Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 200202
AFDLWX

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

.SYNOPSIS...
A warm front will slowly cross the region by midday Tuesday.
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
Davenport, IA. The "wedge" pattern is still very much evident
east of the Blue Ridge and hasn`t changed much since this
afternoon. The lowest visibilities due to fog and drizzle are
now confined over north central Maryland, and this is where
ensembled hi-res guidance depicts them to likely stay
overnight. Fog may spread farther south/west, but a light
southerly flow may result in just low clouds. While patches of
dense fog are certainly possible, am thinking persistence of
visibilities 1/2 to 1 mile will be most common.

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, nearly steady 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. Southerly flow of 20+ kt below the subsidence
inversion should be able to accomplish this, but it may take
well into the midday hours before clouds fully break. 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. Such temperatures are more 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 /02Z TUESDAY THROUGH SATURDAY/...
IFR/LIFR cigs have been persistent across the metro areas into
this evening, though obs are beginning to confirm select hi-res
models that show the IFR cigs lifting north of DCA/IAD for at
least several hours. Am not certain of the timing of any
transition of categories, as IFR cigs could redevelop later
tonight. CHO is solidly in the MVFR environment, but is wafting
in and out of ceiling.

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.

Have conditions improving between 15-18Z across the area,
although there is certainly low confidence in exact timing.
Afterward, southerly wind gusts to 20 kt will develop. 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...
Light southerly flow will continue through Tuesday morning, with
dense fog and drizzle being the main concern to mariners. After
that, warm front should bring improved visibility overall,
though will still need to watch for lingering patchy fog/haze.
Areas near shore have strong potential for SCA gusts around 20
knots by Tuesday afternoon, so have issued SCA. The onset of the
SCA may be too early, but didn`t feel confident to make
adjustments at this time. 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...ADS/RCM
SHORT TERM...RCM
LONG TERM...BKF
AVIATION...ADS/BKF/RCM
MARINE...ADS/BKF/RCM
CLIMATE...LWX



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