Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 111949

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

A warm front will gradually move north of the area tonight. A
cold front will then drop back south across the region Friday. The
front will stall to the south and a few waves of low pressure will
ride east along the front over the weekend into early next week.


As of 215 pm, the next wave of rainfall has overspread Virginia
per regional radars. However, surface obs suggest that its not
much more than sprinkles at this time. This is in step with
recent meso/synoptic scale model runs, which suggest that the
focus of the rainfall will be northwest of the area. The
associated shortwave will be crossing the area this evening, with
the axis of rainfall shifting to the north by midnight. Will be
holding onto chance overnight PoPs, mainly for residual showers.
All temperatures will be above freezing, so there are no precip
type/icing concerns.


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.


VFR flight conditions continue this afternoon. However, radar
suggests that rain has already overspread the Virginia terminals.
Coupled with surface observations, looks like it just sprinklesfor
now. Ceiling and vsbys will lower after dark as low levels
continue to saturate. IFR is possible, but not highly likely due
to the limited amount of rainfall. Have confined those
restrictions to right along the coast (DCA/MTN).

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
there. Am forecasting a return to VFR for the overnight, but its a
lower confidence forecast.

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.


Light south flow over the waters this afternoon, but a low level
jet will increase tonight. A Small Craft Advisory is already in
effect for tonight and Thursday; have extended it into Thursday
night for most of the waters. While mixing will be poor, 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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