Winter Storm Warning 3, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
246 PM EST Thu Jan 17 2019

.SYNOPSIS...
Low pressure will slide by to our west through tonight, bringing
light snow to the region. A cold front will move through on
Friday, with a brief warm up ahead of it. High pressure builds
in Friday night into early Saturday. Stronger low pressure will track
through the Tennessee Valley Saturday, before passing through
our area Saturday night into Sunday. Arctic high pressure will
build overhead later Sunday through Monday, ushering in very
cold air into the region. High pressure moves offshore Tuesday

&&

.NEAR TERM /THROUGH TONIGHT/...
Low pressure will track up the Ohio Valley this afternoon and
into the lower Great Lakes tonight. Snow should begin in the
next couple of hours along the Allegheny Front, then move east
throughout the afternoon/evening hours. So, a Winter Weather
Advisory is in effect for the Allegheny Front starting at 3 PM
and going through tonight. Snow spreads east into the
DC/Baltimore metropolitan areas by early evening, right around
the evening rush hour. So, Winter Weather Advisories spread
east into northern VA and central MD at 4PM, then to the
DC/Baltimore metropolitan area starting at 6PM and going through
tonight. Given the timing of the onset of precipitation, and
temperatures dipping below freezing at roughly the same time,
roads could become slippery this evening.

Widespread totals of 1-2 inches are expected across the
advisory area through tonight, with the onset occurring right
around the evening rush hour. Amounts generally around an inch
or less will be found for southern portions of the CWA in this
modest QPF event. Along the Allegheny Front, expecting higher
totals of 2-4 inches due to upslope influence.

Temperatures will drop through the evening as sunset occurs, but
then temperatures in most areas will either hold steady or rise
a few degrees through the night as low level flow becomes
southeasterly. For the most part, the column will rise above
freezing from warm air advection aloft nearly coincident with
where surface temperatures will be above freezing. So while some
sleet pellets or drizzle/freezing drizzle could occur as
precipitation tapers off, it will largely be a snow (with some
rain in the southeast) event. All precipitation east of the
Allegheny Front will likely have ended by daybreak.

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
The cold front associated with the aforementioned surface low
pressure system will swing through the area early tomorrow.
Warmer temperatures are expected tomorrow as a result, as the
front is fairly weak, and has no real push of cold air advection
to come along with it. Accordingly, low-level moisture will be
slow to leave the area, as flow is very weak behind the front.
So, could see some clouds linger tomorrow across the area. In
fact, recent guidance seems to like this solution more than
previous runs. This will be the difference between high temps
into the upper 40s/low 50s, or high temps barely reaching 40
again. So, a bit of uncertainty in that aspect of the forecast
tomorrow, but if things are able clear out, it should be a
seasonably mild day. The once certainty is that it will remain
dry east of the Allegheny Front tomorrow, so at least it will
be dry.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
A strong area of low pressure will approach the region from the
Tennessee Valley on Saturday. Warm advection aloft will overspread
the area late Saturday morning through early Saturday afternoon
allowing precipitation to breakout. There is the potential for
precipitation to briefly start out as snow or a wintry mix anywhere
except far southeastern portions of the forecast area, but most
locations are expected to quickly transition over to rain by
Saturday evening. With high pressure situated to the north, and
model guidance indicating a small wedge of higher pressure holding
strong to the west of the Blue Ridge/East of the Allegheny Front,
there is some concern that cold air may remain locked in at the
surface much longer across portions of West Virginia Panhandle,
Western Maryland and the far northern Shenandoah Valley. All model
guidance has a strong southerly low-level jet (on the order of 50
kts) blasting warm air in aloft Saturday evening across the entire
CWA. If cold air remains locked in at the surface for the
aforementioned areas, a prolonged period of freezing rain or sleet
could ensue. The Euro is currently most aggressive in keeping the
cold air locked in at the surface, and the NAM keeps temperatures
near freezing in those areas for much of the event. A majority of
EPS members keep temperatures near or below freezing through much of
the event as well. The 12z GFS erodes the cold air very quickly, but
is often way too fast in eroding cold air at the surface in these
types of situations. As a result we're currently leaning toward the
Euro/NAM temps and resultant precipitation types.

