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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
1018 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 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 /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
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 /02z Monday through Friday/...
at kclt...still a chance for some lingering MVFR clouds for the
first hour...but once the sun sets and cooling sets in...the low
cloud deck should continue to scatter and dissipate. Broken band of
showers running across northern upstate SC should remain S of the
airfield and should dissipate by 02z. Thereafter... expect
conditions to be VFR most likely for the overnight and morning. That
being said...some of the guidance develops some fog across metropolitan clt
during the pre-dawn hours. Will leave it out for now as dewpt
depressions on the lamp remain fairly high and the surface pressure
condensation deficit on the NAM also remains high. The NE wind this
evening is expected to back to northerly and then northwest on Monday.
Expect VFR Monday after sunrise with northwest wind and clear sky.

Elsewhere...a few showers to deal with across kgmu/kgsp at issuance
time. Will include this as a vcsh as intensity is not expected to
impact flight category. The only location with a restriction will be
khky which still has lingering MVFR cloud ceiling. The low clouds
should scatter out by 02z at the latest as heating is lost. Expect
VFR conditions after 02z at the latest. Similar to kclt...khky has a
chance at seeing some fog in the pre-dawn hours...but perhaps more
likely than kclt. The fog should dissipate quickly after sunrise.
Winds will come around to north...and then northwest by morning. All locations
should be mostly clear with a light northwest wind on Monday.

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

Confidence table...

02-08z 08-14z 14-20z 20-00z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 83% high 98% high 100% high 100%
khky high 90% 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
short term...jdl
long term...cdg

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