Scientific Forecaster Discussion
fxus64 kbmx 181153
Area forecast discussion
National Weather Service Birmingham Alabama
553 am CST Sat Nov 18 2017
for 12z aviation.
today and tonight.
A wide range of temperatures early this morning with lower 60s
across West Alabama and mid 40s across east Alabama. Warm air
advection has increased significantly for areas along and west of
Interstate 65 where southerly surface winds have been running 5 to
10 mph. Low clouds have also developed for areas across east
Mississippi and western Alabama. No rain was currently indicated
across the region as the air mass is still relatively dry with
precipitable water values around one inch. A few showers may develop
by late morning across northwest Alabama due to strong warm air
advection and increasing low level moisture. Most of the shower and
thunderstorm activity will be confined to near the surface front due
to a strong pre-frontal upper level inversion. The surface cold
front will enter Marion and Lamar counties late this afternoon and
quickly track southeast, reaching Troy near midnight tonight.
Thunderstorm activity looks very limited ahead of the surface front
due to weak mid level lapse rates and cape values struggling to
reach 500 j/kg. Any thunderstorms that may develop will likely be
north of I-59. The rain will end within one or two hours after
fropa, so rainfall amounts will be mostly under one-quarter of an
Sunday through Saturday.
The cold front is well to our southeast by Sunday morning, so expect
clear skies as high pressure builds in with the colder air mass. The
center of the surface high slides eastward through the arklatex and
into the Tennessee River valley Sunday night into Monday morning. Models
are in pretty good agreement with the center moving across north-
central Alabama by sunrise Monday morning. This will lead to clear skies
and calm winds across the area, allowing for very efficient
radiational cooling. Expect much of the area to drop to near or
below freezing early Monday morning with the coldest temperatures
(mid to upper 20s) in the northern counties. With increasing model
agreement in the setup, I will continue mention cold weather impacts
in the severe weather potential statement and increase the confidence. After discussions with
neighboring offices, will hold off any freeze watch or warning
products for now, as we're still 48 hours out.
The high pressure shifts eastward towards the Carolinas during the
day Monday. Another longwave trough deepens across the eastern
plains Monday night into Tuesday. Both the GFS and European model (ecmwf) pick up on
a weak impulse ejecting out of the base of the trough Tuesday
morning. This impulse moves through central Alabama fairly quickly, and
without much moisture return, i'm not convince we'll see any rain
activity out of it. However, have included at least a slight chance
of showers south and east of the I-85 corridor Tuesday just in case.
As we get into the second half of the week, the forecast becomes
somewhat of a headache. Overall, the synoptic setup is a highly
amplified pattern with a deep trough over the eastern US and a ridge
over the western US. An impulse moves through the trough into the
southeast US on Wednesday, and the models have not handled the
evolution of the trough and this impulse very well. The GFS, which
had been showing a phased trough moving through now shows a cutoff
low developing in the northern Gulf of Mexico and sliding eastward
into southern Georgia. The ECMWF, which had been showing something
similar now shows a phased trough moving through with high pressure
quickly building in behind. Now, remember that most of this week
we're under northerly or northeasterly flow, which limits our
moisture return. Under normal circumstances, this would make ME lean
on a dry forecast. The GFS solution, however, has a fairly strong
surface low developing in the eastern Gulf of Mexico, which as it
slides into southern GA, could lead to enough wrap-around moisture
to cause light shower activity in our southeastern counties.
Essentially, the forecast comes down to (1.) If (or where) a low
develops along the coast and (2.) How strong the low is if it forms.
If a surface low does not form, or if it is weak, expect mostly dry
conditions through the Holiday weekend. If a surface low does form
and can pull in enough moisture, expect some light rain showers
Wednesday night through Thursday before high pressure again builds
in Friday into Saturday leading to a dry start to the weekend. For
my forecast this morning, I have edged drier given the lack of
substantial moisture return, but kept a slight chance of showers
south and east of the I-85 corridor on Thanksgiving just in case.
Confidence remains low in the forecast from Wednesday night through
Saturday, so expect changes as models hopefully become more
12z taf discussion.
MVFR cigs developed overnight for areas mainly along and west of I-65.
Increasing southwest flow will advect in Gulf moisture through 00z,
although a strong inversion aloft will hinder shower development
during the day. Cigs near 1000 feet agl will impact ktcl thru
15z, with MVFR cigs at kbhm/keet. Due to strong mixing ahead of
fropa, cigs should stay aoa 3000 feet agl this afternoon until the
cold front arrives. A band of showers will accompany the front
with vsbys 3-5 miles lasting for several hours after fropa.
Instability will be limited, so tstm chances too low for inclusion
in forecast. South to southwest winds will increase to 12-18
knots with gusts to 25 knots ahead of fropa. After fropa, sfc
winds will become NW at 15-20 knots with gusts to 28 knots. Even
after fropa, cigs should stay above 1000 feet agl.
Windy conditions today ahead of fast-moving cold front. Rain
chances increase Saturday afternoon and evening as the front moves
through the area. There are no fire weather concerns at this
time. Dry and colder air sets up for the Sunday and Monday.
Preliminary point temps/pops...
Gadsden 69 39 52 27 57 / 50 80 0 0 0
Anniston 70 40 53 29 59 / 30 70 0 0 0
Birmingham 72 42 53 32 58 / 40 80 0 0 0
Tuscaloosa 73 41 56 30 59 / 40 80 0 0 0
Calera 72 40 54 30 58 / 30 70 0 0 0
Auburn 70 43 55 32 59 / 10 50 0 0 0
Montgomery 75 44 58 33 60 / 10 60 0 0 0
Troy 74 45 59 33 61 / 10 50 0 0 0