Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 201154
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
654 AM EST Wed Feb 20 2019

.SYNOPSIS...
Canadian high pressure will ridge down over the forecast area
into today as a series of low pressure systems moves up from the
Gulf. After a period of wintry precipitation in the mountains this
morning, temperatures will warm as rain chances continue. The moist
pattern will continue well into the weekend until a cold front
finally moves through the region Sunday. After a brief break early
next week, another system will approach by mid-week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 650 AM: Weak radar returns continue to blossom within
increasing isent lift this morning. Still not much of any active ice
nuclei, and temps are about steady state with most of the Winter
Storm Warning area still below freezing. Most of the advisory area
has bottomed out just above freezing or is starting to warm from
earlier freezing temps. So much of that may be able to be cancelled
soon. We also trimmed back the end time to noon for all our WSW
products right after the 4 AM update. Overall, no significant
changes needed at this time.

As of 330 AM: Upper level right entrance region divergence and band
of mid-lvl frontogenesis have lifted north of the area. Moisture
depth is shallower behind this forcing, as seen with IR cloud temps.
Despite this, still seeing a few reports of sleet/PL in some of the
METARS, including KAVL, KTNB (as UP). I still expect the sleet to
end, leaving just rain and freezing rain. Also, with some mid-lvl
drying, we may have areas of DZ/FZDZ. The diminishing precip rates
may be halting the cooling of the wedge, so concerns are growing
that much of the areas away from the SELY upslope regions may not be
able to get any ice accretions. With all that said, I will not trim
away any of the Winter Wx Advisory with the 4 AM update. Will wait
till daybreak to see if temps can fall any further.

Otherwise, it will be another chilly, dreary day for the area. The
strong CAD will begin to weaken as the parent high drifts off the
East Coast, but moist isent lift will persist atop the in-situ wedge
and keep areas of light rain and drizzle around thru the day. Temps
will warm a few degrees from morning lows within the wedge, which
fortunately, should end the freezing rain threat by early aftn.
Highs will range from the upper 30s in the northern NC foothills to
the mid 50s in the SW mountains outside the wedge.

Tonight, an area of low pressure will quickly track NE across the
Great Lakes dragging a trailing cold front across the OH/TN valleys
and into the southern Appalachians overnight. Both the NAM and GFS
show fairly steep mid-lvl lapse rates, producing 500-1000 J/kg of
MUCAPE above a strong inversion. The front will be accompanied by a
return of upper jet dynamics and some frontal forcing to sustain a
band of disorganized convection just to our west. I think given the
elevated instability some of this activity will produce some rumbles
of thunder across the area. So I bumped up thunder mention to chc
associated with this axis of MUCAPE, reaching the I-85 corridor
around daybreak Thursday. There is some concern that rain rates will
be a little higher with the convective nature of the precip in areas
that have elevated stream levels. For now, QPF totals overnight of
generally 1.5" or less. This still seems to be under FFG enough that
I will hold off on any kind of Flood Watch for now. But one may be
needed for portions of the SW and central NC mountains, and possibly
NE GA and SC Mountains, if QPF trends increase for tonight. Temps
should actually rise a little overnight, as wedge continues to
weaken.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 300 AM EST Wednesday: The short term begins Thursday morning,
with the residual damming still in control of the area with the
scouring cold front on our doorstep. After daybreak, the heaviest
precip will be diminishing rapidly, though with sort of a high-
pop/low-QPF situation during the day. The difficulty with Thursday
is the significant differences in guidance. NAM wants to keep the
cool pool firmly in place, in spite of the scouring front, whereas
the GFS lifts the damming out and is back to showing better airmass
recovery in the afternoon especially across southern zones. We tend
to forecast CAD erosion a little quickly, so with that have leaned
closer to the NAM and cooled afternoon highs for Thursday (just a
tad below normal), but with the front across the area, lapse rates
aloft above the strong surface inversion lead to some elevated
instability. With that, and given the isolated thunder we saw
yesterday (Tuesday) afternoon, have opted to expand the slight
chance thunder coverage across the area, despite the low-level
stability.

