Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1035 PM EDT Tuesday Jul 29 2014
Canadian high pressure will dominate the surface pattern through
midweek...leading to well below normal temperatures more typical of
late Summer or early autumn. As the surface high weakens...moisture
will return for the weekend increasing the chances of widespread
showers and thunderstorms.
Near term /through Wednesday/...
evening update...the forecast is in good shape overnight. Still
anticipate a reduction in cloud cover allowing good radiational
cooling most locales. Winds are remaining a little high at kavl
which may keep their temperature above the forecast/d record low...but a
gradual decreasing wind speed is expected over the next few hours.
Some morning br/fog is expected...but not too dense...across the NC
730 PM EDT update...updated pop/sky grids to account for lingering
Lee wave clouds across the upstate and brief -shra just south of the
upstate Escarpment. Temperatures tweaked as hour/lys running 2-3 degrees
above the forecast curve...mainly non/mtns.
430 PM EDT update...made minor changes to the T/dew point and sky grids.
Rest of forecast is in good shape with cumulus becoming a little more
enhanced along the Escarpment which could support a few brief -shra
over the next couple hours.
As of 200 PM Tuesday...the afternoon forecast still looks on
track...especially with the idea of minimizing the mention of any
showers in the mountains. No echoes yet on radar and the Storm Prediction Center mesoscale
anal Page still shows no SBCAPE. Hence...still think that current
slight chance probability of precipitation over the highest terrain in the mountains will hold
US despite the considerable quantitative precipitation forecast response in the 12z NAM and GFS. Any
isolated showers that do develop should dissipate quickly with the
loss of heating.
In regard to the big picture....the anomalous upper trough will
remain in place over the eastern U.S through Wednesday. At the
surface...cool high pressure will remain to the north of the region.
These ingredients will produce a cool night across the area for late
July. In fact...our forecast is reflecting tying the record low for
the date at avl which is 54 set in 1897. Elevations above 4000 feet or
so will see lows in the 40s. Outside the mountains...lows will be
mainly in the 60-65 degree range...except for some 50s in the NC
foothills/northern NC Piedmont. No records outside the mountains
appear to be in play tonight. The record low is 58 at clt/63
forecast and 60 at gsp/62 forecast.
Wednesday should be a similar day to Tuesday with partly to mostly sunny
conditions. Forecast soundings are essentially capped so the chance
of afternoon convection looks to be even lower on Wednesday. Will only
increase probability of precipitation to slight chance over the SW mountains in the
afternoon where models show some light quantitative precipitation forecast response. Maximum temperatures will
continue to be a couple categories below climatology.
Short term /Wednesday night through Friday/...
as of 145 PM Tuesday...the upper trough over the eastern part of North
America will remain the dominant weather feature through the short
range...but its evolution will have important consequences. The axis
of the upper trough should be located over the middle-Atlantic and
Carolinas on Wednesday night...but this will shift to the MS
river/Great Lakes on Thursday in response to new short waves digging
down into the trough. A weak 850mb low will also move west with the
upper trough...and in doing so...the low level flow will come around
to southeast for Thursday and Friday. Although the flow will be light and
moisture transport will be weak...there should be enough moisture
return to fuel an increase in shower coverage both days. Precipitation
probability climbs up to the chance range mainly over the mountains and
foothills for Thursday...and then solidly in the chance range for
Friday. Bouyancy will also increase...so thunderstorms are possible
again Friday afternoon. Not ready to jump into the likely range for
Friday yet. The position and timing of a vorticity center moving through
the region on the GFS is viewed with a suspicious eye at this time.
Temperatures will remain below normal.
Long term /Friday night through Tuesday/...
as of 130 PM EDT Tuesday...the medium range forecast picks up at 00z on
Saturday with the persistent upper trough axis over the Great Lakes
and miss River Valley and upper ridging over the western Continental U.S.. the
long range models suggest that the trough will begin to deamplify on
Sunday and continue to lift through the rest of the period. This would
result in a split flow pattern over much of the southeast region sandwiched
between upper ridging to the west and the Bermuda high to the east.
At the surface...deeper layer moisture is expected to be spread over the
County warning forecast area by the start of the period and remain over the region through day
7. By Sat evening...the bndy layer flow is expected to become more
southeasterly and remain southerly to southeasterly for the remainder of the forecast period.
Precipitation chances will remain good Thursday the weekend with probability of precipitation tapering
off some for days 6 and 7 as some drier air could work its way in
from the northwest. Temperatures should remain well below normal for early August
with values expected to warm a few degrees by the end of the period.
Aviation /03z Wednesday through Sunday/...
at kclt...very weak flow will continue through the taf period with
generally a north/ly direction. The low level flow by the way h95-850 mb does back during the
day however...so will expect a more northwest/ly trend in the afternoon.
VFR cumulus field will likely develop around noon with coverage few to
perhaps scattered by late afternoon.
Elsewhere...with good decoupling and surface cooling...kavl could see
some MVFR visibility around daybreak...burning off quickly after 12z. A
weakening p/gradient is in store...so mainly north/ly winds will become
occasionally vrb/calm in the afternoon with more of a northwest/ly direction
overall. A VFR cumulus field will be few to scattered.
Outlook...high pressure will prevail into Thursday morning with the only
possible restrictions being early morning fog/stratus in the
mountain valleys. Moisture will be increasing during Thursday
into Friday which will yield unsettled weather with enhanced
precipitation/fog/stratus chances through the weekend.
02-08z 08-14z 14-20z 20-02z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 97% high 89% 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)