Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 191959
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
259 PM EST Mon Feb 19 2018

.SYNOPSIS...
A warm front will slowly cross the region late tonight through
Tuesday morning. Record warmth is possible behind this system
for Tuesday and Wednesday. A cold front pass through the area
Thursday before stalling out nearby during the end of the week.

&&

.NEAR TERM /THROUGH TUESDAY/...
Latest surface analysis places cool high pressure over the
western Atlantic south of Nova Scotia. A warm front snakes its
way inland near Virginia Beach, dives south into northeastern
Georgia, then wraps back north along the Appalachians through
Tennessee, western Virginia, West Virginia and southwestern
Pennsylvania, then turns west towards a low pressure near Kansas
City. The boundary continues westward towards another low in the
Rockies, then finally dives south towards Baja California. This
is a classic wedge pattern for our region, and we are seeing the
classic results... low clouds, mist and drizzle east of the Blue
Ridge/Catoctins. Warmer air and some sun have managed to reach
portions of the Shenandoah Valley and eastern West Virginia,
with these areas recently sneaking up into the 60s.

The wedge will gradually be eroded away tonight and tomorrow,
unable to resist the onslaught of warm, moist air moving
northward from the Gulf of Mexico. However, this erosion will be
slow, and especially east of the mountains, temperatures will
only gradually rise. The ongoing warm advection/isentropic lift
will promote continued low clouds, mist and drizzle at times,
especially closer to the Chesapeake Bay. West of the Blue
Ridge/Catoctins, skies will be clearer, but some patchy
radiation fog could develop late tonight. In these areas,
temperatures will actually fall tonight, but east of the
mountains, steady or rising temperatures are expected.

While odds of measurable precipitation are low, the strong warm
advection combined with the approaching warm front may spark
off a few showers closer to the Chesapeake Bay in southern
Maryland later tonight into early Tuesday.

Tuesday, the warm front essentially dissolves to our south and
reforms over Pennsylvania as the cool air is expected to mix out
across our region, unable to resist the strong warm advection
accompanying the 40 knot low level jet late tonight and early
Tuesday. Assuming this happens, temperatures should reach the
low 70s in most areas. Did nudge temps down a little bit given
uncertainties about timing of this mix out, and it must be noted
that some guidance wants to keep the mix out slow enough to
limit our warming notably, but at this time feel reasonably
confident that we will still see 70 across most of our region,
albeit perhaps not til late in the day. Any low clouds, fog and
drizzle in the morning should dissipate by afternoon, resulting
in a day much more like what is normal for early May than late
February.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
Southerly flow beneath an exceptionally strong ridge of high
pressure aloft will continue advecting warm moist air across
the region Tuesday night. We should have mixed out by this
point, but any place that can go light and variable could get
patchy fog, and fog may be an issue near the bay as well as the
warm air crosses the cool waters. Otherwise, lows should be
quite mild, with 50s common.

Cold front starts to encroach on the region later Wednesday and
Wednesday night, but most showers should hold off until
Thursday, especially east of the Blue Ridge and Catoctins. That
should allow Wednesday to be the warmest day of the stretch and
likely one of the warmest days on record during the month of
February. See climate section below for some details. With the
front not reaching us until Thursday, Wednesday night should
remain fairly mild, with 40s and 50s remaining common. Regarding
the showers later Wednesday into Wednesday night, there are
hints of instability that might allow for some thunder, but at
this time did not insert any into the forecast.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
A frontal boundary will be laying along the western periphery
of our CWA on Thursday, with rain likely as low pressure rides
along the boundary and into our region. This frontal boundary
may sink south of our region and stall Thursday night, and with
high pressure settling in to our north. Guidance does drop
temperatures back to near or slightly above normal through
Friday. Another area of low pressure moves quickly northeastward
toward the Great Lakes and across northern Pennsylvania on
Friday, which will result in increased rain chances along the
northern portion of our area.

