Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
609 am EST Friday Feb 27 2015
cold Canadian high pressure will remain across the area through the
weekend. Above normal temperatures should return to the area
Monday...before falling again Tuesday in the wake of a cold front. A
more substantial cold front is expected to cross the region during
Near term /through tonight/...
1115 UTC update...winds were updated form the latest adjmav and NAM.
Probability of precipitation were raised slightly near the Tennessee border per radar trends. Areas
of fog were adjusted per surface observations.
At 0330 EST Friday...a broad upper trough was over the USA this
morning...with nearly zonal flow over the southern tier of states.
An upper low will dive into the western portion of the trough over
California today and tonight.
The models vary on the timing of various impulses crossing our area
through tonight. Model time heights suggest lingering low level
moisture through tonight...while upper level moisture decreases as a
cold front over the western Atlantic drifts to the southeast. Gradually
diminishing northwest flow along the Tennessee border may produce some light snow
showers early this morning. Otherwise...precipitation is not
expected due to a lack of lift east of the mountains. Low level flow
will veer from northwest to north to NE today...as high pressure builds in from
the west. High temperatures will run around 15 degrees below normal
as the cool air mass builds into the region. Low temperatures
tonight will run around 10 degrees below normal.
The Winter Weather Advisory will remain in effect for widespread black
ice and freezing fog for all but our southern tier counties this
morning. An Special Weather Statement for patchy black ice will be remain in effect for
the remaining zones.
Short term /Saturday through Sunday night/...
as of 305 am Thursday...the short term period will see
establishment of a hybrid cold air damming event...not classical
due to the location of the parent high across eastern Virginia/northern
NC. In fact...there will be a dry onset to the event...as the center
of the high builds far enough south to send dry low level air
through the forecast area. Nevertheless...Saturday is expected to be
quite cool...as NE flow is aided by an area of substantial middle/high
level clouds depicted by a model consensus within the entrance of a
strong upper jet maximum. Therefore...temperatures will struggle to get out of
the 30s in all but southern Piedmont areas Sat afternoon.
Southerly flow developing above the cool dome...in response to
central Continental U.S. Height falls...will result in quick transport of lower
level moisture into the area Sat night into early Sunday. There
remains timing differences regarding the arrival of the moisture and
associated light precipitation...with the NAM remaining as the fastest
model...with only slight support from the sref system. It is also
noteworthy that only the NAM depicts moisture deep enough to support
ice nucleation during this time...which explains its more robust
quantitative precipitation forecast...while the other models would suggest moisture deep enough to
only support a drizzle or --ra event. Sunday morning. We have opted
to run with slight/low chance probability of precipitation for light precipitation/dz by daybreak
Sunday. The timing of this will be quite important...as thermal
profiles would be supportive of wintry precipitation across much of the
area during this time. Depending upon the degree of ice
nucleation...a brief period of snow/sleet would be possible near
onset time...before rapid warming aloft within the strong warm air advection regime
would result in a change to rain/freezing rain. The concern is that
with the potential for rather strong hybrid cad...some locations
could lock in freezing temperatures long enough to allow for some ice
accretion...especially along the Escarpment and the I-40 corridor in the
Piedmont. On the good news side...duration and coverage of precipitation is
still very much in doubt...and if freezing rain were to develop and
persist in some areas...The Mount of precipitation expected would keep the
event solidly in the advisory category.
Temperatures will gradually warm late Sunday into Sunday night as the
parent high migrates off shore...and a cold front approaches from
the tenn valley. While probability of precipitation gradually ramp up during this time...
the model consensus is that this frontal zone will be very weakly
forced...even in the lower levels...where veered westerly/weakly
convergent flow is anticipated by late Sunday.
Long term /Monday through Thursday/...
as of 325 am...the large scale pattern change only hinted at during
the short term will be completed during the medium range...as major
western Continental U.S. Height falls force height rises across the east...with
a fairly substantial ridge anticipated along the East Coast by day
7. However...next week is expected to remain somewhat unsettled...as
this pattern change will result in a quasi-stationary baroclinic
zone lingering over the southern Continental U.S. Through much of the period.
In fact...by early in the work week...the western trough is expected
to migrate into the central Continental U.S....resulting in moisture return
over another transient/hybrid cad air mass. Thus...probability of precipitation for upglide
precipitation will be in place during much of the Sunday night through Tuesday
night period... although it certainly will not be raining non-stop
during this time...and precipitation will be mostly on the light side.
A more substantial precipitation event is quite possible during middle-
week...when the trough and associated cold front move into the
eastern states...interacting with a wide open Gulf. In fact...
heavy rainfall may become a concern during this time. Temperatures will be
quite variable during the medium range...with above to well-above
normal temperatures expected during non-cad periods...and below normal
conditions expected on days of transient/hybrid cad. However...
temperatures should be warm enough during periods of precipitation to
support rain as the lone p-type.
Aviation /11z Friday through Tuesday/...
at kclt...MVFR visibility is holding...despite low restrictions to the
east. Visibility should improve by middle morning. The cit will reflect the
model time height preference for low VFR through much of the next 24
hours...although breaks will be possible from time to time. Winds
will veer north and then NE today as high pressure builds in from the
northwest. With dry low level Saturday morning...a visibility restriction is not
Elsewhere...taf sites remain in good shape...with MVFR ceilings at kgsp
and kgmu...MVFR ceilings at kavl and kand....and no ceiling at khky. VFR
visibilities prevail...despite some low restrictions to the east. Snow
showers northwest of kavl are not expected to reach the field...and should
diminish by midday. Model time heights suggest low VFR ceilings for much
of the next 24 hours...except high end MVFR at kavl...however drying
from aloft may introduce breaks for much of the period. Winds will
veer from north to NE today...except at kavl where they will remain from
the north until late this evening...when they veer southeast. Dry low level
will preclude visibility restrictions Saturday morning.
Outlook...low level moisture will linger through the remainder of
the week and into the weekend as cold air damming builds from
the north on Saturday. Flight restrictions will be possible during
this timeframe...especially on Sunday once moisture spreads in from
the south and ceilings lower.
13-19z 19-01z 01-07z 07-12z
kclt high 95% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl medium 74% high 100% high 100% high 100%
khky medium 77% 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)
Georgia...Winter Weather Advisory until 10 am EST this morning for
NC...Winter Weather Advisory until noon EST today for ncz033-
Winter Weather Advisory until 10 am EST this morning for
SC...Winter Weather Advisory until 10 am EST this morning for