Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 200923

Area Forecast Discussion
National Weather Service Blacksburg VA
523 AM EDT Sat May 20 2017

Unsettled weather is anticipated through early Monday as an
upper level trough of low pressure over the central U.S. moves
east, erroding the warm subtropical ridge over the eastern
United States.


As of 430 AM EDT Saturday...

Today will be the last day of the unseasonable warmth,
before cooler air arrives tonight.

Busy weather regime with mid-CONUS trough continuing to pound
the center of the country with thunderstorms. Farther east,
subtropical ridge of high pressure has been acting as a road
block to the upper trough, but still providing the heat and
humidity needed to support scattered showers and storms per
either daytime surface generated instability or nocturnal mid
level instability. Either way the coverage has not been that
extensive, mainly confined to the higher terrain, although we
are now beginning to see an eastward drift into the piedmont
from any activity that develops per increasing westerlies aloft.
Today will be no different, showers/storms developing during
the peak heating of the day. Only caveat today will be a better
focusing mechanism, a surface front that will enter the CWA from
the northeast.

A surface front extends from northern VA/WV west across the OH
valley and into an area of low pressure over Kansas. The low
over Kansas is associated with the parent upper level trough.
Cool high pressure was moving east across eastern Canada, this
High poised to build over New England and wedge down the east
side of the northern Appalachians. This will give the surface
front over northern VA/WV a nudge to the south today and tonight
with this feature entering our northeast CWA this afternoon and
evening, then southwest (backdoor style) down the east side of
the central Appalachians. This front will be converging with the
warm southwesterly flow aloft which will persist per the
subtropical ridge just off the southeast Atlantic coast holding
its ground. Net result will be an area of convergence along the
front which will provide the focus for showers and thunderstorms
this afternoon and evening.

Attm it appears the axis of greatest instability will bisect
the CWA from northwest to southeast just ahead of the front and
right through the center of the CWA vicinity of an LWB-ROA-DAN
sort of corridor during the peak heating part of today. As such,
expect the greatest CAPE to pool within this region providing
the fuel for strong to potentially severe storms. Similar to
yesterday, the main severe threat would be from wind (40-60 mph
sort of max gusts), in addition to some hail. Another concern
would be potential for isolated flash flooding per orientation
of the front which would support backbuilding along the
boundary. On the average, anticipating between 0.50 to 1.00
inch of rain, but the potential will exist for locally higher
amounts, with some of the hires models suggesting isolated cores
of 2 to 3 inches associated with storms which track over the
same area.

Temperatures today will once again head for the 80s, but with
more cloud cover around, these numbers will be a few degrees
lower than Friday. Areas northeast of Lynchburg will also begin
to encounter a cooler northeast wind as the surface front
enters the forecast area from the northeast, the cool advection
negating some of the daytime insolation.

For tonight, the surface front will continue to make inroads to
the CWA, and is expected to push cooler air into all areas by
daybreak Sunday. Showers will continue along the frontal
boundary which will favor the southern and western sections of
the CWA with time. Overrunning of this airmass by southwest wind
flow aloft will maintain mid-level instability, so not eager to
pull pops completely, even after midnight. Temperatures tonight
will retreat into the upper 50s-lower 60s...the coolest readings
northeast of the Roanoke valley into northern VA.


As of 400 AM EDT Saturday...

It continues to be unsettled for Sunday and into the first half of
Monday. 00z global model guidance continues to display reasonably
good agreement on the idea that the mid-level subtropical ridge axis
builds eastward, being replaced by increasing southwest flow aloft.
A surface cold front trailing from the eastern Ohio Valley southward
to the western Alleghanies then south-southwest into the Tennessee
Valley early Sunday morning is apt to make slow eastward progress.
With a continued feed of moisture ahead of the front from the
western Atlantic and the eastern Gulf, there may be multiple rounds
of showers and thunderstorms through the Sunday late morning into
Monday afternoon timeframe. Abundant cloud cover appears likely to
limit potential for stronger storms through this period. Flow aloft
is sufficiently strong enough to keep individual showers and storms
moving along. However given we have had several periods of showers
and thunderstorms in recent days, rainfall from this frontal system
projected to total 1 to 1.75" based heavily on latest WPC QPF
(locally higher in thunderstorms) opens the door for potential
localized flooding. Rainfall appears to be the steadiest/heaviest
during the Sunday evening into Monday morning timeframe. Will
continue to mention this period as one to bear close watch for
at least minor hydro issues in the HWO.

Front slips far enough southward Monday evening into the central
Carolinas that an weak high pressure builds into the northern two-
thirds of the CWA. Cooler 850 mb temperatures to around +7 to +10C
and at least some radiational cooling makes Monday evening the
coolest of the period. Didn`t include in the grids but potential for
patchy fog if radiational cooling is strong enough and winds light
enough as well.

Into Tuesday, dominant feature in mid-levels is a sprawling
trough over the northern tier of states. Unfortunately what this
means for us is that with spokes of energy rotating around the
trough and interacting with remnant baroclinic zone over the
Gulf states, another shot of wet weather emanating from the Deep
South appears to be in the cards for later into Tuesday.

Compared to climatology, high temperatures trend near to slightly
below normal through Tuesday, coolest earlier in the period. Lows on
the other hand trend near to slightly above normal given several
periods of cloudiness, coolest Monday evening.


As of 400 AM EDT Saturday...

Cold front crosses through on Wednesday leading in a colder air mass
over the area for Wednesday and Thursday. ECMWF shows the potential
for some gusty winds behind Wednesday`s front. Expecting showers and
thunderstorms along each of the front with the potential for heavy
rain. Overall troffing and periods of rain will result in below
normal temperatures for much of the week. Have trended toward cooler
daytime temperatures for Monday through Friday.


As of 500 AM EDT Saturday...

Mainly VFR expected the first half of the day. Debris cloudiness
from last nights showers will dissipate later this morning,
followed by afternoon cumulus buildups and a repeat cycle of
afternoon and evening showers and thunderstorms. Aside from
local sub-vfr conditions associated with the deep convection,
will be watching a backdoor cold front slip southward along the
east side of the Appalachians later today and tonight. Models
suggests a low stratus deck will develop behind this front for
tonight, with potential for IFR CIGS.

Extended Aviation Discussion...

Unsettled weather will continue Sunday and into early Monday
with an opportunity for showers and thunderstorms, especially
during the peak heating hours of the day over the western
mountains by late Sunday, and again Monday associated with a
cold frontal passage from the west. Potential for sub-VFR will
exist until the passage of the front Monday. Improvement is
expected behind Monday`s front with dry weather returning
Monday night into Tuesday. Another front may then bring the next
round of showers and subsequent MVFR by Wednesday.




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