Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
1043 PM EDT Thursday Apr 17 2014

Synopsis...
an unseasonably cold air mass will remain over the region into
Friday. A subtropical low will develop off the central Gulf Coast
Friday and track east off the southeast coast by Sat. This will bring
widespread precipitation to the region Friday night into Saturday.
By early next week...temperatures should range close to normal ahead
of an approaching cold front.

&&

Near term /through Friday/...
as of 1030 PM EDT Thursday...high level cirrus combined with
intruding low level stratus highlight the sky conditions this
evening. Moisture plume associated with Gulf surface low continues
to move eastward spreading low clouds across portions of the deep
south. Per latest observation and satellite difference product...some of
this low stratus has made its way into portions of northeast Georgia and
the western upstate. Latest NAM remains consistent with previous
runs regarding the track of the Gulf low...therefore only changes
made with this update were to increase sky cover across the above
mentioned zones.

Previous discussion...

As of 230 PM EDT Thursday...dry cold air damming has kept it cool
and a little breezy over the forecast area today. Going into
tonight...a blossoming cloud shield associated with cyclogenesis
over the northern gom will overspread the region. This should limit frost
potential to just a few locations across the northern mountains and the northern
foothills. In addition...as pressures fall along the Gulf
Coast...this will strengthen the ageostrophic component of the surface
flow which should result in an increase in mixing late tonight. And
if that/S not enough to limit frost potential...a strato-cumulus deck may
spread in from the southwest late tonight.

There continue to be model differences with the strength and timing
of the low as it moves out of the gom and across western Florida on Friday.
The GFS has a pronounced low level pot vorticity mav that results in a
considerably stronger surface low than the NAM. Considering the
widespread thunderstorm activity that is developing over the gom
at this time just down stream of a southern stream short wave moving across south
Texas...I think the GFS may be on to something. Most of the GFS
ensemble members bring precipitation across the forecast area by late
tomorrow as well. The new HPC quantitative precipitation forecast also follows rather closely to the
GFS precipitation output. All of this lends ME the confidence to increase
probability of precipitation into the likely range across the southern part of the forecast
area by late afternoon.

&&

Short term /Friday night through Saturday night/...
as of midday Thursday...cutoff upper low will cross the deep south late
Friday and track slowly across Georgia/SC through Saturday...finally moving onto
the Atlantic early Sunday. The major global models...I.E. The
GFS...ec and Gem...are basically on the same Page with respect to the surface low
track and evolution. NAM family...including the sref mean and NCEP
hires windows...feature a weaker upper low and a more southern track
which effectively keep US notably drier than the solutions from the
other Camp.

In terms of precipitable water...moisture from the system never gets alarmingly
high as far north as our County warning forecast area even on the operational GFS or naefs
anomaly plots. Convection looks to be limited with the low staying to
our south on all the opnl guidance and almost all ensemble members.
Sref has just one member tracking it far enough north to bring even
modest cape into the Piedmont during the day Saturday. However lapse
rates remain favorable Saturday and Saturday night under the upper
trough...so convective rain rates are not out of the question.

Brisk easterly midlevel flow would likely provide enhancement along
the Blue Ridge. How strong the surface winds get in the higher elevations
will depend on how close the low comes. This bears watching as
solutions converge but at this time it appears gusts would remain
below advisory criteria.

Precipitable waters and deep layer relative humidity diminish through the day Saturday...so rates
should taper off. As the low pulls away Saturday night precipitation and sky
cover will diminish leaving a dry and mostly sunny Sunday.

Maximum temperature forecast is somewhat challenging Saturday. The East Coast
high will have recently departed...but with ongoing precipitation and
another fairly strong high moving into Quebec and New England under
confluent upper height pattern...The Wedge could hold on through
Saturday. Kept temperatures near the lower end of guidance Friday night into
Sat...then allowing a return to near normal temperatures Sunday.

&&

Long term /Sunday through Thursday/...
as of 215 PM EDT Thursday...the global models continue to agree that
an upper low pressure trough will shift offshore Sun night with an
upper ridge of high building across the southeast through Monday. A
positively tilted upper trough will then approach from the central
Continental U.S. Monday night and crosses the region on Tuesday. Another upper ridging
will build in by Wednesday.

At the surface...conditions should dry out by Sun night as a surface
low moves further offshore and a dry high builds in from the north.
The high will weaken and shift offshore by Monday night as a cold front
approaches from the west. Models suggest that the frontal passage will occur
Tuesday afternoon. However...the front will weaken considerably as it
crosses the mountains hence...I/ve inherited the low end chance pop over
the Piedmont with solid chance pop in the mountains also have mentioned
thunder as models continue to highlight some degree of instability
along and ahead of the front. Wednesday/Thursday looks dry as a dry high builds
across the region. Temperatures will be around normal on Monday with a
few degrees above normal Tuesday into Thursday.

&&

Aviation /03z Friday through Tuesday/...
at kclt...VFR conditions will prevail through the majority of the
period with middle level MVFR ceilings possible late afternoon Friday.
High level cirrus will continue through the evening as clouds eject
downstream with the mean upper flow from Gulf convection. Winds
will remain in the 4-7kts range and out of the northeast through the
overnight hours. Low VFR stratus will build in by middle morning as
weak isentropic lift atop the extended high pressure wedge advects
moisture into the region. Decided to hold ceilings at low VFR levels
for this timeframe although a few guidance solutions hinted at MVFR
restricitons. Ceilings will remain at low VFR levels through midday
before lowering as deep layer moisture increases ahead of the
approaching surface cyclone. Prevailed MVFR ceilings at 21z with a
prob30 for -ra br. As mentioned earlier...there is also divergence
in the model solutions regarding onset of precipitation. Went with
the consensus...which was also the previous forecast regarding probability of precipitation
and timing. Winds on Friday will remain at 6-10kts and
northeasterly through the end of the period.

Elsewhere...much the same as kclt above with VFR prevailing through
the first three quarters of the taf cycle at all sites. Due to
proximity of better isentropic upglide...kavl will likely escape any
low VFR ceilings Friday morning while all other sites have prevailing
groups for such. These ceilings will hold through the day before
deteriorating to MVFR levels as deep layer moisture advection
prevails and The Wedge retreats. Added from groups at all sites
except for khky for -ra late in the period as the above mentioned
surface cyclone passes to the south. Winds through the period will
remain in 5-10kts range and northeasterly with the exception of kavl
where winds back to the north northwest as the surface low moves
southeast.

Outlook...ceiling and visibility restrictions will return Friday
afternoon/evening through Saturday as a cold front approaches from
the west and a low pressure system develops along the coast. Fair
and drier weather with no restrictions should return for Sunday and
Monday.

Confidence table...

02-08z 08-14z 14-20z 20-02z
kclt high 100% high 100% high 100% medium 71%
kgsp high 100% high 95% high 100% medium 78%
kavl high 100% high 97% high 100% medium 75%
khky high 100% high 97% high 100% high 100%
kgmu high 100% high 95% high 100% high 80%
kand high 100% high 90% high 94% medium 76%

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...joh
near term...cdg/McAvoy
short term...Wimberley
long term...joh
aviation...cdg