Special Marine Warning 317, NWS Mobile, AL

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
755 AM CDT Sun Aug 29 2021

.UPDATE...Hurricane Ida is a dangerous category four hurricane and
will make landfall along the southeast LA coast in a few hours. As
Ida gains latitude, local impacts will increase. Outer spiral rain
bands rotating in off the Gulf will increase in magnitude and
coverage through the course of the day. Higher momentum air will
mix down in these bands likely to produce tropical storm force
wind gusts in the tropical storm warning area. Excessive rains
will increase the threat of flooding including river flooding.
Isolated tornadoes are a threat and forecasters are seeing
evidence of this in radar velocity couplets. Storm surge threat
will also increase. For all of the latest, specific information on
the multitude of impacts, please refer to the WFO Mobile Webpage
at weather.gov/mob

/10

&&

.PREV DISCUSSION... /issued 600 AM CDT Sun Aug 29 2021/

NEAR TERM UPDATE /Now Through Monday/...Today is a great period
of transition\ as Ida is forecast to make landfall in SE Louisiana
later today as a major hurricane. Rain chances are currently
increasing from the south and east as some outer rainbands have
already arrived and the precipitation shield from Ida overspreads
the southwest half of our region.

During this period, our region quickly goes from being to the north
of Ida to being located within the northeast quadrant of the storm
this afternoon and this will further increase the low level vertical
wind shear locally, thus promoting the development of potentially
deep mesocyclones with tornadoes today. Modified hodographs using
observed storm motion vectors and sfc winds locally already show
0-500 m SrHel to be approaching 250 m-2 s-2. Mixed layer CAPEs are
hovering around 500 J kg-1. Our area will increasingly become
classically situated in the favorable quadrant relative to the
storm as it moves along, and at a favorable distance for
impressive deep outer rainband mesocyclones to form. We are
already seeing some mesocyclones with intense rotations, and that
was a few hours ago so that will only increase.

Also, we will have to really pay attention to where Ida's outer
rainbands set up with regard to flooding, as training individual
cells along these bands will cause the rainfall rates and amounts to
ramp up in a big way to flash flood levels. We are still forecasting
5 to 10" east of the storm's center, with locally up to 15" plus. We
have though for quite some time that a dominant rainband will set up
with training cells. Of course the higher rainfall amounts will be
across coastal Alabama, southeast MS and western portions of inland
SW Alabama. WE ARE ENCOURAGING EVERYONE TO HAVE A WAY TO GET TORNADO
AND FLASH FLOOD WARNINGS IF YOUR POWER GOES OUT!  /23 JMM

AVIATION...
12Z issuance...As Ida's very first rainband approaches coastal areas
just after midnight, scattered showers and thunderstorms with
gusty squalls will be the main message over the south of I-10 and
gradually developing northward through Noon. MVFR to occasionally
IFR conditions through the remainder of the period, especially
west of the I-65 corridor and south of Highway 84. As today
progresses, again, shower and thunderstorm coverage will become
near continuous regionwide. Expect on average, after brief LIFR in
squalls as Ida's outer rainband coverage increases after 29.15
UTC. Low level wind shear also increases throughout the day. MVFR
will likely become re-established this afternoon and evening,
except in the vicinity of stationary rainbands where IFR to LIFR
conditions may persist for several hours at a time. /23 JMM

PREV DISCUSSION... /issued 458 AM CDT Sun Aug 29 2021/

NEAR TERM UPDATE /Now Through Monday/...This is a period of
transition\ as Ida is forecast to make landfall in SE Louisiana
later today as a major hurricane. Rain chances are currently
increasing from the south and east as some outer rainbands have
already arrived and the precipitation shield from Ida overspreads
the southwest half of our region. Water levels continue to rise
locally and are already running anywhere from 1.5 to 2 feet above
astronomical predictions. Bayou La Batre at this hour is already
at 2.53' MHHW and rising as swell packets continue to arrive, and
this is BEFORE, the full surge envelope of water arrives.

During this period, our region quickly goes from being to the north
of Ida to being located within the northeast quadrant of the storm
this afternoon and this will further increase the low level vertical
wind shear locally, thus promoting the development of potentially
deep mesocyclones with tornadoes today. Modified hodographs using
observed storm motion vectors and sfc winds locally already show
0-500 m SrHel to be approaching 250 m-2 s-2. Mixed layer CAPEs are
hovering around 500 J kg-1. Our area will increasingly become
classically situated in the favorable quadrant relative to the
storm as it moves along, and at a favorable distance for
impressive deep outer rainband mesocyclones to form. We are
already seeing some mesocyclones with intense rotations, and that
was a few hours ago so that will only increase.

