Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170749

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
349 AM EDT Sat Mar 17 2018

Weak low pressure will pass by to the south this afternoon into
this evening. High pressure will return for Sunday and Sunday
night. Stronger low pressure will track into the Tennessee
Valley Monday and it will transfer its energy to a coastal low
off the Mid-Atlantic Coast Monday night. The low will track to
the northeast Tuesday but before another area of low pressure
passes by to our south Wednesday. High pressure will return for
late next week.


Surface high pressure remains over the region early this
morning. Light winds and plenty of breaks in the clouds have
allowed for cold conditions due to radiational cooling.

Surface low pressure over the Midwest early this morning will
track east. As of early this is a potent system and
closed off upper-level low pressure is associated with this
system. it progresses east this system should
weaken as it runs into a confluent zone over our area. This will
suppress the low and allow it to pass by to our south this
afternoon into this evening. Having that been said...there will
be enough moisture and forcing for a period of precipitation
across much of the area. Precipitation amounts are expected to
be light across most areas since this system will be weakening
and there is plenty of dry air in place early this morning.

The best chance for precipitation will be during the mid-morning
hours into the midday hours across the Potomac Highlands into
the central Shenandoah Valley...and this afternoon across
central Virginia into southern Maryland. However...did increase
pops this afternoon across northern areas. Latest guidance shows
increased frontogenetical forcing across northern Maryland and
eastern West Virginia. A band of precipitation may develop
across these areas this afternoon.

As for p-type...uncertainty remains high. Warm advection has
caused low-level temps to increase and this is evident looking
at the observation at KHSP with temperatures increasing
overnight. However...the airmass is dry so evaporative cooling
will cause temps to dip after precipitation begins. As of looks like a wintry mix is likely across the Potomac
Highlands and the Blue Ridge Mountains. Left the Winter Weather
Advisory in effect for those areas. The primary threat will be
a light glaze of ice from freezing rain on elevated surfaces.
However...there may be enough cooling and forcing for a band of
snow to develop with localized accumulations around 1-2 inches.

For the northern and central Shenandoah Valley...the advisory
has been cancelled. It looks like precipitation will not move
in until the mid to late morning hours at the earliest. This
will allow for surface temps to rise well above freezing.
Therefore...freezing rain is not a concern for most areas. There
could be enough forcing and cooling with banding precipitation
for a period of snow around midday...and slushy accumulations
are possible. However...most accumulation should be on grassy
surfaces and it appears too localized for an advisory across
the entire area.

For the metro areas...a period of light rain or rain/snow mix is
possible this afternoon. No accumulation is expected since
precipitation rates will be less intense as the system continues
to weaken. Should the banding precipitation hold longer than
expected it is not out of the question for a coating on grassy

High pressure will return for tonight, bringing dry and chilly
conditions across most areas. However...a weakening shortwave
that will be remnants from the closed upper-level low this
morning will pass by to the south. A few showers are possible
across the central Shenanadoah Valley into the Potomac


High pressure will build overhead Sunday through Sunday night.
Dry and seasonable conditions are expected.

Low pressure will track through the Tennessee Valley Monday and
it is expected to transfer its energy to coastal low pressure
off the Mid-Atlantic Coast Monday night. A confluence zone
around upper-level low pressure over the Canadian Maritimes and
ridging over the north-central CONUS will pump high pressure to
our north over New England. This should cause the primary low
to transfer its energy to the coastal low south of our
latitude...putting our area on the cold side of the storm.
Havine that been is the middle of March so there will
be issues with warmer air near the surface.

Precipitation is expected to gradually overspread the area
Monday into Monday evening. Exact timing is still a bit
uncertain...but temps should be well above freezing for most
areas Monday due to warmer air near the surface. This means that
the main p-type should be rain Monday into Monday evening. As
the low transfers its energy to the coastal low Monday
night...north to northeast winds will increase and this will
draw colder air down from the north and east. Rain may mix with
or change to snow...especially across higher elevations and
climatalogically favored areas (northern Maryland and near/west
of Blue Ridge and Catoctin Mountains). Uncertainty remains high
because if the low tracks far enough to the south then colder
air could work its way into the Washington/Baltimore Metro areas
causing rain to change to snow. For now, the latest forecast
takes a blend of the latest guidance which continues to keep the
best chance for snow north and west of the metro areas.
However...confidence remains low at this time.


