Scientific Forecaster Discussion
fxus61 krnk 222017
Area forecast discussion
National Weather Service Blacksburg Virginia
317 PM EST Fri Nov 22 2019
a cold front will stall across the Carolinas tonight and and
upper level wave will spread an area of rain back over this
front during the day Saturday, at the same time pushing this
front back north as a warm front. By Saturday night a stronger
cold front will sweep all the moisture east, with precipitation
ending on the western slopes of the Appalachians early Sunday as
light snow showers and blustery conditions. Dry, high pressure
then moves in for the early part of next week.
Near term /through Saturday/...
as of 315 PM EST Friday...
Cold front to exit east out of the forecast area late this
afternoon and with westerly flow aloft nearly all rain has
dissipated east of the Piedmont. Shallow moisture will remain
banked up against the western slopes of the Appalachians, but
not expected anything more than a sprinkle or two out of this
early this evening before ceilings raise and atmosphere
continues to dry, at least for the overnight. Gradient behind
front to weaken quickly as well so expect any breezes to slowly
The cold front will stall just to our south and east tonight and
as large trough digs over the mid Mississippi Valley it will allow
an upper short wave and good isentropic lift over the stalled
front, spreading steady rain back into our region during the day
Saturday. Perhaps some minor slowing of this timing in the
models, but often this ultimately moves back in faster that
models advertise, so did not slow the trends down too much as
far as timing of precip. Amounts look a little less as lift is
not especially strong, but there could be some precip
enhancement along Blue Ridge especially. Overall, rainfall
amounts should not result in any issues as additional amounts
expected to range from about a half inch to an inch, with
highest amounts near Blue Ridge and across the south.
One minor concern is if precip moves in a bit more quickly in
the morning and temps have dropped to around freezing, which is
more likely if we do get some clearing tonight which looks
likely, then there may be some locations that pick of very light
freezing rain. Model forecast soundings support this possibility
along ridges especially, but it does look like precip will be
very light and the upslope southeast flow is not strong either.
Only have a brief period with a few grids with freezing rain or
chance of snow mixed in Saturday morning, but quantitative precipitation forecast so minor by
this time do not have any accumulation in any of the snow or ice
grids. Will have to monitor this overnight for possible trends
in either colder temps or faster arrival of precip, in which
case we may need to add some very light icing on ridges
Medium confidence in the near term period.
Short term /Saturday night through Monday/...
as of 145 PM EST Friday...
Early Saturday evening, a cold front is expected to be immediately
west of the forecast area, with our region still under the influence
of strong, moist, southwesterly flow with decent isentropic lift
helping to generate widespread coverage of rain across the region.
The cold front is expected to zip across the area during the evening
hours and be east of the forecast area by, or slightly after
midnight. In the wake of the passing cold front, low level winds
will shift abruptly northwest and drier air will start to rush into
the region. The result will be a fairly quickly ending of the
precipitation/cloud cover along and east of the crest of the Blue
Ridge where the increasing northwest wind will yield and
downsloping/subsidence effect. Across parts of southeast West
Virginia, south into the northern mountains of North Carolina, these
same gusty northwest winds will help mainly cloud cover and isolated
to scattered showers thanks to the upslope component there. As
temperatures fall through the night, look for the rain showers to
transition to, or mix with, snow showers. These will continue into
early Sunday morning, but decrease in coverage. A light snowfall is
expected at the highest terrain, but be generally less than 1/2 inch.
While winds are expected to become gusty Saturday night into Sunday,
the probabilities of speeds/gusts reaching advisory level or greater
are not high. However, if there is an area to watch for this
potential it would be the northern mountains of North Carolina.
A shortwave ridge is forecast to build over the lower Ohio Valley by
Sunday afternoon and evening. This will have the influence of
causing the 850mb wind pattern to start to back and weaken, thus
turning off the upslope precipitation/cloud machine across our far
western counties by Sunday afternoon.
High pressure settles over the region by Monday, providing for
limited cloud cover and a dry forecast.
Temperatures during this portion of the forecast will average
around normal for this time of year.
Forecast confidence for this portion of the forecast is moderate to
Long term /Monday night through Friday/...
as of 12:00 PM EST Friday
Tuesday will remain dry and mild as high pressure makes its way
out over the Atlantic. A surface low near the Central Plains
will shift northeast towards the Great Lakes, bringing a front
over our region Tuesday night into Wednesday. Showers will
accompany this front, but moisture appears limited so there is a
question of how much if any precip will make it east of the
Blue Ridge. A strong high moving in to our northwest should
displace the low pressure system on Thursday, returning US to
dry conditions for the end of the week.
Aviation /20z Friday through Wednesday/...
as of 1005 PM EST Friday...
Areas of light rain dissipating quickly early this afternoon
and conditions are generally VFR except for kblf and klwb where
MVFR ceilings will linger for a while as the cold front moves
through, but then should lift to VFR later this evening with
partial clearing at least temporarily. West winds remaining
below 10 kts except for on higher ridges, so there may be some
turbulence but not enough to consider low level wind shear mention in tafs for
remainder of the day.
Front expected to stall just east of the area tonight and
another upper wave will bring moisture back into the region on
Saturday with lowering ceilings again by Saturday morning, and
rain moving back during late morning to early afternoon. May see
MVFR ceilings again by end of taf period...18z...but then will
continue to lowering later in day with IFR likely with more
moderate rain. Cannot rule out some brief IFR conditions at
klyh early Sat morning with fog, but low confidence given higher
clouds may be moving back in toward morning so cooling may not
be enough to produce much fog. Even less chance for any fog at
other sites. Will take a closer look with future updates.
Winds will remain rather light and variable or southeasterly
tonight into Saturday.
Confidence is average on ceilings/vsbys and high on the winds.
Extended aviation discussion...
Winds will become gusty from the northwest as the low pressure
system exits the region on Saturday night and Sunday. Showers
and snow showers will linger into Sunday morning for western
slopes including kblf and possibly klwb with IFR conditions
likely there. Otherwise VFR and dry conditions are expected by
late Sunday. Dry weather is expected for Monday into Tuesday so