This weekend looks unsettled as waves of low pressure continue
to streak northeastward toward the Great Lakes region while
riding along the aforementioned frontal boundary. Guidance wants
to bring that front back northward late in the week, and it
appears to stall at least in the vicinity of our region. Where
this front resides will obviously control temps/precip. GFS
wants to keep our region in the warm sector and bring back 60s
to perhaps 70s for daytime highs. It also keeps the axis of
heaviest precipitation just north of the Mason Dixon Line. The
ECMWF holds the boundary a bit closer to our region, so
temperatures are not as warm, but still residing in the upper
50s to near 60. It is also the much wetter solution with the
proximity of the boundary close by. Moral of the story, this
weekends weather looks unsettled with above normal temperatures,
and we`ll have to wait and see where this boundary decides to
set up shop. A cold front will finally sweep through the area
Sunday night, ushering in drier and cooler conditions.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
MVFR and some IFR cigs/vis will be likely at DCA/IAD/BWI/MTN/CHO
through early Tuesday as warm front gradually lifts north of the
region. Main concerns are low cigs and drizzle reducing
visibility. MRB will be west of this and should stay VFR most of
the time, but may see some patchy fog later tonight. Low level
wind shear is also a concern late tonight and early Tuesday as a
strong low level jet develops across the region, especially
east of the mountains. Conditions improve as we head through
Tuesday morning with VFR likely all terminals by afternoon. Will
need to watch for patchy fog or low clouds redeveloping east of
the mountains Tuesday night/Wednesday morning, but otherwise
expect VFR to continue through Wednesday night. Showers may
encroach on MRB late Wednesday/Wednesday night, but significant
vis/cig reductions are not expected there or elsewhere until
Thursday.

A cold front will sink through our region Thursday and Thursday
night, increasing the likelihood of sub VFR VIS/CIGs as rain
chances also increase. This boundary will reside at least in the
vicinity of our region through weeks end, and as waves of low
pressure continue to ride northeastward along it, the pattern
will be unsettled with rain chances each day and night. This
will result in periods of MVFR/IFR conditions, but to the degree
of flight restrictions will be highly dependent on where this
boundary stalls. Winds will be light throughout this period at
10 knots or less, favoring a southerly trajectory.

&&

.MARINE...
Winds should remain below SCA criteria through early Tuesday
morning, but fog and drizzle will be a concern to mariners.
After that, warm front should bring improved visibility overall,
though will still need to watch for lingering patchy fog. Areas
near shore have strong potential for SCA gusts around 20 knots
by Tuesday afternoon, so have issued SCA. With warm air
temperatures expected over considerably cooler waters, the
strongest gusts will likely be observed along the shorelines,
with lower gusts over the wider and more open parts of the bay
and Potomac. The same situation will occur Wednesday.

A cold front will near the waters on Thursday and Thursday
night, with the best chance of SCA conditions during the day on
Thursday with gusts of 15 to 20 knots possible. The frontal
boundary will stall and then lift northward as we finish the
work week and head in to the weekend. Winds during this period
will be light, with SCA headlines looking unlikely at this time.

&&

.CLIMATE...
Some all-time February highest minimum temp records could be in
jeopardy, but it depends on how quickly a cold front moves
through Wednesday night.

If the Wednesday calendar day low does not drop below 60
degrees, it would be the first time not dropping below 60 at
DCA in February since 1891, and only the sixth time on record in
the entire meteorological winter season. Even a low 56 or
higher at DCA would be the warmest low in the month of February
since 1976.

The all-time February highest minimums are:
61 at DCA/Washington (2/17/1891)
58 at BWI/Baltimore (2/17/1891)
55 at IAD/Dulles (2/17/1976)
- records only go back to 1960 at IAD

It almost goes without saying that daily records are in jeopardy. A
table of those records follows:

Record warm daily maximum temperatures
       Tue 2/20   Wed 2/21
DCA    76 (1930)  75 (1953)
BWI    76 (1930)  74 (1930)
IAD    70 (1971)  70 (1997)

Record warm daily minimum temperatures
       Tue 2/20   Wed 2/21
DCA    59 (1939)  51 (1954)
BWI    57 (1939)  49 (1981)
IAD    46 (1981)  45 (1981)

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 10 AM to 8 PM EST Tuesday for
     ANZ530>543.

&&

$$
SYNOPSIS...RCM
NEAR TERM...RCM
SHORT TERM...RCM
LONG TERM...BKF
AVIATION...BKF/RCM
MARINE...BKF/RCM
CLIMATE...LWX



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