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FXUS64 KBMX 171722

Area Forecast Discussion
National Weather Service Birmingham AL
1118 AM CST Tue Jan 17 2017

For 18Z Aviation.


Today and tonight.

Main focus will be across the northern counties as a cold front
begins to approach the area from the west. The main surface low is
currently near Chicago and will continue to slide east northeast
during the day. As it approaches the ridge it will continue to move
away from the area and the main surface boundary will stall out
across our northern counties. This will allow areas along and north
of I-20 to see the best chances of rain through tonight before the
short wave works through the area by Wednesday morning. While the
best chances will be across the north, there will be just enough
lift around to support an isolated shower or two across the south as
well, so included rain chances here as well. Above normal
temperatures to continue again today although the well above normal
or near record levels will be confined to the far southern counties
due to the additional cloud cover and rain chances. 


Wednesday through Tuesday.

The majority of the extended discussion will focus on the severe
weather potential for Thursday and Saturday/Sunday.

For Wednesday, ridging briefly builds into the Southeastern US. Have
decreased PoPs as models have trended drier and the departing
front staying to our north. 

On Thursday, a deepening upper low moves across the Great Plains as
a surface low develops in southeast Texas and moves into the Ohio
Valley. Both the GFS and ECMWF develop a low level jet ahead of the
trough as it becomes more negatively tilted. This results in a more
southerly low-level flow bringing a more moist/unstable air mass
into Central AL. By Thursday afternoon, dewpoints across the area
are expected to be in the low to mid 60s. The lapse rates over our
area aren't particularly impressive, at around 5-6 C/Km. However,
600-700 J/Kg MUCAPE with 40-45kt 0-6km Bulk Shear does support a
limited severe weather threat. Currently, guidance shows the best
environmental shear and instability overlapping along and south of
I-20, giving this area the greater chance for severe weather. With
that said, will continue to mention a limited threat area- wide
with lower confidence.

Another, potentially more impactful, system makes its way into the
area over the weekend. A longwave trough digs across the Central US
Saturday from the Pacific Northwest. Some timing and model trend
issues continue, but for now, both the GFS and ECMWF show an
embedded low-level shortwave developing across southern Texas and
initiating some coastal convection with the support of strong
upper- level jet nosing into the area. The ECMWF tries to bring
this convection further north into Central AL, while the GFS keeps
it south of our area. By Saturday evening, both models show lee
cyclogenesis occurring with a closed low deepening as it moves
across the Southern Plains, and the trough becoming more
negatively tilted. 00z GFS and ECMWF had the best dynamics
(90-110kt upper jet and significant 500mb vort max) about 9 hours
behind the best instability (800-900 J/kg MUCAPE). However, even
with relative lower instability, the 55-65 kt Bulk Shear is more
than sufficient for severe weather to develop. As I'm writing
this, the 06z GFS came in with the low tracking further south,
providing much higher deep layer shear (70-80 kts). Also, with
more instability (~1500 J/kg MUCAPE) building in on Sunday,
matching up better with the stronger upper-level support. Hence,
run-to-run trends are still casting uncertainty into the forecast.
For now, will continue to mention the elevated threat for severe
weather for the weekend. Expect timing and strength forecast to
change as models begin to trend more consistently one way or the



18Z TAF Discussion.

Cloud cover will remain across the northern sites and gradually
increase across the southern sites this afternoon as a weak cold
front arrives from the northwest. Pushed window for VCSH back 3-6
hrs and dropped thunder mention, could still have an isolated TSRA
but potential too low to include in this cycle. Low level
southerly flow will persist at most sites today with some breezy
conditions at times. Expect a low level northwest flow to develop
from NW to SE tomorrow as the front drifts across the area and
becomes nearly stationary around MGM/TOI. Expect CIGs again to lower
overnight with reduced VIS possible at all terminals toward daybreak
on Wednesday. Slow improvement in CIGs expected Wednesday except for
MGM/TOI if the front stalls nearby.




Unseasonably mild temperatures will continue through the entire
week. Rain chances return today with a wet and chaotic pattern
setting up Wednesday through Sunday. Afternoon relative humidity
values will likely stay above 40 percent due to the southerly winds
bringing gulf moisture northward and rain.


Gadsden     70  56  64  52  66 /  50  60  20  40  60 
Anniston    71  59  67  55  68 /  20  50  20  30  60 
Birmingham  71  59  67  57  69 /  30  50  20  40  80 
Tuscaloosa  74  60  70  58  70 /  30  40  20  40  80 
Calera      72  61  69  57  69 /  20  40  20  30  70 
Auburn      73  61  70  57  68 /  20  20  20  20  50 
Montgomery  77  61  74  59  73 /  20  20  20  20  60 
Troy        77  62  75  59  72 /  20  20  20  20  60 



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