FXUS64 KBMX 272052

Area Forecast Discussion
National Weather Service Birmingham AL
352 PM CDT Thu Apr 27 2017

Tonight through....

Slow eastward progressing cold front remains across east Central
Alabama this afternoon, generally from Wedowee to Fort Deposit.
Latest obs show dewpoints just behind the front in the low 60s.
Ahead of the front, dewpoints are in the 70s. Once the main line
of storms moves east of Barbour Co, over the next hour,
instability will drop, however a few showers and possibly a
thunderstorm will be possible ahead of the front. This activity is
not expected to be severe.

Activity will dissipate this evening as the front slows and
stalls near our southeastern border. Winds will turn to the
southeast, and by Friday morning, the front will begin to lift
northward. Ahead of the front low clouds and fog are possible
across the area. With dewpoints remaining high and the increase in
cloud cover around sunrise, overnight lows will be muggy, with
temps in the mid 50s northwest to upper 60s southeast.

Temps will warm into the mid to upper 80s tomorrow, with
instability increasing south of the warm front, isolated showers
and storms are possible. Upper level ridge axis moves through the
area Friday night and Saturday morning, which will help suppress
most daytime activity. With deep layer southerly flow, there could
be isolated activity around the edge of the ridge, mainly across
far west Central Alabama Saturday afternoon.


Sunday and Sunday Night.

A deep longwave upper-level trough will advance eastward across the
central US Sunday and Sunday night, upstream of a strong 592
decameter 500mb anticyclone centered southeast of Cape Hatteras.
Strong wind fields will be in place on the east side of the trough,
with an 80-100 kt mid-level jet and a 45-55 kt low level jet. A
surface low will be occluding near the Iowa/Missouri border with a
cold front advancing eastward from near the Mississippi River on
Sunday morning to Georgia by Monday. The meridional upper-level flow
between the amplified trough and ridge will bring in tropical
moisture from near the Bay of Campeche with PWATs of 1.8 to near 2
inches. This flow will also bring in very warm 300 mb temperatures
around -35C and fairly warm 500 mb temps around -10C, as the mid-
level cold core will remain northwest of the forecast area. This
will be partially offset by dewpoints in the upper 60s to low 70s
with strong southerly flow out of the Gulf, and temperatures
climbing into the 80s. This will allow moderate instability to
develop with CAPE values in the neighborhood of 1500 J/kg, and 0-6
km bulk shear values will be around 50 kt, increasing overnight.
Strong height falls/upper-level forcing will be present as well.
Helicity values will be around 200 to 300 m2/s2 with curved
hodographs, favorable for rotating storms. There is a mid-level
weakness noted in the hodograph due to some veer-back-veer in mid-
levels, which could result in messier storm modes. Mid-level winds
do veer more to the southwest by Sunday evening.

A squall line will likely be ongoing Sunday morning near the
Mississippi River, and should maintain itself given flow parallel to
the line and plenty of instability ahead of it. There is about a 6
hour timing difference between the global models, with the
GFS/Canadian bringing the squall line into West Alabama late Sunday
afternoon and the ECMWF holding off until late evening. The NAM
meanwhile is an uncharacteristically fast outlier. Have fairly good
confidence that the squall line will be strong to severe with
damaging winds, with a severe hail threat being confined to any
strongly rotating updrafts. At least a brief tornado threat will be
possible given the strong low-level shear. Will keep the tornado
threat in the HWO low-confidence for now given a predominately
linear convective mode and saturated profiles, as well as the model
timing differences. If robust updrafts can develop ahead of the
squall line, especially during the late afternoon, then a higher
tornado threat could evolve, but this remains uncertain this far out
in time. The squall line will continue eastward across the forecast
area Sunday evening and through the overnight hours. Locally heavy
rainfall will also be possible with any training storms. Please
continue to stay tuned to the latest updates.

After this system moves out during the overnight period looks like
we will be drier and a touch cooler as we work into the first week
of May, with another system just on the outside skirts of this




18Z TAF Discussion.

Line of storms is not east of all terminals, with a mix of MVFR and
low end VFR cu and stratocu behind the convective line. Expect these
clouds to remain across the area through much of the afternoon,
diminishing this evening around sunset. West to northwesterly winds
will remain elevated at most locations thru this afternoon, at 8-

Tonight, winds will turn to the south as a warm front lifts back to
the north. Expect clouds to spread back northward with IFR cigs
across the south and MVFR to the north. Given low level moisture,
fog development is also possible.




A warm front will lift northward Friday, resulting in isolated
showers and storms during peak daytime heating. Convection remains
limited to far west Central Alabama on Saturday due to an upper
level ridge axis. Rain and storms return to the area Sunday
afternoon into Sunday night. Otherwise, there are no fire
weather concerns.


Gadsden 56 86 65 89 66 / 0 20 10 10 10
Anniston 59 85 67 89 66 / 0 20 10 10 10
Birmingham 62 87 69 89 70 / 0 20 10 10 10
Tuscaloosa 61 87 70 90 69 / 0 20 10 10 10
Calera 62 86 68 89 68 / 0 20 10 10 10
Auburn 65 84 68 88 68 / 10 20 10 10 10
Montgomery 66 88 68 91 70 / 10 20 10 10 10
Troy 68 87 68 89 68 / 10 20 10 10 10




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