Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 251317

Area Forecast Discussion
National Weather Service Blacksburg VA
917 AM EDT Sun Sep 25 2016

A backdoor front slide south into the Carolinas today, before
stalling. High pressure following the front will wedge south along
the eastern slopes of the mountains Sunday into early Monday.
Another cold front will shift in from the west Monday night into
Tuesday bringing better chances of showers and isolated
thunderstorms for early next week.


As of 917 AM EDT Sunday...

Backdoor cold front continues to drop south today into tonight.
High pressure over the Great lakes Region will move east across
the Northeast and producing a wedge over our area. Moisture
pushing up against the mountains has resulted in plenty of clouds
for our forecast area. KFCX WSR-88d does not show much in the way
of drizzle or showers this morning. For this update of the isc
grids, adjusted temperatures with surface obs and trends, then
blend the late morning to afternoon towards the lamp guidance.
With moisture being quite shallow, we will see how well the wedge
holds. More changes later this morning...

As of 342 AM EDT Sunday...

Backdoor front situated from eastern Kentucky, southeast into the
mountains of NC early this morning will continue to drift slowly
south to the Smokys and toward the coastal region of NC/SC. High
pressure will move across the Northeast setting up a wedge over us.
Satellite showed the stratus layer had advanced as far southwest as
Pulaski, VA to Danbury NC at 330am. Will see these clouds make it to
most of the forecast area by mid morning but confidence not too high
that it will encompass the entire forecast area given shallowness of
the moisture. As far as precip goes, models generating light qpf
across the Blue Ridge but vary on exact location. Given depth of
moisture threat is very low but could see patchy light rain or
drizzle from near the Blue Ridge and Alleghanys east to the piedmont
this morning. As head through the day should see edge of cloud cover
erode somewhat in the west, but looks to hold firm roughly along and
east of the WV/VA border south to the NC mountains. Days getting
shorter with lower sun angle will be a factor on how much erosion

Will hold onto slight chance pops most of the day across the Blue
Ridge into the piedmont. Previous forecast of lower temperatures
today looks on target with readings in the upper 60s to mid 70s east
to lower 80s far southwest.

Tonight, the flow will turn more southeast as previous backdoor
front shifts back to the north across the Southern Appalachians.
Should see better lift enter across the southern forecast area
overnight so kept pops in the chance range but still no higher than
30, with slight chance pops across most of the rest of the region.

Lows are going to be held up by the clouds with upper 50s to lower
60s common throughout southern WV and SW VA into NW NC.


As of 330 AM EDT Sunday...

The backdoor front stalled across western NC will begin to return
north fairly quickly by Monday afternoon as a strong cold, front,
basically our first fall cold front of significance, arrives
Monday night into Tuesday. Early morning low clouds and drizzle
along the Blue Ridge should give way to partly cloudy and
continued fairly warm and humid conditions in advance of the
front. Synoptic scale models are in good agreement on the arrival
and timing of the front, reaching the western part of the CWA
during peak heating, then tracking east across the Piedmont during
the overnight hours. With the wedge gone and a warm humid air
mass, expect some thunderstorms. However, best instability remains
west of the region, diminishing considerably as the front tracks
across the Alleghany front. Have included chance thunder, but do
not expect much in the way of severe. Due to the broad nature of
the parent upper low/trough sinking southward through the Great
Lakes, dynamic forcing is not impressive. Have opted to drop
mention of thunder after 06Z Tue. The front will slow forward
speed Tuesday, but should still track far enough east to allow
most of the precipitation to exist the CWA. However, weak surface low
pressure developing along the front across the southeast states
and the southward sinking upper low will result in potential
retrogression of precipitation across eastern NC Tuesday.
Consequently, kept a slight chance pop along the far southeastern
border to account for this possibility.

Beyond Tuesday, considerable discrepancies are noted among the
various models regarding the evolution of the Great Lakes/Central
Appalachians upper low. Initially the upper low will sag southward
into northern WV/VA, then lift out by Thursday into the northeast
states, per GFS. However, the ECMWF, for the second run, progs an
anomalous situation where the upper low sags southward into the
Carolinas, then retrogrades northwestward back into the Ohio
Valley during the later half of the week. This solution would
bring considerably more rain to the region than the GFS (and most
other model solutions), as well as much colder temperatures. The
pattern appears fairly unrealistic and is not accepted at this
time, although the relative consistency from run-to-run of the
ECMWF regarding this solution is somewhat disconcerting. For now,
have just advertised slight chance pops, focusing mainly across
the western mountains and northern areas closer to the more likely
path of the upper low.

Temperatures will finally trend toward more normal fall readings
after Tuesday. 850mb temps will drop back toward +10C by midweek,
bringing lows in the 40s and 50s and highs in the 60s and 70s.
Something we have not seen since last spring, yet only near normal
for late September.


As of 400 AM EDT Sunday...

As noted above, the extended periods hinge largely on the
progression of a deep closed low progged to be somewhere in the
Appalachians or Mid-Atlantic region. ECMWF solution has been
thrown out at this juncture, opting for the more progressive
solution of the GFS and other models. Thus, expect a drying trend
with increasing amounts of sunshine during the later half of the
week. Giving some small credence to the ECMWF solution, however,
given its run-to-run consistency, have opted to increase pops into
the slight chance category in the Wed-Thu time frame, especially
north and west, until we can obtain a better handle on the
eventual pattern that will evolve.

Temperatures will remain near seasonal normals during the period,
but could be considerably cooler if the ECMWF solution verifies
and the area remains in a cloudy, wet pattern for several days
under the upper low. Really just to many uncertainties at this
point to make any wholesale changes.


As of 726 AM EDT Sunday...

Low clouds and fog will be prevalent over most of the terminals
early this morning, exception BLF where scattered low clouds may
occur but appears bkn/ovc should stay east. Any dense fog will
begin to lessen by mid-late morning, 13-15z.

Overall should see some improving ceilings though will be slow
with most going VFR for a few hours after 18-20z.

Then tonight, the flow turns more southeast and lower cigs will
again be encountered at all terminals, with most experiencing MVFR
with some IFR possible. Light rain is possible but best chance
resides south of ROA/BCB/DAN.

Extended aviation discussion...

Overrunning will begin in earnest by early Monday in advance of a
strong, but slow moving, cold front that will approach the area
from the west late Monday. A healthy line of showers can be
expected in advance of the front as it moves through the region
during this time frame. Widespread sub- VFR cigs/vsbys are
expected in the wedge air mass Monday before the front arrives,
which will also be accompanied by a period of sub- VFR cigs/vsbys.
Conditions will improve by midweek, although as an upper low sinks
southward over the Appalachians, conditions could become unsettled
enough to continue periods of sub-VFR cigs and scattered showers.




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