Ice Storm in North Carolina, February 17 to 18, 2021

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1027 PM EST Wed Feb 17 2021

.SYNOPSIS...
High pressure will retreat to the northeast in response to a low
pressure system tracking northeast out of the Gulf of Mexico. This
low pressure system will bring widespread precipitation into the
area tonight into Friday. Wintry precipitation can be expected late
tonight into tomorrow afternoon and again tomorrow night across
parts of western North Carolina and the higher elevations. This
weekend high pressure moves in leading to drier weather and cool
temperatures.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 955 pm: No changes will be made to current W/W/A products at
this time, as we're very much in the 'wait and see' part of the
forecast process. Widespread warm conveyor belt precip currently
covers much of the TN Valley, primarily in the form of snow within
the deeper cold air mass in place there. This precip is beginning to
ooze into the southern Appalachians late this evening, and will
likely initially fall as snow and/or sleet before strong warm
advection aloft changes the p-type to RA/FZRA...and indeed it still
appears that the bulk of the precip in our area will fall as one of
these. Therefore, as per usual, impacts will depend almost
exclusively on temp trends in the lowest 10 meters of the
atmosphere...always a forecasting adventure even when wintry weather
is not on the table.

Currently, the surface wet bulb freezing line extends from the
southern Blue Ridge escarpment, through the northern NC foothills
and far northern NC Piedmont. This has been oozing steadily
southward across the Piedmont since sunset, and should progress to
the I-40 corridor and vicinity by the time precip begins there after
midnight. As such, rain/possible sleet across the NC foothills and
northwest Piedmont should change over to FZRA after the first hour
or two. Relatively weak cold air damming is currently in place, in
response to elongated 1030+ mb surface high covering much of the NE
quarter of the country. Once the precip begins falling into this
regime, CAD will strengthen somewhat into a respectable hybrid
event. At this point, we are most confident in warning accumulation
ice being realized along the southern and eastern Blue Ridge
escarpment, where upslope flow should allow for a sufficient cooling
mechanism to keep temps locked in at or below freezing. Ice
accretion of .25-.5 inch appears quite reasonable in these areas per
the current forecast. East of the escarpment, confidence is moderate
at best due to concerns about whether or not the developing hybrid
CAD will result in sufficient cold wet bulb temp advection to
overcome the self-limiting nature of freezing rain. Incoming 00Z
guidance is offering no surprises and giving us little-to-no
additional insight into these matters. All that's to say that
there's nothing in our toolbox currently that will help us improve
the going forecast, and the only logical path is to stay the course,
and again...wait and see.

Precip coverage and intensity tapers off from west to east Thursday
as better forcing moves east of the area. Slow warming takes place
Thursday will most locations going above freezing during the
afternoon. Most additional ice accretion will be during the morning
with only minimal amounts for the afternoon. No surface based
instability expected Thursday, but there will be some elevated CAPE.
The H85 jet is not especially strong, so no strong storms expected,
but isolated elevated storms are expected south of the I-85
corridor. Better chances for heavy rain look to be south and east of
the area as well; however, can't rule out an isolated creek or
stream coming out of its banks given the recent rainfall.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 200 PM EST Wednesday: Broad upper troughing will continue to
run overhead the cwa at the start of the short-term. A secondary sfc
low will follow up the initial low and ride the Carolina coastline
Thursday night into Friday. This will allow for the QPF response
to linger into the area during the first 12 hours or so of the
forecast period. Temperatures will be below freezing across the
mountains and north foothills, so wintry p-types could enter back
into these areas before shutting off by mid-Friday morning. No to
little accumulations are expected, but shallow low-level moisture
will allow for NW flow snow to develop along the TN Border during
the day Friday before the trough axis slides across the cwa by
Friday evening. This will gradually dwindle snow chances through
the late evening hours, with up to an inch expected, mainly in the
northern mountains. Wind gusts will likely occur Friday into Friday
night for the mountains as the sfc pressure gradient tightens.

Broad sfc high follows in behind the low pressure system and
allows for the area to remain dry through the day Saturday as
thicknesses begin to rebound and flat, upper ridging moves over
the eastern CONUS by the end of the period. Temperatures will
likely run below normal for both max and min temps through the
short-term. CAA filters in behind the system with the sfc high and
winds will mainly come out of the N to NW, while shifting to the
NE by Saturday evening. This will keep temperatures below-normal
despite mostly clear skies Saturday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 200 PM EST Wednesday: The extended is overall fairly quiet,
starting off on Saturday night with the upper trough pushing
offshore and surface high pressure dominating the area. Expect clear
and cold Saturday night into Sunday, with temperatures 5-8 degrees
below seasonal normals. A deepening shortwave will approach the area
Sunday night into Monday, with some differences in how guidance is
handling the attendant moisture, but kept with the blend and mid-
range chance pops. Cold temperatures/thicknesses aloft, for now,
support a rain/snow forecast depending on surface temperatures, so
have kept previous trend of some light snow possibly mainly across
NC zones, but some mix possible even into the Upstate, though not
enough moisture at this time to support much in the way of accums,
so this will be refined as we get closer. We'll see a warming trend
as we push into Tuesday and especially Wednesday as modified high
pressure builds in from the south and west with nearly zonal flow
aloft trending toward weak ridging.

