Severe Thunderstorm Warning 1, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
217 PM EST Fri Jan 12 2018

.SYNOPSIS...
Low pressure and its associated cold front will pass through the
area tonight. Canadian high pressure will build overhead
Saturday through Monday. Low pressure and a reinforcing cold
front may impact the area Tuesday into Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
A southerly flow will has continue to usher in unusually warm
and moist air through this evening. Max temps this afternoon will
be well into the 60s for most areas and a few locations may
even reach 70 degrees...especially across northern and central
Virginia into portions of eastern West Virginia.

For locations along the western shore of the Bay and the shore
of the Tidal Potomac River...areas of dense fog are possible
through this evening. This is because of warm and moist air over
the colder waters. Will continue to monitor visibilities through
this evening...but confidence is too low for a Dense Fog
Advisory at this time since it appears to be localized.
Also...incoming rain may help keep vsbys up just a bit for most
areas. Having that been said...vsbys around or below a quarter
mile are possible especially right by the water.

The warm and moist air will cause plenty of clouds along with
some showers. Showers will increase in coverage late this
afternoon due to the strong southerly steering flow causing
convection upstream over southern Virginia/North Carolina to
move into the area. Latest mesoanalysis does show limited
instability over the area. In fact...some of that instability
has become surface based. Therefore...an isolated thunderstorm
or gusty shower cannot be ruled out late this afternoon into
this evening. The best chance for that will be across northern
and central Virginia where instability is highest.

A strong cold front located over the Ohio Valley and western
Pennsylvania this afternoon will move into Potomac Highlands
late this evening...before passing through the rest of the area
overnight. An upper-level low in the southern stream will also
pass through our area during this time...and this will cause
surface low pressure to develop along the cold front. The
developing low will cause a swath of rain/isolated thunder to
accompany the frontal passage. A stronger low-level jet just a
few thousand feet from the surface suggests that heavier showers
or isolated thunder may contain gusty winds.
However...instability will be limited and also elevated for most
areas during the frontal passage. Therefore...the threat for
severe storms remains low.

Strong northwest winds will develop right behind the frontal
passage and it will usher in much colder air. Temperature drops
around 15 to 25 degrees are likely within just a few hours of
the frontal passage. The rush of cold air may cause rain to end
as wintry precipitation. There is still low confidence for most
areas on how much moisture will be left by the time that cold
air arrives to change the ptype to a wintry mix. The one
exception is along/west of the Allegheny Front. Rain will change
to a wintry mix late this evening before changing to snow
overnight. A Winter Weather Advisory is in effect for these
areas. For other locations...rain may change to a light wintry
mix toward morning. Despite low confidence in accumulating
wintry precipitation...it is important to note that temperatures
rapidly falling below freezing will cause any wet surfaces to
freeze quickly. Therefore...icy conditions are possible toward
Saturday morning especially from the Washington and Baltimore
Metropolitan areas north and westward. The Winter Weather
Advisory may need to be expanded farther south and east from
where it is now should confidence increase for wintry
precipitation tonight.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
The cold front will move away from the area Saturday morning
while Canadian high pressure approaches from the north and west.
A strong northwest wind will usher in much colder conditions
with temps falling into the 20s to lower 30s for most areas.
Temps in the single digits and teens are expected along the
ridge tops. A longwave upper-level trough will sharpen a bit
overhead Saturday as a northern stream system digs into the
Great Lakes. This system will cause the upper-level jet to
strengthen overhead and the trough axis should remain to our
west. Our area will be in the left-exit region of the upper-
level jet and this may provide just enough lift for light wintry
precipitation Saturday morning. Again...confidence is too low at
this time but the Winter Weather Advisory may need to be
extended in area to account for this. Regardless...a rapid
freeze is possible Saturday morning and this may result in icy
spots on paved surfaces. Highest confidence is from the
Washington/Baltimore Metro areas north and westward.

By Saturday afternoon...there should be enough dry air and
subsidence from the approaching high for just dry and
cold/blustery conditions.

