Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 190610

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
210 AM EDT Thu Apr 19 2018

A cold front will move through the area tonight and bring rain
showers across the mountains. On Thursday, dry high pressure will
build in from the northwest and linger, before another moist
frontal system approaches the region late Sunday.


As of 145 AM Thursday: conditions remain mostly quite across the
fcst area except for the continued gusty southwest winds. A line
of showers associated with an approaching cold front is still moving
eastward across Eastern TN, yet they appear to be gradually weakening.
The latest CAMs have the line of showers reaching the NC/TENN Border
over the next few hours, yet they still dissipate most of the precip
as it crosses the mountains.

Otherwise, a sfc low will move east toward Chesapeake Bay.
Accordingly, its cold front will trail behind, pushing through
our area in the early morning hours. Strong gusts will develop at
high elevations of the northern mountains perhaps as soon as late
evening, so we will hoist a Wind Advisory at 02z. Any moisture
associated with the front will be shallow, and most guidance
produces PoPs only over the NC mountains; east thereof, models seem
to dissipate any precip due to downsloping. A couple WRF-based
models do redevelop some very light QPF over the southern Upstate,
but for now we will go in favor of the dry majority. As cold
advection will get underway before dawn over the NW half of the
area, min temps will be near climo there, flirting with freezing at
some high elevations. Contrast that with mins as much as 10 degrees
above in the lower Piedmont. Moisture is expected to diminish before
temperatures cool enough to support types other than rain. It is not
out of the question that a few very high elevation spots would see
snow or perhaps freezing drizzle before the event comes to an end.
Skies will be mostly sunny today. Wind gusts will continue under
persistent cold advection, though subsidence is progged to limit
mixing and moderate gusts over most of the area. Our northern
mountains however may continue to see some advisory level gusts
so we will maintain that headline until 22z. Dewpoints will fall
once again in the wake of the front. While temperatures will be
notably cooler, peaking 7 to 15 degrees below normal, mixing should
be sufficient to bring dewpoints below 25 percent in localized
areas. With continuing gusts a Fire Danger Statement is warranted
again for NC today.


As of 130 PM EDT Wednesday: Fair and cool will be the rule for
the end of the week as high pressure slowly builds in from the
west thru Saturday. The axis of the upper ridge will remain to
our west, which should spell dry weather across the region. The
main concern will be the possibility of frost/freeze early Friday
morning. Min temps over the mtns will be down into the lower/middle
30s, so a Freeze Watch might be required for those zones. Elsewhere,
cannot rule out some frost, but min temps are expected to stay in
the upper 30s/lower 40s. The center of the surface high over Iowa
Thursday night will keep enough pressure gradient over the region
to maintain a light N/NW wind, so only sheltered locations that
manage the mid 30s, such as the nrn foothills/NW Piedmont of NC,
stand much of a chance of frost. The rest of the period will feature
sunny/clear sky with temps running about five degrees below normal.


As of 240 PM EDT Wednesday: The 12Z model guidance has returned to a
wet and cool forecast for the first part of next week. Have trended
that way as well. The main difference in the models is at the
beginning where the GFS is faster with precip onset than either the
Canadian or ECMWF. Have slowed chance precip onset to late in the
day mainly over the western CWFA. Sunday will be the warmest day of
the period as well with lows near to slightly below normal and
highs around 5 degrees below normal. An upper low crosses the mid MS
Valley late Sunday into Monday. The low then slowly crosses the TN
valley into the southern Appalachians Tuesday before being kicked
out or absorbed by another upper low diving into the area Wednesday
from the Great Lakes. At the surface, cool high pressure builds in
from the north in a cold air damming pattern Sunday. A low pressure
center associated with the upper low moves east along the gulf coast
bringing a moist southerly low level flow across the area and over
the dome of high pressure. This leads to increasing precip chances
Sunday night and Monday. There is some discrepancy about the speed
of the surface low. However, even the faster GFS only slowly moves
the low up the southeastern coast Monday night and Tuesday which
keeps a moist easterly isentropic upglide in place both periods.
Therefore, have at least good chance PoP across the area through the
period, but limited the likely PoP to the western CWFA and south and
southeast upslope flow areas where there is good agreement on precip
continuing. The guidance has also kept the wet trend in place. The
GEFS mean is between 2 and 3 inches across the CWFA for the event
with the ECMWF in the 3 to 4 inch range. While most of this precip
is spread out over time and not particularly concentrated in a short
time windy, will still have to keep an eye on these trends for flood
potential. With the strong damming in place, precip should be in the
form of rain. There will be some cold temps across the higher
elevations, but precip should remain liquid rain even there. Lows
will be near to slightly above normal while highs will be around 10
degrees below normal. Highs could be even colder depending on the
amount and duration of the precip falling into the wedge.

The secondary upper low on Wednesday brings a clipper type system or
cold front across the area. This will bring another round of showers
and possibly thunderstorms as the wedge erodes and weak instability
develops. Lows rise slightly and highs rebound but remain around 5
degrees below normal.


At KCLT and elsewhere: expect VFR conditions to prevail thru the 06z
taf period. Gusty west to southwest winds will linger thru the overnight
and into the early morning across the area. A relatively dry cold front
will cross the region later this morning, shifting winds to NWLY first
at KAVL, and then elsewhere by the early afternoon. The front will be
accompanied by shallow moisture, however most of the MVFR to low-VFR
cigs should remain confined to the western slopes of the NC mountains.
Some bkn low-VFR cigs are possible over KAVL this morning, but they
shouldn`t linger for more than a few hours. Elsewhere, only sct to few
clouds are expected thru the period. In the wake of the front, wind
speeds will increase again along with higher gusts. Speeds should diminish
a bit later on tonight as the pressure gradient weakens and high pressure
begins to spread over the area.

Outlook: broad high pressure will overspread the region on Friday
leading to lighter winds and pleasant conditions through Saturday.
Restrictions may develop late Sunday and into Monday as a warm
front lifts northward over the area, bringing deeper moisture up
from the Gulf of Mexico.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     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:


A nearly dry front will push through tonight, with cooler but much
drier conditions expected through the end of the work week. RH
values will slowly drop Thursday to around 30 percent, with locally
drier RH as low as 20 percent possible. Furthermore, NW winds
Thursday will combine with the drying airmass and will result in
elevated fire danger across the area again. Winds will decrease on
Friday below critical thresholds but will have to be monitored,
since RH values will remain critically low.


NC...Freeze Watch from late tonight through Friday morning for
     Fire Danger Statement from noon EDT today through this evening
     for NCZ033-035>037-048>053-056>059-062>065-068>072-082-
     Wind Advisory until 6 PM EDT this evening for NCZ033-049-050.


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