306
FXUS62 KCAE 231430
AFDCAE

Area Forecast Discussion
National Weather Service Columbia SC
930 AM EST Mon Dec 23 2024

.SYNOPSIS...
Strong high pressure sliding to our north brings chilly conditions
again today, with both daytime highs and overnight lows expected to
be below normal. A weak coastal low will bring a chance of
precipitation to the region tonight into Tuesday, especially across
the eastern Midlands. More seasonal temperatures follow for the mid
to late week period, with increasing chances for rain towards the
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Key Message(s):

- Below normal temperatures continue.
- A coastal low brings the potential for rain, mainly across the
eastern Midlands tonight into Tuesday morning.
- Low chance for a period of freezing rain early Tuesday morning,
mainly across portions of Lancaster and Chesterfield counties.

9:30 AM Update: Satellite imagery shows a coastal low is in the
beginning stages of developing off the coast of Georgia this
morning. This had brought some cloud cover to the Low Country
and it should enter the far eastern Midlands in the next couple
of hours. Any precipitation associated with this low is expected
to hold off until tonight for our area, however. Latest
temperatures readings remain on the chilly side, as expected,
generally in the lower to mid 30s. The rest of the forecast
remains generally the same as discussed in the early morning
discussion below.

Early morning discussion: Much of the rest of today will be
similar to Sunday. High pressure remains entrenched over the
Eastern Seaboard, maintaining a cold, dry air mass over the
forecast area. The ridge will slowly lift offshore through the
day, and temperatures will once again only reach the mid to
upper 40s due to ongoing CAD and northeasterly surface winds.

An upper shortwave will push across the state this evening with
strengthening southwest flow aloft, further enhancing the wedge. As
a result, cloud cover will increase as a coastal surface low
develops offshore to our southeast. Moisture levels quickly rise as
the low tracks off the coast, with PWATs jumping from around
0.25" this morning to around 0.75" by midnight. Isentropic lift
will then promote the possibility for isolated to scattered
showers, particularly from the coastal plan inland to the
eastern Midlands, mainly east of I-20. Guidance has trended
drier and further east with the latest runs, so PoPs were
trimmed back slightly with this package, particularly west and
north of I-20.

The final tricky piece to this forecast period remains the threat
for some brief freezing rain across our northern counties for just a
few hours before dawn on Christmas Eve. Model soundings
indicate there will be a very strong and deep warm nose above
the surface. MOS temperatures guidance for tonight is much
colder than blended guidance, particularly toward the western
Midlands and the Upstate, likely due to the eastward shift in
precip potential. This should result in a tight temperature
gradient somewhere across the forecast area extending from
southwest to northeast, though it is difficult to pinpoint
exactly where the freezing line will lie. The problem then
becomes the timing of any freezing temperatures at the surface
in conjunction with precip development. At this point, believe
the threat is quite low, but the official forecast does mention
slight chances for freezing rain across mainly Lancaster and
Chesterfield Counties for a few hours before sunrise Tuesday.
Ground temperatures and the relative brief period that this
could occur will generally preclude any ice accumulations.
Therefore, no winter headlines have been issued at this time.
Precip elsewhere will remain of the liquid variety. Overnight
lows are expected to range from the upper 20s northwest to mid
30s south and east.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
Key Message(s):

- A brief period of freezing rain is possible across the Pee
  Dee and northern Midlands early Tuesday morning, but
  confidence is low.

- A return to more seasonal temps for mid-week with dry high
pressure to our north.

As the aforementioned coastal low shifts northeastward, WAA and
moisture advection aloft will continue across the northeast Midlands
and Pee Dee into Tuesday morning. A brief period of freezing
rain (see near term section) is possible, until ~9am in these
locations but it should be noted that confidence is very low.
Wet bulb temps in the boundary layer over the last two hi-res
suites has trended warmer, near or above 32 F. Precip intensity
has also trended east, which combined with warmer wet bulb temps
will make any freezing rain very difficult to achieve.
Essentially it would require the coincidence of the very low end
of guidance for temps and dew points, along with the highest
precip solutions which is unlikely. So only a 5-10% chance of
marginal freezing rain continues across the extreme northeast
portion of the FA. Overall, this setup would be fairly atypical
for freezing rain in our area; we typically see freezing rain
from more mature CAD events and with WAA-moisture advection
approaching from the southwest as opposed to a coastal low.

For the remainder of Tuesday, the coastal low will shift northeast
and northerly surface flow will keep relatively dry air in place.
The entire airmass across the eastern CONUS will slowly start to
moderate through mid-week as ridging aloft builds in, even as the
surface high pressure center remains quasi-stationary over
southeastern Canada. So temps will bounce back into the 50`s
Tuesday and again on Wednesday; increasing southwesterly 850mb
flow ahead of a weak shortwave Wednesday may push temps back
into the 60`s across the CSRA. While this shortwave and
associated surface front has consistently trended drier, a
shower or two overnight Wednesday night into Thursday morning is
possible in the CSRA; the most recent 00z suite, especially in
the deterministic members was a bit more aggressive with this
precip.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Key Message(s):

- Near average temps for late week with a warmer and wetter
  pattern expected over the weekend.

The nearly stationary surface high pressure center across the NE
CONUS will dominate the pattern for Thursday and Friday, with
consistent northeast flow keeping temps and moisture near or
slightly below average. As mentioned in the short term, the only
feature of note until later in the weekend is a few showers may
linger across the CSRA and southern Midlands on Thursday as a
weak shortwave pushes through from the northwest. Deep ridging
across the eastern US will then build into the area for
Saturday. GEFS and ECE guidance is consistent in then showing a
sharp trough passage with an associated moisture plume.
However, timing is extremely inconsistent across guidance which
would have some notable influence of the potential impacts. Rain
chances are therefore increasing regardless from late Saturday
through Sunday. ECE members are also showing a modest amount of
instability developing late Saturday and into Sunday, but there
are significant inconsistencies about how the diurnal timing and
synoptic forcing will line.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
High confidence in VFR conditions through much of the TAF
period.

SKC into this afternoon. Clouds gradually increase from the south
and east, but bases should remain VFR into this evening. MVFR CIGS
may then start to develop toward the end of the TAF period,
particularly at OGB/AGS/DNL. Winds expected to be light from the
northeast to calm at times through daybreak before picking up to
around 6-9 kts. Winds then returning to light northeast tonight. A
few -SHRA may start to move in from the south after 00z this
evening, but confidence too low for TAF inclusion at this time.

EXTENDED AVIATION OUTLOOK...There is potential for a few rain
showers and associated restrictions late tonight through the end of
the week.

&&

.CAE WATCHES/WARNINGS/ADVISORIES...
SC...None.
GA...None.

&&

$$