Scientific Forecaster Discussion
fxus62 kgsp 241946
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
246 PM EST Fri Feb 24 2017
a cold front will move through the area tonight with showers and
thunderstorms mainly for the mountains. Temperatures cool to near
normal late in the weekend then warm up again early next week. A
warm front lifts north through the area around midweek, bringing
increased rain chances.
Near term /through Saturday/...
as of 230 pm: a robust sfc/upper low will move from near Chicago to
the North Shore of Lake Huron by daybreak Saturday. We initially
will remain under the influence of offshore high pressure, seeing
only fair-weather cumulus and light southerly winds. Overnight,
a cold front will move through the lower Ohio and Tennessee
valleys. The airmass preceding the front is characterized
by springlike warmth and humidity, and accordingly a line of
thunderstorms is expected to develop in that area this evening
and progress east toward the Appalachians.
The overall expectations from this activity have not changed much
based on the latest model runs. The incumbent airmass over our
area still features poor low to mid level lapse rates and/or a
distinct cap, each of which should hinder deep convection; though
some elevated cape is still present it does not look particularly
viable. Pops begin to ramp up late evening in the far western cwfa,
reaching the likely range along the tenn border around dawn. As
the line of storms enters the County warning forecast area hi-res models show the cells
weakening and becoming disorganized. The best cape comes as the
front erodes the cap. The potent shortwave trough associated
with the system will bring very strong mid-upper level winds and
accordingly Eyebrow-raising deep shear parameters (0-3km bulk shear
of 40-50 kt, 0-6km 60-70 kt). However, with the instability being a
significant limiting factor, Storm Prediction Center continues to feature our mountain
zones in a marginal risk only for day 1, which seems appropriate.
As the front proceeds across the area winds will shift to
westerly, gusts will pick up, and a dramatic drop in dewpoints is
expected. While models depict instability redeveloping downstream
of the mountains, high LCLs and downsloping seem to be the
reason that models are not depicting any convection beyond late
morning. Afternoon Max temps should still reach the lower-mid
70s in the Piedmont--still shy of records. On the other hand,
over the high terrain, they should start off in the low 40s in
the morning and fall through the day. The much lower dewpoints
and gusty winds will create some degree of fire danger; please
see fire weather discussion below.
Short term /Saturday night through Tuesday/...
as of 215 PM EST friday: we begin the short term in the wake of the
weekend front, with modified Canadian high pressure pushing across
the mid-south toward the southeast, and perhaps some lingering
northwest flow showers (possibly some snow showers) in the mountains
with the upper trough axis still over the Appalachians. Yes, you
read that right - I said snow, because with the deep-layer cold air advection
behind the front, temperatures Sunday night will drop down to
downright frigid temperatures...which in this regime means about 3-5
degrees below seasonal normals. Might even see some teens at the
higher elevations of the mountains, but most likely remaining in the
20s as what little moisture is associated with the upper trough
quickly exits. We are left with a sunny day on Sunday with highs
right at seasonal normals. That doesn't last long though as the
surface high moves off the southeast coast and we get return flow
from the Atlantic around the western periphery of the high.
Temperatures will gradually warm through the remainder of the short
term as warm air advection and moisture flux continues across the area, despite
increasing cloud cover and (generally light) precipitation.
A shortwave pushing off the southern rockies into the Southern
Plains Sunday night will tap into Gulf moisture lifting north into
the southern states, with precipitation developing across the lower
Mississippi Valley Monday, spreading east during the day. Weak
ridging along the Appalachians will remain, almost like a residual
cool pool but not very impressive, as precipitation overspreads our
area Monday afternoon. The European model (ecmwf) continues to advertise a bit of a
split in the rain as the mountains break up some of the features,
but a secondary (weaker) shortwave lifts out of the deep south
Tuesday night to bring another round of light precip to the area.
All in all generally less than an inch. Expect that we'll see some
thunder especially on Tuesday with surface temperatures in the 60s
and lower 70s allowing for at least moderate instability; however
shear is a little on the low side...not bad, but 30-50kt 0-6km.
Severe threat looks minimal.
Long term /Tuesday night through Friday/...
as of 230 PM EST friday: longwave trough deepening over The Rockies
will push into the plains early Wednesday, providing another kick in
the form of a subtropical jet and upper diffluence to the already-
moist atmosphere across the southeast. Timing differences between
the operational models are problematic, with the GFS on the faster
side vs. The European model (ecmwf). There is at least some ensemble support for the
faster solution (which frankly doesn't really help narrow down the
decision), so have continued with a blend but leaning with wpc
slightly more toward the European model (ecmwf). Concern with the GFS is that with
the faster upper trough, while slowly weakening, timing would put
the greatest lift and shear collocated with best instability
Wednesday afternoon, thus with 300-500j/kg SBCAPE with an impressive
70-80kt 0-6km shear, which would be a bit concerning. With the
slower ECMWF, the threat would be slightly later, but still non-
zero. Will have to continue to refine the forecast as new guidance
Behind the front, again depending on the model of choice, we could
see a brief period of northwest flow snow in the mountains Wednesday
night as the cold air advection moves in in the wake of the front. Should see a
couple of days of near-normal temperatures at the end of the
period, so it might actually feel like early March as it should.
Aviation /20z Friday through Wednesday/...
at kclt and elsewhere: a low VFR cu field will be seen over all
sites through late afternoon, with light southerly winds. Tonight,
a low pressure system moving thru the Great Lakes region will bring
a cold front into the Tennessee Valley. This front will move across
the western Carolinas between daybreak and noon Saturday. Moistening
conditions and weak warm upglide preceding the front will generate
MVFR stratus over our area before dawn. A line of thunderstorms and rain is expected
to accompany the front into the mountains of western NC, but they
will be moving into an increasingly unfavorable airmass as they
push toward the taf terminals. A small chance of thunder does exist
but rain showers look much more likely. These may bring IFR cigs/vsby for
a time. Winds will shift to westerly and become gusty behind the
front, but except at kavl, the shift to northwest should be after 18z.
Outlook: drier air will advect in behind the front Saturday
afternoon as high pressure builds in from the west, maintaining
VFR thru Sunday. Another wave approaches on Monday, once again
increasing chances for precipitation/restrictions.
19-01z 01-07z 07-13z 13-18z
kclt high 100% high 100% high 89% high 87%
kgsp high 100% high 96% Med 71% Med 79%
kavl high 100% high 100% Med 75% high 93%
khky high 100% high 100% Med 73% high 85%
kgmu high 100% high 100% Med 68% high 81%
kand high 100% high 100% Med 74% high 88%
The percentage reflects the number of guidance members agreeing
with the scheduled taf issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:
gusty west/northwest winds and much drier air will overspread the area in the
wake of a cold front Saturday. Minimum relative humidity of 25 to 30 percent is
expected across the Piedmont and foothills Sat afternoon, and may
dip below 25 percent in some areas, especially across upstate SC.
Meanwhile, sustained winds of 10 to 20, with gusts of 20 to 30 mph
are also expected. These critical conditions could overlap for an
hour or two Sat afternoon, especially across the SC Piedmont. If
these trends hold, coordination on a potential fire danger
statement may be required overnight.