Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 190217

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

High pressure will move offshore overnight. A warm front will
then lift northward through the area on Monday, ushering in
another round of unseasonably warm temperatures that will last
through mid week. A cold front will approach the region
Wednesday night into Thursday before stalling nearby through the
end of the work week.


High pressure is centered over the Jersey shore per the latest
surface analysis. Cirrus is already wafting overhead, and
satellite imagery detects a patch of low clouds developing over
Nelson County...a consequence of onshore flow around the
periphery of the high.

Surface flow should be veering south overnight as strong warm
advection ensues aloft. Thus, expect clouds to be thickening
and lowering. However, high-res guidance (HREF/NMM/ARW) has
generally be holding precip off through dawn, a trend that
recent RAP/HRRR runs continue to show. Will hold off likely PoPs
to south of Charlottesville/Harrisonburg by sunrise. Will have
some chance PoPs creeping into central Virginia and into the
Potomac Highlands after midnight though.

There is a low-end probability in some datasets that there
could be freezing rain across the ridge tops at onset. However,
believe that warm advection will offset any wet-bulb cooling.
Most guidance sets continue to indicate lows above freezing
across all but MD/PA border east of the Catoctins-- where PoPs
will be at or below 15%. Therefore, am going with an all rain
solution. Consensus temperature forecasts suggest that
temperatures will only be dropping a few more degrees from
current values; accepted this solution in light of warm
advection aloft and increasing clouds.


Warm front lifts through our region on Monday and by Monday
evening will reside to our north. This will deliver increasing
rain chances area wide during the day, and allow cloudy skies
to prevail. Global model consensus not too keen on much in the
way of QPF, maintaining a tenth of an inch or less through
Monday evening, with our southern zones having the best chance
to see the most rainfall. High res guidance shows light rain
trying to make its way through the metro areas during and after
the morning rush, with lowering rain chances in the afternoon,
with the exception of western Maryland and the eastern WV
Panhandle where the warm front will be nearby.

Warm air advection will be ongoing throughout the day on
Monday, and by Monday evening, 850 temps will have increased by
around 12C from what our upper air sounding will observe this
evening (19/00z). Highs Monday will be in the 50s for all,
maybe even cracking 60 degrees over the Potomac Highlands.

With the warm front to our north Monday evening, expecting
drying conditions and continued cloudiness with warm air
advection continuing. Lows Monday night will settle in the upper
40s to near 50 degrees, or nearly 20 degrees above normal

Unseasonably warm conditions will continue Tuesday and Tuesday
night, as strong ridging persists over the Mid Atlantic region,
as 500mb heights will be near the 99th percentile for this time
of the year. Deep south southwesterly flow will also be
realized throughout the atmospheric column. Highs on Tuesday
will surpass 70 degrees once again, which will be 25 to perhaps
30 degrees above normal for some locations. Tuesday night, some
spots in and around the cities may not even drop below 60
degrees, but mid 50s to around 60 degrees can be expected area
wide. One consequence as a result the much warmer temperatures will
be the increase in dewpoints or moisture in the air. By Tuesday
afternoon, dewpoints will reach the lower 60s, bringing a
feeling of stickiness to the air. On the other hand, with dry
air and subsidence dominating aloft, rain chances will be near
nil Tuesday and Tuesday night.


The warming trend will continue into Wednesday with a strong
upper level ridge... and at the surface we remain between a high
pressure settled off of the southeast coast and a cold front to
our west. Temperatures are expected to reach the mid to upper
70s... that is about 25 to 30 degrees above normal. Wednesday
should start dry with chance of showers increasing as the slow
moving front approaches.

The front will move across our CWA late Wednesday or early Thursday,
before stalling to our south Thursday into Friday. The front will
then push north as a warm front Friday night or Saturday, with
another boundary pushing through on Sunday. Chance of showers will
remain for most the long term period with temperatures remaining
above normal.


Clouds have already begun to increase once again...a trend that
will continue through daybreak Monday, but remaining VFR. CIGs
will approach MVFR to potentially IFR categories midday Monday
as light rain slides across the terminals. Could see some VIS
reductions as well. Confidence in occurrence of rain is too low
at this time to explicitly include in the TAF, but is is implied
via cigs and vsbys.

Conditions improve Monday evening as a warm front lifts north of
the terminals, and VFR CIGs and VIS return through Tuesday
night under the influence of deep ridging. South southwesterly
winds AOA 10 knots can be expected during this period, and they will
be gusty during the day on Tuesday, upwards of 20 knots.

VFR conditions expected part of the day Wednesday before sub
VFR periods possible late on Wednesday and into Friday when
unsettled weather could affect our area as a front moves across
and stalls nearby.


High pressure will move offshore tonight, supporting light south
flow. As a warm front pushes through the area by Monday
evening, southerly breezes will increase across our waters,
likely reaching SCA criteria by Tuesday.

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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