Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
114 am EST Friday Mar 7 2014
a coastal low will strengthen and move from Florida up the Atlantic
coast tonight and Friday. Drier and warmer high pressure is then
expected to return to our region this weekend and persist through
early next week.
Near term /through today/...
as of 110 am...tricky forecast continues as surface temperatures and wet bulbs
have warmed across much of the area. Still snowing across the northern
mountains and balsams with little snow elsehwere. Mainly rain is falling
outside of the mountains...except for the northwest Piedmont where temperatures and wet
bulbs are hovering around freezing. Freezing rain could be more
significant across that area. Snow levels from the Hankins National oceanic and atmospheric administration
hmt-southeast profiler show snow levels rising to 5300 feet. This
shows warm nose continues to move in and strengthen across the area.
Have used the latest temperatures and guidance to update the p-types across
the County warning forecast area. Expect snow/sleet to continue across the northern mountains/northern Blue
Ridge and the balsams into the morning before warming kicks in there
quickly changing precipitation to rain. Freezing rain may linger a little
longer over the northwest Piedmont as well before warming quickly after
daybreak. Will leave west-southwest products in place until a full forecast
package is developed.
As of 940 PM EST Thursday...based on social media...spotter and ping
reports it appears that mainly freezing rain is falling over the
North Carolina Piedmont roughly along and north of a line from
Shelby to Lincolnton and Kannapolis. Reports indicate that elevated
objects are starting to get a coating of ice in these areas.
Unfortunately...precipitation rates are about to increase as
stronger low level ascent develops...which shows up nicely as an
expanding area of cold cloud tops over the upstate and southern NC
Piedmont. We Haven/T seen many power outages yet...but that/S likely
to start to change over the next couple hours. The sleet line
appears to run from eastern Buncombe County to central McDowell and
Burke and southeast Caldwell. A few areas north of this line...right
along the Blue Ridge...are likely to be seeing all snow at this
time. Still...we are expecting a changeover to at least a snow sleet
mix even in these areas. Interestingly...the latest GFS/NAM
soundings indicate that we could see a changeover back to snow
across parts of the northern mountains and foothills around sunrise. So...at
this point the going warnings and advisories look good...especially
for the foothills and Piedmont.
Another interesting thing is that the NAM pivots a band of heavy
rain across the western Piedmont through Friday afternoon. The model
has nearly 4 inches of rain along a line from Raleigh to just east
of Charlotte. Aside from significant ice accumulations...flooding
issues could also be a concern.
As of 825 PM EST Thursday...the 00 UTC gso sounding came in looking
more like the colder GFS...though the ffc sounding looked more like
the NAM. Therefore...there is a sharp 850mb front over the region.
It would appear that the warm nose has stopped advancing northward
along a line from northern McDowell to central Burke and southeast
Caldwell counties. This includes the northern mountains as
well...though locations near the immediate Tennessee line may be seeing
mainly rain. I kept the heaviest snow totals in these areas when
updating the precipitation accumulations...though I did lower snow totals a
little in this area as I expect more sleet to mix in over the next
couple hours. The freezing rain line from the day shift still looks
good. Very little adjustment was made...though I did increase ice
accumulations just a hair to the north and west of Charlotte. So
overall the warnings and advisories look on track as a period of
freezing rain is possible later tonight all the way down to the
NC/SC line over the western Piedmont even though the damaging ice
accumulations are expected to be north of there. I Haven/T heard
much from Transylvania or Henderson counties...but mesonet
observations are hovering around freezing and they should be seeing
sleet and freezing rain in many locations.
As of 655 PM EST Thursday...the grids are doing a good job of
capturing the precipitation transition across the region. We are getting
reports of snow over the NC foothills near the Blue Ridge and into
the central French Broad valley. The 0.5 degree cirrocumulus dual-pol data on the
wsr88d shows a line of very low cirrocumulus/S advancing to the northwest
across Transylvania...Henderson and central McDowell counties. This
appears to be the snow/sleet line. If this is indeed the case...then
the more aggressive warm nose of the NAM would be verifying. This
would mean more of a sleet than snow threat and a warmer thermal
profile in general which could severe to limit...at least to an
extent...the winter storm threat of all three wintry precipitation types.
