Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 182018

Area Forecast Discussion
National Weather Service Blacksburg VA
418 PM EDT Tue Apr 18 2017

Cool high pressure over eastern Canada will remain
wedged east of the Appalachians through Wednesday, allowing
precipitation to spread across the region from the southwest
tonight, and continuing into Wednesday. A cold front will then
approach from the west later in the week, stall and bring a chance
for showers and thunderstorms Thursday into Saturday.


As of 330 PM EDT Tuesday...

A surface wedge will continue to move south into the Carolinas
tonight. Meanwhile, a short wave trough and warm front over the
Tennessee Valley will try to advance northward overnight. The battle
front for these two systems will be over the Blacksburg CWA tonight.

The main differences between the two are dew points and wind
direction. The wedge has dew points in the upper 30s along the Blue
Ridge to mid 40s across the piedmont. Low level wind direction has
been out of the northeast today but will veer to the southeast this
evening. The short wave trough storm motion is tracking northeast
with dew points in the upper 50s to lower 60s. So far, the wedge has
won the battle of precipitation keeping the area dry. The short wave
trough has won the cloud category continuing to advance thick high
clouds northward. The trough will win out tonight as it has more
energy to push over the wedge. Isentropic lift increases and along
with upslope easterly flow, rain will start spreading across the
mountains this evening, then everyone should see rain in the
morning. Rainfall amounts will vary from a quarter to a half inch
/025-0.50/ across the mountains to a tenth to a quarter of an inch
/.10-0.25/ east. The higher end of the range will occur across the
south along the VA/NC border.

Despite the short wave winning the battle tonight with rain, the
wedge will gain strength due to evaporative cooling. Overnight
temperatures will drop into the upper 40s to lower 50s and will only
warm into the mid to upper 50s tomorrow. Areas that may see the
wedge erode tomorrow (Bluefield-Richlands) could see temperatures
warm into the mid to upper 60s.


As of 345 PM EDT Tuesday...

Increasing southwest wind flow ahead of an approaching front
from the Ohio Valley will bring warming temperatures and partial
clearing for Thursday. Can`t rule out a stray pop-up shower or
thundershower Thursday, but think day as a whole will be dry
with primary focus for showers residing along the front which
will be over the Ohio Valley.

Better threat for showers will accompany the front as it enters
the CWA late Thursday night, crossing the forecast area Friday.
Forward progress of the front is expected to slow as main area
of upper support passes from the Great Lakes into Ontario then
exits to the northeast across New England. This will leave us in
a situation where the front will either stall over or just south
of the VA/NC border Friday night.

With front stalled across the Mid-Atlantic, this will set the
stage for a wet weekend, models indicating a strong area of low
pressure developing over the central Plains and moving east
along the front.


As of 415 PM EDT Tuesday...

An area of low pressure is forecast to move east along a stalled
front which will be aligned east west from the Mid-Atlantic
Coast to the central plains. The low will cross the Mid-MS
valley Saturday, then directly across the Mid-Atlantic Region
Sunday. Primary focus for wetting rain will be along the stalled
front with increasing southwest wind flow aloft over-running the
front. Dynamic lift will also increase, especially Sunday as the
area of low pressure passes overhead, yielding 2 day rain
amounts (Saturday-Sunday) of 1-2 inches, with isolated higher
amounts possible pending thunderstorm development.

High pressure will build into the area Monday into Tuesday, dry
weather returning for the middle of next week.

Maximum temperatures will average near to slightly below normal
this weekend thanks to abundant cloud cover, and the forecast
area residing on the cool side of the front. A transition to
warmer than normal temperatures will return by the middle of
next week after a couple of near normal days Monday and Tuesday.


As of 135 PM EDT Tuesday...

Cool wedge of high pressure continues to build in from the north
this afternoon. Dry northeast low level flow will keep areas
along and east of the Blue Ridge (BCB-ROA-LYH-DAN) rain free
into this evening. Flow across the North Carolina High Country
(TNB-GEV) and along western slopes of SE WV (BLF-LWB) and SW VA
is more southeasterly. This downslope flow is fighting off rain
trying to advance northeastward from the Tennessee Valley.
Eventually, this rain will over come this flow and move over the
area tonight.

VFR conditions will prevail into the evening for the mountains
and into early Wednesday morning east of the Blue Ridge.
Deteriorating conditions (sub MVFR) will advance from the southwest
to northeast tonight, then remain low into Wednesday night.

Extended Aviation Discussion...

By Thursday, the front will return north as a warm front just
as a new front approaches from the northwest. Showers and
thunderstorms will be on the increase Thu-Fri as this front also
drifts into the area and then stalls Friday just south of the
CWA, most likely near the NC/SC border. This front will return
to the north as a warm front Saturday as a much stronger weather
system moves into the southeast U.S. While there may be some
periods of VFR cigs Thursday as the area comes into the warm
sector, expect MVFR or worse ceilings to return at times Fri-
Sat, with widespread sub-VFR ceilings Sunday with widespread
heavy rainfall and some storms in the KDAN area during the
afternoon Sunday. Winds may be a factor late in the weekend as




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