Severe Thunderstorm Warning 121, NWS Birmingham, AL

Area Forecast Discussion
National Weather Service Birmingham AL
211 PM CDT Sat Jun 22 2019

.UPDATE...
Midday Update and 18Z Aviation.

&&

.SHORT TERM...

Like mentioned previously, there are a few smaller scale boundaries
south. These are leftover from previous nights convection and will
be a focusing mechanism for storm development this afternoon and
evening. Due to the microburst index being high and atmospheric
conditions conducive for downburst, will leave mention of severe
storms far south. The best shear, although on the weaker side,
remains north and east. The activity northwest was building on
the remnant outflow from last nights MCS, and this area has the
potential to organize somewhat and produce damaging winds. Large
hail also remains possible due to cold temperatures aloft, good
lapse rates and dew point depression in the lowest levels. A
Severe Thunderstorms Watch is in effect covering this activity.
Due to an increasing number of boundaries and potential focusing
areas for storms, some storms will last into the evening hours
especially north and east.

After making adjustments on the temperatures and dew points, the
heat indices will top out in the 100-105 degree range this
afternoon. Therefore, its going to be hot but not to any advisory
criteria. Still exercise caution since this is the first real
shot at these humidity levels.

75

Previous short-term discussion:Today through Sunday.

Last night a remnant outflow boundary was left from near Pickens
County to Lee County from a weakening segment of an MCS that moved
into GA/SC. This boundary is likely to provide the spatial axis
favorable for convective initiation this afternoon, though sporadic
showers and thunderstorms have developed as a result of isentropic
ascent just along the boundary's edge throughout the overnight
period. As of now, looking at threat potential for today with
overnight activity, a couple scenarios for severe weather potential
exist today.

A favorable thermodynamic parameter space will reside across a good
portion of the Southeast this afternoon. Though effects from the
ridge will act to resist development of thunderstorms with height
rises and confluent flow aloft, the remnant boundary will help
create a theta-e gradient for convective development as MLCAPE rises
back into the 3,000-4,000 J/kg range. With mid-level lapse rates ~8
C/km & 500 mb temperatures from -9 to -11 C, robust updrafts are
possible within areas across Central Alabama, especially with low-
level moisture availability. Concern for today's severe threat is
that discrete cells will contain potential for hail and downbursts
given indication that DCAPE will range 1,200-1,500 J/kg with
relatively cool temperatures aloft, though the weak shear
environment (eff. bulk shear <20 kts) & synoptic scale subsidence
suggest lifespan of storms may be short. Thus, uncertainty exists in
whether lift will be sufficient to overcome inhibiting factors for
vertical development in the afternoon. If lift is sufficient, areas
generally along and north of the remnant boundary will see best
chances for thunderstorms.

Another scenario brings a MCS across portions of Central Alabama
tomorrow evening (can happen whether isolated development takes place
this afternoon or not). As of 04 Z, this MCS was positioned in
southern IL & IN, heading south-southeastward. If this system can
maintain intensity through this morning, it will move into further
destabilizing environmental conditions and have a chance at
impacting our north/east areas with damaging winds. This complex
will also compete against effects of the ridge, including steering
mechanisms that may keep it to our east as last night's threat did
and unfavorable alignment of shear vectors. Nonetheless, severe
weather has found ways to outdo expectations the past few days
despite limiting factors, thus we're hedging with caution for today
and outlining areas from Pickens to Lee County & northward in a
Slight Risk for severe weather.

This afternoon will be hot and muggy in many areas with heat indices
100-105 F. If you're outside today, take it easy and limit exposure
and dehydration. Sunday's weather looks to be hot and humid as
well, though chances for rain/thunderstorms are on the decrease.
This is suggested as the low/mid-level ridge nudges more
northwestward across our area allowing for flow aloft to steer
potential storm complexes around our area, as well as more
efficiently mixing the boundary layer & limiting shear values.
Though we're bound to see some showers and storms somewhere
Sunday, have left best chances in the northeast where better PWs
(~1.5") are expected. Otherwise, stay tuned to for forecast
updates today as we assess trends with observations throughout the
morning.

40/Sizemore

40

.LONG TERM...
/Issued at 345 AM CT/

Monday through Friday.

The ridge will begin to slide east on Monday allowing for southerly
flow to increase across the area. This will again increase the rain
chances, especially across the northwest as a front will try to work
into the area or at least be nearby. The ridge will begin to build
back into the region on Tuesday, however, moisture will remain and
we will likely see scattered showers/storms on both Tuesday and
Wednesday.

On Thursday, the main ridge will break down and we will see
moistures move into the region from the southeast. This will provide
a little bit more in bulk moisture in place across the area and
better rain chances. Friday looks to be the best chances and will
likely need to raise these pops as we get closer and models begin to
converge on an even better solution. With the summer time
temperatures and afternoon heating, any storm good be on the strong
side. But predictability is too low to mention anything for next
week at this time. On a whole look for near normal temperatures
through next week, with above normal rain chances.

16

&&

.AVIATION...
18Z TAF Discussion.

Convection starting across Central Alabama early this afternoon.
Increased rain chances over the region and therefore added mention
of thunder to all terminals. Scattered activity was near some
terminals now and added mention, while upstream activity may
become more organized and move through later. Some of the storms
will be capable of gusty winds and hail. Several additional
smaller scale boundaries were around the region to provide focus
for storm development. Otherwise, it does not appears that we have
quite the moist environment like last night. At this time, did
not mention any lower restrictive ceilings except at TOI. This
may need some adjusting depending on how widespread the rain
becomes. Winds will generally be west southwest 5-10kts.

75

&&

.FIRE WEATHER...

A hot and humid airmass will remain in place with the potential
for scattered showers and storms each day. Rain chances may become
enhanced by an approaching front on Monday. There are no fire
weather concerns.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     70  92  70  89  68 /  40  30  10  40  40
Anniston    71  92  71  90  69 /  50  30  10  40  40
Birmingham  73  95  73  91  71 /  30  20  10  50  40
Tuscaloosa  74  96  75  91  72 /  20  10  10  50  40
Calera      72  93  72  90  70 /  30  20  10  40  40
Auburn      72  92  72  91  72 /  50  30  10  20  30
Montgomery  73  96  73  94  72 /  30  20  10  30  30
Troy        72  94  72  93  70 /  20  10  10  20  30

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

Hazardous Weather Outlook
National Weather Service Birmingham AL
408 AM CDT Sat Jun 22 2019

ALZ011>015-017>050-231215-
Marion-Lamar-Fayette-Winston-Walker-Blount-Etowah-Calhoun-Cherokee-
Cleburne-Pickens-Tuscaloosa-Jefferson-Shelby-St. Clair-Talladega-
Clay-Randolph-Sumter-Greene-Hale-Perry-Bibb-Chilton-Coosa-Tallapoosa-
Chambers-Marengo-Dallas-Autauga-Lowndes-Elmore-Montgomery-Macon-
Bullock-Lee-Russell-Pike-Barbour-
408 AM CDT Sat Jun 22 2019

This Hazardous Weather Outlook is for the counties served by the
National Weather Service office in Birmingham.

.DAY ONE...Today and Tonight.

There is a Slight Risk for severe thunderstorms capable of producing
quarter size hail and damaging winds through this evening, generally
along and north of a line from Pickens County to Lee County.
Additional strong to severe thunderstorms south of this line cannot
be ruled out.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Activation of storm spotters and emergency management may be needed
through this evening.

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