However...I/M certainly not about to back away from any warning or
advisory products yet. While early March in not a climatologically
favored time of year for ice storms...the gefs soundings have a lot
of ice in them and we are going to have to wait and see where the 32
degree surface wet bulbs set up under the heavier precipitation. We certainly still
have the potential for significant icing across the Piedmont and
foothills north and west of Charlotte.
the Winter Weather Advisory has been upgraded to a warning along the
eastern Blue Ridge in North Carolina...and along the I-40 corridor
east of the mountains...with a new Winter Weather Advisory posted for the I-85
corridor in North Carolina.
Extensive high clouds and decent low level cold air advection has resulted in temperatures
remaining quite chilly across much of the area this afternoon
(except for the exposed areas of the southwest NC mountains...where temperatures
are near 50 this pm)...and most areas have not managed to escape the
30s. As a result...model guidance is too warm in comparison to
observed wet bulb temperatures...which is not a good sign. Precipitation
associated with ongoing NE greater Mexico cyclogenesis extends from northern
Alabama to the I-85 corridor in SC...and will continue to overspread
the forecast area through the afternoon/evening. Based upon wet bulb
temperatures...it appears that most areas south of I-85 will see mostly
rain through much of the evening...with perhaps some sleet thrown in
as temperatures wet bulb below 40. In fact...most portions of the forecast
area will see precipitation begin as rain or a rain/sleet mix.
The fun begins later this evening into the overnight...as the
surface low begins to deepen off the southeast coast...strengthening
the damming ridge and increasing advection of cold/low wet bulb
surface air into the forecast area. The NAM remains an outlier in
developing a very prominent warm nose (of +5 to +6) in the 3-5 kft
layer this evening...as easterly flow accelerates (to 70-80 kts)
above the cold dome north of the developing cyclone. Meanwhile...the
GFS and European model (ecmwf)...both featuring a more /off-coast/ surface low
track...develop very little in the way of a warm nose...certainly
one that would suggest more of a sleet than a freezing rain concern.
We remain quite skeptical of the NAM...as the winds/advection
patterns seem overdone...and feel that warm air advection will have difficulty
overwhelming cooling induced by intense upward vertical velocity in this layer. In
addition...this is not a climatologically favored time of year for
ice storms. However...there are enough members of the sref ensemble
supporting the op NAM that I/M not prepared to eliminate it
altogether. Therefore...the forecast is largely based upon a blend
of partial thickness guidance with a blend of the GFS and NAM (but
weighted toward the gfs). This results in mainly a sleet/snow
scenario along the Blue Ridge...with a sleet/freezing rain situation over the
Total accums of sleet/snow are expected to range from 2-6 inches
along the Blue Ridge. However...much will depend upon how much sleet
falls vs the amount of snow...and the forecast is generally based
upon an expectation of about 50/50 snow and sleet. More snow than
sleet would equate to quite a bit more snowfall accumulate. Over the northwest
Piedmont...will forecast around a half inch of sleet along with a
quarter inch of ice...but latest guidance trickling in suggests we
may see more sleet than ice in these areas. Meanwhile...around a
tenth of an inch or sleet/ice is expected across the southern NC
foothills and Piedmont.
As the cyclone moves up the coast Friday morning...the cold dome will
begin to drain...allowing precipitation to change to all rain as the upper
low passes south and east of the area...and deformation zone precipitation
remains wrapped into the area into the early afternoon.
Short term /tonight through Sunday/...
at 230 PM EST Thursday...the upper low and associated surface
cyclone will be off the Carolina coast at the beginning of the period
and will move quickly NE away from the area Friday night. This will
allow upper heights to quickly build over the region Friday night. A
flat upper ridge will then persist over the southeast U.S. Through Sunday.
A short wave moving through the northern stream will push a weak
backdoor cold front into the area on Sunday.
In regard to sensible weather...clouds will quickly pull out to the
east Friday night as surface high pressure builds east over the
area. Generally clear conditions will persist through Saturday. As the
weak front approaches...clouds will be on the increase early sun.
However...with downslope flow on Sunday a mix of sun and clouds
looks to be a good bet. Can't completely rule out some light showers
reaching the mountain counties adjacent to Tennessee on Sunday as
the front arrives. However...the chance for measurable precipitation appears
to be less than 30 percent.
