Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 200557

Area Forecast Discussion
National Weather Service Birmingham AL
1157 PM CST Thu Jan 19 2017

For 06Z Aviation.


This Afternoon through Friday.

--Significant severe weather potential on the board for this

Central Alabama was generally in the warm sector this afternoon. A
developing low was located along the Mississippi River south of
Memphis with a cold front extending southward. A warm front was
located across northeast Alabama into central Georgia. A trough was
analyzed with the more organized line of storms stretching from
central to southwest Alabama.

The current rap analysis has plenty of atmospheric shear near the
line of storms and low LCL`s. Therefore, rotation is possible and
weak rotation has already been noted in the lower levels. This area
of storms did produce a tornado back in Mississippi earlier this
morning. Surface based instability has been limited today due to the
low clouds, fog and drizzle/showers. But the latest water vapor
imagery has the upper trough beginning to turn negative with some
good heating noted south. Upper level divergence will increase as
well as the wind speeds. Therefore, some destabilization is
anticipated the next few hours, especially south. Will continue to
mention a threat of a brief tornado or some damaging winds along and
ahead of the line of storms, and south of Interstate 20. This area
is moving rather slowly but may pick up some speed. Due to
uncertainty in speed, held the threat through midnight east.

Rain chances very slowly decrease from west to east into Friday
morning. Did hold onto some chance pops south a leftover boundary
from the convection hangs over the area. Overnight lows will be very
similar to last night mainly in the 50s. With little change in the
airmass, went near record highs on Friday especially south.


Friday Night through Weekend.

--Instances of severe weather this weekend. Significant severe
weather possible across southern portions of the service area--

*Headline: Rounds of thunderstorms are expected to move through the
region this weekend. Friday night to Sunday morning holds the
greatest potential for severe weather. Details on threats, timing,
and areas are outlined below.

*Weather setup: Several impulses will emanate from a southern U.S.
longwave trough, while a very strong ~125-knot jet streak moves into
northern Mexico, instigating the development of an upper-level low.
Our severe weather potential will be tied to the broad southwesterly
flow preceding the upper low, and with associated upper- and low-
level lobe impulses translating through the region. Dynamics-wise,
the low-level wind field will essentially be a mishmash of
comparatively laxed winds and surges of ~35-45 knots (key factors in
subsequent severe activity), southwest at 850mb. Instability-wise,
CAPE will be available alongside a plume of steeper lapse rates.

*Forecast changes: There are no changes to threat areas or hazards.
As of Thursday afternoon, we are maintaining a swath of `elevated`
severe weather risk through roughly the southern-half of the service
area, with a `limited` risk points north. Refer to the severe
weather threat graphic on our homepage.

*Impacts_Severe: There will be a risk of tornadoes, severe-
caliber hail (1"+), and severe straight-line winds.

Multiple rounds of thunderstorms are expected:

1) Friday night through Saturday morning as storms advance into
the area from Mississippi and southern Alabama, associated with an
inland- moving boundary. This could end up being the main show,
focused on roughly the southern-half of the service area (and
toward the immediate Gulf coast). Supercells and thunderstorms
segments are expected. Forecast models are suggesting a zone of
backed surface and low-level winds in the vicinity of a northward-
moving maritime air mass, locally enhancing hodographs and SRH as
the first low-level jet surge occurs. This will be a principal
area to watch as storms interact with the boundary. A best
estimate `window of concern` (this will be adjusted) appears to
line up from 09Z/3AM Saturday to 18Z/noon, or so;

2) A questionable period Saturday afternoon when renewed
thunderstorm development is uncertain (pending morning storm
characteristics/progression, trigger), but would have severe
potential if recovery is realized to the extent of some model
projections; and

3) Saturday night as another impulse moves through, especially
toward southeastern counties. This will be another period to watch
closely as it`ll be associated with another low-level jet impulse.

*Impacts_Hydro: Analysis from our senior hydrologist indicates that
streamflow remains low across area rivers. It is anticipated that
basins will be able to handle forecast storm total rainfall amounts
of 2-4 inches. However, localized flooding could occur in poor-
drainage areas or small streams, associated with any heavy downpours.



06Z TAF Discussion.

The bulk of the rain has pushed east of the terminals with only a
few showers remaining. With abundant low-level moisture IFR to
LIFR conditions are expected to develop by sunrise at all sites.
There is some uncertainty as to whether low clouds or fog will be
the main impact with satellite indicating a mix of low clouds and
clear skies. Have opted to hit the low cigs more than low
visibilities for this TAF issuance, with TCL potentially seeing
the best chance at fog based on upstream obs. Low clouds will last
for much of the day Friday with improvement to VFR not occurring
until late in the afternoon. This improvement will be temporary as
flight conditions will begin to deteriorate towards the end of
the TAF cycle, which will be addressed in the next TAF issuance.




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.


Gadsden     59  72  56  68  54 /  30  30  40  70  80
Anniston    60  74  59  69  56 /  30  30  50  70  80
Birmingham  60  74  59  69  56 /  30  20  60  70  80
Tuscaloosa  60  76  60  72  57 /  20  20  60  70  70
Calera      60  75  60  70  57 /  30  20  60  70  80
Auburn      62  75  61  70  59 /  90  30  60  90  80
Montgomery  61  79  62  73  58 /  50  30  90  80  80
Troy        62  78  63  73  60 /  90  30  80  90  80




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