Moving onto Friday, the front will push south as a surface high over
the Great Lakes shifts east and wedges down into the area (again).
Another slug of moisture lifted in the SW flow aloft will overspread
the area (again), with highest amounts expected across the mountains
(again). The good news is that amounts continue to trend down, so
with this event current thinking has only about 1/2-1" max across
those extreme western zones through the short term period. With the
damming, temperatures on Friday will be below normal, but the low-
level CAA will not be nearly strong enough to bring cold enough
overnight lows for any p-type issues across the area.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 315 AM EST Wednesday: Damming in place, strong upper ridge
over the Bahamas and western Atlantic, moisture lifting up into the
area in the SW flow aloft - all sounds very familiar - will be in
place to start the extended. A potent little cutoff upper low
ejecting into the Plains will help to finally kick some of the
moisture out (at least briefly), with a cold front approaching the
area Saturday night into Sunday. With the speed of the front and the
initial rain early in the period over the mountains, could see
another quick 1/2-1" in the mountains, and of course by this point
soils will be quite saturated and streams will likely respond fairly
quickly. The 00z guidance has sped the front up substantially, and
we are no longer looking at a favorable peak heating timing of the
frontal passage Sunday; rather it looks like the whole thing could
be cleared the area by 18z Sunday. With this, have removed thunder
wording for now. Of note though, the upper low will be absorbed into
the longwave trough to our north, leaving us in mainly zonal flow
aloft with very little in the way of CAA, so despite the "cold"
front, temperatures on Sunday will soar to near 70 across the
Piedmont (really quite a nice day, albeit with a decent breeze).

We get a break in the rain behind the front Monday as we will be in
the near-zonal flow between the northern/southern stream systems. A
wave in the southern stream will lift up into the Southeast on
Tuesday, with some uncertainty as to how far north the moisture will
spread. If nothing else, increasing cloud cover ahead of the system
and low-level flow shifting more out of the N will help to moderate
afternoon highs Monday, but for Tuesday the ECMWF has trended a
little closer to the GFS, keeping the bulk of the moisture to our
south. Will continue with slight to chance pops and a cooling trend
advertised for the end of the period, back below seasonal normals.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: Areas of -RA and -DZ continue to develop
across the area within persistent SWLY upglide flow atop a strong
wedge today. Temps at all the TAF sites are hovering above 32, so no
more mention of frozen precip in the 12z TAFs. There may be a
minimum of precip around this aftn, but periods of -DZ will continue
thru the day. Cigs should stay IFR thru the day and into tonight,
but vsby may be variable, depending on the precip rates. Tonight,
the wedge will weaken further and cigs are expected to lower to LIFR
at most sites this evening. Then a band of elevated convection is
expected to cross the area overnight with TS possible, especially in
the mountains and western Upstate. Confidence in the TS surviving to
the east is lower, so no mention for now. This activity won`t
completely mix out the wedge, as the inversion will be very strong.
But improvement in cigs/vsby is expected not long after the 12z TAF
period. Winds will continue in the CAD configuration, but weaken
thru the period, becoming light and variable late tonight. Winds may
start favoring a SW direction east of the mountains around the end
of the TAF period.

Outlook: A series of low pressure systems will continue to move
through the region through the end of the week. This will produce
precipitation and associated restrictions for long periods of time
with each system.

Confidence Table...

            12-18Z        18-24Z        00-06Z        06-12Z
KCLT       High  84%     High  86%     High  84%     High  92%
KGSP       Med   72%     High  87%     High  87%     High  86%
KAVL       High  96%     High  89%     High  84%     Med   78%
KHKY       High  87%     High  94%     High  90%     High  89%
KGMU       Med   75%     High  87%     High  86%     High  86%
KAND       Med   78%     High  87%     High  90%     High  89%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Winter Weather Advisory until noon EST today for GAZ010.
NC...Winter Weather Advisory until noon EST today for NCZ035>037-
     048-052-053-056-057-059-063>065-502-504-506>510.
     Winter Storm Warning until noon EST today for NCZ033-049-050-
     501-503-505.
SC...Winter Weather Advisory until noon EST today for SCZ001>003.

&&

$$
SYNOPSIS...TDP
NEAR TERM...ARK
SHORT TERM...TDP
LONG TERM...HG/TDP
AVIATION...ARK


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