The big story will be the warm up which will commence on Sat. Low
level thickness supports maximum temperatures of 65 to 70 both Sat and sun
outside the moutnains...with 60s in the mountains. This looks
reasonable considering the general downslope flow that will
Long term /Sunday night through Thursday/...
as of 205 PM Thursday...the medium range forecast picks up at 00z on
Monday with a weak shortwave passing across our area Sunday evening.
We will then be under the influence of a low amplitude upper ridge
centered over the eastern Gulf into Monday. Low level wind direction
to start the week will remain generally from the west.
The new GFS run is fairly different from the previous with a shorter
duration of precipitation in middle week. All models keep our region either
dry or almost dry through at least Monday night. The GFS has a cut
off 500mb low over SW Texas at 12z Monday which tracks to Corpus
Christi at 12z Tuesday as it opens up. The European model (ecmwf) has a somewhat
similar feature. Both models bring this shortwave with
accompanying surface low over our area Tuesday night into Wednesday.
Almost no instability as the main feature comes over Wednesday morning.
Big difference between the models is the broad upper trough moving
in behind the initial shortwave is dry on the GFS but has a low to
form on the inflection Point of the deeper European model (ecmwf) trough. The ec
actually has the first feature wrapping into and merging with the
bigger trough moving southeast. The trough and the surface low on the ec
merge and both swing NE with most energy moving north of our
forecast area. The GFS ends all precipitation over our area before midnight
Wednesday night and the ec ends the wrap around northwest flow precipitation for the northern
mountains of NC Thursday afternoon. All precipitation should be rain as we
expect precipitation to move east of area before colder air filters in but
may be brief transition to snow as it ends for the NC mountains trends
are that temperatures in the wake of the departing low and under the
upper trough will be cooler than previously forecast. Temperatures
early in the week being above normal and dropping to below normal
under the upper trough and cool air advection behind the surface low.
Aviation /06z Friday through Tuesday/...
at kclt...wet bulb temperatures have risen above freezing across most of
the southern Piedmont...and do not expect them to fall overnight.
Therefore...have removed freezing rain from the taf. That said...cannot rule
out a few sleet pellets or even a very brief period of freezing rain for the
next few hours. Otherwise...expect rain to continue into the morning
with continued IFR ceilings and MVFR visibility. Cannot rule out a period of
LIFR ceilings toward daybreak though. Gusty north-northeast winds continue through the
morning as well. Rain tapers off through the afternoon with ceilings slowly
rising through MVFR...while visibility becomes VFR quicker. Gusts diminish
through the afternoon with northerly wind developing. Low VFR clouds expected
by evening with light northeasterly wind returning.
At kavl...although precipitation has been rain for the bulk of the
evening...nearby soundings show some sleet could mix in during the
next few hours. Therefore...have kept a brief period of rapl in the
taf. Expect rain for the most of the night into the morning before
precipitation tapers off around noon. MVFR conditions will become IFR
before daybreak then return to MVFR ceilings and VFR visibility around noon.
Clouds should scatter out around sunset. Light southerly winds become northerly
before daybreak and continue through the period.
Elsehwere...the upstate taf sites will see rain with IFR conditions
as a heavy band of precipitation rotates in and lingers into the morning.
Conditions improve to VFR around noon then low VFR by late afternoon
as precipitation tampers off. Gusty north to NE wind continues into the morning
before tapering off around noon when wind gradually becomes light
northeasterly. Khky should see mostly rain...but cannot rule out some sleet
pellets or even a brief period of freezing rain before daybreak. That said...
wet bulb temperatures have risen above freezing there as well. Otherwise...
conditions should be similar to the rest of the area.
Outlook...dry high pressure and no restrictions are expected
Saturday through Monday. Rain and associated restrictions may spread
back in from the south Tuesday night.
06-12z 12-18z 18-24z 00-06z
kclt medium 74% medium 70% medium 75% high 100%
kgsp medium 71% medium 70% medium 78% high 100%
kavl high 90% medium 71% medium 71% high 100%
khky medium 76% medium 72% medium 77% high 100%
kgmu medium 60% medium 71% high 84% high 100%
kand medium 74% medium 69% medium 79% 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...Winter Weather Advisory until noon EST today for ncz048-052-
Winter Storm Warning until noon EST today for ncz033-035>037-
SC...Winter Weather Advisory until noon EST today for scz003.