Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
443 PM EST Thursday Jan 29 2015
in the wake of a cold frontal passage...high pressure will build
into the region later on Friday. The high will linger over the area
into Sunday before moisture returns out ahead of a developing
low pressure system. Dry Canadian high moves back into the area by
Near term /through Friday/...
as of 430 PM...tweaked probability of precipitation to remove mention for the rest of the
afternoon...as radar shows band of showers exiting to the east...with no
returns seen upstream at this time. A second shortwave and accompanying cold
front will cross the area tonight...bringing a return of showers along
the Tennessee border...as winds turn out of the northwest and increase. The rest
of the forecast looks on track.
As of 230 PM EST Thursday...this afternoon and evening...a rather
fast moving light band of precipitation associated with an upper wave was
moving eastward across the NC Piedmont and upstate SC. There have
been numerous reports of a mix of rain and sleet as this band moved
across the NC mountains/valleys and into the Piedmont due to wet-bulb
cooling effect. Hence...have carried a chance of rain and sleet
across the Piedmont where the band was pushing eastward. As this
feature pushes off to the east...there will be a lull in precipitation
through this evening.
Tonight and tomorrow...another round of precipitation is expected mainly
across the NC mountains zones as an upper level trough axis associated
with a surface cold front pushes through the region. As overnight
temperatures plummet well below freezing...snow should be a dominant
p-type. Latest mesoscale-scale models agree that the best chance of northwest
snow will occur between 3z-12z when the best upper forcing...moist
low level layer (surface to 700mb) will be present within strong cold air advection northwest
upslope flow regime. Therefore...using a blend of campop and other
guidance...categorical probability of precipitation were mentioned along the Tennessee line with
sold chance to likely probability of precipitation elsewhere in the mtns/valleys.
However...given relatively short duration of snow and less favoring
850 mb west-southwesterly winds...total snow accums will be less than 2 inches which
is below the advisory criteria. So no west-southwest will be issued at this
time. Strong cold air advection northwesterly winds will develop by late tonight and
continue through tomorrow afternoon as a tight surface pressure
gradient sets up. Will issue a Wind Advisory across the northern NC
mountains tonight into tomorrow afternoon where gusty northwest winds of
45-50mph will occur. Conditions should dry out along the Tennessee line
toward Friday midday as dry high pressure builds in from the northwest.
Elsewhere will see continued dry weather. Temperatures overnight will be
around near climatology for most locations. Daytime temperatures on Friday will stay
5-7 degrees below normal over the mountains/valleys with 2-3 degrees
below normal elsewhere.
Short term /Friday night through Sunday/...
as of 230 PM Thursday...forecast trends improving for the weekend
weather. Short range models agree that the center of a Canadian high
pressure will approach the region early Saturday...with the center
passing over the southern Appalachians Saturday afternoon and
evening. A blend of MOS guidance favors temperatures 5 to 7 degrees
below normal for Saturday. Afternoon relative humidity values are forecast to range
from the middle 20s to lower 30s.
On Sunday...models have trended significantly slower with arrival of
deep moisture and forcing...now arriving during the late afternoon.
NAM...GFS...and European model (ecmwf) indicate that greatest potential for the onset
of measurable precipitation will occur between 18z to 0z. Temperatures
across the mountains are forecast to range generally in the middle 30s to
low 40s. I will indicate in the forecast that precipitation will fall as
rain with high elevation snow...little to no accumulate before 0z Monday.
Sun highs will likely range from low to middle 40s across the mountains to
around 50 east of I-85.
Long term /Sunday night through Thursday/...
as of 255 PM Thursday...good agreement in the overall patter for the
medium range...but considerable disagreement remains in the details
and expected sensible weather. A northern stream short wave crosses the area
Monday with a flat trough in place on Tuesday. An upper low that starts
the period over the Baja California peninsula opens up and moves east into the
area Wednesday or Wednesday night with some degree of phasing with a northern stream
short wave dropping into the trough at that same time. The wave then
moves east of the area Thursday.
At the surface...precipitation will likely be ongoing at the start of the
period in the moist southerly flow ahead of a low pressure system moving
NE to the west of the Appalachians. The low then slides east near
the Virginia/NC border and drags a cold front across the area. This will
keep the area outside of the mountains too warm for anything but rain.
However...temperatures will be cold enough for snow to develop across the
high elevations...then fall to the valley floors toward daybreak
Monday. Precipitation ends as a brief northwest flow snow Monday afternoon. Quantitative precipitation forecast amounts
will be high enough for a high end advisory across the higher
elevations and a low end advisory in the valleys from Haywood north.
However...the temperatures remaining question as the cooling could be
delayed depending on the movement and speed of the low and
associated cold front. Cold and dry high pressure builds in Monday night
and Tuesday. Temperatures Monday night above normal with below normal temperatures through
The next weather system moves in during the Wednesday time frame. The European model (ecmwf) is
faster and has a more coherent Miller a type Gulf low...while the
GFS is slower and has more of an elongated frontal zone crossing the
area with a low in Quebec and a weaker Miller a farther south in the
Gulf. Have used a model blend for timing. If the precipitation moved in
fast enough and out slow enough...then there could be some snow outside
of the mountains at onset and before dissipation. However...this remains
highly uncertain. It is also questionable how much snow would fall
across the high elevations of the mountains...right now it looks sub-
advisory. Dry conditions expected outside of the mountains Thursday as high
pressure builds in. Temperatures remain below normal from Tuesday night through
Aviation /22z Thursday through Tuesday/...
at kclt...expect VFR conditions to persist through 18z Friday. A
light band of precipitation associated with a weak upper wave will cross
the NC mountains through this afternoon. However...latest mesoscale-models
indicate that this feature will weaken considerably as it enters the
NC foothills/Piedmont by early this afternoon. So no precipitation was
mentioned in the taf. SW winds should persist with occasional gusts
to around 15-20kts possible through this afternoon. Tonight...SW winds
should to veer to the northwest as a surface cold front pushes through the
region. As surface pressure tightens up behind the front...gusty northwest
winds of 15-20kts will develop by around 15z. Cloud ceilings will bottom
out around 8k feet during the frontal passage tonight.
Elsewhere...VFR through the period except kavl where MVFR clouds
will spread in from the north tonight due to the frontal passage. A light band
of precipitation will move across the NC mountain zones through this afternoon.
However...the precipitation appears to be very light and it will be mainly
confined to the ridge tops of the mountains hence...no precipitation was
mentioned in kavl. Otherwise...expect mostly cloudy skies with SW
winds persisting through late this evening across much of the
region. Winds will veer the northwest tonight as a surface cold front
pushes through the area. Again...precipitation chances are too low to
mention in the taf. Tight pressure gradient behind the front will
result in gusty northwest winds across much of the region by Friday morning.
Outlook...a pair of clipper systems will bring chances for rain or
snow showers to the NC mountains Friday. Most of these showers will remain
west and north of kavl...but brief periods of restrictions cannot be
ruled out. Otherwise...mainly VFR conditions are expected across the
area until moisture begins to return Sunday. Drying is expected on
21-03z 03-09z 09-15z 15-18z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% medium 78% low 59%
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)
NC...Wind Advisory from midnight tonight to midnight EST Friday
night for ncz033-049-050.