570 
FXUS64 KBMX 292153
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
353 PM CST Thu Nov 29 2018

.SHORT TERM...
Through Tonight.

Westerly flow aloft and southwesterly low level flow around 
surface high pressure centered to our southeast has resulted in an
increase in clouds and light rain showers today. Most of this
activity has been across the northern half of the area, further
from the influence of the surface ridge. Tonight, a weakening
surface low will move out of the Central Plains toward the Ohio
River Valley, leaving a surface trough/convergent zone to our
north. With increasing moisture and lift south of this zone, 
shower coverage will increase overnight, especially across the 
north and west. Temperatures will be noticeably warmer, with lows 
in the upper 40s east to upper 50s west. 

14

.LONG TERM...

Friday and Friday Night.

Low level ridging is in place over the Florida Peninsula on Friday 
as a trough digs into the Rocky Mountains. A low level jet stretches 
through much of North MS, into the TN Valley and Eastern TN. The 
presences of this jet, in conjunction with isentropic lift, will 
allow for scattered to numerous rain showers to develop, mainly 
north of the I-85 corridor. The best chances will remain north of I-
59 early during the day on Friday. As the trough deepens into the 
Southern Rockies Friday afternoon, the pattern becomes more 
amplified, leading to temporary height rises over Central AL in 
response. This amplifying pattern lifts the low level jet 
northeastward, which could lead to a brief period of rain-free 
conditions sometime Friday evening, but confidence in an exact time 
of this happening is low at this time, so for now I've kept at least 
chance PoPs in the forecast through the evening hours Friday. 

At this same time, a surface low develops on the lee side of the 
Rockies and begins moving eastward through the Central Plains. This 
low will lift an effective warm front west to east across Central AL 
late Friday night into Saturday (after midnight Friday) leading to 
widespread rain with a few elevated thunderstorms.

25/Owen

Saturday and Saturday Night.

A potent upper level trough will become negatively tilted Friday 
night over north Texas and western Oklahoma then will swing east 
over the rest of Oklahoma along with into Kansas, Missouri and 
Arkansas by Saturday morning. The trough will rapidly move 
northeast into the Ohio River valley by Saturday evening with the 
upper low positioned over southern Iowa after sunset.

Toward the surface, low pressure low will develop in response to 
the upper trough and strong forcing aloft across northwest Texas 
and western Oklahoma Friday night and will continue to deepen as 
it tracks northeast, becoming centered over eastern Kansas by 
Saturday morning and begins to occlude over southern Iowa by Early
Sunday morning as a new low pressure area develops near Chicago.

Most global models generate a convective complex near or offshore
of the Louisiana coastline by 06z Saturday which moves northeast 
into Southern Alabama and the Western Florida Panhandle by 12z and
propagates further northeast into the Central Florida Panhandle 
and into southwest Georgia by 18z.

We will have the Low Level Jet (LLJ) present over our northwest 
counties as early as 09Z Saturday and exits by 15Z followed by 
another LLJ that overspreads our western and northwest counties by
21z and will move east across our north-central counties before 
exiting the state by 00z Sunday.

Wind profiles including bulk shear and helicity values are more 
than supportive of rotating thunderstorms and poses a tornado 
threat. Hail may also be produced by the stronger storms Saturday 
afternoon as height falls begin to be realized with the 
approaching upper level trough.

Surface-based instability remains in question with heavy coastal 
convection expected to our south along the Northern Gulf Coast 
through 15z and this will help limit low-level instability over 
our forecast area for Saturday morning and will confine the window
for destabilization roughly from 18z through 00z for conditional 
convective development along and ahead of a surface trough that 
will move into the state by 00z Sunday and should exit the state 
before 12z Sunday.

A substantial dry slot is expected to overspread much of our CWA 
by 21z and this will help suppress notable convective development 
for the remainder of the evening.

Despite very favorable wind profiles, wind shear and helicity 
values, the tight time frame for low-level destabilization coupled
with the lower sun angle along with reduced daytime hours coupled
with expected cloud cover over our area and points south will 
hamper solar insolation and make low-level destabilization largely
dependent upon low-level advection which will be aided by the Low
Level Jet but confidence is low that sufficient instability will 
materialize to support more than the potential for a few strong to
severe storms Saturday late morning through early evening.

If coastal convection is not as robust as expected the severe 
weather risk would substantially increase across our forecast area
while if coastal convection is more substantial and persists 
later the severe weather risk would further decrease across our 
forecast area. The feature to watch will be the timing and 
strength of the shortwave trough that lifts that coastal 
convective complex northeastward from the northern Gulf of Mexico 
into south Georgia by Saturday night.

05

Late Saturday Night through Wednesday.

By late Saturday night, the surface low has moved into the Great 
Lakes region and progressive forcing has diminished, which 
substantially slows the forward speed of the main cold front as it 
approaches Central AL. Guidance hasn't handled this aspect of the 
system very well over the last few model runs, so the question will 
be how far southeastward does the cold front push before it stalls. 
I expect it at least push through a portion of the area, so I'm 
keeping rain chances high in the southern half of Central AL, and 
lowering PoPs generally north of I-20/22 on Sunday. This won't last 
long, as a shortwave impulse slides through the main upper level 
trough and develops a weak surface low in the Gulf States that lifts 
the cold front northward as an effective warm front Sunday night 
into Monday, leading to at least a chance of rain area-wide. By 
early Tuesday, the main 850mb trough axis has slid southeast of our 
area, which should push the cold front out of Central AL by Tuesday 
afternoon, ending rain chances from northwest to southeast. 

A cooler airmass dips into the Southeastern CONUS Wednesday into 
Thursday as low level ridging builds in ahead of another deepening 
trough in the Pacific Northwest. This should lead to clearer skies 
and cooler weather for Central AL as we get through the middle of 
next week. 

25/Owen

&&

.AVIATION...
18Z TAF Discussion.

Scattered light showers/sprinkles will move across north Central 
Alabama through the afternoon. These should not cause aviation 
impacts at terminals. Expect MVFR cigs to spread from west to east 
across the area late this afternoon through the early evening. Rain 
showers will become more widespread overnight, reducing visibilities 
to MVFR, with cigs falling to 1000ft by Friday morning. 

Generally, winds will have a southerly component through this 
afternoon, varying from southwest to southeast at times, and cannot 
rule out a few gusts to 15-17kts. The one exception will be ANB that 
could maintain a more persistent northeast wind due to terrain. 
Overnight, winds will become consistently southerly, at 7-10kts 
through tomorrow morning.

14


&&

.FIRE WEATHER...

Low level moisture will increase overnight with clouds deck at or
below 1kft Friday morning. The main focus for rain showers 
through tomorrow afternoon is across the west and north. This 
activity will become widespread Friday night through Saturday. 
Cannot rule out thunderstorms on Saturday. No fire weather 
concerns are expected at this time.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     50  62  54  65  58 /  60  70  70 100  50 
Anniston    49  64  54  67  60 /  40  50  70 100  40 
Birmingham  55  66  59  69  59 /  60  60  90 100  30 
Tuscaloosa  57  70  61  73  58 /  60  60  90  90  10 
Calera      53  66  58  69  61 /  40  50  90 100  20 
Auburn      48  65  55  67  63 /  20  20  70 100  30 
Montgomery  52  70  60  73  64 /  20  20  90 100  30 
Troy        51  69  59  71  66 /  10  10  90 100  40 

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None. 

&&

$$