to the area from
the southwest. Snow could be briefly heavy in bands. The system
quickly exits the terminals by 04z, resulting in a return to VFR
conditions, which is expected to prevail through Sunday night.

Next weather system is a warm front that will most likely bring
some period of MVFR on Monday.

Predominantly VFR expected Tuesday through Wednesday with gusty
southwest flow. Chances for showers will be increasing late
Wednesday and Wednesday night.

&&

.MARINE...
Gusty winds over the main stem of the Potomac and Bay subsided
earlier this morning, thus the SCA that was in effect was
allowed to expire. Winds will remain below advisory criteria
through Monday as high pressure traverses our region and moves
offshore.

Small Craft Advisories may be needed Tuesday through Wednesday with
gusty southwest winds. With warm air temperatures expected over
considerably cooler waters, the strongest gusts will likely be
observed along the shorelines.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Winter Weather Advisory until midnight EST tonight for MDZ501.
     Winter Weather Advisory from 2 PM this afternoon to midnight
     EST tonight for MDZ003>006-502-503-505-507-508.
VA...Winter Weather Advisory until midnight EST tonight for
     VAZ025>027-029-030-040-501-503-504-507-508.
     Winter Weather Advisory from 2 PM this afternoon to midnight
     EST tonight for VAZ028-031-505.
WV...Winter Weather Advisory until midnight EST tonight for WVZ055-
     502-505-506.
     Winter Weather Advisory from 2 PM this afternoon to midnight
     EST tonight for WVZ050>053-504.
     Winter Storm Warning from 2 PM this afternoon to 2 AM EST
     Sunday for WVZ501-503.
MARINE...None.

&&

$$
SYNOPSIS...BKF
NEAR TERM...BKF
SHORT TERM...BKF/CS
LONG TERM...MM
AVIATION...MM/CS/BKF
MARINE...MM/CS/BKF



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