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> Dec 27-28, 2017 SE Ice Threat OBS, Forecasts and OBS
post Dec 27 2017, 07:07 AM
Post #1

Rank: F5 Superstorm

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Joined: 10-August 09
From: SW Coastal CT
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A thread for each day this month? What an active month.

National Weather Service Peachtree City GA
640 AM EST Wed Dec 27 2017

.SHORT TERM /Today through Thursday/...
The short term forecast is complex this morning. We are dealing with
two potential mixed precip events...this morning and again Thursday
morning. A short wave in the Gulf was currently spreading rain
across AL and GA.
Most of the temperatures remained at or above
freezing but the dew points over the north half of GA were mostly
below freezing.

7amEST map.. Cold rain in some areas..

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Ice Threat Areas

Attached Image

This post has been edited by NorEaster07: Dec 27 2017, 07:10 AM
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post Dec 27 2017, 07:13 AM
Post #2

Rank: F5 Superstorm

Group: Member
Posts: 21,104
Joined: 10-August 09
From: SW Coastal CT
Member No.: 18,864

More technical from NWS Raleigh from yesterday. Sounds like it's a Gulf shortwave, a front, an inverted trough, and a coastal low that's causing all this.

National Weather Service Raleigh NC
220 AM EST Wed Dec 27 2017

As of 940 PM Tuesday...

...Winter weather advisory for a light freezing rain threat
continues over the SE Sandhills and SE Coastal Plain late tonight
into Wed morning...

Have expanded the advisory area slightly based on the latest hi-res
guidance to include Wilson and Harnett counties, although confidence
remains moderate to low regarding the surface ptype. The coastal
inverted trough is slowly sharpening along the SE coast, and it is
along this boundary that a baggy surface low will form late tonight
before moving out over the Atlantic Wed morning
. It is still
believed that passage of a weak mid level perturbation and a 25-30
kt SSW low level jetlet nosing into S-central and E NC overnight
will prompt light precip to break out east of I-95 from SW to NE
tonight into early Wed. Factors which might favor a significant
wintry weather event are absent in this case: The surface high (cold
air source) is still well off to our NW; dewpoints are already
climbing into the teens and 20s from their afternoon lows; forcing
for ascent will not be especially strong or deep; and the surface
low will be too weak and fleeting to provide a strong low level
advection pattern that would allow for a feed of lower dewpoints
into the area to counteract the latent heat released by the freezing
process. Precip should actually be mostly light and patchy until
expanding in coverage and intensity over E NC Wed morning as the
right entrance region of an upper jet passes over the area, but by
this time temps should be warming. Also, current wet bulb temps are
already above freezing in our far SE, and the latest hi-res models
indicate wet bulbs hovering near the freezing mark as the precip
develops and through Wed morning before rising. A few small
adjustments were made to the anticipated pattern of precip, and as
such, there now appears to be enough of a chance of light icing in E
Harnett county and in Wilson county that I felt compelled to add
them to the advisory. While this advisory is likely to be just a
minor event at most, even a small amount of icing on the roads can
create significant travel headaches if travelers are not cautious,
so better safe than sorry in this case, given the envelope of
possible outcomes. Temps in a few spots across the N and W have
dropped a bit faster than indicated in the earlier forecast, so have
adjusted lows downward a degree or two, mainly over the N and W. But
temps should start leveling off shortly, if they haven`t already, as
clouds overtake the area. lows 25-31. -GIH
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