Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 240507

Area Forecast Discussion
National Weather Service Birmingham AL
1207 AM CDT Thu May 24 2018

For 06Z Aviation.



The most concentrated convergence zone evident this morning has
migrated to the southern extent of Central Alabama this afternoon.
As mentioned in previous discussion, numerous boundaries remain
over the area and expect additional showers/storms to develop as
the afternoon progresses. Due to the low clouds and fog east, this
area will be the last to have some development, but nonetheless,
at least some scattered activity. The highest rain chances remain
south and west with likely to categorical until sunset.

Rather weak low pressure surface and aloft will remain from
central Georgia into southern Mississippi into Thursday. An area
of upward motion rotates around the low overnight, which should
kick off some activity after midnight in southeast areas. The
highest rain chances will be close to where we experienced 3-6
inches last night, and will need to be monitored closely due to
saturated ground conditions.

Slightly drier air and a shift in the pattern will push the
highest rain chances farther south on Thursday, with an axis near
I-85 and US 80 and southward.

The National Hurricane Center has increased the chances of
tropical to subtropical development in the northern gulf to 60
percent. The GFS is the outlier with this system so far, but at
this time, it still looks like there will be a heavy rain threat.
Will add isolated mention Friday and Saturday.


Previous short-term discussion:
Numerous boundaries are located over Central Alabama this
morning. These boundaries range from a mid level trough, a low
level trough, rain/no rain boundary, cloud no cloud (differential
heating) area, fog/no fog, and finally storm scale outflows. The
latest RUC analysis has much of the area in the relative humidity
maximum, with precipitable water values 1.8 inches or higher. Due
to this moisture content, microburst indices are low today. But
SBCape values are around 2500 with an instability maximum also
near us. 850-300 mean winds are rather light and lower level
convergence is not hugely outstanding, but enhanced near the
boundaries. Therefore, some of the stronger storms will be
capable of brief very heavy rainfall on the order of a few inches
in a short period of time and some gusty winds from precip
loading. The coverage of the heavy rain potential is should remain
relatively small, but where it rains, it will rain. Increased rain
chances west and south with likely to categorical rain chances
south of I-20.


Previous short-term discussion:Today and Tonight.

A concentrated area of heavy rainfall fell over Lee County overnight
with radar rainfall estimates of 4-6 inches over the far southeast
portion of Lee County. This activity was along a low level trof axis
and decent convergence enhancing rainfall rates. The low level trof
axis is forecast to push slightly southward today, with convergence
weakening due to a decrease in low level inflow south of the
boundary. Do not expect a repeat of excessive rainfall rates today
or tonight, but the areas along and south of I-85 will have the best
chance of rain today and tonight. There also appears to be a zone of
mid level convergence setting up across west Alabama by early
afternoon, so there is a chance of showers and a few thunderstorms
for a majority of central Alabama. Better chances for thunderstorm
activity today will be west of I-65 where surface temperatures will
be warmer. Shower and thunderstorm activity will diminish in areal
coverage this evening, but some activity will likely linger
overnight south of I-20.


Thursday through Tuesday.

High rain chances continue on Thursday as the moist air mass
remains in place. The low to mid-level circulation/remnant MCV
that has been hanging around the past couple days will initially
be over Georgia, but some high-res models retrograde its remnants
westward during the day. Drier air will briefly filter in across
the far northern/northwest counties.

A very stagnant upper-level pattern will be in place across the
Southeast and the Gulf throughout the forecast period, as a
southern-stream anomalous trough becomes trapped underneath mean
ridging in the northern stream. At the surface, low pressure is
expected to develop near the Yucatan by the end of the week and
lift northward towards the northern Gulf Coast this weekend. NHC`s
latest outlook indicates a 50% chance that this will become a
subtropical or tropical system. The GFS remains an eastern outlier
with this system tracking this system along the Florida
Peninsula. The ECMWF and its ensembles as well as the Canadian are
in better agreement that this system will lift towards the
northern Gulf Coast, putting much if not all of Central Alabama in
the moist eastern side of the system. Once it moves inland, it is
expected to stall out as a cutoff low next week. While wind shear
associated with the upper-level trough will probably prevent a
strong system from forming, slow-moving tropical depressions and
weak tropical storms have historically been heavy rainfall
producers. A tropical-like air mass is already in place and deeper
tropical moisture will move in by this weekend. Isolated flash
flooding will be possible almost any day. Daily high rain chances
will have a cumulative effect, and the best chance for more
organized flooding will come when 850mb moisture transport
associated with the low pressure system arrives by Sunday. There`s
still plenty of uncertainty regarding rainfall amounts and
timing, keeping confidence on the lower end at this time, but it
seems prudent to add a mention of flooding to the HWO for the
Sunday-Tuesday timeframe. Latest 7-day QPF from WPC indicates 3-6
inch area-average amounts across the area. Low-level wind fields
appear too weak to support any tropical tornado threat at this



06Z TAF Discussion.

Convection is mainly across the far eastern part of the state as
of now. Current thinking is that it will remain away from TAFs
during remainder of the overnight, but will monitor for changes.
Do not have any convection mentioned in TAFs until MGM/TOI for
Thursday afternoon. Could see some better chances for MVFR fog and
cigs as cirrus is thinning better than previously thought. May also
see some IFR in spots. We remain in a moist environment for the
foreseeable future.




A moist pattern is expected over the next seven days and should
result in daily rain chances. Patchy fog and low clouds will be
possible each morning particularly where rainfall occurs during
the prior afternoon and evening.


Gadsden     86  67  82  66  82 /  40  30  60  40  70
Anniston    85  68  82  66  83 /  40  40  60  40  70
Birmingham  87  69  85  69  83 /  40  30  60  40  70
Tuscaloosa  89  69  86  68  85 /  40  40  60  40  70
Calera      86  68  84  67  82 /  40  40  60  40  70
Auburn      82  68  81  67  79 /  60  40  70  50  70
Montgomery  86  69  85  68  84 /  60  40  70  50  70
Troy        85  68  84  68  82 /  70  40  70  50  70




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