Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 231155
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
555 AM CST Sat Feb 23 2019

.UPDATE...
For 12Z Aviation.

&&

.SHORT TERM...
Today and Tonight.

* A significant threat for severe weather exists for the
  northwestern half of Central Alabama this afternoon & overnight
  period. Threats include strong tornadoes, damaging wind gusts
  up to 60 mph, and quarter-size hail.

* Greatest potential for all modes of severe weather will primarily
  reside in areas currently outlined by the Enhanced Risk (3/5) &
  Moderate (4/5) Risk areas northwest of Interstate 59.

* A Flood Watch remains in effect for the northern half of Central
  Alabama through 6 AM Sunday morning.

Synopsis...

GOES-East water vapor imagery & RAP mesoanalysis depict a dynamic &
strengthening mid/upper-level disturbance near the Southern Rockies.
This upper trough is exhibiting a neutral tilt with 500mb heights
~544 decameters. Ridging to our southwest has established a broad
swath of southwesterly 700-300mb flow with southeast to southerly
surface winds (depending on location relative to the surface front).
South of the front, southerly surface winds along with a 30-40 kt
850mb low-level jet have advected tropical air of Caribbean origins
(PWs 1.5-1.6" per 06Z VortexSE soundings) into the Southern Gulf
Coast states. The warm front is now approaching Tuscaloosa
southeastward towards Eufaula as of 09Z. Surface dewpoints south
of the front have generally ranged in the mid to upper 60s
overnight, though observation stations across southern MS &
southwestern AL have shown dewpoints ~70-72 degrees (F) supportive
of SBCAPE already up to ~1,500 J/kg. Areas of moderate to heavy
rainfall with embedded thunderstorms have lingered to our
northwest overnight across northern MS, northern AL, & southern TN
where a dangerous flooding event is ongoing.

Discussion...

The primary upper-level feature to our west is progged to eject
northeastward into the Great Plains today & phase with the sub-
tropical jet. A strong H5 jet streak with winds in excess of 100 kts
is expected to develop at the base of the trough north of the
ArkLaTex region. Despite not having the best jet streak orientation,
height falls & deep-layer forcing along with an eastward moving
surface front/dry line feature will provide focus for downstream
convective initiation across the lower Mississippi River Valley
(MRV). Severe convective weather is expected across this region up
to an area generally where the warm front/triple point sets up near
western KY/TN. Afternoon boundary layer destabilization with surface
dewpoints in the upper 60s to near 70 is expected to increase SBCAPE
values to ~750-1,500 J/kg, especially across the western & southern
portions of our forecast area where temperatures in the upper 70s &
lower 80s are possible given sufficient insolation & mixing. With
deep-layer shear values (50-60 kts 0-6km bulk shear, 400-500 0-3km
SRH) & their orientation to the front/trough, any stronger
storms/updrafts will likely become supercellular across Mississippi
& eventually into our western & northwestern counties in the late
afternoon/evening. Low LCL heights (400-700 meters) & low-level wind
shear (300-400 0-1km SRH) w/ broad hodograph curvature suggest any
organized thunderstorms/supercells will carry a significant tornado
threat whether they remain discrete cells or manage to congeal into
a squall line or QLCS. Forecast soundings also suggest more robust
CAPE in the hail growth zone, so stronger storms will also carry an
isolated severe hail threat.

Currently forecast timing progs the threat for severe weather to
enter the northwest around 5 PM at which time severe parameters will
be high. Trends after sunset point to a general weakening of
instability as we lose diabatic heating, though the tropical airmass
will still support strong updrafts with sufficient SBCAPE values
~500-1,000 J/kg. As this system moves eastward/southeastward
towards the I-59 corridor (transition to Slight Risk area) its
orientation will become more parallel to shear vectors. This
eventually favors a diminishing tornado threat, though expect a
tornado threat through midnight & early morning. Damaging winds will
also carry a threat into this region if any stronger downdrafts can
transport LLJ energy to the surface. Loss of deep-layer forcing &
instability signal more weakening as the system approaches the I-85
corridor ~3 AM Sunday where a transition to sub-severe conditions is
expected. Scattered pre-frontal showers & thunderstorms are also
expected throughout the morning & early afternoon, though latest
forecast soundings suggest an elevated mixed layer & associated
inversion aloft which would help to limit this activity.

