Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 171443

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1043 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.


Low pressure over southern Ohio will move east the remainder of
this morning. Its upper level support will catch up to it this
afternoon and bring a light wintry mix to parts of the region
this afternoon, but not before a light wintry mix affects the
Appalachian Mountains late this morning. Current radar shows
multiple bands of precipitation but most of this is not reaching
the ground considering dewpoint temperatures are in the teens
across much of the northern half of our zones. There is
currently a Winter Weather Advisory in effect for the Appalachian
Mountains and the Blue Ridge Mountains through late afternoon.

As for precipitation type overall, uncertainty remains high
considering the amount of dry air in the lower levels and the
steady rise in temperatures this morning into the afternoon.
Warm advection continues to increase temperatures. As of looks like a wintry mix is likely across the Potomac
Highlands and the Blue Ridge Mountains. 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.

There could be enough mid level 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 Shenandoah 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.
Having 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
climatologically 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 until 5 PM EDT this afternoon for
VA...Winter Weather Advisory until 5 PM EDT this afternoon for
WV...Winter Weather Advisory until 5 PM EDT this afternoon for
MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ530-


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