Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180208

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1008 PM EDT Mon Apr 17 2017

A weak cold front will push into the area from the north tonight,
enhancing the possibility of showers and thunderstorms. This front
is expected to drop just south of the forecast area Tuesday and then
return north as a warm front on Wednesday. This will keep unsettled
weather in the forecast through midweek. Another storm system will
impact the region next weekend.


As of 1000 PM EDT: One area of showers and TSTMS continues to sag
southeastward across the Upstate, while another area of convection
approaches from the west. The upstream activity is generally
weakening, with only a few hundred J/kg of CAPE at most to work
with. The CAMs that showers will continue into the overnight, but
should decrease in coverage. PoPs were tweaked to line up with these

Otherwise, an area of high pressure ridge down the Eastern Seaboard
overnight, pushing the cold front thru the southern edge of the
CWFA. Cold air damming will develop, with easterly upslope flow and
upglide overnight allowing precip to expand again in the early
morning (either as light showers or drizzle). Isolated TSTMS remain
possible thru roughly midnight over the fcst area, but otherwise
profiles don`t look supportive of elevated convection. Overnight,
min temps should be about 10 degrees above normal with high temps
tomorrow about a category below normal where the CAD is in place.


As of 245 PM EDT Monday...a kind of tricky forecast through the
middle part of the week because of what should be an ongoing cold
air damming wedge in place Tuesday night and into Wednesday. The
guidance shows plenty to maintain the wedge, with low level
isentropic lift and warm advection over the cold dome, along with
easterly upslope flow to maintain precip production centered on the
Blue Ridge overnight and early Wednesday. A high precip probability
is warranted, so chances have been raised into the categorical range
on the Blue Ridge and likely in most other places. The isentropic
lift gradually weakens early in the day and the warm advection
late in the day, so precip chances go down, however it is entirely
likely that patches of light rain/drizzle will be more widespread
and persistent than indicated. It is the high temps that are really
problematic with a high bust potential. Have nudged downward a
degree or two so we range from the mid-60s in the I-40 corridor to
the mid-70s across the Lakelands. However, it is at least as likely
that temps won`t get out of the 50s in the usual damming locations.

The weakening trend of the forcing continues into Wednesday night
while the remaining wedge cool-pool drains away, so in spite of the
model guidance showing some response, I think we will see a period
of inactivity, thus the dry forecast by early Thursday morning. We
are left with a nearly zonal upper flow pattern for Thursday and
into Thursday night, with little in the way of forcing. Although,
we should retain enough low level moisture to fuel the development
of convection with some diurnal heating. This suggests accepting
the model portrayal of what looks like a typical diurnal convective
precip scenario, thus the chance over the higher terrain and slight
chance to the east. Temps will be warmer on Thursday...well above
normal. A front should approach from the west early Friday morning,
but will most likely remain to our west through sunrise.


As of 240 PM EDT Monday: Guidance remains in general agreement
through the period. An upper low moves from the Great Lakes on
Friday to new England on Saturday as short wave ridging builds over
our area in its wake. A cold front associated with the upper low
moves across the area Friday with high pressure building in from the
north Friday night. There will be enough moisture, forcing, and
instability for scattered diurnal convection to develop Friday.
Precip returns for Saturday as the surface high quickly translates
east allowing a moist southerly flow to develop. Temps around 10
degrees above normal Friday drop to near or slightly above normal
for Saturday with a cooler air mass and clouds.

A stronger upper low then moves into or across the area Sunday and
Monday. The GFS keeps the low more open and faster with a deep trof
developing on Sunday. The axis of this trough reaches the spine of
the Appalachians by the end of the day then quickly moves east of
the area Monday. The ECMWF keeps the low closed and slower bringing
it to the Middle TN/KY border by the end of the day Sunday then
slowly moving it across the Carolinas Monday. This, of course,
creates differences in the surface pattern. The GFS spreads precip
across the area on Sunday with a hybrid damming pattern as the
surface low takes on a Miller B type configuration. The cold front
and lows move east of the area Sunday night ending precip with a
relatively dry Monday. The ECMWF has a stronger low to our west with
a strong cold front. Precip still spreads over our area Sunday but
with some in situ damming. A triple point low forms along the front
and the developing TMB and generally moves along the I-85 corridor
Sunday and Sunday night. The complex low and frontal systems only
slowly move east of the area Monday. Of course, these differences
could have a profound effect on the amount of precip and or any
significant thunderstorm development. Therefore, have gone with a
blend for the forecast which highlights the agreements. PoP
increases to likely through Sunday then diminishes on Monday. Do
show thunderstorms for a portion of the area as well. Temps remain
nearly steady a little below normal both days.


At KCLT and elsewhere: An area of showers with a couple of isolated
TSTMS continue to cross the NC mountains and into the Upstate.
Instability looks limited, but highest in the Upstate. Using the
HRRR as the preferred guidance with respect to convection, I will go
with -SHRA or VCSH at the NC TAF sites, and VCTS for the first 1-2
hours in the Upstate TAFs this evening. There should then be a lull
in precip late tonight thru at least mid-morning Tuesday. Meanwhile,
a backdoor cold front will push thru the area late this evening,
bringing winds around out of the NE, except switching to SE at KAVL,
with gusts expected during the day. Low clouds should begin to
develop as cold air damming (CAD) sets up. Guidance is in good
agreement on MVFR to IFR cigs by daybreak. If the wedge deepens,
cigs may improve, especially across the NC piedmont. But another
round of showers is expected to develop atop the wedge, and may help
lock in or lower cigs further during the day. Deeper convection
should stay south and west of all the TAF sites, due to the stable
air within the wedge.

Outlook: CAD should persist Tuesday night and most of Wednesday as
well.  Low clouds and showers are expected to be widespread under
the CAD. The wedge should erode by early Thursday with a return to
more typical diurnal thunderstorm potential for Thursday and Friday.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High  87%     Low   41%     Med   76%
KGSP       High 100%     Med   79%     Low   51%     Med   77%
KAVL       High 100%     High  91%     Med   70%     High  83%
KHKY       High 100%     High  86%     Low   55%     High  81%
KGMU       High 100%     High  86%     Low   51%     High  81%
KAND       High 100%     High  94%     Med   65%     High  97%

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:




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