Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1249 PM EST Sat Dec 8 2018
.SYNOPSIS...
With cold Canadian high pressure to our north and low pressure
passing to our south, we are having a major winter storm. The
low pressure moves off the Carolina coast Sunday evening taking
most of the wintry weather with it. The last of the light
precipitation finally ends Monday night when the upper trough
passes. Cool high pressure will be in control Tuesday and Wednesday,
then the next storm system arrives at the end of the week.
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.NEAR TERM /THROUGH TONIGHT/...
As of 1245 PM: No additional changes have been made to the current
hazard suite pending full inspection of the 12Z model data. The
morning adjustments were made in light of slightly warmer 850 mb
temperatures on the 12Z FFC raob than apparent in the numerical
models, and slightly colder Sunday AM surface temperatures
penetrating farther south in the cold wedge in the incoming model
solutions. These adjustments gave slightly more confidence to
upgrading the northern portion of the lingering Winter Storm Watch
to a Warning, and posting a Winter Weather Advisory for the southern
piedmont where light icing is looking a bit more likely early Sunday.
Otherwise, precip rates will steadily increase this afternoon into
the evening as low-to-mid level frontogenetical forcing intensifies,
and forcing deepens with the approach of a potent short wave trough
from the lower MS and TN Valleys. Meanwhile, surface ridge
associated with expansive 1035+ mb Arctic high pressure spilling
east of the central Appalachians will continue to nose down the
Eastern Seaboard, with a strengthening gradient between the surface
high and Gulf Coast cyclone and diabatic effects from falling
precipitation resulting in an intensifying cold air damming regime,
which is forecast to become quite strong from late afternoon into
this evening.
Advection of sub-freezing wet bulb surface air will therefore
increase substantially into our forecast area during this time,
which will allow for a steady transition from rain to snow from
northeast to southwest across the Piedmont/foothills this evening
through the overnight. This transition is expected to occur along
the I-40 corridor early this evening, pushing as far south as the
I-85 corridor between midnight and daybreak. It is during this time
that the operational model guidance diverges with respect to their
thermal fields, as the NAM is insistent in pushing a substantial
warm nose into the Piedmont, suggesting a transition to sleet and
rain/freezing rain along and south/east of the I-85 corridor, while
the GFS paints mostly a rain/snow scenario for the entire area, with
perhaps some sleet mixing in across the Piedmont. The NAM often
tends to do a fairly decent job in handling warm nose features
during coastal cyclone scenarios, while the other operational
guidance tends to under-do them. That being the case, the NAM
thermal fields were blended in heavily with other guidance to
produce tonight`s p-type forecast. The result is a swath of warning-
level ice accretion forecast along I-85 and locations within about a
county to the southeast, while sleet and FZRA mixing in with snow
substantially cut into forecast snowfall totals along and near I85.