Scientific Forecaster Discussion
fxus61 krnk 221347
Area forecast discussion
National Weather Service Blacksburg Virginia
947 am EDT Tue Oct 22 2019
a cold front will move from west to east across the area today,
crossing the mountains this morning and then across the
Piedmont this afternoon. Widespread showers are expected.
Thunderstorms are possible for eastern Virginia and North
Carolina. High pressure will build in behind the front for
tonight and Wednesday, providing several days of fair weather.
The next chance of rain is expected for the weekend as low
pressure develops over the southeastern United States.
Near term /through tonight/...
as of 947 am EDT Tuesday...
A cold front will slide east across our area this afternoon into
tonight. Scattered showers and a few thunderstorms will develop
along the boundary as it progresses east. Some of the storms
could be strong to severe in the southeast portion of the
forecast area as highlighted in Storm Prediction Center day 1 convective outlook.
The placement is where there is increasing low level moisture
combining with deep layer shear and possibly some heating or
Richer instability in the warm sector. The main threat will be
damaging winds. The namnest develops a few strong individual
cells before a line forms in eastern Virginia. The latest hrrr
forms a line of storms around 18z and moves it east this
afternoon. Decided to expand our chances of thunderstorms
further north and increase in coverage for this afternoon.
Adjusted late morning temperatures into this afternoon utilizing
the current surface obs, their trends and a blend of conshort
and nbm. Elected to adjusted quantitative precipitation forecast up a little for this
afternoon. Also slowed the exit of the rain this afternoon. More
adjustments later this morning...
As of 400 am EDT Tuesday...
Passage of a cold front will bring widespread showers today.
Main focus for these showers will be across the mountains this
morning, then across the Piedmont this afternoon...rainfall
amounts averaging about a half inch /0.50/. Potential for
strong to severe thunderstorms will exist across eastern North
Carolina into southeastern virgina.
A deep upper trough covering much of the central/eastern Continental U.S. Is
expected to progress slowly eastward today. While the primary
occluding cyclone is expected to move from the upper Great Lakes
into Ontario, a secondary frontal wave is expected to move from
upstate SC this morning into the eastern mid Atlantic this
evening and southern New England overnight. As this occurs,
relatively rich low-level moisture is expected to stream
northward over portions of the central/eastern Carolinas into
the Tidewater region in advance of the eastward-moving cold
Most of our forecast area will escape the severe thunderstorm
threat. There is a marginal risk for the Danville area, but
timing of the front suggest it will have cleared areas west of
Highway 29 by the time the deeper convection has a chance to
develop. That said, if storms do manage to develop before
exiting the cwa, the main threat would be from damaging wind
Ahead of the front winds were out of the southeast. This
upslope component to the wind was resulting in a persistent
layer of stratus and fog up against the Blue Ridge. This fog may
restrict visibility attms to less than a mile, and potentially
as low as a quarter mile where the cloud base intersects the
terrain (e.G. The Blue Ridge parkway). Once the front passes,
expect rapid improvement.
Model timing of the front, brings it through the mountains this
morning, greatest rain threat west of the Blue Ridge before
noon, followed by clearing during the afternoon. For areas east
of the Blue Ridge, expect the rain to linger longer, with the
clearing delayed until evening. All areas should be clear by
tonight with a much drier airmass advecting into the region from
Temperatures today will remain mild with highs in the 60s, and
possibly into the lower 70s. There may actually be a brief spike
in the temperature with the actual frontal passage, the clouds
ahead of the front holding the temperature down until clearing
can materialize with the frontal mixing. Any clearing prior to
the arrival of the Post frontal cold air advection will
contribute to solar insolation and an uptick in temperature.
Once the sun sets, the cold air advection, in addition to a
rapid drop in dewpoints, will allow for the temperature to
quickly drop after sunset. Lows tonight should be closer to
normal with lows ranging from the 30s in the mountains to the
40s in the Piedmont.
Short term /Wednesday through Friday/...
as of 330 am EDT Tuesday...
High pressure will build over the mid Atlantic on Wednesday and
persist through Thursday. Confidence is excellent that it should
remain dry during both days thanks to good agreement in the models.
While calm winds and mainly clear skies should allow good
radiational cooling during Wednesday night, low temperatures by
Thursday morning do not seem to quite get close enough to freezing.
Thus, it seems only patchy frost may be the main issue along and
west of the Blue Ridge. With the mountains having its second chance
in less than a week to get near freezing and/or have frost occuring,
the 2019 growing season will more than likely be pronounced done
afterward as it is getting so late in the year.
Details significantly diverge in the models by Thursday night and
Friday. The latest GFS shows a fast and progressive pattern with an
approaching upper level trough in the form of an open wave crossing
the plains and pushing a cold front over the Appalachian Mountains
by late Friday. That scenario would result in moisture streaming
from the Gulf to provide rainfall south of Route 460. Conversely,
the latest ECMWF, Canadian, and other international models are
slower with this upper level trough as they cut it off over New
Mexico. This scenario would result in a cold front only reaching the
southern Mississippi Valley by Friday and keeping high pressure over
the mid Atlantic. Because the GFS has been trending slower and
slower with this cold front during the past several days, this
forecast has been steered toward the drier and slower solution from
the global model consensus. Consequently, pops were lowered, sky
cover was decreased, and high temperatures were raised for
Long term /Friday night through Monday/...
as of 210 PM EDT Monday...
An active weather pattern is expected during the long term period.
Pattern evolution uncertainty result in low forecast confidence in
details of the forecast. The uncertainties Stem from when the upper
trough approaches from the west, which will depend on whether or not
it remains attached to the larger scale westerly flow or developed a
cutoff low in the base of the trough.
On the ecmwf(similar on UKMET and cmc), the low becomes cutoff over
Texas and Oklahoma around Saturday and lifts northeast into early
next week, while the GFS holds an open trough that progresses east.
The new wpc prognostic charts showed a less prominent frontal wave
moving across the southeast during the weekend compared with
previous progs. Utilized a blend of hpcguide and nbm to capture
a progressive solution.
A cold front will move southeast across the region Friday and
be located to our south Friday night. A wave of low pressure will
develop along the boundary around Friday night and ride along the
boundary Saturday into Saturday night. A weak shortwave will rotate
east across the region Sunday morning. High pressure will build
across the region on Sunday afternoon into Sunday night. A cold
front will approach from the west on Monday. Details on the long
range will become clearer over time.
Aviation /14z Tuesday through Saturday/...
as of 500 am EDT Tuesday...
Poor aviation conditions expected through roughly 18z today
before conditions return to VFR by mid or late afternoon.
A cold front will move from west to east across the area
today, passing through the mountains before noon, then across
the Piedmont during the afternoon. Isentropic lift ahead of the
front will promote widespread stratus and fog with Mountain
Ridge obscurations, favored flight categories IFR-LIFR. As the
front crosses the area a quasi-linear band of deep convection
will provide a downpour of rain. This qlcs is expected to
strengthen and become more organized as it crosses the Piedmont
this afternoon, with the potential for strong to severe storms
from southeastern Virginia into the eastern Carolinas.
Passage of the front will bring a wind shift, the winds becoming
westerly. The airmass behind the front is quite dry, and
confident this will lead to rapid clearing and a quick
transition to VFR.
Extended aviation discussion...
Dry weather and VFR conditions are expected Wednesday through
Friday. Moisture is expected to return by Saturday associated
with a slow moving area of low pressure that will approach from
the southern U.S.