Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
447 PM EST Wednesday Jan 28 2015
dry high pressure will remain in place tonight through Thursday. A
cold front will cross the forecast area Thursday night with another
high pressure system building in Friday through Saturday. A moist
low pressure system is expected to affect the region on Sunday into
Near term /through Thursday/...
as of 430 PM...forecast is on track. Sunny skies and light winds with
seasonably cool temperatures heading into the evening. Temperatures and dewpoints
were adjusted with the latest observation...blending into the evening hours.
As of 230 PM EST Wednesday...rising heights aloft and building high
pressure at the surface continue to yield a fairly nice...yet cool
day across northeast Georgia and the western Carolinas. Light and
northerly surface winds will continue to slowly veer southerly as
above mentioned surface ridge slides overhead eventually taking up root
just off the Carolina coast overnight. Cold/dry advection over the
last 12-24 hours combined with low level mixing this afternoon has
allowed dewpoints to fall into the low teens to single digits at
some locations. Warming temperatures amongst abundant incoming
solar radiation have led to relative humidity levels falling into the low 20s to
upper teens across much of the NC/SC Piedmont regions and also
portions of northeast Georgia. Although these are critical relative humidity levels as
it pertains to fire weather concerns...as mentioned above winds across
the region remain light with some sites even reporting calm winds.
Thus...caution is urged if planning to burn today...however no fire
weather products are planned at this time.
Meanwhile further west...an series of upper short wave impulses are
sliding through the central/northern plains with the first of which
yielding weak surface cyclogenesis. A cold front associated with this
feature will approach the southern/central Appalachians through the
day on Thursday with prefrontal precipitation spreading into the western NC
mountains around noon. At that time...temperatures will have
recovered enough such that any ptype concerns are not expected. As
the front slides through the high terrain late in the day towards
evening...snow levels will fall across the high terrain with a brief
period of rain/snow mix possible across the northern mountains
before periods end. Thus the forecast features increasing chance level
probability of precipitation through midday across much of the western NC high terrain with
a dry forecast prevailing over the Piedmont regions. Temperatures on
Thursday will approach normal levels as southerly flow prevails
amidst mostly sunny...although increasingly cloudy skies.
Short term /Thursday night through Saturday/...
as of 230 PM Wednesday...the main focus for the short range will be
the passage of a short wave and cold front Thursday night. Precipitation
chances will already exist over the mountains at 00z Friday and see
little reason in the new guidance to change that. Forcing and the
deepest moisture still maximize around 06z Friday...so a likely pop
looks good on the Tennessee border. The forcing and moisture slowly wane
from that point Onward. Will gradually taper off the pop through
Friday...and will end it by the end of the daytime. There is some
question about the degree of cold air as the front is moving across
the mountains some of the initial precipitation could be more in the form of
rain...although the forecast favors a larger coverage of snow than might be
supported directly from the model data. This will be a limiting
factor in keeping snow amounts on the Tennessee border well within the
advisory range. This will be included in the severe weather potential statement as a possibility of
light snow. The main accumulate potential will be the second half of the
third forecast period...thus it is still too far in the future to issue
and advisory. The model guidance also continues to support strong
winds across the ridgetops behind the front late Thursday night and
Friday morning...although the trend is for slightly less wind at the
top of the mixed layer. The trend suggests more of an advisory-level
event apart from elevations above 5k feet...and the severe weather potential statement will be
The rest of the forecast will be relatively benign. We should see the
upper flow flatten and become more zonal through Friday night and
Saturday...which will support surface high pressure moving across
the mountains and mid-Atlc/Carolinas. That should keep temperatures about a
category or so below normal...with clear sky.
Long term /Saturday night through Wednesday/...
as of 250 PM...uncertainty reigns during the medium range period
with little run to run model consistency. That said...they do agree
on an unsettled period with 2 weather systems affecting the area...one
early in the period and one late. Aloft...the European model (ecmwf) and the GFS show
a northern stream short wave crossing the area during the Sun night-
Monday time frame as a closed low moves south in the Baja California California
area. At the surface however...the European model (ecmwf) moves a dry cold front into the
area Sunday...with a low pressure forming to our west and moving
across the area Sun night. This system then moves east of the during
the day Monday. The GFS also brings a cold front in but it is slower
with the front coming in Sun night and the main low pressure center
remaining to our north. The GFS also develops a pre-frontal trough
across the area Sunday...developing precipitation earlier than the European model (ecmwf).
The GFS is also warmer indicating a mainly liquid precipitation event until
northwest flow snow develops Sun night and Monday as the main area of precipitation
moves east. The European model (ecmwf) is colder showing snow as far south as the I-
85 corridor. The GFS ensemble mean is not especially cold...keeping
the coldest temperatures across the mountains until after the precipitation exits.
Given all the uncertainty...have used a model blend which brings the
best precipitation chance across the area sun and Sun night...then tapering off
to a northwest flow snow for Monday afternoon. Used a conservative rain/snow
temperature for the weather which keeps snow limited mainly to the mountains...
with some snow across the I-40 corridor at onset. Should the European model (ecmwf)
prove right...then snow would fall much farther south.
Good model agreement that dry and cold weather expected Monday night and
Tuesday as very cold high pressure moves in. In fact...the GFS
ensemble mean 850 mb temperature is as cold as 2 Standard deviations below
normal at its coldest Tuesday morning.
The guidance then differs again on the handling of the afore
mentioned closed low over Baja California California. They both open the low
and move it east into the southeastern Continental U.S.. however...the European model (ecmwf) is faster
and farther south than the GFS. At the surface...both models agree on a
Miller-a type scenario. However...the European model (ecmwf) keeps the low closer to
the Gulf Coast and faster while the GFS is farther north and slower.
Despite the cold high pressure moving in ahead of this system...it
is not a classical type cad. The parent high is over the middle-
Atlantic and transient. Still...if precipitation were to start early
enough...then a wintry mix would develop even across the southern County warning forecast area
before warming to all rain for the afternoon...except across the NC
mountains given the uncertainty...have limited pop to chance...and used a
conservative rain/snow temperature for weather.
Aviation /22z Wednesday through Monday/...
at kclt and elsewhere...VFR conditions will prevail at all taf sites
through the period. With the exception of 18-20kt gusts over the
next few hours at kavl...light north to northeasterly flow will
prevail under sky clear through the remainder of the afternoon as surface
high pressure slides overhead. Once the ridge takes root to the
east by early evening...winds will gradually back southerly before
becoming light/calm through the overnight. Southerly winds and
middle/high level clouds will increase near daybreak and through
morning on Thursday as upper trough and associated surface front
approach from the west. No weather mentioned at any taf site through
this taf cycle.
Outlook...a pair of clipper systems will bring chances for rain or
snow showers to the NC mountains Thursday afternoon/evening and 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
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% 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)