Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
139 am EDT Thursday may 28 2015
a Bermuda high pattern will remain across the region into the
weekend before a cold front pushes in from the north late Sunday.
This front will then weaken and become stationary over the area
through early next week.
Near term /through today/...
as of 135 am...convection to the west is dissipating and taking on a
significant northerly trajectory. Therefore...do not expect any
significant rain showers overnight...but a sprinkle or two is possible over
the western County warning forecast area. Should see Mountain Valley fog with patchy fog and low
clouds elsewhere. Otherwise...going forecast on track with updates mainly
for current conditions.
1015 PM update...the convection which had set up over the mountains along
some sort of mesoscale boundary has rapidly diminished over the past
30 mins as instability was exhausted. Isolated activity along outflow
boundary over the csra has struggled to move northward into our srnmost
zones. While these areas should continue to see coverage diminish
over the next couple hours...weakening mesoscale convective system with leading edge now over
the northern Alabama/Georgia line is timed to arrive in the SW NC/NE Georgia mountains around
1230-100 am. At least some light-moderate stratiform precipitation perhaps
with a couple rumbles of thunder could make it into those
areas...but it will be increasingly less potent as it works northeastward
particularly into the areas of the NC mountains which got soaked this
evening. Revised probability of precipitation to handle the arrival of the remnant activity
into the western half of the County warning forecast area late tonight.
Previous discussion...a short wave trough will approach the area
from the tenn valley this evening. There is a considerable amount of
convection ahead of this feature across the miss valley this
afternoon. Much of this activity will push across the tenn valley
into the evening hours...but mesoscale and short term guidance are
in general agreement that this activity will tend to lose steam as
it approaches our area tonight in the weakly sheared/stabilizing
environment. Thus...only token slight/low chance probability of precipitation will be
retained through the overnight.
By Thursday afternoon...precipitable waters will return to above climatology levels...as
deeper moisture pushes into the southern Appalachians and
surrounding areas in association with dampening short wave trough.
On the other hand...instability may end up being limited by
considerable middle-level cloud cover tomorrow afternoon.
Additionally...the deep layer flow will be extremely weak. In
fact...2-8km mean winds are less than 5 kts in the latest NAM
forecast soundings. Therefore...any cells that do develop tomorrow
afternoon will not cover much area. We will continue to advertise
higher-than-climo probability of precipitation in light of the high precipitable waters ...but convection
is not expected to be especially robust in the weak lapse rate/
modestly unstable regime. Localized heavy rainfall will be of some
concern in light of the high moisture content and weak steering
Short term /tonight through Saturday/...
at 200 PM EDT Wednesday...on Friday morning an upper ridge will be
along the eastern Seaboard...while a trough will be over The
Rockies. The eastern ridge begins to break down on Saturday.
Meanwhile...the deamplifying upper trough progresses to the Southern
Plains...reaching the lower MS River Valley on Saturday night.
At the surface...Atlantic ridging will over the Carolinas and Georgia
will be reinforced by high pressure centered over eastern Canada.
Low level moisture will remain banked up against the southern
Appalachians...with only weak isentropic upglide and upslope flow
into the Blue Ridge. The surface high moves offshore on Saturday...
but the ridge maintains a foothold on Georgia and the Carolinas. Limited
instability will be present both days...chiefly over the mountains
on the western periphery of the upper ridge...but shear is expected
to be relatively tame. Temperatures will exhibit a reduced diurnal
range due to clouds and moisture.
Long term /Saturday night through Wednesday/...
as of 230 PM Wednesday...models remain in good agreement with a
broad upper trough crossing the northeastern Continental U.S. And eroding the
upper ridge over the eastern Seaboard on Sunday. A broad surface
high will build into the Midwest...slowly pushing a cold front
towards the southeast late Sunday and into Monday. The boundary will
then stall out over the Appalachians and weaken slightly early in
the work week.
As the frontal boundary approaches Sunday and Monday...probability of precipitation increase
once again...with the GFS driving the boundary in slightly faster
than the European model (ecmwf). While not particularly impressive at this
time...weak shear and instability look to be present Monday through
the end of the extended period...suggesting storms with some
organization are possible with a diurnal peak. Confidence is lower
Tuesday-Wednesday...but models suggest the boundary will be slow to
clear the area...so probability of precipitation will remain above climatology through the end of
the period. Moist south/southwesterly flow will will keep highs and
lows well above climatology on Sunday...with cloud cover dropping highs to
near normal and lows remaining above average for the work week.
Aviation /06z Thursday through Monday/...
at kclt...guidance is homing in on low clouds and not fog creating
the MVFR restrictions early this morning. Have gone with MVFR by 10z
then lifting to low VFR by 14z and higher VFR by 17z. Prob30
continues with diurnal convection expected. Light southerly wind becomes
calm this morning then back to southerly after daybreak.
Elsewhere...convection to the west dissipating rapidly and taking on
a northward trajectory. Do not expect any precipitation this morning. MVFR
clouds have developed at kavl...and expect conditions to deteriorate
overnight given the evening rain and forecast soundings. Still a
question of how low will conditions fall...but IFR seems likely.
Have limited it to 1sm and ovc005 for now...but LIFR or even vlifr
possible. Elsewhere...forecast soundings look like fog so have gone
with MVFR for now...but IFR possible. Better chance at kand...so have
tempo IFR near daybreak. Should be a relatively quick return to VFR
after daybreak for all but kavl where MVFR ceilings linger through the
morning. Prob30 continues with diurnal convection expected.
Generally calm winds this morning become southerly or southwesterly through the day.
Kavl the exception where calm wind becomes northerly after daybreak then
southerly for the afternoon.
Outlook...moist SW flow will continue to affect the terminal
forecast area through the upcoming weekend...as a cold front lays
over to the north of the area. Expect pockets of morning fog and/or
stratus most days...with continued chances of afternoon/evening rain showers/thunderstorms and rain
05-11z 11-17z 17-23z 23-00z
kclt high 100% medium 78% high 100% high 100%
kgsp high 100% medium 61% high 100% high 100%
kavl high 100% high 94% high 100% high 100%
khky high 100% high 83% high 100% high 100%
kgmu high 100% medium 61% high 100% high 100%
kand high 100% high 94% 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)