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Area forecast discussion
National Weather Service Birmingham Alabama
1148 PM CDT Wed Jun 19 2019

Update...
mesoscale update.

&&

Short term...

A convective line of strong/severe storms are currently moving
eastward across Mississippi at ~60 mph, on track to enter western
portions of central Alabama shortly after midnight. Severe
characteristics have been maintained by the mesoscale convective system thus far with
damaging straight-line winds the primary threat. 04 z rap
mesoanalysis depicts a broad region of MLCAPE upwards of 2,000 j/kg
with ~40 kts eff. Bulk shear as the mesoscale convective system moves eastward in
association with the convectively induced shortwave. 50-60 kts of
westerly flow at 500 mb was also analyzed, with 1,100-1,200 j/kg
dcape already in place up to the I-59 corridor. 925-850 mb
southwesterly flow of ~30 kts appears to be aiding in replenishment
of instability and moisture to the system, though there is a cape
and moisture deficit in front of the line which may act to weaken it
as it moves eastward, as well as an eastward weakening trend in mid-
level lapse rates.

Regardless, a buoyant lower-troposphere and active low-level jet
suggests the mesoscale convective system will pose a severe threat as it moves into portions
of the forecast area given lightning and remote sensing trends
showing a relatively healthy system as of 0430 z. Uncertainties
remain as to when the line weakens below severe limits tonight, and
where the resultant boundary develops for severe potential again on
Thursday afternoon. Best forecast thinking as of now suggest severe
threats remain generally along and west of I-65 and north of I-59 in
the coming hours, areas outlined by Severe Thunderstorm Watch #406.

40/sizemore

Previous short-term discussion:

Have removed the chance of any strong storm over central Alabama
this evening. An exiting short wave trough has produced some
subsidence in its wake and much of the cloud cover has dissipated
north. Some convection still exist along a localized boundary in
south central Alabama, but this activity was rather limited.

Convection has begun in earnest near The Ark-la-tex early this
evening. This is the beginning of the system that will affect
central Alabama later tonight in one fashion or another. Models
are still split in whether the convection holds together as strong
to severe as the activity moves in after 1am. It certainly appears
there will be a chance this activity keeps some strength,
especially northwest as it moves in. Therefore, have held the
severe weather outlooks in place with an ever so slightly drop
south and east of the slight risk zone. As mentioned previously,
the discussions below due handle the conditional situation quite
well.

75



Long term...
/issued at 430 am CDT/

* a slight risk for severe weather on Thursday.

* Heat indices of 100-105 f possible Friday through Sunday.

Thursday.

The advertised shortwave moving across the southern/Central Plains
eastward into the eastern Continental U.S. Will bring two possible rounds of
severe weather to central Alabama on Thursday.

The first possible threat window opens ~1 am Thursday morning
with a potential ongoing mesoscale convective system advancing eastward across northern
Mississippi and into northern Alabama. This particular threat will
carry the chance for damaging straight-line winds up to 60 mph as
it moves into the forecast area. This scenario remains at low
confidence at this time as the system could very well weaken
before it arrives here, though MLCAPE ~2,000 j/kg and 30-40 kts
eff. Bulk shear across its inflow region suggest maintenance of
strong to severe characteristics if the system doesn't become too
outflow dominate overnight. Nocturnal 925-850 mb winds ~30-40 kts
also suggest the convective system will be actively replenished
throughout its journey into our area. As a result, a slight risk
has been placed across our northwest forecast area as a damaging
straight-line wind threat may manifest in the early morning hours
Thursday.

