Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 191148
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
548 AM CST Thu Jan 19 2017

.UPDATE...
For 12Z Aviation.

&&

.SHORT TERM...
Today and Tonight.

Increasing broad scale lift will occur today as the first of many
s/wv trofs move through the central Gulf Coast region.  Mostly light
showers expected this morning as the air mass is still relatively
dry and stable. PWATS will increase significantly by 18z today west
of I-65, with numerous showers and thunderstorms across Mississippi
in zone of large-scale lift ahead of s/wv trof. This activity will
spread slowly eastward due to activity nearly parallel to upper
level flow.  While cell movement will be be close to 50 knots, the
slow eastward progression of the convective activity will result in
decent rainfall amounts, especially across the northwest counties.
Rainfall amounts will easily reach 1-2 inches across the northwest
counties with local amounts near 2.5 inches. Rainfall totals along
the I-59 corridor will be the in 1 to 1.5 inch range with amounts
generally less than 1 inch south of I-20.  Severe potential still
looks rather limited today across central Alabama. The limiting
factors are poor mid level lapse rates (<6 Celsius) and
unidirectional wind flow in the 925-500mb layer.  The axis of
maximum lift will move across the area tonight, and maintained high
rain chances for the entire area.  With the forcing and instability
waning by the late evening, thunderstorm activity will also diminish
and will end severe threat by midnight.

58/rose

.LONG TERM...
Friday through Thursday.

For Friday, a strong upper jet moves across the Southwestern US and
Northern Mexico behind a fairly broad upper trough pattern. Some
wedging along the Gulf Coast could spark some showers on Friday,
especially for our southern counties.

Late Friday night through Sunday morning look to have the best
chances for severe weather. Overall, sufficient deep-layer shear is
in place throughout the weekend to support supercell development
when and where other ingredients (forcing and instability) fall into
place.

Friday night into Saturday morning, before sunrise, both the GFS and
ECMWF show a low level jet in place across the southern portions of
Central AL with a weakening upper-level jet and an embedded 850mb
shortwave moving through. This could potentially spark an early
round of storms for our southwestern counties where better
instability exists. This lines up with the SPC Day 2 outlook
covering our extreme southwestern counties in a Slight Risk.

Saturday afternoon, the low level jet shifts eastward and the upper-
level jet weakens. However, models still indicate the possibility of
of some thunderstorms moving through. The question for Saturday
afternoon is how much instability is able to build in behind the
coastal convection earlier in the day. If the rain and storms to our
south continue into the afternoon, this could limit thunderstorm
activity. The NAM and GFS have more substantial instability building
in (NAM having >1500 J/Kg, GFS having >2000 J/kg), while the Euro
has remained consistent on the lower end of the spectrum at around
700 J/kg MUCAPE.

Saturday night into Sunday, an upper low develops on the lee side of
the Rockies, setting up an upper-level diffluent pattern across the
region. This upper low begins to pull in the strong jet streak
across the area, providing ample upper-level support for any
thunderstorms that develop. Models are showing another shortwave
moving through late Saturday night into early Sunday morning, which
could initiate another round of thunderstorms. Models keep
sufficient instability and shear in place that would support large
hail, damaging winds, and tornadoes.

Threats: We want to continue to emphasize the multiple rounds of
potential severe weather. With the environmental setup and dynamics
in place, all modes of severe weather (hail, damaging winds, and
tornadoes) are possible. Exact timings are still uncertain as many
pieces have to come together. For now:

1)Friday into Saturday morning (before sunrise) mainly for our
southwestern counties as shortwave trough moves through Southern
MS and into Southwestern AL.

2)Another round is still questionable for Saturday afternoon as it
would be highly dependent on how much lingering convection is in
place from the morning storms just south of our area. This round
could be significant if it can develop, considering the ample wind
shear, instability, and low-level lapse rates.

3)Saturday evening into Sunday morning could also be on the
significant side with stronger upper-level dynamics, wind fields,and
instability.

By Sunday afternoon and evening, the deepening low pressure system
moves across the Southeastern US, providing a chance of rain.
However, models have scaled back on the instability during this
time, so we`ll likely see mostly rain showers lingering into Monday
evening. Ridging builds in Tuesday, clearing out rain chances. Later
on Tuesday, an upper low develops over the Great Plains and moves
northeastward over the Great Lakes. This could increase rain chances
for Wednesday evening into Thursday as the frontal boundary moves
in.

25/Owen


&&

.AVIATION...
12Z TAF Discussion.

Widespread LIFR cigs/fog have developed across south Alabama and
this area was spreading rapidly northward. The LIFR conds will
likely reach KEET and KTCL by 14z, with cigs at KBHM, KASN, KANB
likely becoming IFR between 15z and 18z. Cigs will improve to
MVFR at KMGM and KTOI by 18z as drier air aloft in that area
mixes downward. Cigs across the northern TAF sites will not
improve much during the afternoon as rain moves into the area from
the west. The activity will mostly be shwrs thru 18z, but tstm
activity will increase after 18z as upper lift associated with a
short/wave trof approaches the area. Rainfall intensities may be
heavy at times reducing vsbys to less than 1 mile. Widespread
showers and tstms will continue across all of central Alabama
overnight, generally diminishing from west to east after
midnight.


58/rose

&&

.FIRE WEATHER...

Unseasonably mild temperatures will continue through the entire
week. A wet and chaotic pattern sets up today through Sunday.
Afternoon relative humidity values will generally remain above 40
percent due to the southerly winds bringing gulf moisture
northward. Some strong to severe storms can be expected today,
Saturday and Sunday morning. Rain amounts through Sunday will be
3 to 4 inches.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     68  55  70  55  68 /  80  90  30  40  70
Anniston    70  57  72  57  69 /  70  90  40  40  70
Birmingham  69  59  72  58  69 /  90  90  30  40  70
Tuscaloosa  68  58  73  58  72 /  90  90  20  50  70
Calera      70  58  72  58  70 /  80  90  30  50  70
Auburn      72  59  72  61  70 /  60  90  40  60  80
Montgomery  72  60  76  60  73 /  60  90  40  70  80
Troy        75  60  76  60  73 /  60  90  40  70  80

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Dense Fog Advisory until 9 AM CST this morning for the following
counties: Autauga...Barbour...Bullock...Dallas...Elmore...Lee...
Lowndes...Macon...Marengo...Montgomery...Pike...Russell.

&&

$$


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