The remainder of the area, including the immediate I-95 corridor,
will receive a dose of heavy rainfall Saturday Night into early
Sunday. In excess of an inch of rainfall is expected to fall area-
wide. All of that rain falling on top of remnant snow pack from last
weekend's storm could lead to instances of flooding Saturday Night
(smaller stem streams/rivers) through Monday (main-stem rivers).
Precipitation should come to an end from west to east late Sunday
morning through early Sunday afternoon as a cold front pushes
through the area.

Behind the front, conditions will become very windy and turn much
colder Sunday afternoon. Temperatures will drop from the 40s to the
low 20s by evening. As a result, any leftover standing water will
freeze over Sunday afternoon and could result in hazardous travel
conditions. Wind advisories may be needed Sunday afternoon as gusty
winds move in behind the front. Those gusty winds combined with cold
temperatures will lead to bitterly cold wind chills Sunday Night.
Wind chill headlines will likely be needed Sunday Night as wind
chill values drop well below zero everywhere. Wind chill values may
bottom out below minus 10 in the metro areas and below minus 20 in
the mountains. Monday will be bitterly cold as well, with high
temperatures in the teens to low 20s and wind chills remaining below
zero for much if not all of the day.

High pressure will build in from the Ohio Valley for Monday Night
into Tuesday, allowing winds to gradually relax. Temperatures will
remain well below normal (lows in the single digits/teens, highs in
the 30s), but it will feel much warmer with light winds. Dry weather
conditions are expected Monday Night through Tuesday Night.

The next system will approach the area from the west by Wednesday
afternoon. Currently it appears as though any precipitation would
fall in the form of rain, but forecast uncertainty is still rather
high that far out.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
Periods of light snow will begin from west to east this
afternoon/early evening, with the steadiest snow expected during
the evening hours before tapering off late tonight. Visibility
and ceiling restrictions will likely fall to MVFR and could be
IFR at times. As the cold front clears the area on Friday,
conditions should gradually improve. However, flow will be weak
behind the front, so low level moisture may have difficulty
scouring out, especially over the Baltimore terminals. Otherwise
VFR conditions and light winds are expected through Friday
night as weak high pressure build into the area.

Sub-VFR conditions appear likely Saturday Night into early Sunday
Morning as a storm system approaches the area then tracks nearly
overhead. Gusty northwesterly winds will be possible Sunday
afternoon through Monday in the wake of the departing system.
Visibility/Cigs should improve back to VFR by late Sunday afternoon
and persist through Tuesday.&&

.MARINE...
Winds will then turn southeasterly tonight as an area of low
pressure tracks to our northwest, but winds will remain below
SCA criteria. Snow and rain are possible over the waters,
especially this evening. The associated cold front will cross
the waters early Friday, but the flow behind it is weak, so no
advisories are expected through Friday night as weak high
pressure builds in.

A SCA may be needed Saturday Night through Sunday morning in
southerly flow ahead of an area of low pressure. Gale level
northwesterly winds appear likely Sunday afternoon through Sunday
Night, and possibly even extending into Monday as the system departs
off to the northeast.

&&

.TIDES/COASTAL FLOODING...
An onshore flow will increase later Friday through Saturday
night as low pressure passes through the area. Minor tidal
flooding for sensitive areas near high tide is possible during
this time. A strong offshore flow will develop later Sunday and
Monday. Tidal blowout conditions are possible during this time.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Winter Weather Advisory from 6 PM this evening to 4 AM EST
     Friday for DCZ001.
MD...Winter Weather Advisory from 6 PM this evening to 4 AM EST
     Friday for MDZ005-006-011-013-014-504>508.
     Winter Weather Advisory until 4 AM EST Friday for MDZ003-004-
     503.
     Winter Weather Advisory until 6 AM EST Friday for MDZ501.
VA...Winter Weather Advisory from 6 PM this evening to 4 AM EST
     Friday for VAZ052>054.
     Winter Weather Advisory until 4 AM EST Friday for VAZ501-502-
     505-506.
     Winter Weather Advisory until 6 AM EST Friday for VAZ503.
WV...Winter Weather Advisory until 4 AM EST Friday for WVZ051>053.
     Winter Weather Advisory until 6 AM EST Friday for WVZ501-503-
     505.
MARINE...None.

&&

SYNOPSIS...CJL
NEAR TERM...CJL/ADS
SHORT TERM...CJL
LONG TERM...KJP
AVIATION...CJL/KJP
MARINE...CJL/KJP
TIDES/COASTAL FLOODING...BJL

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