Tuesday morning, low pressure will be emerging off the VA/NC
coastline as strong high pressure resides north of the Great
Lakes region. Precipitation will be observed areawide Tuesday
morning, with differing of opinions between model guidance as to
how thermal profiles shake out aloft. As always, this will be a
major factor in determining p-type across the area. The ECMWF
is the colder solution, but also indicates a more suppressed
track southward with a stronger high pressure area to our north,
and keeping the axis of heaviest moisture just to our south.
GFS favors a more northward extent of precipitation, but also
not as cold aloft, thus p-type issues result. Regardless, odds
of developing coastal low pressure to our south and east are
increasing, and sufficient cold air does look like it will be in
place for at least portion of our CWA, favoring north and west
of the metros at this time. As precip continues during the day
on Tuesday, we will be fighting the higher March sun angle which
will play a role in boundary layer temperatures and any
accumulation potential.

Low pressure intensifies off the coast and moves quickly out in
to the western Atlantic Tuesday evening. The pattern remains
unsettled through Wednesday as mid to upper level troughing over
the Mid Atlantic region takes on a negative tilt, and another
area of low pressure looks to develop just east of the Outer
Banks. GFS depicts a bit more moisture over the region on
Wednesday with weak low pressure along the Appalachians, while
the ECMWF highlights an even weaker area of low pressure, thus
less precipitation. Global guidance does agree on low pressure
emerging off the Carolinas early Wednesday, rapidly intensifying
and pressing eastward, bringing an end to any precipitation
overnight Wednesday. Temperatures Tuesday through Wednesday will
remain well below normal for mid-March, with highs in the 30s
to near 40, and lows in the 20s to near 30 degrees around the

A surface ridge of high pressure extending southward from Canada
will stretch over the region Thursday and Friday, as the mid to
upper level trough starts to exit the eastern seaboard and
ridging aloft takes hold. This will bring dry and unseasonably
cold yet moderating temperatures to end the work week with highs
in the 40s, and lows in the 20s to near freezing.


A period of light rain or rain/snow mix is possible across the
terminals late this morning into this afternoon. Precipitation
amounts should be light but brief reductions to vsbys toward
MVFR levels are possible. High pressure will return for tonight
through Sunday night.

Low pressure will track through the Tennessee Valley before
transferring its energy to a coastal low off the Mid-Atlantic
Coast Monday night. Rain will likely overspread the terminals
later Monday into Monday night. Rain could mix with or change to
snow Monday night as colder air works its way into the area.
SubVFR cigs/vsbys are likely later Monday through Monday night.

Periods of MVFR/IFR conditions expected to persist Tuesday
through Wednesday as an unsettled weather pattern affects the
terminals with a range of precipitation types. Low pressure
passing just south of the area early Tuesday will redevelop off
the VA/NC coast, increasing chances of VIS/CIG reductions at all
terminals. A secondary area of low pressure looks poised to form
off the Outer Banks early Wednesday, favoring potential flight
restrictions at DCA/CHO but could have impacts further north
depending on how much precipitation overspreads the area and the
track of the low. North to northwest winds will remain at 10
knots or less through the period.


Low pressure will weaken as it passes south of the waters this
afternoon into tonight. A Small Craft Advisory is in effect for
the northern Chesapeake Bay this afternoon, but it will be
marginal. High pressure will build over the waters tonight into
Sunday. The pressure surge may cause a period of SCA wind gusts
late tonight into early Sunday across middle portions of the Bay
but confidence was too low for an SCA at this time.

Low pressure will track through the Tennessee Valley before
transferring its energy to a coastal low off the Mid-Atlantic
Coast Monday night. A Small Craft Advisory will likely be needed
for the waters Monday night.

SCA conditions appear likely Tuesday through Wednesday night for
at least a portion if not all of our waters as a series of low
pressure areas bring precipitation, potentially of the winter
variety to the region. Winds will favor a north northwest
trajectory during this period, gusting upwards of 20 knots, with
gusts in excess of 20 knots over the lower Chesapeake Bay.


MD...Winter Weather Advisory from 10 AM this morning to 5 PM EDT
     this afternoon for MDZ501-502.
VA...Winter Weather Advisory from 10 AM this morning to 5 PM EDT
     this afternoon for VAZ507-508.
     Winter Weather Advisory from 8 AM this morning to 5 PM EDT
     this afternoon for VAZ503-504.
WV...Winter Weather Advisory from 10 AM this morning to 5 PM EDT
     this afternoon for WVZ050-055-501>504.
     Winter Weather Advisory from 8 AM this morning to 5 PM EDT
     this afternoon for WVZ505-506.
MARINE...Small Craft Advisory from noon today to 6 PM EDT this evening
     for ANZ530-531-538-539.


MARINE...BJL/BKF is the U.S. government's official web portal to all federal, state and local government web resources and services.