Thereafter, effects from the possible early-morning mesoscale convective system will have
implications for the second round of potential severe weather in the
afternoon & evening. As the overall synoptic shortwave amplifies
with eastward progression, height falls & cooling temperatures
aloft will overspread our area with a remnant outflow/Theta-E
gradient across central Alabama. Wherever this gradient
establishes will generally determine the spatial area favorable for
convective development in the afternoon. Convergence along this axis
will need to be sufficient enough to overcome unfavorable synoptic
scale subsidence as confluent flow aloft establishes behind the
shortwave trough axis. If lift is sufficient, guidance has certainly
maintained favorable thermodynamic variables (mlcape upwards of
3,500 j/kg and ~7 c/km mid-level lapse rates) with deep-layer shear
~30-40 kts for severe thunderstorm organization. Thunderstorms will
be capable of large hail and damaging convective downbursts
considering dcape 1,100-1,300 j/kg & 500 mb temperatures -9 to -10
c. As the shortwave continues eastward into Georgia and the
Carolinas, flow aloft will veer more northwesterly as upstream
ridging amplifies. Because of varying scenarios of how severe
threats may manifest in the next 24-48 hours, forecast confidence
remains low with a marginal risk being maintained for Thursday
afternoon & evening in our graphical products. Stay tuned for
future forecast updates.

Friday through Tuesday.

In wake of the exiting shortwave Friday morning, the mid/upper-level
pattern will transition to a longwave trough building across the
intermountain west with resultant ridging developing along the
Mississippi River valley. This ridge will continue to
strengthen/amplify into the weekend as hot temperatures develop in
the afternoons. To make matters worse, a humid, Gulf airmass will
linger across the southeast. As such, heat indices are expected to
increase to the 100-105 f range on the afternoons of Fri/Sat/sun. It
appears the hottest afternoons will be Fri/Sat with some locations
likely approaching criteria for a heat advisory. With this
possibility now likely, will be mentioning in the severe weather potential statement and monitoring
trends in temperatures & dewpoints in the next day or so. Looking
ahead in to next week shows a relatively active pattern for this time
of year to continue with the ridge axis now approaching the East
Coast. Rain chances will be on the increase with the upstream trough
and associated surface frontal boundary moving in from the northwest.

40/sizemore

&&

Aviation...
00z taf discussion.

Slightly tricky forecast the next 24 hours with 3 separate
convective chances and some MVFR ceilings. Most of the area is
under some subsidence from the exiting short wave trough, but a
few showers/storms were near mgm/toi. This activity will move
eastward the next couple hours and potentially impact these
sites. Then there will be a lull in the convective activity and
VFR conditions. Another disturbance enters the picture after 06z.
Mentioned thunderstorms in the vicinity starting at 06-08z for the northern sites due and
a prob30 for prevailing conditions due to time in the future. Did
not mention this activity south. Winds variable to south southwest
to start around 5kts and increasing to around 8kts as
storms/outflow nears. There should be a time frame near and behind
the storms/outflow that experience MVFR ceilings. Lastly, another
shot of storms is possible at all terminals in the afternoon and
handled this with another prob30. Winds increase again by
afternoon with southwest winds around 10-12kts.

75



&&

Fire weather...

Rain and thunderstorm chances remain high through Thursday with
an active upper-level pattern. Southerly flow and tropical air
will keep relative humidity values at or above 60% percent with
further recovery after peak heating subsides. Kbdi values remain
high, but increased low-level moisture & recent rainfall will keep
US from reaching critical fire weather criteria. Drier weather
with hot afternoon temperatures are expected Friday through Sunday
with heat indices likely exceeding 100 degrees.

&&

Preliminary point temps/pops...
Gadsden 71 87 68 91 70 / 70 70 30 30 20
Anniston 71 89 70 92 72 / 70 60 40 30 20
Birmingham 70 90 72 93 74 / 80 60 40 30 20
Tuscaloosa 70 92 73 95 75 / 80 50 40 30 10
Calera 71 90 71 92 72 / 80 50 40 30 10
Auburn 73 89 72 91 73 / 20 40 40 30 10
Montgomery 75 93 73 95 74 / 30 40 40 30 10
Troy 74 92 72 93 73 / 20 40 40 30 10

&&

Bmx watches/warnings/advisories/...
none.

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