Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
1037 PM EDT Friday Apr 18 2014

Synopsis...
a moist low pressure system will move slowly across southern Georgia
and off the South Carolina coast through Sunday. High pressure will
briefly build in Monday...before a cold front crosses east on
Tuesday. A broad and dry area of high pressure will then build south
Wednesday and linger through the end of next week.

&&

Near term /through Saturday/...
as of 1030 PM EDT Friday...forecast remains in good shape overall
for this update. Did opt to tweak hourly temperatures overnight as
latest trends continue to undercut forecast trends by a degree or two on
average. Also made minor adjustment to probability of precipitation through the evening
which consisted of increasing to solid categorical levels across the
i40 corridor in NC. Otherwise left the remainder of the forecast as
is.

Previous discussion...

As of 230 PM EDT Friday...based on the latest buoy reports...it
appears the center of the surface low over the Gulf is a couple hundred
miles south of the western most Florida Panhandle. An expansive area of
rain has spread across most of Georgia and western SC. Dry air associated
with a dry wedge of surface high pressure has kept precipitation at Bay over most
of the forecast area...though rain has recently begun to fall over the southern
upstate. The latest NAM has gone from being the driest model over
the forecast area to the wettest...with a 3 to 4 inch quantitative precipitation forecast bullseye
over the western upstate and NE Georgia. The GFS continues to be much more
consistent with the quantitative precipitation forecast and general mass fields associated with the
low. It/S current low level pot vorticity maximum lines up well where the heaviest
precipitation is falling...and local vwp show that Ely winds have increased
to around 40-45 kts over southern Georgia.

The 12 UTC 800 mb zonal winds on the GFS increase to 4 to 5 Standard
deviations above normal by 0600 UTC tonight. This should result is
strong moisture transport and upglide across the low level warm front.
The GFS ensemble quantitative precipitation forecast averages around 2 inches over the upstate and
southern NC Piedmont and this seems like a reasonable amount for the
event. Interestingly...the Canadian has a 3 inch quantitative precipitation forecast maximum near
Charlotte for the event. That/S to say...some of the models really
go to town with the deformation zone precipitation...but I feel much more
confident staying closer to an ensemble blend. My rainfall amounts
are not high enough to warrant a Flood Watch...but some urban
flooding is certainly possible for the event. For example...the GFS
800 mb zonal wind anomaly remains at or above 3 Standard deviations into Sunday.
So while the model dozen/T have much quantitative precipitation forecast beyond middle-day Saturday...I
would not be surprised to see a narrow corridor of rain last
considerably longer over some part of the forecast area.

Went much lower than the NAM maximum temperatures over parts of the forecast area
tmrw...more in line with the GFS and what one would expect with
clouds and easterly flow.

&&

Short term /Saturday night through Monday/...
as of 200 PM Friday...stacked surface and upper low will depart the southern
Atlantic coast Saturday evening...slowly progressing east and further
offshore through Sunday. While a fair amount of low level moisture will
remain over the area Saturday night...the best forcing will have
moved east with the system. However northeasterly 925-850mb winds will still
be strong...bringing some very weak lift from warm advection but
perhaps more significant upslope forcing into the Blue Ridge. Various
models continue to produce light quantitative precipitation forecast overnight. Thus probability of precipitation and cloud
cover are slow to trail off...lasting into the morning hours Sunday. Min
temperatures Sun morning will be near normal.

Thicknesses rebound somewhat through the day as short upper ridge starts
to build in the wake of the low. However East Coast high pressure
will maintain northeasterly winds and maximum temperatures a category or two below
normal. The upper ridge builds overhead Sunday night though it is
soon impinged upon by height falls over the Mississippi Valley
Monday. Between that trough and the Atlantic low the degree of return
flow is limited but some moisture does arrive Sunday night in the
mountains...enough to bring back cloud cover. Some guidance develops quantitative precipitation forecast
response Monday...along with some diurnal instability...though ascent
will be hindered by poor low level lapse rates under the ridge. Have
kept precipitation chances below mentionable values Monday. Maximum temperatures return
to slightly above climatology.

&&

Long term /Monday night through Friday/...
as of 215 PM Friday...there is fairly good agreeement in the medium
range amongst the global models in regard to the upper air pattern
and resultant surface features. An upper ridge over the region
initially will translate eastward Monday night with a short wave
trough traversing the area on Tuesday along with a surface cold front.
Model soundings do produce some SBCAPE on the order of 500j/kg Tuesday
afternoon so will continue to mention thunder. Wind fields do not
look particularily strong...so severe threat looks minimal at this
juncture. Another upper ridge is forecast to build over the area by
late next week. In the mean time...surface high pressure will
migrate from the upper Midwest on Wednesday...to off the Carolina coast by
late Thursday. After Tuesday...dry and mild weather is expected into
Friday...with just a low end chance for some scattered mountain showers
Friday afternoon with increasing upslope flow.

&&

Aviation /03z Saturday through Wednesday/...
at kclt...a mixture of MVFR/IFR conditions will highlight this taf
cycle. MVFR conditions will prevail for the first few hours of the
taf cycle due to visb/ceiling restrictions as lowered cloud deck advects
in from the midlands of SC in association with enhanced precipitation
shield. Expecting further deterioration to IFR levels after
midnight as moisture flux maximizes due to surface cyclone passing
by to the south. Likewise...enhanced pressure gradient forces
between the approaching surface high to the north...and the above
mentioned surface low will yield increasing northeasterly winds with
sustained flow in the 10-15kts range and gusts nearing 26kts. These
winds will likely persist through much of this taf period until the
surface cyclone ejects to the east allowing the pressure gradient to
relax. Held onto mention of weather through 18z Saturday with ceilings
recovering to MVFR levels a few hours before that. Ended taf with
MVFR ceilings and a prob30 for -ra as the system ejects out of the
region late in the period.

SC sites...much the same as kclt above with slight adjustments to
timing with MVFR/IFR conditions highlighting the taf cycle. Winds
will increase as pressure gradient forces ramp up when the surface
low passes to the south by early/middle morning. Sustained
northeasterly flow in the 12-15kts range with gusts nearing 28kts is
expected. All weather mention is removed as the system pulls away
Saturday afternoon around the 18-19z timeframe.

Kavl and khky...flight conditions will remain above IFR levels at
these locations as low level moisture flux is not as significant due
to proximity to parent surface low. However...did keep mention of
MVFR ceilings and -ra/rain with br ahead of increasing surface winds.
Winds will remain northerly with no significant gusting expected
through the period...however cannot rule out an occasional gust.

Outlook...ceiling and visibility restrictions will linger well into
Sat as a southeast coastal low pressure system pulls slowly away. Dry high
pressure will return Sunday into Monday...with a fairly wet frontal
system expected on Tuesday.

Confidence table...

02-08z 08-14z 14-20z 20-02z
kclt high 80% medium 65% high 91% high 96%
kgsp medium 66% low 58% high 85% high 100%
kavl high 88% medium 68% high 81% high 92%
khky high 87% medium 69% medium 78% medium 79%
kgmu medium 64% low 57% high 81% high 90%
kand medium 74% low 56% high 82% 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)

Www.Weather.Gov/gsp/aviation_tables

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.

&&

$$
Synopsis...sbk
near term...cdg/McAvoy
short term...Wimberley
long term...lg
aviation...cdg