Also, we will have to really pay attention to where Ida's outer
rainbands set up with regard to flooding, as training individual
cells along these bands will cause the rainfall rates and amounts to
ramp up in a big way to flash flood levels. We are still forecasting
5 to 10" east of the storm's center, with locally up to 15" plus. We
have though for quite some time that a dominant rainband will set up
with training cells. Of course the higher rainfall amounts will be
across coastal Alabama, southeast MS and western portions of inland
SW Alabama. WE ARE ENCOURAGING EVERYONE TO HAVE A WAY TO GET TORNADO
AND FLASH FLOOD WARNINGS IF YOUR POWER GOES OUT!  /23 JMM

SHORT TERM /Monday Through Tuesday night/...On Monday Hurricane
Ida will be weakening as it moves north across western Mississippi.
A large rainband will likely be set up across our forecast area,
with convection moving north along the band over our area, and
this will likely continue into Monday night. Even into Tuesday
this band will continue across parts of the forecast area, but
progressing into the eastern portions of the area and the
associated heavy rainfall threat will be a little less. With
this setup, a pattern of widespread heavy rainfall will continue
through at least Monday night, and a Flash Flood Watch will
continue in effect for most of the forecast area (except for our
far eastern tier of counties). An additional 3 to 5 inches of
rainfall, which will come on top of the rain that fell during the
near term period is possible over the Flash Flood Watch area. Heavy
rainfall may continue beyond this period, and local extensions of
the Watch in space and time may be necessary.

A tornado threat will will continue over the area on Monday as
Ida, which will be west of our area, lifts northward. Strong low
to mid-level winds will persist throughout the eastern periphery
of Ida. Given the anticipated storm track, at least a low-
probability tornado threat will extend from southeast LA across MS
and AL into the TN Valley. The main question regarding the
tornado threat is whether or not there will be enough buoyancy to
maintain relatively deeper updrafts for a long enough duration to
result in tornadogenesis. Highest likelihood for enough buoyancy
is current expected to be where dewpoints reach the mid 70s, which
is forecast to occur from far southeast LA and coastal AL/MS
northward into southeast MS and southwest AL.

A coastal flood threat will continue over the next several days as
well due to the continued moderately strong onshore flow. We
still expect a long duration coastal flood event lasting at least
two more high tide cycles in the short term period (Monday morning
and Tuesday morning). Peak water levels will occur during the near
term period (Sunday) into Monday morning. There have already been
a number of reports of flooded roads and other coastal flooding
this morning, and similar conditions (but mostly to a lesser
extent) Monday into Tuesday.

In summary, a multitude of hazards and a very wet and storm
pattern will continue to impact the area through Tuesday as Ida
lifts north to our west. /12

EXTENDED TERM /Wednesday Through Saturday/...The remnants of Ida
will have moved to the mid Atlantic seaboard by Wednesday, with
trailing moisture back into our area. High PoPs will continue into
Wednesday, gradually diminishing late in the extended period as
high pressure pushes much of the moisture down to the south of our
area (and into the Gulf). Highs through the period will generally
be in the upper 80s and low 90s. Lows in the low to mid 70s. /12

MARINE...No changes to the marine forecast this package. Tropical
storm conditions develop throughout the day and will be slow to
subside this evening through sunrise Monday.  /23 JMM

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...Flash Flood Watch through late Monday night for ALZ051>056-059-
     261>266.

     Tropical Storm Warning for ALZ052-261>266.

     Wind Advisory until 10 PM CDT Monday for ALZ051-053-055-056-059.

     Storm Surge Warning for ALZ263>266.

     High Rip Current Risk through late Wednesday night for ALZ265-
     266.

     High Surf Warning until 7 AM CDT Tuesday for ALZ265-266.

FL...Flash Flood Watch through late Monday night for FLZ201>206.

     Wind Advisory until 10 PM CDT Monday for FLZ201>206.

     High Rip Current Risk through late Wednesday night for FLZ202-
     204-206.

     Coastal Flood Advisory until 7 PM CDT Monday for FLZ202-204-206.

     High Surf Warning until 7 AM CDT Tuesday for FLZ202-204-206.

MS...Flash Flood Watch through late Monday night for MSZ067-075-076-
     078-079.

     Tropical Storm Warning for MSZ067-075-076-078-079.

GM...Tropical Storm Warning for GMZ630>633-650-655-670-675.

     Small Craft Advisory until 3 PM CDT Tuesday for GMZ634>636.

&&

This product is also available on the web at:
http://weather.gov/mob