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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
129 am EDT Monday Apr 27 2015

cool and dry high pressure ridges down from near the Great
Lakes and be the main influence in our weather through most
of Tuesday. Another area of low pressure will move east from
Texas in middle week giving our region a cool rain. A drying
trend returns for the weekend.


Near term /through today/...
as of 130 significant changes to the near term forecast at
this time. Biggest adjustment was to increase cloud cover across the
area over the next couple of hours...but continued trend of
dissipating cloud cover through the morning.

As of 930 PM...a boundary continues to work its way south and west
across the upstate of South Carolina and NE Georgia. This will
require some re-shuffling of the temperature/dewpt grids to account for
places where the front was supposed to have passed already. Widely
scattered showers near the boundary should pass south of the clt
metropolitan area and should dissipate during the late evening.

Previous discussion...

At 230 PM upper low was located near Nova Scotia...with
broad troughing to its south over the middle Atlantic coast...and flat
upper flow farther south over the Gulf states. A stationary front
marking the perimeter of a cold air damming wedge extended from the
Interstate 26 corridor in SC to the NC Blue Ridge. Heating and
instability this afternoon will be concentrated south of the
boundary in upstate SC and NE Georgia...with the convective window rather
brief...either side of sunset. Afternoon temperatures will exhibit a
marked difference north and south of The Wedge.

The models suggest a decrease in cloud cover The
Wedge front eventually moves south and out of our area. Northerly
downslope flow and falling dew points behind the front should keep
fog from forming overnight. Monday features dry conditions. Below
normal temperatures are expected tonight and Monday in cold
advection behind the front.


Short term /tonight through Wednesday night/...
as of 155 PM Sunday...model agreement has improved drastically over
the past 24 hours regarding southern stream upper level low pressure
wobbling out of the southwest Continental U.S. Early in the period...and its
subsequent interaction and eventual phasing with the northern
stream. There are still somewhat small disagreements regarding the
timing and placement of key synoptic features during mid-week... but
a strong consensus is evolving that cyclogenesis will be underway
near the mouth of the miss river Tuesday morning...with resultant
surface cyclone gradually deepening as it moves east/NE across the
southeast coastal states before pushing off the Carolina coast by
Thursday morning.

The upshot of this pattern will be increasing frontogenetical
forcing and solid rain chances across our area beginning Tuesday night
and continuing through Wednesday before tapering off Wednesday night. Temperatures will
remain below climatology through the period...and in fact will be well
below climatology on widespread clouds and possibly rain
persist through much of the day.


Long term /Thursday through Sunday/...
as of 200 PM EDT Sunday...the medium range forecast period initializes
Thursday morning with global models continuing to indicate departing
surface cyclone sliding up the Carolina coastline beneath a
positively tilted...nearly full latitude phased trough. Both the
European model (ecmwf)/GFS indicate a reinforcing shortwave rounding the base of the
trough axis at forecast initialization that quickly phases with the
upstream wave supporting the above mentioned atl coastal low by
early afternoon. The details on evolution of this system remain
unclear...albeit slightly better than previous runs. Leaned heavily
on HPC guide which relied on the GFS/gefs/ECMWF ensemble means which
lend a solution similar to that of previous European model (ecmwf) runs.
Thus...expecting the coastal low to eject east over the Outer Banks
into Thursday evening while reinforcing 500 mb shortwave dives sharply
south from the Great Lakes. Modest northwest flow associated with said
wave will warrant sustained probability of precipitation through Thursday night with the
highest of which residing along the upslope favored regions of the
NC high terrain.

Further west an upper ridge will build over the central
Continental U.S....eventually sliding east reinforcing high pressure at the surface
over the southeast United States through the day on Friday.
Therefore the forecast dries out for the weekend with mostly sunny skies
and moderating diurnal temperatures prevailing. Expect temperatures
to return to...or exceed normal levels by Saturday into Sunday. The
surface ridge center is prognosticated to shift east on Sunday night
allowing for modest return flow over the region on Monday. In
response...dewpoints will rise leading to increasing diurnal
instability as the warm sector rebuilds. However...expecting deep
layer ridging to prevail thus leading to a dry forecast.


Aviation /06z Monday through Friday/...
at kclt...expect VFR through the period. Clouds at taf time
range from scattered to broken 6-8kft and these will clear out through the
morning as the front continues pushing south. Cannot rule out some
brief few-scattered 5-6kft cumulus this afternoon. Otherwise just high clouds
through the period. Only caveat to prevailing VFR conditions remains
that some guidance...namely the RUC...does still want to develop IFR
to LIFR fog across the NC Piedmont this morning including at kclt.
Based on dewpoint depressions and condensation pressure deficit...
not inclined to believe this outlier solution but cannot rule out
some patchy MVFR fog. Northwest winds generally through the period but
picking up to between 5-10kt this afternoon...with potential for a
brief very low-end gusts.

Elsewhere...very similar trend to clt. Some MVFR ceilings still hanging
out across the upstate...but ceilings should lift and begin to scatter
in the next couple of hours. Generally north-northwest to briefly north-northeast winds
everywhere between 5-10kt this afternoon...but will probably see
slightly stronger winds at kavl...10kt or just above with gusting
potential to 20kt or so. Otherwise just high clouds through the

Outlook...dry high pressure will continue through Tuesday. Another
low pressure system will track across the deep south Wednesday and
Thursday...with ceiling and visibility restrictions possible.

Confidence table...

05-11z 11-17z 17-23z 23-00z
kclt high 100% high 100% high 100% high 100%
kgsp high 83% high 100% high 100% high 100%
kavl high 91% high 94% 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: (must be lower case)



Gsp watches/warnings/advisories...


near term...jat/PM/tdp
short term...jdl
long term...cdg

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