Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191153

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
653 AM EST Tue Feb 19 2019

Canadian high pressure will ridge down over the forecast area
today and tonight as a series of low pressure systems begin to
move up from the Gulf. The combination of cold air and abundant
moisture will result in an area of wintry precipitation for the
mountains and foothills early Wednesday. The moist pattern will
continue well into the weekend until a cold front finally moves
through the region and dries things out.


As of 645 AM: Mid-lvl frontogenesis has really intensified and is
producing enough precip to reach the ground in spots and wet-bulb
into some sleet mixing in with the -RA. I have bumped up PoPs to try
to line up with the radar. Also added a mention of sleet within this
band. No accums are expected, fortunately.

Later today, a strong area of high pressure (~1040 mb) will continue
to build in across most of the eastern half of the CONUS. Meanwhile,
a deep upper level trough will begin to shift east across the
Rockies, inducing a broad area of strong isent lift across most of
the Southeast US. Copious moisture will moisten profiles from the
top down, and wet-bulb effects in the low-level will weaken a warm
nose aloft. Strong, classic cold air damming will be diabatically
enhanced by the precip falling in the dry air. So it`s no surprise
that guidance continues to trend colder with CAD temps starting this
evening. Precip today should be mostly rain, except a mix of
snow/sleet/fzra in the northern NC mountains. QPF looks light thru
the daylight hours. Highs will be 10-15 deg below normal (mainly in
the low to mid 40s).

Tonight, the CAD will be at its strongest, with a strong easterly
upslope flow and wet-bulbing bringing temps down below freezing
along the eastern escarpment, possibly all the way down into the SC
mountains and the Balsams. Profiles show a rapidly strengthening
warm nose, but the strong CAD keeps a deep sfc-based cold layer,
resulting in freezing rain (and sleet on the onset in the northern
mountains). Temps will be tricky away from the escarpment, however.
As usual, the NAM is one of the coldest on sfc temps. It usually
does well with CAD temps, but can be a bit overdone. So I only
blended about 50% of the NAM for temps tonight thru Wed morning.
This still results in a brief period of 31-32 deg readings across
the NW Piedmont around daybreak Wed. Models fortunately agree that
the strongest deep-layer forcing and highest PWAT values will stay
west of the FA. So QPF doesn`t look as heavy as previous fcsts.
However, several hours of sub-freezing temps will be enough to
produce up to a 0.25" of ice accums in the Northern Mountains and
along the eastern escarpment down to the SC/NC border. Will hold off
on expanding the warning into Buncombe or Henderson, as most of
those counties shouldn`t see the quarter inch ice. Overall, the
current Winter Storm Warning and Winter Wx Advisory look well
placed. Have gone and expanded the advisory to include northern
Greenville County and the NW Piedmont counties from Catawba to
Rowan. The rest of the area will see a cold rain with temps in the

Wednesday, I continued to favor the NAM on a very slow warming trend
thru the day. Temps should warm above freezing everywhere by early
aftn. Precip rates overall decrease somewhat as an 850 mb front
lifts north and upglide weakens. Highs will be able to get into the
50s outside the wedge in the extreme western NC mountains, but stay
in the 30s to lower 40s for rest of the FA.


As of 230 AM EST Tuesday: The short-term period will begin with
lingering damming/residual cool pool in place across the Piedmont,
with the incoming front right up against the western border. Expect
a non-diurnal trend for lows Wednesday night, with a very slow
warming trend toward daybreak as the damming begins to erode.
Temperatures should remain all above freezing so no lingering
concerns of p-type issues, though with the mechanical lift along the
front cannot completely rule out some embedded rumbles of thunder
early in the overnight period.

Timing of the frontal passage...or rather not quite passage but
front-moving-into-the-forecast-area will be critical for afternoon
highs on Thursday. NAM is way cooler than other guidance, but given
how often it tends to handle damming a little better, and we`re
typically too fast to erode, have blended in the raw NAM temps on
Thursday, going quite a bit lower than the consensus. Definite bust
potential there of course. The front lays out across the area
Thursday afternoon as the strong upper ridge over the
Bahamas/Western Atlantic really prevents any forward progress.
Meanwhile yet another deep upper system in the Rockies will enhance
the moist SW flow aloft, as a shortwave riding up the flow from the
NW Gulf will bring another slug of moisture to our area. GFS seems
to think on the southern side of the nearly-stationary front we
might get some airmass recovery and brings some low-end CAPE values
into the area, though less than last night, though in an unusual
twist the NAM keeps us stable. Have continued slight chance thunder
wording for southern zones but confidence not particularly high.

