Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210203

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1003 PM EDT FRI MAY 20 2016

A wedge of cool high pressure will remain entrenched over the
Carolinas and northeast Georgia through early tomorrow. A cold front
will then sweep across the area later in the day, leading to partial
clearing. Drier high pressure slowly builds into the region early
next week.


As of 1000 PM...The axis of deep moisture is beginning to shift east
of the area...while the main upper short wave TROF is still to our
west. It will be interesting to see if the passage of the short wave
will be able to trigger any deeper convection overnight atop the there is still some MUCAPE on the SPC mesoanalysis page.
Meanwhile...the sfc was analyzed in NW AL...with a warm front
extending east along the edge of the CAD wedge across central GA.
Guidance still in decent agreement that the sfc wave will cross the
area like a Miller "B" system. This will "pinch off" the wedge, and
may result in some dense fog for portions of the area, as stratus
lowers to the ground. This is not a slam-dunk DFA event, but I will
mention fog in the HWO, and beef up the wording to "areas of fog" in
the grids. As for additional rain, I still think PoPs will taper off
to mainly low-end chance overnight.

Saturday morning, the low pressure and associated cold front will
exit east of the area, switching the winds around out of the NW.
As is typical with CAD erosion, temperatures should be warmer
*behind* the cold front than ahead of it, with actually a fairly
pleasant day in store for Saturday, at least for the most part.

However, lapse rates will steepen through the day as the upper
trough continues to dive south, with intermittent vorticity lobes
rotating around it. With the steepening lapse rates plus breaks in
the clouds leading to increased sunshine and thus increased
instability, we should see a corresponding increase in thunderstorm
chances Saturday afternoon as well. Best chances will be either over
the mountains (closer to the upper trough) or across generally
eastern Upstate SC and the I-77 corridor ahead of the mesolow that
is progged to form in the lee of the Appalachians sometime midday.
Both the GFS and NAM increase sbCAPE across those eastern zones to
up to 1500 J/kg, with corresponding deep-layer shear of 45kt. SPC
Day2 outlook only has us in general thunder but thinking that
chances for strong to even severe thunderstorms are a little higher
than that, though WRF ARW/NMM reflectivity forecasts only develop
isolated convection over our area. The concern here is that the best
chances of thunder will be near the Charlotte metro area where there
is most definitely a public safety concern over the weekend, so will
further refine this forecast with the overnight package.


As of 230 PM looks like the second half of the weekend
and the first part of next week will be dominated by an upper low
that closes off over VA and then rotates down across the Tidewater
region Sunday night and off the Mid-Atlantic coast on Monday. Within
this flow regime, a fairly strong short wave will rotate down across
the mountains late Saturday night, bringing a swath of DPVA
overhead. For that reason, a precip chance will be retained across
the mountains with a larger expanse than usual through early Sunday
morning. Should be breezy over the higher terrain early Sunday with
the passage of the wave, but not to the point where an advisory
would be needed. Once the wave passes, we should see the usual N/NW
flow upslope cloudiness and shower activity on the TN border thru
Sunday. East of the Blue Ridge, strong downslope should put a damper
on most of the potential for convection, but an isolated shower will
not be ruled out over the western Piedmont of NC. The upslope
showers on the TN border should end after sunset, along with any
activity over the Piedmont. The rest of the period...through
Monday...should be dry. The upper low should keep temps below normal
through the period...altho warming up almost back to normal Monday


As of 100 PM Friday, still expecting a return to more summerlike
conditions as next week wears on.  Temperatures should rebound to
above normal levels on Tuesday, but with only nominal return flow
developing and ridging aloft, deep convection should still be
suppressed. A bit warmer and moist on Wednesday with the suppressive
effects of upper ridging beginning to lessen. Small and/or slight
chances for diurnal tstms will remain limited to the mountains.

For the latter half of the week...the pattern atop the SE conus does
not change very much with the return flow around Bermuda high
pressure continuing.  The above climo temperatures should remain for
Thursday and Friday with daily diurnal deep convective chances each
day cwfa wide.


At KCLT and elsewhere: Cold air damming (CAD) will continue this
evening into the overnight, but will gradually weaken, as a weak
area of low pressure approaches from the west. This should result in
very low cigs and possibly some dense fog around late tonight into
early Saturday morning. Guidance generally agrees on the CIG trends
to LIFR (and even some VLIFR). I will not go with anything below
airport minimums in the TAFs. An area of showers will hit KCLT in
the 0040 UTC TO 0200 UTC time frame. Behind that, all sites should
see mainly spotty light showers for most of the evening.

By around 1200 UTC Saturday, the low pressure center should be
crossing the KCLT area, with winds switching around out of the N to
NW at all sites. The dissipating CAD wedge should allow for
improving CIGS and VSBY after daybreak. Guidance often is too fast
with trends, but I expect decent downslope and CAA to scour out the
low stratus. Moisture will linger, however, for a decent MVFR to low
VFR cumulus deck with a few AFTN SHRA and TSRA possible, highest
chance at KHKY and KCLT.

Outlook: Conditions should gradually dry out Sunday and Monday. AFTN
showers and TSTM chances may return on Wednesday.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High  88%     High  92%     High  83%     High 100%
KGSP       High  86%     Med   74%     High  82%     High 100%
KAVL       Med   79%     Med   68%     High  82%     High 100%
KHKY       High  86%     High  86%     High  86%     High 100%
KGMU       High  80%     Med   73%     High  83%     High 100%
KAND       High 100%     High  81%     High  86%     High 100%

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




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