Canadian high pressure will continue to settle over the region
Saturday night through Sunday night...bringing dry and cold
conditions. The combination of cold air and blustery northwest
winds will cause wind chills to drop into the single digits and
lower teens for most areas. Wind Chills below zero are expected
along the ridge tops of the Blue Ridge and Allegheny Highlands
Saturday night into Sunday morning. A Wind Chill Advisory may be
needed.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Surface high pressure will be located over eastern Canada on
Monday as broad upper level troughing encompasses most areas
east of the Rockies. After a cold start, highs will struggle to
reach freezing across northern parts of the area. Ahead of the
main trough axis, a clipper low will be moving through the Great
Lakes Monday night into Tuesday. There remains some timing
uncertainty with the trailing cold front and how much moisture
will be associated -- although overall it should be light.
Temperatures will be cold enough to support snow. Besides the
snow potential, the front will reinforce the cold weather. After
a slight warm up on Tuesday, high temperatures may struggle to
reach freezing once again on Wednesday.

The mid/upper level trough will sharpen and cross the area
Wednesday night. At a minimum, some upslope snow showers will
be possible, but the trough will also likely interact with the
baroclinic zone to our southeast and induce cyclogenesis. Will
have to keep an eye on this period to see how close the low is
to the coast.

High pressure will begin building from the west late Thursday
into Friday as heights rise aloft. Temperatures will be
moderating toward climatology.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
Southerly winds will continue to usher in unusually warm and
moist conditions through this evening. The warm and moist air
over the Bay has resulted in low clouds and fog for KBWI and
KMTN. IFR/LIFR conditions are expected through this evening.
IFR/LIFR conditions are possible for KDCA as the surface wind
comes off the Potomac River. Elsewhere...MVFR to IFR conditions
are expected through this afternoon.

Showers will become more widespread late this afternoon and
evening as a cold front approaches. The cold front will pass
through overnight. A strong wind shift to the northwest winds
gusts around 25 to 35 knots are expected behind the cold front.
The wind shift is most likely between 03 and 05z for KMRB and
06-09z for the other terminals. Rain may end as a light wintry
mix Saturday morning. Any wintry precipitation amounts appear
that they will be light...but a wet ground may quickly freeze
toward Saturday morning with temperatures rapidly falling below
freezing.

Gusty northwest winds through Saturday evening will diminish a
little overnight Saturday through Sunday. VFR conditions are
expected Saturday afternoon through Sunday night.

No issues expected Monday with high pressure. A period of snow
is possible Tuesday as a clipper system crosses the area. There
are still plenty of details that need to be refined with this
system.

&&

.MARINE...
A southerly flow will continue through this evening. A Small
Craft Advisory is in effect for the waters...but it will be
marginal due to relatively cooler waters. A wind shift to the
northwest is expected behind the cold front...generally between
07 and 11z and winds will be gusty. A Gale Warning is in effect
for the waters overnight through Saturday morning. Winds will
diminish a little for Saturday afternoon and Saturday
night...but a Small Craft Advisory remains in effect for the
waters during this time.

High pressure will continue to build over the waters Sunday
through Sunday night. A Small Craft Advisory may be needed for
portions of the waters during this time.

High pressure will be north of the region on Monday resulting in
light southerly winds. A cold front will cross the waters on
Tuesday, and Small Craft Advisory conditions will be possible in
its wake. If the front moves far enough away, winds should
lessen by Wednesday.

&&

.HYDROLOGY...
Rain is expected late this afternoon into tonight. Break up of
ice may cause localized issues, but confidence is currently too
low and threat too isolated to include in the HWO at the moment,
but we will continue to monitor.

Around 0.75-1.50 inches of rainfall is anticipated through
tonight. While precipitable water is well above climatology,
suggesting that flooding is possible, relatively quick movement
of individual elements and dry antecedent conditions should
preclude a widespread hydrologic issue. Limited rainfall up to
this point has helped. Even though soils are warming, absorption
will still be limited, thus substantial ponding is possible,
but not enough issues are expected to need a Flood Watch at this
time.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Winter Weather Advisory from 10 PM this evening to 10 AM EST
     Saturday for MDZ501.
VA...Winter Weather Advisory from 10 PM this evening to 10 AM EST
     Saturday for VAZ503.
WV...Winter Weather Advisory from 10 PM this evening to 10 AM EST
     Saturday for WVZ501-503-505.
MARINE...Small Craft Advisory until midnight EST tonight for ANZ530>543.
     Small Craft Advisory from noon Saturday to 6 AM EST Sunday for
     ANZ530>543.
     Gale Warning from midnight tonight to noon EST Saturday for
     ANZ530>543.

&&

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

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