Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Birmingham Alabama
547 PM CST Monday Dec 22 2014
for 00z aviation.
tonight through Tuesday.
Challenging forecast remains ahead of US as a very dynamic system
takes shape overnight tonight and moves through central Alabama
Tuesday night through the day on Wednesday. First...lots of activity
on radar this afternoon associated mainly from two factors. The
first is isentropic lift and ahead of the warm front to our south
that is currently located along the Gulf Coast. The second is due to
lift from a shortwave rotating around the base of the longwave
trough currently located in the arklatex. Steep lapse rates aloft
have actually allowed for a few elevated updrafts. Nickel size hail
was reported in Lowndes County...and a few more small hail producers
won't be out of the question through the rest of the afternoon and
evening with 500mb temperatures around -17 to 18c.
As the warm front continues to lift northward overnight
tonight...temperatures should rise with warm air advection on the
increase. Showers with a few storms will continue to be scattered to
numerous in nature. The best chance of more widespread rain appears
to be in the northern half of central Alabama tonight as the best
lift should remain north of the warm front.
Taking a look at the highly advertised system coming through on
Tuesday and Wednesday...it now appears that the greatest impact from
this system may be heavy rainfall more than severe storms. Precipitable waters
advertised from 1.6 to 1.7 inches are close to the 99th percentile
if you look at precipitable water climatology. Forecast soundings also indicate
that thunderstorms will be quite proficient rainfall producers with
tall skinny cape profiles and plenty of divergence aloft to sustain
updrafts. Although we've been relatively dry so far this fall and
are still under drought categories...we're concerned about training
thunderstorms given the continued southwesterly flow aloft and
additional storms expected to develop from tonight through Wednesday
morning. We're going to go ahead and issue a Flash Flood Watch for
generally our southeastern counties where confidence of rainfall
amounts of 2.5 to 3 inches is the highest. Localized amounts of up
to 4 inches can be possible where storms move repeatedly over the
same areas. We'll closely monitor ongoing trends to see if the watch
area would need to be expanded northward if necessary.
Getting into the details about the severe weather threat...it
appears the overall threat should remain rather limited with this
system. The amount of clouds and widespread rains that have been
consistently advertised will be the big factor to help prevent more
widespread severe storms to develop. As showers and storms develop
along the Gulf Coast...the best moisture and instabilities are
expected to stay shunted off to the south. Given the dynamic nature
of this system...we're still going to highlight the low-confidence
severe threat generally along and south of I-20 starting Tuesday
afternoon through early Tuesday evening...then shifting farther
southeast during the overnight and early morning hours on
Wednesday. During this timeframe the 850mb jet really Cranks up to
around 50 knots Tuesday evening and remains around 40 knots
overnight into Wednesday morning. With the current instability
values that are expected...a low threat of severe storms will be
possible. An isolated weak tornado can't be ruled out...along with
a few damaging wind gusts. Of course...if more instability can be
realized we might be looking at a different situation and we'll
continue to analyze the latest trends as they evolve. Surprises
can occur with these cold season dynamic systems.
Wednesday through Monday...
Rainfall begins to push out of the area on Wednesday morning with
showers ending across east Alabama before noon. The high temperature
will come in the morning with cooling temperatures through the day.
Christmas day looks cool but dry with morning lows in the 30s and
afternoon temperatures in the 48 to 53 range.
Warmer Friday and Saturday but a fast moving shortwave will approach
the southeast Saturday evening with a chance of rain late Saturday
through Sunday. Went with a general Euro/Canadian blend...
however...there is some considerable spread from the 12z models on
this feature and details will have to be worked out a little later
in the week.
Just beyond the forecast period...another round of heavy rain may be
possible through the middle of next week.
00z taf discussion.
IFR to LIFR conditions will prevail throughout the taf period.
Scattered light showers and drizzle will continue overnight with a
warm front currently just inland along the Gulf Coast. Isentropic
upglide and an increase in low level moisture will produce low
ceilings and visibilities throughout much of the taf cycle. There
are already some reports of 1/4sm visibilities along the front and
expect these conditions will move in along with the front during
the overnight hours. Improvements will be slow after sunrise with
the possibility of MVFR conditions after 18z. However...numerous
showers are expected at the terminals with thunderstorms possible
at kmgm and ktoi during this time.
Flash Flood Watch from Tuesday morning through late Tuesday
night for the following counties: Autauga...Barbour...Bullock...