Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 251829

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
229 PM EDT Thu May 25 2017

An upper level low pressure system will cross the region today, with
showers persisting over parts of the area as a result. Following
the departure of this system, drier air will return tonight and
linger into early Saturday. Moisture and unstable conditions will
then build over the Southeast over the weekend, ahead of a slow
moving cold front.


As of 1100 AM EDT Thursday: More minor adjustments to precip prob to
acct for convection developing across the area. Could see a few more
thunderstorms with the cold air aloft. Low freezing level and wet
bulb zero suggests that stronger cores will produce small hail.
Temperatures look good for now.

Otherwise, much quieter today in the wake of yesterday`s cold front.
The parent upper low will push up the Ohio Valley today, with a
shortwave extension over the Tennessee Valley progged to damp and be
re-absorbed by the primary low. Of greater note will be the main low
that rides up the Central Appalachians toward the eastern Great
Lakes today. Lee troughing induced just east of the Appalachians
will keep a firm hold on the area today, with a brief but notable
increase in the pressure gradient through the day, leading to some
breezy winds across the area. Should see a redevelopment of showers
across the northern tier as the DPVA moves across, and some low-end
sbCAPE is progged across portions of the northwest Piedmont. While
deep layer shear isn`t impressive (30-40kt), that combined with the
increased lapse rates from the CAA aloft plus the surface
instability may be enough for some scattered thunderstorms as well.
Gusty winds and small hail would be the main concerns. SPC`s Day1
outlook includes a sliver of Davie and Rowan in the Marginal Risk,
so cannot rule out a brief strong to low-end severe storm as well,
but chances seem pretty minimal across western NC (a little better
farther east).

Should be another very nice day with the aforementioned CAA, with
high temperatures today a good 10 degrees or so below seasonal
normals. Another cool night tonight, about 5 degrees below normal,
with clearing skies and pops shrinking to only the extreme northern
mountains before ending by the end of the period.


As of 230 AM Thu: A low-amplitude ridge will remain over our region
Friday and Saturday. Initially westerly low-level flow will back
to southwesterly as a warm front lifts north thru the Tenn Valley
and western Carolinas. This will bring back seasonably warm temps
Friday, that trend further upward for Saturday.

Strong subsidence and relatively low dewpoints will preclude
convection on Friday, but by Saturday conditions will be more
favorable. Thus slight chance to chance PoPs will be advertised
northwest of I-85; a subsidence inversion most likely will keep
a lid on things to the southeast. In fact model SBCAPEs are
quite impressive aside from the capping. Furthermore, the height
gradient aloft will be enough to produce some modest upper winds
and deep shear. So Saturday will be a day where we may struggle
to initiate an updraft, but if that does occur the cells quickly
could become troublesome. SREF probs of DCAPE > 1000 J/kg are
in the "chance" range, with the operational GFS generally in the
500-1000 range. Multicell storms producing robust cold pools and
wind damage are not out of the question.

Small PoPs will be retained in the NW half of the CWFA Saturday
night for the possibility that convective debris may linger and/or
a remnant MCV move in. High clouds will increase ahead of the next
frontal system.


As of 300 AM Thu: Another deep upper low will settle into the Great
Lakes during the medium range period. As heights fall to our west and
northwest, low pressure will track through the Ohio Valley.  Our area
will be in the warm sector of this system Sunday before the cold
front drives southeastward Monday. Lapse rates aloft suggest some
convection may develop ahead of the front, though the degree of
low-level capping is in question, given that the front is displacing
a subtropical ridge. As noted for Saturday, relatively robust CAPE,
DCAPE and shear appear likely to be present Sunday, so where the cap
is unable to hinder convection, severe storms easily could develop.
GFS suggests deeper moisture Monday will reduce instability, but it
is too early to say with certainty that the threat will be lower
then. Max temps will remain a few degrees above normal Sunday,
falling back to near normal Monday.

While it looks like the front will have passed by early Tuesday, the
24/12z EC develops a frontal wave that could allow precip to linger
over the area; this is plausible given the upper pattern. A secondary
cold front will push in late Tuesday or Wednesday, reinforcing
slightly below normal temps, but bringing only mediocre forcing for
precip. Dry conditions return following that front.


At KCLT and elsewhere: VFR through the period. SCT/BKN clouds
continue to develop generally with bases in the 040-050 range as
colder air aloft moves overhead. Think SCT will prevail with
temporary BKN conditions. A few showers may develop, more likely
over the mtns near the TN border, and over the wrn Piedmont of NC.
Have included a TEMPO at KCLT for MVFR in passing showers through
22Z. The clouds should diminish in coverage with loss of heating,
and any remaining showers after sunset should be confined to the TN
border area. Wind will remain WSW with occasional gusts through
sunset, then diminishing overnight as the boundary layer decouples
and weak high pressure ridges up from the S. On Friday, only sct
cirrus. Wind will be SW or WSW again outside the mtns, with
occasional gusts beginning midday as the boundary layer deepens
again with heating.

Outlook: Continued VFR Friday afternoon and night, and into the day
on Saturday. Thunderstorms and associated restrictions may develop
across NC Saturday afternoon, but will be more likely Sunday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     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 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     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:




SHORT TERM...Wimberley
LONG TERM...Wimberley
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