Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 161130

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
630 AM EST Fri Feb 16 2018

Warm high pressure will move out over the Atlantic today as a cold
front crosses bringing much cooler air to start the weekend. A wave
of low pressure will develop along the front over the weekend,
keeping rain in the forecast Saturday and Saturday night. The next
cold front will arrive in mid week bringing some thunderstorms.


As of 630 AM EST Friday: Showers still isolated and very light near
the TN border this morning with better coverage closer to the
approaching cold front. Still looks like only a slow increase in
precip chances early this morning, so have trended toward this trend
for the update. Gusty SW winds continuing as well. There will be
breaks in the low clouds this morning before they become widespread
by afternoon. Basically only minor changes for current conditions.

The strong, flattened upper high over the Gulf of Mexico weakens
slightly and the center moves east through the period. This allows
heights to fall slightly as a series of short waves pass by to our
north. The waves push a relatively strong cold front across the area
late this afternoon or early this evening. Despite the strength of
the front, synoptic scale forcing is relatively weak with the
strongest portions of the short waves and associated upper jet
streaks remaining to our north. There is some deep moisture
associated with the front, but the low level flow remains WSW
keeping any inflow from the Gulf of Mexico limited at best. Temps
and dew points are quite high for this time of year, but instability
is limited with warm mid level temps. CAPE values max out around 200
J/kg at most, so no thunder is expected. The guidance agrees that
the best precip chances will be across the mountains, with a
secondary max along and east of the I-77 corridor where the weak
instability is best. Chances across the western Upstate and piedmont
of NE GA is limited and these locations may even remain dry. Gusty
SW to W winds continue through the day ahead of the front. Gusty NW
winds develop behind the front. Highs will again be well above
normal with record high forecast at CLT and near record at AVL.
Should a westerly downslope flow develop again at AVL then the
record would be smashed. GSP is a few degrees below a record but
that can`t be ruled out.

Showers end early in the evening as the front moves southeast of the
area taking the deep moisture and forcing with it. There may be a
few lingering showers across the mountains in the lingering low
level moisture and developing NW flow. Rain returns toward daybreak
as a cold air damming high builds in from the north and southerly
low level flow develops. Low level moisture and insentropic lift
increase by daybreak bringing a chance of rain across most of the
forecast area. Temps drop behind the front as a relatively cold air
mass moves in. Lows will still be 10 to 15 degrees above normal
which is significantly cooler than the previous 2 mornings. Temps do
drop to near freezing over portions of Avery county and the northern
Blue Ridge. However, do not expect any freezing precip at this time.
A deep warm nose remains in place precluding any snow.


As of 330 AM EST Friday: A hybrid/in-situ cold air damming will
dominate the weather on Saturday, with strong isentropic lift atop
the CWFA. The parent high will quickly translate east and a weak low
will streak along the wedge front, then deepen over the Mid-Atlantic
coastal waters Saturday night. This will bring the low-level flow
around out of the NW and cut off PoPs by daybreak Sunday. The
rainfall shouldn`t be too heavy, with the axis of highest QPF across
the TN Valley, where the upper divergence will be strongest. The
SWLY to WLY upslope areas will see the most rain with locally 1-2"
possible. Most of the area will see just less than 0.5". Temps will
be held down by the CAD, mainly in the 40s to mid 50s, then cool
into the 30s to lower 40s Saturday night.

Sunday still looks like a dry day with guidance even trending a
little less cloudy. So we should see a little more sun than we`ve
seen in a few days with a weak, transient high pressure crossing the
area. Temps rebound to the 50s to mid 60s, which is a couple
categories above normal.

The upper flow starts to amplify Sunday night, with low pressure
deepening over the Central Plains. Meanwhile, high pressure will
exit off the East Coast, allowing return flow to spread moisture
back into the area. Guidance is in good agreement on light precip
breaking out before daybreak Monday, as a weak in-situ CAD develops.
Temps will be held up well above normal overnight by the increasing
clouds and LLVL WAA flow.


As of 130 AM EST Friday...The extended begins at 12Z Monday with
500mb high centered east of the Bahamas and trough over the West.  A
low level southerly flow will enhance showers in upslope areas from
NE GA over toward Caesars Head area.  This upslope flow should be
maintained into Tuesday night then 925mb wind becomes more parallel
to the mountain range.  The cold front should extend from Kansas to
the Great Lakes early Monday and crossing KY and TN Tuesday night.
The front will slow down quite a bit going into Wednesday as waves
of low pressure move NE along the front from the northern Gulf. Also
instability will increase ahead of the front. GFS has CAPE values
over 500 Wed PM over the lower piedmont and the ECMWF has 1000 CAPE
south of I-85 same time. The front should clear the mountains early
Thursday then CAPE values in lower piedmont rise above 600 for
Thursday afternoon. The front moves south of our area on Friday and
we get into a cooler and more stable airmass with a high to our NE
for a wedge configuration. Max Temps in the lower 70s Tuesday,
Wednesday and Thursday. Min Temps well above normal due to the cloud
cover not getting near normal until Thursday night north of the
front over NC.


At KCLT and elsewhere: Gusty SW winds continue this morning ahead of
the approaching cold front except for a brief period of NW wind at
KAVL. Winds will turn NW late this afternoon or early evening as the
front moves through. Winds then diminish overnight and turn N to NE.
MVFR clouds this morning will lift to low VFR by afternoon as
heating develops. Latest guidance still shows best precip chances at
KAVL with KCLT second. Have included VCSH these areas but chance too
low to include elsewhere. Low VFR continues overnight with MVFR
possible toward daybreak as precip chances increase.

Outlook: The weather will remain periodically unsettled, with
periods of restrictions likely into at least early next week.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High  85%     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  93%     High 100%     High 100%     High 100%
KAND       High 100%     High  87%     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:



  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      72 2001     18 1958     51 1990      2 1905
   KCLT      76 1976     28 1969     60 1990     11 1905
   KGSP      80 1911     32 1958     59 1990      9 1958




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