A flooding threat persists across the northern half of Central
Alabama during this short-term period. Conditions have been much
worse as you head into northern MS/AL as well as TN, though many
locations across our north & northwest have experienced areal
flooding issues throughout the past several days including last
night & this morning. Though additional rainfall of 1-2" is forecast
in the short-term, especially in the far northwest, the transient
nature of this evening`s system should limit flash flooding.
However, any training showers & elevated thunderstorms north of the
warm front this morning will only exacerbate any ongoing local
flooding in low-lying/poor drainage areas as well as local streams &
rivers which are already at or above their banks & into flood stage.
Please refer to our hydrologic & flooding products for more specific
& detailed information.

Maintain multiple ways to receive vital information on watches or
warnings today & into early Sunday morning. Stay tuned to local news
or a NOAA Weather Radio & refer back frequently for forecast
updates. Take immediate action if you are placed under any severe
thunderstorm or tornado warnings.

40/Sizemore

.LONG TERM...
Sunday through Friday.

We will continue to see some of the leftover rain in the far
southeast through 9 to 10 am Sunday, but then we can say goodbye to
the rain for a few days. Sunday and Monday will be dry as high
pressure will be in control. Much of Tuesday will be dry as well,
but we will begin to see an increase in clouds as the high begins to
to exit the region and the southerly flow begins to increase across
the area once again.

By Tuesday night, low pressure develops along the gulf coast and
slides slowly across the area. This will be the first wave of
several upper level wave that will slide through the area through
Friday ahead of the next cold front that will work into the area
Friday night and move out by Saturday morning.

Synoptically this pattern makes sense and will be the overall
forecast for the time being, but there are some model discrepancies
in regards to position of the main vorticity max and support of the
system. The more bullish system, the GFS, has the main support in
the north and drags the front through with very little wrap around
behind it. This would entail a fairly dry weekend for the extended.
The Euro, brings things a touch further south and therefore brings
the upper level low a little further south. This would provide more
clouds and some linger light rain into Saturday. Right now either
scenario is plausible and will need to monitor over the next few
days  the trends of each solution. But for now the main focus will
be on the system moving the area through tonight.

16


&&

.AVIATION...
12Z TAF Discussion.

Winds will start increasing and veering around sunrise. This will
also trigger the ceiling/vis improvement with MVFR by 16z and VFR
by 20z. A cold front will move into western areas around the 00z
time frame with thunderstorms/ceiling restrictions developing
again.

A warm front still located between BHM/MGM has the northern sites
in lower ceilings, while southern sites have some clearing. The
shower activity will generally be scattered at the terminal
locations and most likely will amend for specific timing of any
rain at any particular site, but overall chance look low.

Ahead of an approaching front on Saturday, winds will increase
into the 10-20kt range and possibly higher. These winds will
linger into the evening hours ahead of the front. This will occur
outside the rain areas.

16

&&

.FIRE WEATHER...

A prolonged period of rainfall will continue through Sunday
morning, with a strong storm system expected to move through
Central Alabama today and tonight. Very wet conditions are
expected, especially across the northern half of the area. Drier
conditions return to the area for at least a couple days to start
next week. There are no fire weather concerns at this time.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     72  52  60  34  59 /  60  90  10   0   0
Anniston    75  53  62  35  60 /  40  80  10   0   0
Birmingham  76  52  61  36  61 /  60  90  10   0   0
Tuscaloosa  78  51  62  36  62 /  60  90   0   0   0
Calera      78  53  62  38  61 /  50  80  10   0   0
Auburn      78  58  65  39  62 /  20  80  20   0   0
Montgomery  83  58  67  39  66 /  30  80  10   0   0
Troy        83  59  67  40  65 /  20  80  20   0   0

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Flood Watch through late tonight for the following counties:
Bibb...Blount...Calhoun...Cherokee...Clay...Cleburne...Etowah...
Fayette...Jefferson...Lamar...Marion...Pickens...Shelby...St.
Clair...Talladega...Tuscaloosa...Walker...Winston.

&&

$$


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