Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
236 PM EDT Monday Apr 27 2015
a large cool and dry air mass will dominate the weather across the
southern appalachian region through Tuesday. A well organized low
pressure system will lift across the deep south...tracking along the
Atlantic coast during the middle week. Another Canadian air mass will
settle back over the region during the late week and will remain
into the weekend.
Near term /through Tuesday/...
as of 230 PM...a short wave rotates across the area tonight in the
cyclonic flow around an upper low moving east away from the New
England coast. At the surface...high pressure noses in from the northwest
overnight. Expect the cumulus outside of the mountains to diminish through this
evening...but will linger a little longer across the mountains until the
northwest flow abates after midnight. There will also be quite a bit of
cirrus overnight as upper level moisture moves in with a jet streak.
This cirrus will help keep temperatures from bottoming out across the area.
However...temperatures will still fall enough for frost to develop across the
NC mountains the frost should be widespread enough for a frost advisory
along the Tennessee border counties from Swain north...including the northern
Blue Ridge portions of the foothills. Expect patch frost across the
rest of the NC mountains...and possibly normally colder locations of the
I-40 corridor. Lows will be around 5 degrees below normal.
The flow becomes more westerly Tuesday as the New England upper low moves
farther off shore and a Southern Plains upper low moves into the
mid-south. At the surface...high pressure continues to build into the
area as a low pressure system moves along the central Gulf Coast.
Clouds will increase through the day ahead of this system...but all
guidance has backed off on the speed of the precipitation with the low.
Therefore...have slowed precipitation onset as well until after the Tuesday
period. Although middle and upper level clouds increase through the day...
there may be little in the way of low clouds as low level moisture
is limited and isentropic lift is delayed even with low level flow
turning southeasterly. The clouds and lower thicknesses will limit heating
and keep highs 5 to 10 degrees below normal.
Short term /Tuesday night through Thursday/...
of 200 PM Monday...main focus for the short term is the southern
stream upper low moving eastward across the deep south. An attendant
surface cyclone is forecast to be just east of the Mississippi Delta
Region at 00z Wednesday and is forecast to track eastward to the SC coast
by 00z Thursday and then turn northward up the East Coast on Thursday in
response to another upper low digging rapidly southward through the
All this portends several forcing mechanisms affecting the western
Carolinas and NE Georgia beginning Tuesday night. Upper divergence combined with
isentropic upglide and upslope flow in combination with accompanying
deep moisture will result in a rapid increase in rainfall coverage
by early Wednesday. The transitory nature of the weather system and
lack of convective elements should preclude excessive rainfall
totals that would present Hydro issues. However...a soaking rain is
expected with rainfall totals generally around 1 inch or so. The
southern track of the low will keep the area under a cool NE flow at
the surface. Therefore...temperatures on Wednesday will show little diurnal change.
Will lean toward the rawblend guidance for maximum temperatures which will tamp
down highs into generally the low to middle 50s across the region.
The bulk of the precipitation will move east of the area Wednesday night as the
surface low tracks off the Carolina coast. Our attention then turns to
an anomalous upper low plunging quickly southeast from the Great Lakes. The
upper low will reach the western Carolinas and NE Georgia late Thursday
and it will be accompanied by a surface cold front. The 12z NAM and
GFS respond with showers Thursday afternoon as the upper forcing and
cold front arrive. They also develop a few hundred joules of
cape...so some rumbles of thunder may occur. With freezing levels
falling to around 5000 feet or so in the mountains some graupel may
occur with some of the convective cells. Temperatures will remain below
climatology on Thursday with 50s in the mountains and 60s elsewhere.
Long term /Thursday night through Monday/...
as of 200 PM EDT Monday...the medium range forecast period initializes
Thursday evening amidst a departing surface low over the western
atl...while a secondary upper shortwave dives south from the Great
Lakes region. Not much change from previous forecast regarding said
upper wave as models continue to warrant low end chance to slight
chance probability of precipitation over the forecast area through Friday morning with the
highest probability of precipitation residing over the NC high terrain where northwest flow
upsloping will aid shower development. The forecast dries out through
Friday...then remains dry through the weekend as deep layer ridging
prevails. That said...models do hint at low chances for precipitation on
Sunday afternoon across the NC mountains with maximum heating...however kept
any mention out of forecast as upper ridge is expected to provide ample
subsidence. On Monday said ridge begins to flatten as a more potent
upper wave develops over the central/northern plains. Thus did
introduce slight chance probability of precipitation across the NC mountains and northern
foothills as any capping could be weakened amidst diurnal induced
instability. Temperatures through the period will moderate to above
Aviation /19z Monday through Saturday/...
at kclt...expect VFR conditions through the period. Scattered VFR cumulus this
afternoon will become few this evening with cirrus becoming broken. Broken
cirrus and few cumulus will continue overnight and through the morning Tuesday.
North-northwest wind this afternoon becomes light northerly this evening...then north-northeast
with an increase in speed Tuesday morning.
Elsewhere...very similar trend to clt. Main exception is the gusty
northerly wind at kavl this afternoon which remains northerly through the period...
and westerly wind at kand this afternoon which becomes northerly this
evening...northeasterly overnight and east-northeast Tuesday morning.
Outlook...dry high pressure will continue Tuesday afternoon. A low
pressure system will track across the deep south Wednesday and
Thursday then up the Atlantic coast Friday...with ceiling and visibility
18-24z 00-06z 06-12z 12-18z
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 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)
NC...frost advisory from 2 am to 9 am EDT Tuesday for ncz033-