890 
FXUS62 KGSP 232327
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
727 PM EDT Fri Apr 23 2021

.SYNOPSIS...
High pressure sticks around this afternoon into this evening. A low 
pressure system increases rain chances late tonight into tomorrow 
morning. By tomorrow afternoon thunderstorms are likely across much 
of the forecast area. Some of these could be strong to severe. 
Shower and thunderstorm chances will continue into the evening 
hours. The low pressure system tracks off the East Coast Saturday 
night into Sunday morning. Another cold front should track through 
the forecast area late Sunday. Early to middle of next week high 
pressure returns leading to gradually warming temps and drier 
weather.

&&

.NEAR TERM /THROUGH SATURDAY/...
600 PM Update...The fcst is in good shape with temps and tds 
changing slowly over the past few hrs. Some sprinkles and brief -ra 
will occur across the nrn NC mtns this evening, otherwise the FA 
will remain dry and cool. 

As of 230 PM...A vigorous shortwave trough will track from the
Southern Plains to the Lower MS Valley tonight. An embedded vort max
will ride out slightly ahead of the main trough axis and provide
strong DPVA atop the FA from 12-18z Saturday. In the meantime,
plenty of mid and high clouds will stream across the area within
a flat ridge that is downstream of the main trough. Areas of
convection will provide even more cloudiness, and will go with
a mostly cloudy forecast for the most part tonight. Lows will be
held up near to slightly above normal under the clouds.

Guidance is in good agreement on a fairly quick onset of deep
moisture and lift, right around 12z Saturday, peaking around
15-16z, then starting to shift east in the aftn. A sfc high will
be positioned east of the Carolinas, but dry air in place will
immediately induce some in-situ CAD across the area with widespread
rain moving in. Nocturnal convection to our west should be strongest
to our south Saturday morning, and may help keep the highest QPF
out of the area. With that said, the strongest synoptic forcing
will be over the area, and PWATS will approach 1.5" along our
southern zones. I think that the short duration and our current
dry antecedent conditions should preclude a flood threat. But
some areas in the southern third of the area could see some decent
downpours with 1-2" of QPF.

A dry slot should punch in from the west in the aftn, as a sfc
low tracks across the Great TN Valley to northwestern NC. However,
there will be a second shot of DPVA with the main upper trough that
guidance has triggering strong to severe storms across AL/GA. The
NAM looks like a worst case scenario with lifting the in-situ wedge
boundary north to about I-85 and uncovering 1500 J/kg of SBCAPE and
60-70 kt of 0-6 km shear. Sure enough, the NAMnest has the most deep
convection across northern GA and tries to organize it into a sort
of QLCS along the I-85 corridor and east by 22z Saturday. The other
CAMs have a similar idea, but keep the better CAPE and convection to
our south. With all the early precip and a decent wedge in place,
I'm skeptical of the NAM, but the latest SPC Day 2 outlook keeps
a slight risk up to I-85 and marginal to I-40. So will continue to
mention the severe threat in the HWO. Otherwise, the wedge should
keep temps in the 50s to lower 60s, and mid to upper 60s will be
possible along the edge of the wedge southeast of I-85.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 300 PM EDT Friday: At the start of the short term the low 
pressure system will be directly over the Carolinas and NE Georgia 
leading to ongoing showers and thunderstorms. Instability should 
wane throughout the evening hours as the forcing progresses 
eastward. Saturday night the low pressure system tracks northeast 
and ends up just off the Mid-Atlantic coast early Sunday morning. 
Rain chances should decrease overnight Saturday into Sunday morning 
for most of the forecast area. Wrap around moisture from the low 
pressure system could lead to continued rain showers along the NC/TN 
border into Sunday morning. 

Another FROPA is expected to cross the CWA late Sunday, but this 
cold front will be dry. Sunday night temps drop into the upper 30s 
and low 40s across the mtns with lows elsewhere ranging from the mid 
to upper 40s. Highs Sunday will be much warmer compared to Saturday 
and will be closer to normal for April. Monday Canadian high 
pressure will extend into the southeastern CONUS leading to building 
heights and dry conditions. Highs Monday will be even warmer with 
temps slightly above normal across the forecast area.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 141 PM Friday: The upper pattern looks slowly progressive
through the rest of next week. The middle part of the week looks
quiet enough. There is good agreement among the models as to an
upper ridge axis moving across the region Monday night and Tuesday,
which should push sfc high pressure off the Carolina coast on
Tuesday. Once that happens, expect a warm-up as the upper pattern
amplifies as a deep trof digs over the Plains. Temps are expected to
be on the order of 5-10 deg above normal for Tuesday and Wednesday
then into Thursday. The trend in the model guidance has been to
slow down the eastward progression of the deepening upper trof
during the middle part of the week, suggesting that precip will
not arrive from the west on Wednesday, and probably not until some
time on Thursday morning. Timing differences expand between model
solutions late in the week, with the ECMWF a full day slower than
the GFS solution with the arrival and passage of a strong cold
front. The fcst blends the solutions together and keeps a good
chance of showers for Thursday and Friday, while we wait for more
clarification on which day the front will move through. At the
moment, the guidance is suggesting that severe weather potential
will be relatively low, but from pattern recognition and climo,
we will want to keep monitoring the fcst trends.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere: VFR conds will persist overnight across all 
TAF sites then begin to deteriorate quickly arnd sunrise once a 
moist low pressure precip system reaches the area. The low will 
interact with an insitu wedge which will be maintained thru most of 
the day. Thus, low IFR to LIFR CIGs will linger along with MVFR/IFR 
VSBY in heavier rain showers into the evening. Thunder chances will 
affect areas along and south of the I-85 corridor, including KCLT. 
Some of these storms could become quite strong to severe as deep 
layered shear will remain quite high. Winds within the 2 Kft layer 
will be strong in the afternoon, but not enuf change with height to 
include LLWS mention attm, but may be included in subsequent TAF 
sets.  

Outlook: Dry high pressure returns on Sunday and lingers thru the 
middle of next week.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z 
KCLT       High 100%     High 100%     High 100%     High  83%     
KGSP       High 100%     High 100%     High 100%     High  94%     
KAVL       High 100%     High 100%     High 100%     High 100%     
KHKY       High 100%     High 100%     High 100%     High  94%     
KGMU       High 100%     High 100%     High 100%     High 100%     
KAND       High 100%     High 100%     Med   60%     High  89%     

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...AP
NEAR TERM...ARK/SBK
SHORT TERM...AP
LONG TERM...PM
AVIATION...SBK