Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
435 PM EDT sun Apr 26 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 Monday/...
as of 430 PM...surface analysis showed a boundary working its way
south and west across the Piedmont of the Carolinas. 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 continue to develop near the boundary...so the
small chance of showers was kept from the mountains and across mainly
upstate SC. Expect this to fade away with loss of heating.
At 230 PM EDT...an 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 this
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 overnight...as 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 /Monday night 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
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 Wednesday...as 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 /21z Sunday through Friday/...
at kclt...the cold air wedge is holding fast. Guidance does support
ceilings rising to MVFR this afternoon and low VFR this evening...before
completely eroding after midnight. Model visibility improves to VFR even
earlier. NE winds will back to northwest by daybreak. Light showers near
the Blue Ridge are having trouble spreading east...and convective
chances will be best to the south where better heating will occur.
Elsewhere...khky will face much of the same restriction problems
that kclt does..with low clouds sow to lift and erode...but visibility
improving perhaps a bit more quickly. SC sites and kavl should
remain VFR...although kgsp is awfully close to the edge of the low
wedge clouds. SW winds in SC will veer north-northeast as the front moves south
this afternoon...then back northwest by daybreak. Kavl winds remain
channeled up valley from the northwest. Convective chances will be best in
SC...but confidence is too low for a mention in the tafs.
Outlook...dry high pressure will build in through Tuesday. Another
low pressure system may track across the deep south Wednesday and
20-02z 02-08z 08-14z 14-18z
kclt medium 79% high 100% high 91% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl medium 79% high 87% high 98% high 100%
khky high 89% high 84% high 87% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 100% high 100% high 98% 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)