Scientific Forecaster Discussion
fxus64 kbmx 151756
Area forecast discussion
National Weather Service Birmingham Alabama
1156 am CST Sat Dec 15 2018
for 18z aviation.
today and tonight.
Here comes the upper low. Right now the center of the upper low
according to water vapor is near Philadelphia, MS and moving due
east. This is a little further south than a few of the short term
models, but still expecting a more east-northeast, northeast turn
of the low through the day. This would indicate that light showers
may be further south than previously forecast and falls in line
with some the shorter range models. In fact most of the short
range models and ensembles say that the entire forecast area will
at least have a chance of a shower this afternoon and the first
part of tonight. Conceptually, I agree and have included at least
a slight chance of rain showers this afternoon all the way to Pike
and Barbour counties. Still would expected the best chances to be
on the south and west side of the upper low early this morning,
but then as the low slides northeast, we should see the moisture
wrap all the way around and increase in the northeast ahead of the
low by late morning and increase in coverage across the north
through the afternoon. Based on the latest projected position of
the low this afternoon, the best rain coverage will be right along
the I-20 corridor and points northward.
As we move into the evening, we will begin to see the downward
trend of the rain as the upper low begins to exit the area. Even
though the moisture levels will begin to decrease, I can not rule
out some isolated showers early on across any portion of the area,
so continued with at least slight chance for the area. Still
expect best chances to be in the northeast, so went with chance
pops there. Looks like cloud cover should remain fairly thick and
may limit the amount of fog to generally the south where the
clouds are the lowest.
Sunday through Friday.
A closed upper low over the Carolinas will continue to move
eastward on Sunday, while northerly low-level flow and cold
advection prevails across Alabama. Forecast soundings indicate
moisture will become trapped in the surface to 900 mb layer
beneath a subsidence inversion, and a stratus deck is likely to
persist through Sunday afternoon particularly across the eastern
two-thirds of our forecast area. Temperatures in those locations
may struggle to climb out of the 40s, and our forecast leans
toward raw model guidance for afternoon highs.
Dry and mild conditions are expected for Monday and Tuesday with
high pressure in control, but our next system will begin to take
shape as a trough digs across the Southern Plains. Isentropic lift
and an approaching shortwave will probably lead to light to
moderate rain Wednesday night into Thursday. Rain chances have
been raised to 50 percent for this period, and will likely
increase with the next update as confidence increases in timing
and placement. Models generally agree that a strong jet Max will
move southward into the base of the trough, causing significant
amplification and deepening. Low 500 mb heights are indicated by
the European model (ecmwf) and GFS Thursday night into Friday as the trough axis or
upper low moves through. This setup favors shower activity due to
steepening lapse rates, and wrap-around precipitation could end
as a rain/snow mix across the north on Friday.
18z taf discussion.
The bulk of the rain has ended for central Alabama taf locations.
I could foresee a stray sprinkle or bit of drizzle for a short
time, but I don't (at this point) think it will be enough to have
any additional impact to ceilings or visibility.
Satellite imagery does show extensive cloud cover continuing,
although there were a few "sucker holes" mixed into the overcast
as well. Those holes and thin spots in the overcast make for a
difficult ceiling forecast through the afternoon. I think the
predominant ceiling height early on will be around 2500 ft, with
occasional excursions a bit lower. Computer model moisture
profiles do suggest the clouds will fill in any remain holes by
00z, with a lowering ceiling trend as well. So, the forecast will
remain MVFR early on, becoming primarily IFR overnight. Perhaps
some improvement right at the very end of the forecast period
tomorrow, but a more definitive improvement in conditions
shouldn't be expected until after 18z Sunday.
Scattered showers are expected mainly north of I-20 today, and
dry conditions should return for Sunday through Tuesday. Low
clouds are expected to persist through Sunday afternoon. Relative humidity values
will remain above critical values, and there are no fire weather
concerns at this time.