&&

.AVIATION /04Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: Precipitation is expected to quickly
overspread the Terminal Forecast Area between 04 and 07Z. This will
result in deteriorating flt conditions, with IFR cigs and MVFR visby
likely developing at all terminals btw 06 and 09Z. Precipitation
should primarily be -RA at KCLT and the upstate SC terminals, but
surface temps will be close enough to freezing to warrant
continuation of Prob30s for -FZRA at those sites (except KAND),
generally btw 11 and 15Z. Meanwhile, primarily FZRA is expected at
KHKY and KAVL. Precip will persist through the morning, gradually
tapering off from the west during the afternoon. Warming temps
should result in a transition to RA at KAVL/KHKY during late
morning/early afternoon. Minimal improvement in flt conditions is
otherwise expected during the afternoon, with perhaps cigs lifting
to MVFR by the end of the forecast period.

Outlook: Cig restrictions may hang on until Friday morning. Any
lingering precipitation may change to snow again in the mountains
Thu night. Dry and most likely VFR over the weekend. Another frontal
system may bring precip and restrictions to the area on Monday.

Confidence Table...

            03-09Z        09-15Z        15-21Z        21-00Z
KCLT       High  94%     High  84%     High  94%     High  93%
KGSP       High  94%     High  88%     Med   76%     High  93%
KAVL       High  87%     High  89%     Med   71%     Med   79%
KHKY       High  92%     High  91%     High  91%     High  86%
KGMU       High  94%     High  88%     High  89%     High 100%
KAND       High  89%     High  95%     High  87%     High  82%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Winter Weather Advisory until 7 PM EST Thursday for GAZ010.
NC...Winter Weather Advisory until 7 PM EST Thursday for NCZ068>072.
     Winter Storm Warning until 7 PM EST Thursday for NCZ033-
     035>037-048>050-052-053-056-057-064-065-501>510.
     Winter Weather Advisory below 3500 feet until 7 PM EST
     Thursday for NCZ051-058-059-062-063.
     Winter Storm Warning above 3500 feet until 7 PM EST Thursday
     for NCZ051-058-059-062-063.
SC...Winter Weather Advisory until 7 PM EST Thursday for SCZ001>003-
     005>009.

&&

SYNOPSIS...AP
NEAR TERM...JDL/RWH
SHORT TERM...CAC
LONG TERM...TDP
AVIATION...JDL

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
646 PM EST Wed Feb 17 2021

GAZ010-NCZ068>072-SCZ001>003-005>009-181200-
Rabun-Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-Oconee Mountains-
Pickens Mountains-Greenville Mountains-Greater Pickens-
Greater Greenville-Spartanburg-Cherokee-York-
646 PM EST Wed Feb 17 2021

...WINTER WEATHER ADVISORY IN EFFECT UNTIL 7 PM EST THURSDAY...

This Hazardous Weather Outlook is for northeast Georgia, piedmont
North Carolina and upstate South Carolina.

.DAY ONE...Tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Weather Advisory.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Weather Advisory.

.SPOTTER INFORMATION STATEMENT...

Spotters are requested to report snow and ice accumulations to our
office.

NCZ051-058-059-062-063-181200-
Swain-Graham-Northern Jackson-Macon-Southern Jackson-
646 PM EST Wed Feb 17 2021

...WINTER STORM WARNING IN EFFECT UNTIL 7 PM EST THURSDAY ABOVE
3500 FEET...
...WINTER WEATHER ADVISORY IN EFFECT UNTIL 7 PM EST THURSDAY BELOW
3500 FEET...

This Hazardous Weather Outlook is for western North Carolina.

.DAY ONE...Tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Weather Advisory.
   Winter Storm Warning.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Weather Advisory.
   Winter Storm Warning.

.SPOTTER INFORMATION STATEMENT...

Spotters are requested to report snow and ice accumulations to our
office.

NCZ033-035>037-048>050-052-053-056-057-064-065-501>510-181200-
Avery-Alexander-Iredell-Davie-Madison-Yancey-Mitchell-Haywood-
Buncombe-Catawba-Rowan-Transylvania-Henderson-Caldwell Mountains-
Greater Caldwell-Burke Mountains-Greater Burke-McDowell Mountains-
Eastern McDowell-Rutherford Mountains-Greater Rutherford-
Polk Mountains-Eastern Polk-
646 PM EST Wed Feb 17 2021

...WINTER STORM WARNING IN EFFECT UNTIL 7 PM EST THURSDAY...

This Hazardous Weather Outlook is for piedmont North Carolina and
western North Carolina.

.DAY ONE...Tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Storm Warning.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Storm Warning.

.SPOTTER INFORMATION STATEMENT...

Spotters are requested to report snow and ice accumulations to our
office.

GAZ017-018-026-028-029-NCZ082-SCZ004-010>014-019-181200-
Habersham-Stephens-Franklin-Hart-Elbert-Union NC-Greater Oconee-
Anderson-Abbeville-Laurens-Union SC-Chester-Greenwood-
646 PM EST Wed Feb 17 2021

This Hazardous Weather Outlook is for northeast Georgia, piedmont
North Carolina and upstate South Carolina.

.DAY ONE...Tonight.

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

JMP