Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 170448

Area Forecast Discussion
National Weather Service Blacksburg VA
1148 PM EST Mon Jan 16 2017

Strong area of low pressure over the central United States will
move northeast and into the Great Lakes by Tuesday. A cold
front trailing south from the low will move east, and cross the
Mid Atlantic Region Tuesday night.


As of 955 PM EST Monday...

Will be making no changes this update period to the timing of
the approaching precipitation. A late night/early morning timing
of arrival in the western portions of the area is still looking
promising based upon the latest radar trends and mesoscale model

The biggest adjustment made was to the forecast low
temperatures. The region of the forecast area deepest in the
cold air damming, the Lynchburg area, has already experienced
temperatures falling to below forecast low temperature values.
Readings the past couple of hours have been in the upper 30s.
Suspect these readings will hold nearly steady the remainder of
the night, and have adjusted the forecast low to similar values.
Not as drastic, but slightly cooler nonetheless, are values
heading south into Southside Virginia and neighboring north
central North Carolina. Have adjusted slightly lower overnight
low forecasts for the Danville to Reidsville regions as well.

Patchy dense fog continues over parts of southwest Virginia and
northwest North Carolina. Coverage do not appear broad enough or
persistent enough to warrant a Dense Fog Advisory. Will continue
to highlight the potential in a Special Weather Statement.
Current Hazardous Weather Outlook also highlights the fog
potential tonight in general.

As of 645 PM EST Monday...

The biggest forecast adjustment this evening is a delay in the
arrival time of the main area of precipitation across western
portions of the region. Based upon the latest guidance and radar
trends, will have most of the area precipitation-free until
almost daybreak Tuesday. At this point, areas between
Richlands, VA and Lewisburg, WV can expect the arrival of the
main line of showers in advance of the cold front.

Also, some area between roughly Danville and Lynchburg, VA saw
some clearing of the low clouds right before sunset. Low clouds
are expected to redevelop this evening, and this trend has been
reflected in the forecast.

AS OF 345 PM EST Monday...

Vigorous upper level low is spiraling northeast through the
Central Plains and into the upper Mid-West. As of 18Z/1PM the
surface low was near Kansas City with a cold front trailing
south into east Texas. A warm front extended east from the low
into the lower Ohio Valley. Southerly winds ahead of this system
was advecting warm moist air northward from the Gulf Coast
Region and into the Tennessee Valley where temperatures were in
the 60s/70s. This warm air was trying to spill across the
Appalachian Divide from the west, but was only making it as far
east Bluefield WV...Marion VA...and Boone NC. East of the
Divide, the airmass was cool and stable with temperatures in the
40s. The only exception was a sucker hole in the clouds
vicinity of Martinsville...Greensboro...and Danville where
temperatures squeaked into the 50s.

For tonight, the cool wedge east of the divide (which covers
3/4ths of our CWA) will remain in place with stratus and areas
of fog persisting through the overnight. Temperatures will
change very little within this airmass. West of the
divide, Tazewell and Smyth Counties, and for the higher
mountain peaks (which will be above the stable air), the warming
southwest winds from the Tennessee valley will keep temperatures
mild through the overnight with readings not dropping much lower
than 50.

Models are in relatively good agreement for the mid-CONUS surface
front to move east Tonight and Tuesday with deeper moisture
arriving across our mountain counties by daybreak Tuesday, then
overspreading the rest of the forecast area during the morning.
Greatest POPs and QPF are expected across the northern CWA
closer to the surface low which is headed for the Great Lakes.
Rainfall amounts of a half inch or so are expected across the
northern CWA with generally a quarter inch or less across the
south. The cool wedge is expected to finally get displaced by
Tuesday afternoon, so in spite of the increasing rain threat,
temperatures should be 5 to 15 degrees warmer.


As of 330 PM EST Monday...

A vigorous northern stream short wave will move through the upper
midwest and penetrate the broad southern stream ridge over the eastern
US. This will allow a surface low moving through the Great Lakes region
to push a cold front through the area Tuesday night, with a good chance
of precipitation mainly west of the Blue Ridge. The low level wind
field will become amplified and with the passage of the cold front,
cold air advection will aid downward momentum transfer and create quite
windy conditions Tuesday night into Wednesday.

The upper pattern will then begin to amplify substantially with a sharp
southern stream ridge taking shape in the eastern US. This will allow
high pressure to build over the region, and after some lingering
upslope showers west of the Blue Ridge Wednesday we can expect
generally quiet weather through Thursday. However by Thursday night,
the ridge axis will pass to our east and allow low pressure moving up
the Mississippi valley to push increasing isentropic lift over the
region with the chance for precipitation increasing substantially.

Despite the passage of the front on Tuesday night the air behind the
front is only modestly colder, resulting in a continuation of above
normal temperatures through midweek.


As of 330 PM EST Monday...

The upper pattern will be in a state of flux for the end of the week
and through the weekend, as one closed low over the upper midwest
dissipates on Friday and is replaced by a very large closed low which
dominates the eastern US by Sunday night. This will keep a progression
of weather system s affecting our region through Monday.

First, a dissipating low pressure system moving northward into the
Great Lakes region will push a warm front through the Appalachians and
central mid Atlantic region with a good chance of precipitation for
Friday. Then in response to the changing upper pattern, another low
will develop over the mid Mississippi valley and push what appears to
be a dynamic occluded front into the area on Sunday. By this time deep
moist southerly flow will be well established above the boundary layer,
and when coupled with the dynamics of the front has the potential to
produce a significant amount of rainfall. Guidance also indicates the
presence of some instability which would support a chance for some
thunder in an environment of robust low level winds. This is still
quite a ways off but well worth keeping an eye on in the coming days.
We eventually get into the dry slot, but moisture and colder air
wrapping well around the vertically stacked system will keep a good
chance of rain around through Monday.

Temperatures will remain well above normal through the weekend, but
colder air wrapping around the vigorous low on Monday will bring
temperatures down a bit west of the Blue Ridge.


As of 1145 PM EST Monday...

The coverage of the IFR/MVFR ceilings remains a void from
roughly KLYH-KDAN. Indications are that this area will return to
sub-VFR conditions as the night progresses, obviously later than
earlier anticipated. The latest TAF package delays the return
for a few more hours. Most of the rest of the region already is
experiencing sub-VFR conditions, and this trend will continue
through the night. About the only region that may see more VFR
conditions than sub-VFR will be areas near and west of KMKJ.

Rain showers will move into the region late tonight in advance
of an approaching cold front. Coverage will continue to
increase during Tuesday morning, especially across the western
and northern parts of the region. Increasing southwest flow in
advance of the cold front will advect warm air over the cold air
damming, helping to slowly erode it from the top to bottom.
However, this feature that will terminate the wedge will be the
passage of the cold front in the afternoon. VFR conditions are
expected at all terminals by the mid afternoon. The initial
round of precipitation will likely be pre-frontal. The 850mb
front may bring an additional round of showers to the region
Tuesday evening.

A brief period of low level wind shear is expected just in
advance of the cold front near a KBLF-KLWB line Tuesday morning.

Extended Aviation Discussion...

Clearing may take place behind the front on Wednesday with
potential period of VFR Wednesday into early Thursday. However,
this is expected to be short-lived per return of unsettled
weather by weeks end as moisture returns from the southwest. No
precipitation type issues are expected through the end of this
week...just plain ol rain.




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