Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181527

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1027 AM EST Sun Feb 18 2018

High pressure builds overhead today before shifting offshore
tonight. A warm front will then lift northward through the area
on Monday, ushering in another round of unseasonably warm
temperatures. A cold front will approach the region during the
middle portion of the work week.


Ample stratocumulus across the CWA this morning with GOES-East
picking up a healthy field of mountain wave clouds north and
west of the metro areas. Patchy dense fog was also observed
early this morning over portions of our area, thanks to the
added ground moisture from yesterdays precipitation, but
conditions have been improving the last couple of hours. High
pressure will build overhead today, and as the ~800mb inversion
measured by the 12z KIAD sounding weakens, clouds are expected
to erode this afternoon with skies turning partly to mostly
sunny. Temperatures will rebound nicely, topping out in the
upper 40s to near 50 degrees, which is a couple degrees above
normal for this time of year.

The aforementioned high pressure will move offshore tonight,
helping turn winds to a more southerly trajectory. Clouds will
make a return once again as a warm front moves toward our region
from the southwest, and warm air advection commences. This
could spark some light rain showers over our far western and
southwestern zones by daybreak Monday, however only minor
accumulations of less than a tenth of an inch would result.
Overnight lows will hold generally above freezing, with the
middle 30s observed for most of our area, while the lower 30s
will be measured in the coldest spots.


The new work week begins with a warm front swinging northward
through the area. Although I hate using a 50 PoP because it
feels like a hedging of bets, I`m looking at a pretty even mix
of model guidance that shows virtually nothing and guidance
that paints a period of light rain. Will keep likely PoPs west
of the Blue Ridge, where there`s just slightly more moisture
available. Regardless, I think we are talking about less than a
tenth of an inch of rain in most areas -- so nothing like the
precip events we`ve had recently. Can`t rule out a bit of light
lingering rain into Monday night, mainly near the Bay.
Temperatures Monday night probably don`t move much, and might
even perhaps rise slightly during the night.

Tuesday begins a two-day period of significant heat for mid-
February, reminiscent of the warm spell in the second half of
February last year. 00 UTC GFS MOS guidance is a jaw-dropper in
our western CWA and points west, giving Cumberland MD a high of
79 and Petersburg WV a high of 81. I`m not willing to go there
quite yet but it`s not out of the realm of possibility either.
The notable difference between this upcoming Tuesday and last
Thursday (which was similarly warm) is that it will be even more
humid. Given good model consensus on expected dewpoints and
wind, I think many places may struggle to drop below 60 Tuesday
night. I went several degrees above guidance, and my gut feeling
is I may still be on the cool side of reality.


An anomalously strong upper level ridge (heights 2-3 SD above
climatology for February) will be positioned near the southeastern
US coastline through the end of the week and into the weekend. This
will provide for strong and persistent southwest flow aloft and
continued bouts of moisture/warm air advection, which will lead to
an active period with above normal temperatures and precipitation

A cold front will be on the approach during the day Wednesday, and
models are beginning to form more of a consensus with frontal
passage likely late Wednesday or Wednesday night, and therefore
increasing chances for rain showers by Wednesday afternoon and
Wednesday night. Out ahead of the front, southwest flow and very
warm conditions are expected. After a morning start in the 50s to
around 60F, with 850mb temperatures progged to be from 12-14C and
925mb temperatures from 14-17C, widespread 70s are expected for
highs across the region. These temperatures will be in  (something
that is incredible for me to type)the realm of 25-30 degrees above
climatological normals for the day (49/32F at DCA).

As the front shifts southward Wednesday night and Thursday strong
high pressure will build into southeast Canada and New England. This
will create a cold air damming set-up across the Mid-Atlantic, as
low level northeasterly flow is overrun by continued southwest flow
and additional warm air advection aloft. This will create an
unsettled day weather-wise for Thursday with cloudy skies and rain
chances. Temperatures will be significantly cooler than Wednesday,
although still near or above daily normals.

The frontal zone will remain nearby through Friday and Saturday, and
current model guidance depicts the boundary surging back northward
with the region back in the warm sector by Saturday. But with the
zone still nearby, a wet and active pattern will be in place with
numerous chances for rain showers.


Earlier low clouds and fog that plagued IAD/BWI, which resulted
in LIFR/IFR conditions, have since lifted and conditions are
improving, heading to VFR status. CHO is still locked in to LIFR
CIGs and IFR VIS. Satellite imagery indicates we should see
improving conditions there over the next hour or two, with a
return to VFR by 17z. BKN clouds AOA 050 across all terminals
this morning will dissipate this afternoon, with dry conditions
and light winds prevailing. VFR conditions forecast overnight
and through 18z Monday for all terminals.

Monday afternoon, there is a 50-60 percent chance of rain, and
an even higher likelihood of lowering ceilings, with MVFR likely
and IFR possible. Sticking with MVFR in the forecast for now.
If the rain materializes, visibility could be similarly
restricted. Those restrictions should improve as a warm front
passes late in the day. Tuesday will be extremely warm and a bit
breezy, but dry.

Predominantly VFR expected Wednesday with gusty southwest flow.
Chances for showers will be on the increase by late Wednesday, and
periods of sub-VFR conditions become possible from Wednesday evening
through Thursday with rain and low cloud potential.


High pressure traversing the region today, shifting offshore
tonight will promote light winds across our waters, thus no
headlines expected during this period.

As the high shifts offshore and a warm front pushes through
Monday afternoon (perhaps accompanied by some light rain),
southerly winds will pick up, with gusts to SCA criteria
possible Tuesday and Tuesday night.

Small Craft Advisories may be needed Wednesday with gusty southwest
winds. With warm air temperatures expected over considerably cooler
waters, the strongest gusts will likely be observed along the
shorelines. SCA potential continues into Thursday behind the frontal
boundary as well as winds turn to the north.


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 morning "low" that we are currently forecasting
ends up being the Wednesday calendar day low (60 at DCA), 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)




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