Area Forecast Discussion
Issued by NWS Greer, SC

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039
FXUS62 KGSP 221806
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
106 PM EST Sun Jan 22 2017

.SYNOPSIS...
Numerous waves of energy will bring rounds of heavy rain and
thunderstorms to the area today and tonight. A vigorous low pressure
system will sweep towards the Carolinas today and move northeast of
the Carolinas on Monday, bringing abundant moisture and the
potential for storms today and tonight. Some of the storms could be
severe. Mild high pressure will move in Tuesday and Wednesday until
a cold front crosses the region Wednesday night.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 945 AM, early morning convection has cleared the area, and it
appears that we will be seeing an extended lull in significant
precip, until stronger deep layer synoptic-scale forcing overspreads
the area late this afternoon and evening. Still, low levels will
remain moist, and there will be an increasing low level S/SE upslope
flow throughout the late morning and afternoon, so slight to low
chance pops will be maintained into the early PM.

As far as the potential for severe storms, the latest output from
mesoscale model guidance regarding the next wave of widespread
precip in the late-afternoon/early paints a pretty similar scenario
to this morning`s activity in that the stronger convective elements
tend to remain in the more unstable air to our south. In fact, the
latest HRRR really doesn`t develop anything scary-looking
convective-wise in our forecast area until mid/late evening, when
triple-point surface low approaches from central GA. The primary
sticking point as usual is how fast and how far north the wedge
boundary will push ahead of the stronger lift. So far, (as usual)
short term guidance has been too eager to erode the wedge along the
periphery of the CWA, and it remains quite uncertain as to how far
north the boundary will push later today. With extreme levels of
low-level shear and SRH expected, it won`t take much in the way of
surface-based buoyancy to produce a threat for high shear/low CAPE
severe convection producing damaging winds and isolated tornadoes,
with the highest threat being along and south of the I-85
corridor.

There will again be break late in the evening as the storms move
east of the area. However, showers move back in from the west as the
upper low slides across the area. Do not expect any severe storms
with the convection after midnight. Highs today and lows tonight
will be well above normal, but the lows will be cooler than the past
several days.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 100 PM EST Sunday: Fairly robust cyclonic flow will remain
atop the region Monday night in the wake of departing closed upper
low. Additional pcpn chances will increasingly be limited to NW NC
as the night wears on given that showers should become forced solely
by upslope/CAA along with the depth of moisture becoming
progressively shallow. Model llvl wind progs 45-50 kts at 85H
leading to the ongoing probability of strong high elevation wind
gusts into Tuesday morning. The snow shower threat should be highly
elevation dependent as it looks to be a race between lowering
freezing levels and lessening moisture.  At this point, feel that
high elevation snowfall of an inch or two is not out of the question
throughout the Tenn border counties.

Expect the upper flow to deamplify on Tuesday and sfc pressure
gradient to loosen considerably. Model crosssections are progging
dry conditions throughout the depth of atmosphere, so expect
sunshine with max temperatures 8-10 deg f above climo.  Lower level
return flow/lee troughing looks to develop on Wednesday with a
questionable amount of moisture return ahead of encroaching yet
weakly forced cold front. Sunshine should progressively fade behind
increasing clouds that day, but there should be a downslope aided
boost to max temperatures, maybe even category or two warmer than
Tuesday`s readings.  Any small later day shower chances still should
be limited to the mountains.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 255 AM Sunday...for the period Wednesday morning through
Saturday.  The upper level ridge moves off the Atlantic coast Wed
morning as the broad upper trough over the nations mid section moves
east across the Mississippi Valley. This upper trough slides east
through the end of the week with thickness values lowering each day.
This will coincide with temperatures going from roughly 15 degrees
above normal Wednesday to around 5 degrees below normal Saturday.
The broad trough will create a zonal west to east 500mb flow from
New mexico to the Carolinas Friday.

At the surface, the cold front will be crossing Tennessee Wednesday
morning, crossing our area late Wed and Wed night then reaching the
Carolina coast by Thursday morning. Moisture will be somewhat
limited along the front as flow off the Gulf will be rather weak.
Northwest flow will become persistent from near the Great Lakes
Thursday into the start of the weekend. This will result in mountain
snow showers mainly along the Tennessee and NC border areas for
roughly a couple days. Even the ECMWF is leaning more toward the GFS
forecast with this NW Flow. The GFS has the NW Flow snow showers
starting Thursday evening, continuing through Friday then breaking
down and ending Saturday. The EC has the NW Flow Friday but at least
it now strongly hints at this weather feature. Also of note, low
instability of 30 to 40 CAPE over eastern TN early Thursday night
would enhance snow rates a bit. Most of the snow will be late
Thursday night in the higher elevations along the TN and NC border.
Snow amounts were derived from QPF through Thursday night.
Confidence decreases by late Friday into Saturday as to whether it
will happen or dissipate earlier.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
At KCLT and Elsewhere: Complicated forecast for this TAF period with
widespread MVFR to IFR cigs/vsbys this afternoon. Could see
intermittent improvement to VFR/MVFR before the first round of
SHRA/TSRA moves in. Timing is still uncertain but should see
increase in SHRA toward 00z, with rounds of TSRA off and on through
about 06z. Cigs should remain low for most of that period, but
behind the TSRA, guidance is indicating some improvement in cigs
before the surface low approaches after 06z. Have not depicted this
improvement in the TAFs due to low confidence plus lack of extra
lines. May see some improvement again toward the end of the period
but line restrictions strike again, so only hinted at this. Winds
should be SE to begin, possibly VRB and gusty in TSRA, veering to S
and SW and occasionally gusty. Winds should continue to veer toward
W by the end of the period but will refine those details with later
issuances.

Outlook: The very unsettled pattern will continue through Monday as
the upper low pressure system moves slowly east of the area. This
may allow periods of rain and/or restrictions to continue. Expect
gradual improvement by Tuesday but with moisture returning ahead of
the next cold front Wednesday into Thursday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  84%     Med   65%     Med   72%     High  89%
KGSP       High  87%     High  81%     High  84%     Med   77%
KAVL       High  90%     High  80%     Med   72%     High  83%
KHKY       High  91%     High  86%     Med   76%     High  94%
KGMU       Med   77%     Med   79%     Med   71%     High  83%
KAND       Med   78%     Med   77%     Med   71%     High  87%

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...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...DEO
NEAR TERM...RWH
SHORT TERM...CDG
LONG TERM...DEO
AVIATION...RWH/TDP



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