Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 120222

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
922 PM EST Wed Jan 11 2017

A cold front moves southward across the region Friday. The front
will stall to the south and a few waves of low pressure will ride
eastward along the front over the weekend into early next week.


As of 920 PM, main batch of steady light showers associated with
weak mid-level vorticity lobe and WAA in the low/mid levels
continues to push northeast across the NE corner of the forecast
area. Amounts have been rather underwhelming across much of the
region...with a paltry ~0.06" at our office in Sterling...with the
steady rain having just ended. A little more in other
places...though generally less than a quarter inch. Steady rain
clears the remainder of the area by 11 PM. Though, will be holding
onto slight chance overnight PoPs from I-66 northward, mainly for
a few residual advertised by hi-res guidance. Lows
tonight holding steady from their current values...with M40s
across much of the area.


Have not made many substantive changes to the Thursday forecast.
The day will be spent in the warm sector of the low pressure
system which by that time will be moving up the St. Lawrence
Valley. A few showers may brush by the Highlands, but otherwise it
should be dry. High temperature forecasts are warmer than previous
cycles, which already were soaring. Have raised high temperature
forecasts based on better performing guidance packages and the
current temperature observations.

The associated cold front will push south through the area late
Thursday night or Friday morning. Will lean on the earlier side,
to support cold advection and dropping temps Thursday night. (No
temperature guidance supports extremely warm low temps.) The
precipitation coverage in doubt too, and am keeping much of the
area dry through frontal passage.

Believe that high pressure will be seeping south on Friday. That
will support the drainage of low level cold air. Meanwhile, the
mid-level flow gradient is tightly packed, with the upper jet
just north of the area. That would support waves of precipitation,
especially Friday night1. How much precip would fall is still to
be determined, and precip type is highly questionable. Just about
the only thing that (perhaps) could be ruled out is pure rain or
pure snow; ie: it would be a mixed bag of some kind. That makes
forecasting any specifics problematic at best.


Strong Arctic high builds over New England Saturday as precipitation
moves over the area. Low-level cold air wedged beneath warm air
advection aloft favors a wintry mix of freezing rain...sleet and
snow. Precipitation likely transitions to rain for southeastern
areas but remains freezing rain for the northwestern half or so of
the CWA by late Saturday or Saturday evening. There is uncertainty
in exactly when and where precipitation changes from snow to sleet
to freezing rain and eventually plain rain, but as is typical in
these setups, expect the northwestern corner of the CWA to hold on
to cold air for quite a while (possibly through Sunday night). This
could lead to a prolonged period of freezing precipitation with
multiple rounds of steady light precipitation and drizzle in between.

A more substantial warming trend should ensue early next week, with
conditions expected to be plenty warm enough for plain rain as a
larger area of low pressure moves toward the region during the
middle part of next week.


Rain is beginning to move out of the area. MVFR still possible at
a few sites (e.g., BWI/MTN) until rain pushes eastward over the
next hour or so. Should see improvement to VFR soon after showers

Think that the ground now warm enough to preclude fog development
in the wake of the rain. There would be concerns if skies clear
though. Dont believe that will happen, but the potential still

Of greater certainty is the low level wind shear potential, as a
50 kt low level jet slides north along the I-95 corridor tonight.
That phrasing in TAFs.

Thursday should be VFR, with the cold front to the northwest of
the terminals. Low clouds may redevelop Thursday night. Cold
front will slip south early Friday, but should have little impact
other than the wind shift. The next disturbance could bring wintry
precipitation late Friday night, but confidence in particulars is

Sub-VFR likely over the weekend in freezing/frozen precipitation.
May transition to -RA from CHO to DCA by Sun AM. Very light NELY
flow during this time.


Low level jet increases tonight. A Small Craft Advisory begins at
1 AM and goes through Thursday night. While mixing will be poor
tomorrow night, the magnitude of wind close to the waters surface
can`t be ignored.

A cold front will move through early Friday. It`s a little
uncertain if the northerly surge behind the front will be strong
enough to warrant an SCA.

A wedge of high pressure should keep winds light over the
waters over the weekend.


MARINE...Small Craft Advisory from 1 AM Thursday to 3 AM EST Friday for
     Small Craft Advisory from 1 AM to 6 PM EST Thursday for ANZ542.
     Small Craft Advisory from 8 AM to 6 PM EST Thursday for ANZ535.


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