Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
417 am EDT Monday Apr 27 2015
an upper disturbance moves across the area today into this
evening keeping the region in a cool air mass. Low pressure over
Texas will shift east into the Gulf Coast states and Tennessee Valley to
bring our next round of rain by midweek.
Near term /through tonight/...
as of 345 am EDT Monday...
Upper pattern shows amplified yet split flow with low off the coast
of Nova Scotia this morning...with trough axis extending west-southwest into
the Great Lakes...while Rex block over the middle of the country is
expected with high over the cornbelt and low over the Texas Panhandle.
During the day the trough axis shifts south and across our area.
This will be providing increasing clouds and brisk conditions
especially over the mountains this more like a winter pattern. Not
seeing a good deal of moisture/lift but the upper vorticity seems
sufficient enough in strength to bring showers to the mountains of southeast WV
with sprinkles as far southeast as the New River valley. There could
even be some graupel or ice pellets if enough sun can warm the low
levels to increase lapse rates. At this time...the overall threat is low so no
thunder or wintry weather in the grids.
Tonight...this shortwave exits and showers diminish over the mountains
and winds will subside somewhat. Skies clear so temperatures should be
dropping toward the middle to upper 30s mountains...to around 40 across the
Piedmont. Issue tonight will be frost/freeze potential. Have only
the highest ridges of southeast WV/Montana Rogers close to 32f. While a few
pocket of frost are possible...think enough of a gradient inhibits
frost formation. Those who live in deeper/sheltered valleys will be
prone to frost.
Short term /Tuesday through Wednesday night/...
as of 300 am EDT Monday...
GFS/European model (ecmwf) were similar in bringing the upper low slowly east from
the OK/Texas area Tuesday morning into the Tennessee Valley by Wednesday morning. At
the surface...low pressure will ride along the Gulf Coast Tue-Wed...then
shift off the Georgia coast by Wednesday evening. While this occurs a northern
stream trough moves across the Great Lakes and just north of US.
Models keep these two streams split...but enough of inverted trough
in the area plus Atlantic flow to throw rainfall into area by
Tuesday night...and especially into Wednesday. Nudged probability of precipitation up and
have categorical probability of precipitation along the Blue Ridge south of Roanoke
Wednesday with high chance probability of precipitation from the Greenbrier valley to the
Piedmont northeast of lyh. As we head into Wednesday night the GFS
becomes more amplified and stronger with the northern low than other
guidance...and therefore drives the trough further south and brings
the surface low NE along the NC coast. The European model (ecmwf) more open will take the
low further out to sea. At this time...am compromising the two and keeping
higher probability of precipitation over the southeast County Warning Area Wednesday night.
Highs are expected to stay below normal with Wednesday appearing the
coolest...though the eastern County Warning Area may stay mild enough with middle
60s...while the Blue Ridge south of Roanoke only gets into the upper
40s to middle 50s...and could be cooler if rain moves in faster.
Long term /Thursday through Sunday/...
as of 330 PM EDT Sunday...
While there are still differences in timing...the long range models
show a similar solution of lifting the deep eastern U.S. Trough to
the east and slowly building an upper ridge across the Ohio Valley
by the end of the weekend.
The GFS appears overdone and too far north with its placement of the
axis of heavy rain in our forecast area. Obviously this will need to
be monitored...but leaned toward the European model (ecmwf) and Canadian solutions of
keeping the axis of heaviest rain closer to the forecast position of
the surface low in the southern Carolinas.
Wrap around moisture will keep clouds...much cooler than normal
temperatures...and light precipitation through Thursday and Thursday night. With
the deepening coastal surface low...and passing of a weak upper vorticity
lobe on the west side of the upper trough axis...bumped up winds late
Thursday and Friday...but nothing that appears to warrant headlines
at this time.
As the deep surface low continues to move northeast...we should finally
see some clearing Friday...then temperatures closer to average for early
may by the weekend.
Aviation /08z Monday through Friday/...
as of 125 am EDT Monday...
High pressure entering from the north sending drier air south into
the region. Should see any middle clouds in the Danville vicinity
clear out this morning. Issue will be any fog over the river
valleys...with wet ground. But the drier air will work to race the
temperatures such that dewpoint depression will stay enough to prevent
dense fog at least at taf sites. Will have MVFR at lyh/Dan/lwb and
bcb at times early...tempo over bcb/lwb for IFR. Confidence is
medium of this occurring.
Any fog will be gone after 12z with northwest winds picking up a bit.
Some gusts to 25 kts possible at roa/bcb. Northwest flow and embedded
shortwave will bring clouds into the mountains by midday/early
afternoon. Cannot rule out a shower over blf/lwb but coverage is
low to keep out of tafs. Kept things VFR through the day into this
Winds weaken tonight though the higher terrain could see some
gusts over 15 kts.
Extended aviation discussion...
VFR conditions are expected through early Wednesday before
conditions worsen once again Wednesday afternoon into
Thursday...as a low pressure system passes across the southeast
states. Confidence is growing that this system will result in
rain into most of the forecast along with sub VFR cigs/vsbys. So
much discrepancies in the models at this time for Friday that weather could be
clearing out European model (ecmwf) or sub VFR with surface low situated over the middle