Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
142 PM EDT Wednesday Apr 1 2015

Synopsis...
surface ridging will prevail on Wednesday ahead of increasing
precipitation chances on Thursday as moist southerly flow returns.
Another cold front will push through northeast Georgia and the
western Carolinas on Friday evening ahead of drier high pressure for
the weekend. Moisture will return for the start of the next work
weak as southerly flow once again commences over the region
therefore increasing precipitation chances.

&&

Near term /through tonight/...
as of 1205 PM...clear conditions prevail across the western
Carolinas and northeast Georgia as of midday. Temperatures will maximum out a little
above climatology this afternoon mainly over the western portion of the
forecast area. Current grids are in good shape but made some minor
adjustments to winds/dewpoints primarily for fire weather.

As of 915 am...made some adjustments to temperature trends. Otherwise...the
current forecast looks on target.

As of 630 am...after a near miss from the northern extent of the decaying
precipitation associated with the convection moving across Georgia...we
should remain dry this morning. Still a very small chance of a shower
developing over the lakelands/Savannah River basin with the front
drifting S across that region...but it will not be included in the
forecast. Temperatures continue to run warmer than forecast...giving US a bit of a
milder start to the day.

Over the next 24 hours...a progressive upper pattern will see the
upper trough axis move eastward out over the western Atlantic...and an
upper ridge building eastward over the Appalachians/western Carolinas.
Weak high pressure will build in from the middle-Atlantic region behind
the front this morning. High temperatures should be on the order of five
degrees cooler than yesterday with the subtle air mass change. The
upper ridge axis will move east across the region late tonight and
that should push the surface high offshore...setting the stage for
moisture return after midnight. A dampening short wave riding up the
ridge will approach from the west after 06z Thursday and will bring
some Gulf moisture with it...while the flow around the departed
surface high will allow some Atlantic moisture to return as well.
Prefer the slightly less enthusiastic precipitation development of the GFS
near the Blue Ridge west of I-26 around daybreak Thursday...as it seems
the model guidance develops the upslope precipitation a bit too fast. Will
cut down the precipitation chances a bit. Min temperatures should be at or above
normal.

&&

Short term /Thursday through Friday night/...
as of 300 am EDT Wednesday...the short term forecast period initializes
Thursday morning amidst short wave ridging aloft over the southeast
Atlantic coast...while region of height falls approaches from the
west. At the surface...high pressure will be in control of the
pattern across northeast Georgia and the western Carolinas...however
quickly retreating out to sea as a warm front approaches from the
southwest driven by potent southern stream upper wave and associated
surface cyclone over the Central Plains. A northern stream upper wave
will be sliding through the northern Great Lakes with a surface
trough/front stretched to the south along the Mississippi River
valley. Models continue to agree on improving warm air advection regime with
dewpoints surging across the southern Apps through the day on
Thursday. Therefore the forecast features increasing probability of precipitation through
morning across northeast Georgia...southwest NC...and the western upstate
as upslope/upglide forced precipitation prevails early on ahead of
maximum heating/instability. Probability of precipitation will then spread out further east as
model soundings indicate modest/improving lapse rates aloft as the
upper ridge shifts east...with steepening llv lapse rates with
heating and moisture influx. Soundings also indicate some
directional sheer in the profile Thursday afternoon/evening across
these zones with approx 30km bulk shear present...however speed
shear looks to be lacking as no primary low level jet/mlj is present. All
said...would not be surprised at all to see a few deep/organized
thunderstorms with hail/damaging winds being the primary threats as
fz levels remain below 10kft.

Beyond that...southerly upslope flow will veer southwesterly
somewhat and weaken into the overnight hours. This combined with
heating loss warrants lowered probability of precipitation due to lack of any forcing in the
return. On Friday southerly flow will continue...however according
to model guidance...llv dry air from remnant 850 mb ridge over the Gulf
will be entrained in the mean flow thus advecting a region of lower
dewpoints northward. This drier air at the surface on Friday would
be a substantial limiting factor for any convection ahead of the
frontal passage Friday night. Thus...probability of precipitation actually lower to slight chance
levels across the NC/SC Piedmont as soundings look rather
unimpressive. Models continued to favor at least high end chance
probability of precipitation over the high terrain thus that was accepted in the forecast.
Meanwhile...a line of convection just ahead of the front will likely
be marching east through the Tennessee Valley...eventually arriving into
the western NC and northeast Georgia mountains by the 00z timeframe.
Expecting this line to move through the region overnight...therefore
likely probability of precipitation are feature over most of the forecast area with thunder
mentioned. Due to the above mentioned stable airmass over the
southern Apps...would not be surprised to see any strong/severe
convection along this line to weaken as it enters the forecast area. By
periods end Saturday morning...the primary upper trough axis will
swing through thereby motivating the front along or just east of the
Interstate 77 corridor.

&&

Long term /Saturday through Tuesday/...
as of 345 am EDT Wednesday...the medium range forecast period
initializes Saturday morning amidst a departing cold front and
intruding high pressure beneath quasi zonal flow aloft. Mentionable
probability of precipitation will be in the forecast initially...however will lower quickly to
non mentionable levels by middle afternoon. Skies will clear out as
the high builds in thus allowing for abundant sunshine amongst
falling thicknesses. Temperatures on Saturday will be a few degrees
under climatology...with slightly cooler temperatures prevailing on Sunday
where frost conditions are possible over the mountains during the early
morning hours. The surface ridge will migrate east into Monday allowing
for the return of southerly warm air advection beneath continued zonal flow aloft.
Modest baroclinic zone across the arklatex...east into the midsouth
will build ahead of another potent cold front. Increasing moisture
over the southern Apps Monday into Tuesday will yield reintroduction
of chance probability of precipitation by Monday evening as upglide maxima region advects
overhead along the western periphery of the departing surface
ridge. Probability of precipitation will remain at low end chance levels on Tuesday as
improving warm air advection regime yields dewpoints in the upper 50s to lower 60s
thus providing ample moisture for convection. Temperatures on
Monday will return to normal levels...with warmer temperatures forecast
on Tuesday.

&&

Aviation /18z Wednesday through Monday/...
zczc gspwrktaf 011040
ttaa00 kgsp ddhhmm

At kclt...looks like VFR conditions will prevail through the taf
period. However...moisture will increase across the area Thursday
morning as high pressure moves well off the coast and Gulf moisture
advects into the region. Looks like deeper moisture and best
isentropic lift stays west of clt...so only lowering VFR ceilings
are anticipated after 12z. Light NE winds will shift to southeast circa
00z.

Elsewhere...VFR conditions with light winds will hold for the
overnight. Deepening moisture and isentropic lift will develop circa
12z with VFR ceilings lowering to MVFR and even IFR at kavl by
17z. The exception will be khky which should remain VFR. Scattered rain
showers are also expected to be around the airfields after 12z
except at khky.

Outlook...moist southeast upslope flow will continue Thursday night
into Friday morning which will promote MVFR to IFR ceilings.
Conditions will improve to VFR by Friday afternoon. A cold front
will affect the area Friday night into early Saturday...bringing
another round of rain/possible thunderstorms and restrictions. VFR
will return for the rest of the weekend.

Confidence table...

17-23z 23-05z 05-11z 11-12z
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 90%
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: (must be lower case)

Www.Weather.Gov/gsp/aviation_tables

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.

&&

$$
Synopsis...cdg
near term...lg
short term...cdg
long term...cdg
aviation...lg

National Weather Service Glossary of Abbreviations