NWS Atlanta afternoon disco
.LONG TERM...
(Friday morning through next Tuesday)
Issued at 345 PM EST Wed Jan 8 2025
Key Messages:
- Primary impact continues to be winter weather on Friday into
Friday night. Greatest
probability of impactful
wx will be in
northern Georgia through
metro Atlanta. Areas south of I-20
continue to remain a bit more uncertain.
- Snow looks to be primary precip time in northern Georgia and the
mountains as of this forecast package. Amounts look to be in the
3-6" range. Can`t rule out some mixing in of sleet or freezing
rain that could limit accumulations a bit in some locations.
- Precip type continues to look primarily frozen in the
metro and
points east. Snow would be the onset precip type that could
accumulate and impact morning commute. A switch to sleet and/or
freezing rain would during the afternoon. Can`t rule out a brief
transition to rain.
- South of I-20, uncertain if we can remain below freezing during
the day on Friday. We will
likely start during the morning hours
with snow, but this would quickly transition to rain.
Forecast:
Not too much change in the overall forecast with the update to this
forecast package. Starting to get access to some of the higher
resolution guidance, and it is painting a similar picture to what
the forecast has been thus far. Snow expected in the north, a
transition area in the
metro and points east, including Athens,
and then for central Georgia an initial period of snow before a
quick transition to a cold rain.
Synoptic setup is as follows - cold
air mass continues to linger in
the area during the day on Thursday. We will see another wave pass
by the area on Thursday aloft that will be dry, but will push a
reinforcing shot of cold air in on Thursday into Thursday night.
This will set the stage for winter precipitation on Friday, allowing
our surface
air mass to be cold enough for some sort of frozen
precip. Two significant
upper level features, a cut off low over the
desert southwest, and another
shortwave quickly moving within the
polar
jet, will phase together and eject into the southeast on
Friday into Saturday. Wave will interact with significant
baroclinicity in place across the Gulf coast with cold air settled
in place and generate strong surface low within region of
PVA ahead
of it. This will bring precip and the potential for winter weather
across wide swath of the south, including our
CWA.
Confidence remains high in seeing impactful snow accumulations
across northern Georgia.
Ensemble guidance has come down a little
bit in totals in these areas, and this forecast package reflects
that a bit. A few things going on here - the biggest being models
seeming to be a bit warmer at the surface, as well as some models
seeming to better resolve a warm nose. Precip would
likely start
during the morning hours, around or a bit after sunrise, and remain
snow for a good bit of the day, if not the entire day. Current
forecast amounts would be between 3-6" inches in this area. A
huge caution with looking at 10:1 ratio forecast maps - current
SLRs used in this package are closer to 5 and 6 to 1.
Sounding
profiles are near isothermal for a good chunk of low levels and
approaching mid levels. This is
likely to be a wet,
heavy snow.
There is a chance that would could see some sleet or freezing rain
mix in, especially if some of the more aggressive guidance with
the warm nose occurs, which would act to limit accumulations to
the lower end of projections.
In
metro Atlanta and points eastward, including Athens, we are
expecting to see several different phases of precipitation. The
initial onset during the morning hours looks like it should be
snow.
Hi res models show significant drying in the low levels just
off the surface that will
likely quickly adjust
thermal profiles
to be below freezing in the entire column as initial snowfall
sublimates while falling. From there, a transition to sleet and
possibly freezing rain is expected to happen moving into the
afternoon. Big model discrepancy as to how quickly this transition
will happen. Latest HRRR (18Z), for instance, has come in with an
impressive solution that would
likely pile up an inch or two of
snow in these areas before transitioning in the late afternoon as
the warm nose pushes into the area. Other models have even earlier
transition (for instance, 12Z 3km
NAM) as warm nose in these push
in even faster. Right now, forecast expectations are 1-3 inches
of snow and/or sleet with the potential for a tenth to 2 tenths of
ice. Really want to emphasize an important point here - if models
which show earlier transition are right, there is absolutely
potential for a decent
ice storm, with decrease in snow totals and
increase in ice totals. Please stay tuned to this portion of the
forecast.
Other bit of uncertainty is on surface temperatures in the
metro and
areas east (and really south into central GA). We will need to find
a way to offset the
latent heat release required to
freeze liquid
water to objects for any significant
ice storm. Signs of in-situ
wedge being able to form is noted in a lot of the model guidance.
This could certainly be a way for us to "hang on" to even colder
surface temps than many of the models have shown. Dynamic cooling
may also be possible, especially if models showing substantial 925
mb-850mb drying in lead up to storm are accurate. This would allow
us to physically drag some of the colder air aloft down to the
surface, especially in heavier precip.
Finally, getting into central Georgia - initial onset looks like it
may also be snow during morning hours as the initial
WAA pushes
through the area as the surface low starts to progress east. The
expectation is that it will be a bit more difficult to hold on to
the cold air here at the surface. A period of sleet or freezing rain
may be possible in some locations before a quick transition to a
cold rain. This would
likely lead to limited impacts during the
morning hours. A
winter weather advisory may be needed in some
counties that are south of the current winter weather
watch to cover
morning impacts, but given high uncertainty in potential impacts,
will hold off on any
watch issuance for these areas.
Speaking of the winter weather
watch - small southward expansion for
a few counties in eastern Georgia with this update, otherwise no
large changes. The overnight forecast package will be the earliest
we see any upgrades to these, and given onset time, and potential
uncertainties, it is possible this may not occur until the afternoon
tomorrow. Want to emphasize the timing of when impacts may begin in
the current
watch area - snow may begin falling right across the
morning commute for many areas, especially to the west. Surface
temps at onset are in the mid to upper 20s and it will have been
quite cold in the lead up to this storm. Soil temps are cool per UGA
mesonet near surface. Put it all together, and snow will
likely
begin to stick pretty quickly, especially if we get any decent
snowfall rates. This would
likely include roadways. Untreated roads
may become icy and hazardous very quickly. If you can, please
consider staying off the roadways on Friday.
Winter weather looks to come to an end on Friday night into early
Saturday morning. Cold air settles back into the area, and combined
with any potential
snowpack, highs on Saturday are only in the 30s
to 40s for most of the area. Saturday night is looking cold.
Widespread teens are expected including into areas even south of I-
20. Areas in central Georgia will drop into the lower 20s. For the
rest of the long term, temperatures are pretty average for this time
of year with not much to talk about (just yet).