FXUS62 KGSP 180551

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1251 AM EST Mon Feb 18 2019

A moist warm front will continue to move north from Georgia
tonight as low pressure crosses Tennessee. Expect Canadian high
pressure ridge down into our area from North Dakota by late Monday
giving us a short dry period. Rain returns on Tuesday as a series
of low pressure systems will track from southwest to northeast
across the region, resulting in a persistent moist pattern through
most of the week.


As of 1250 AM Monday...Rain is starting to taper off from SW to NE
(except right along the TN border), as flow begins to veer ahead of
the sfc cold front. Rain rates have continued to be light, so stream
levels are behaving. No significant changes with this update, except
for the aviation grids for the 06z TAFs.

As of 945 PM EST Sunday: Regional radars late this evening depict a
fairly solid moderate to heavy rain shield extending to the
southwest across northern Alabama and Georgia in advance of H5
vorticity lobes embedded in the WSW flow aloft, and ahead of a weak
surface cold front. Isentropic lift and moisture are maximizing
across the forecast area at present, but upglide should diminish
from the west after 06Z and the upper dpva should pass by 09Z, so
the duration of the heaviest part of the event will be fairly short-
lived. Some 1 to 1.5 inch rainfall totals will be possible along the
TN border and in southwest mountain upslope flow areas through
daybreak, but stream and river rises have been fairly modest so far.
The current thinking is that any overnight hydro issues in the
southwest mountains will be isolated. Thus, no Flood Watches will be
posted for the remainder of this event. This will certainly produce
moist conditions, however, for subsequent rainfall through the week.

Meanwhile, cold air damming will continue to weaken overnight as the
parent high pressure slips farther offshore. Developing westerly
flow by daybreak will lead to downsloping and scouring of the
surface wedge layer. This will cause temperatures to rise slightly
overnight and then sharply through tomorrow as the low clouds
finally scatter out and insolation improves without any significant
thickness falls from the passing front. The downslope warming and
improving sunshine should permit a nice warmup to 60s high
temperatures in most areas east of the mountains Monday afternoon.
This break in the rainfall for most of Monday and into Monday night
could well be the last significant dry weather of the upcoming week.


As of 255 PM EST Sunday: The fcst will start off dry across the area
as a broad Canadian high builds south across the Glakes region
and begins to ridge down the east coast. Temps will drop to arnd
freezing across the NC mtns while a developing warm front begins to
approach the area arnd daybreak. There is some uncertainty with the
level of moisture with this front as the NAM continues to have very
little qpf response by 12z Tue. Likely a null chance of any wintry
precip Tue morning as any light precip shud not arrive until temps
warm abv freezing. Temps wont rise much during the day...probably
remaining 40s all locales as sfc-based CAA continues and cloud cover

The fcst becomes quite tricky Tue night as a parent 1040 mb high
sets up across the ern Glakes. The high config looks to be classical
in nature and bad for wintry precip across the colder NC mtn areas
as a stg llvl sw/ly jet brings in a sigfnt warm nose aloft. Model
temp guidance has trended down overall so mixed in lower values with
the previous fcst numbers. This gave more areas for accum fzra over
the NC mtns and even a -ra/fzra mix across the nrn fthills and nrn
NC piedmont. Temps look to hover just north of freezing generally
outside the nrn mtns...but a degree or two difference in sfc Tw/s
could create wintry/icy precip.

The amt of precip is not totally certain as there could be robbing
convec south and the overall depth of saturation will likely be
limited to arnd h7. Therefore...ice nuclei may not be activated esp
as the morning progresses...per the latest GFS/NAM soundings.
Thus...will anticipate an onset of freezing precip arnd 02z with
likely a switch to -fzdz...before a changeover to all rain occurs
arnd 14z. Expect a light glaze of ice at most across the
non/mtns...and the best ice accum potential of a couple tenths over
the nrn NC mtns and high elev nrn fthills...and perhaps even further
south across the Black Mtns. Right now...this event is certainty adv
level and as the fcst changes warning level conds could develop. A
light cold rain will continue thru period with temps only reaching
the L40s north and L50s over the far sw/rn zones as the wedge is
slow to lift out of the area.

Another issue Tue night into Wed will be the potential for stream
flooding across the sw/rn NC mtns. WPC has the area in a Day 3
slight risk of flooding the favored upslope sw-facing regions will
likely recieve arnd 2 inches to near 3 inches by then in continued
high saturated soils.


As of 200 PM Sun: An active pattern is still depicted across the CWFA
throughout the medium range, still with a strong ridge centered over
the eastern Caribbean and deep troughing over the southern Rockies
and vicinity. As low pressure traverses the Great Lakes region early
Thursday, a cold front will move thru the lower Mississippi Valley,
then pivot across the Appalachians as the sfc low moves thru New
England. That will promote erosion of the midweek CAD affecting our
area, though with the boundary lingering, unsettled weather will
persist. Furthermore, global models depict yet another seasonably
strong sfc high moving eastward north of the boundary, suggesting CAD
will return by Friday. Temperatures within the weekend CAD event
currently do not look cold enough for wintry precip, so the main
impact initially may just be to prolong the already cloudy/damp

Model consensus shows pattern change finally occurring next weekend
as a shortwave ejects from the Rockies, cyclogenesis occurs somewhere
to our northwest, and a cold front swings thru the Southeast. This
front is most likely to reach our area on Sunday. The European and
Canadian solutions depict a stronger low and better defined cold
front than does the GFS, though all of them show at least modest
instability in the warm sector preceding the front, while CAD
persists for some of our area. These trends are worth watching as
they imply some severe weather threat may result for those parts of
our area along/south of the wedge boundary.

All in all it is difficult to identify any meaningful break in precip
chances between Wednesday evening and Saturday morning, so we will
advertise at least likely PoPs for nearly the whole area during that
timeframe. A limited diurnal range is expected due to cloud cover
and/or wedging, but overall temps will remain near climo. Chances
were allowed to drop a bit by Saturday afternoon as warm front shifts
northward ahead of the developing low.


At KCLT and elsewhere: The rain is starting to taper off, as a cold
front pushes into the area and brings the low-level flow around out
of the west/northwest during the next few hours. This makes for a
challenging cig/vis forecast, as the front will help mix out some of
the moisture. However, if the front gets hung up along the Blue
Ridge, conditions may become nearly calm and low cig/vis may fill
back in. Guidance is mixed on the scenario, so tried to show the
current improving trends, but keeping some margin for a little
deterioration thru daybreak. From there, NW winds should scour out
the low moisture and VFR conditions with some potential gustiness is
expected from late morning thru the aftn. KAVL should see the
strongest winds, as flow is channeled south up the valley. While the
low-levels dry out, the upper levels remain moist in SWLY flow
aloft. So sct-bkn cirrus is expected thru the period.

Outlook: After brief improvement last through most of Monday night,
a series of low pressure systems will move through or near the
region through much of the rest of the week. This will produce
precipitation and associated restrictions for long periods of time
during each wave of precipitation.

Confidence Table...

06-12Z 12-18Z 18-24Z 00-06Z
KCLT Med 74% High 82% High 100% High 100%
KGSP Med 67% High 94% High 100% High 100%
KAVL Med 78% High 95% High 100% High 100%
KHKY High 83% High 94% High 100% High 100%
KGMU High 83% High 90% High 100% High 100%
KAND Med 78% High 91% High 100% High 100%

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:





LONG TERM...Wimberley
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