Meanwhile another surface high traversing the northern tier will,
you guessed it, dam down the lee of the Appalachians Thursday night
into Friday. Luckily temperatures are above freezing everywhere so
no winter weather impacts, but the below-normal highs will continue
on Friday with the hybrid damming in place.

The other issue through the short term, and will linger into the
extended, will be the QPF. The lowering trend continues, ever so
slightly, with the highest rates expected Wednesday night, 1/2-3/4"
forecast in 6hr 06-12z Thu, with a max in the SW mountains of 1.5-2"
through the short term period. Of course, this on top of the waves
of rain we`ve been having may certainly begin to lead to some hydro
issues, mainly isolated to the mountains. A relative lull on
Thursday and Thursday night as the front lays out across the area
and some of the moisture dissipates a little will be short-lived as
another wave of moisture lifts up on Friday. As we get closer, and
as we see how the near-term QPF shakes out, may have to consider
flood watches later in the week.


As of 300 AM EST Tuesday: The hybrid damming will still be in place
Friday night with the slug of moisture moving NE over the upper
ridge toward the central Appalachians. The Rockies low will kick out
into the southern High Plains Friday night, lifting toward the Great
Lakes Saturday. The attendant surface low will occlude, with warm
sector moisture remaining over our area with low-end QPF values on
Saturday, while the front approaches for Sunday. Extended guidance
seems to be coming into better agreement with timing of the front,
kind of a favorable peak-heating passage across the area. GFS and
ECWMF continue to bring some low-end CAPE values to the Piedmont
Sunday afternoon (less than 500 J/kg), with 60-70kt deep-layer
shear. Highs ahead of the front will soar to near 70 across the
Piedmont, and upper 50s-mid 60s in the mountains. Continued trend of
thunder wording in the grids and will continue to monitor. Not
concerned about any one period for QPF, but rounds of continued
light to moderate rain until the frontal passage may exacerbate any
lingering conditions that develop during the short-term period.

We have a break behind the front on Monday with only a minor cool-
down, but perhaps more importantly, some sunshine! Alas, this is
short-lived as the SW flow aloft persists, and yet another slug of
moisture lifts up from the Gulf into the Deep South late
Monday...with what looks like yet another onset of damming Monday
night. No wintry precip in the forecast at this time but that could
easily change.


At KCLT and elsewhere: A narrow band of precip has broken out from
northern AL/GA thru the Upstate and just south of KCLT early this
morning. Some of this has been reaching the ground as -RAPL mix.
Guidance not handling this all that well, but I expect the band to
move a little to the north and expand in coverage thru the daylight
hours today. Precip will be light and temps should warm enough to
end the sleet by 14-15z. But from there, wintry mix may occur in
spots in the mountains. CIGS will gradually lower to MVFR, then IFR.
Guidance has trended quicker with this, as profiles are moistening a
little faster than expected. Precip will pick up in intensity this
evening. The precip will combine with cold, dry high pressure to the
north to produce strong cold air damming tonight thru Wednesday.
Temps will continue to fall, and freezing rain is expected across
most of the NC mountains and adjacent foothills overnight thru Wed
morning. Expect several hours of -FZRA at KAVL. Have also added -
FZRA at KHKY, as the fcst has trended coler. Not expectig the -FZRA
to get as far south as KCLT. So a cold rain is expected elsewhere.
Winds will take on a typical wedge configuration with breezy NE
winds across the Piedmont sites and SE at KAVL.

Outlook: Freezing rain in the mountains should gradually change to
all rain during the day on Wednesday. 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 100%     High  94%     Med   62%     Med   68%
KGSP       High 100%     High  87%     Med   70%     High  87%
KAVL       High 100%     High  86%     Med   71%     Med   77%
KHKY       High 100%     High  91%     Med   73%     High  81%
KGMU       High 100%     High  82%     Med   71%     High  80%
KAND       High 100%     Med   79%     Med   62%     Med   78%

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:


NC...Winter Weather Advisory from 6 PM this evening to 4 PM EST
     Wednesday for NCZ035>037-048-052-053-056-057-059-063>065-
     Winter Storm Warning from 6 PM this evening to 4 PM EST
     Wednesday for NCZ033-049-050-501-503-505.
SC...Winter Weather Advisory from 6 PM this evening to 4 PM EST
     Wednesday for SCZ003.


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