Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
801 PM EDT Wednesday may 27 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 Thursday/...
745 PM update...a few small cells continue to fire in vicinity of the outflow
boundary southeast of Charlotte...but Sat/radar trends suggest this
activity is short-lived. Over the mountains a new round of convection has
fired...likely as dewpoints rebound and local/S lower after the loss of
heating. This activity is not expected to grow too much but might
persist another hour or two. Looking upstream...the line of cells over
the Cumberland Plateau has fizzled as it encountered more stable air
and also moved into an area with weaker shear...as suggested by cam
trends. Mesoscale convective system over Alabama is holding its own...but propagating more north
than east. It too will start to encounter more stable air soon. This
activity however is not well handled by the cam guidance. Hrrr shows
a similar feature losing strength as it pushes northward into Tennessee...but
radar observation indicate it is more organized than what the hrrr thinks.
So in summary...generally lowered probability of precipitation overnight with most guidance
suggesting we will be too stable to support widespread activity.
However in southwesterly flow I retained low chances over the mountains where the
best forcing will be present. Also could see some remnant/weak cells
from the Alabama mesoscale convective system cold pool move into this area later 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 /Thursday night 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 /00z Thursday through Monday/...
at kclt...low VFR cumulus should diminish in coverage this evening. A
nearby outflow boundary S and east of the field could drive a few
showers...but impact would be low...so have just handled this with
vcsh. The loss of the cumulus suggests there will not be enough moisture
to become trapped under subsidence inversion and create a low ceiling
overnight. Most guidance features cirrus ceilings...mainly convective
debris from convection over al/TN/GA...spreading overhead tonight.
This and dry surface dewpoints should preclude fog. Convective chances
are a bit better tomorrow with more available moisture. Low VFR cumulus
will develop middle-morning with a prob30 during the afternoon for thunderstorms and rain.
Winds will be light but favor S to SW through the period.
Elsewhere...kavl is the only site which stands a mentionable chance
of precipitation this evening...handled with vcsh...but SW flow could kick
off some showers or even an isolated thunderstorms and rain over SW NC or adjacent NC/SC
areas through midnight or so. Cirrus debris fill in overhead tonight
and flow is unfavorable for stratus to creep up from the coast...but
we may need to monitor ceilings in the rain soaked areas upstream. Only
MVFR fog is expected tonight and only in areas which remained more
moist today. Low VFR cumulus field will develop by middle-late morning Thursday and
thunderstorms and rain chances are in prob30 range at all sites for most of 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
00-06z 06-12z 12-18z 18-00z
kclt high 100% high 96% high 98% high 100%
kgsp high 100% high 98% high 88% high 100%
kavl high 97% high 99% high 81% high 100%
khky high 100% high 97% high 100% high 100%
kgmu high 100% high 98% high 88% high 100%
kand high 100% high 90% high 85% 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)