Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 221131

Area Forecast Discussion
National Weather Service Birmingham AL
531 AM CST Wed Jan 22 2020

For 12Z Aviation.


/Updated at 0229 AM CST Wed Jan 22 2020/

Today and Tonight.

Transition day across the area as a surface high pushes off to the
east of Alabama and a gradual warming trend ensues as winds become
southeast. Highs today will range from the upper 40s north to lower
50s south. Cirrus will overspread Alabama today as the next upstream
short wave trof approaches the Mississippi River. Temperatures
tonight will be much warmer than previous nights due to cloud cover
and increasing low level pressure gradient. May see a few sleet
pellets after midnight as mid levels become saturated and light
precipitation falls into a much drier air mass below 850mb. Rain
should hold off until after sunrise Thursday.


/Updated at 0229 AM CST Wed Jan 22 2020/
Thursday through Wednesday.

A complex mid/upper-level trough with multiple embedded vorticity
maxima will continue to approach the area on Thursday. A lead vort
max in the southwest flow aloft, most identifiable as a 700mb
speed max, will be crossing the area tonight and Thursday morning
accompanied by the left exit region of a subtropical jet streak.
Initially with saturated isentropic lift at mid/upper levels but
very dry air at low levels, precipitation associated with this
feature overnight will be associated with virga and possibly a
a sprinkle. But guidance is starting to suggest enough low-level
moisture return wrapping around low-level ridging along the East
Coast (and perhaps also originating from the mid-latitude cyclone
northeast of the Bahamas) may occur over East Alabama for some
light showers to begin reaching the ground Thursday morning. Some
PoPs have been added there and may need to be increased in later
updates since model QPF parameterizations often over-do
evaporation in these situations. Forecast soundings indicate a
warm nose developing from strong low-level warm air advection.
However, the wet bulb temperature profile in soundings across
some of the east-central/northeast counties is slower to go above
freezing due to the low-level dry air initially in place.
Therefore, would not be surprised to get a few ice pellet reports
Thursday morning in these areas which is not uncommon in these
situations, but no impacts are expected at this time since
temperatures are expected to be above freezing and activity will
be light. As the warm nose continues to moisten, profiles will
quickly become supportive of only rain.

Further to the west, a southern stream shortwave trough will begin
to approach the area by Thursday afternoon as it begins to be
absorbed by a stronger northern stream wave coming out of the
Central Plains, which closes off into an upper low over the Ozarks
by Thursday evening. This upper low will move eastward through the
Ohio Valley on Friday as it undergoes some binary interaction with
another upper low over the Northern Plains, and eventually reaches
the Northeast by Sunday. Meanwhile at the surface a CAD wedge will
be in place along the lee of the Appalachians with a warm front
just off the Gulf coast. A double barrel low pressure system will
approach from the west with a northern low following the path of
the upper low, and a weaker southern low developing initially as
a lee cyclone over the High Plains and then re-developing along
the coastal front along the Texas/Louisiana Gulf Coasts. This
southern low will then track northeastward across southeast
Alabama and Georgia up the Piedmont region of the East Coast,
eventually merging with the northern low over the Northeast this
weekend. This southern low and wedging ahead of it will help keep
60F dew points and any surface-based instability near the coast.

Isentropic lift and synoptic forcing from the upper trough acting
on a saturated air mass will result in widespread showers moving
in from the west Thursday afternoon and continuing Thursday night.
Model QPFs continue to play catch-up to the moisture and lift
fields and continued to trend faster with the onset of
precipitation on Thursday. Continued to mention a slight chance
of thunder across the southern counties Thursday night with the
NAM indicating some weak MUCAPE from steep mid-level lapse rates.
This will be elevated instability due to the cool air mass at the
surface so nothing strong/severe expected. With PWATs only around
1 inch, rainfall amounts will only be around a half inch to a
little over an inch, so no significant flooding issues are
expected. Precipitation quickly ends from southwest to northeast
late Thursday night through Friday morning behind the cold front
as a mid-level dry slot wraps around the upper low. Low-level
moisture/low clouds wrap back in Friday afternoon/Friday night
across the north. Forcing with the upper low and steep low-level
lapse rates could result in some drizzle but will hold off
mentioning at this time. Can`t rule out flurries Friday night as
low-level thicknesses drop, but cloud top temperatures are only
marginally cold enough for ice nucleation so will hold off
mentioning these as well.

Cooler air continue to hang around Saturday but it will be nowhere
near as cold as the most recent cold air mass. A shortwave trough
in the northwest flow aloft moves through Sunday night while
partially phasing with a southern stream wave. Models are
trending towards a little more moisture with this system, though
the better moisture remains well to the south as a weak surface
low moves through the Central Gulf. Did add in some slight chance
PoPs across the south Sunday night. Temperature profiles are
supportive of only rain at this time. A ridge builds in for the
first half of next week with warming temperatures, ahead of
another trough that impacts the area around the middle of next
week. Low-level theta-e return/instability appear to be very
limited with this system at this time.



12Z TAF Discussion.

VFR conds expected thru the period with no cig or vsby issues. A
surface high will shift east of Alabama today and sfc winds will
become east-southeast 5-8 kts. High clouds will thicken today as
a short wave trof approaches the MS River. Cigs will slowly lower
overnight, but likely stay above 5000 feet agl.




Temperatures will be milder today while the air mass remains dry,
and relative humidity values will fall to or just below 25
percent for a few hours during the afternoon. However, winds will
be light, and KBDI values remain less than 100 due to recent wet
conditions. Moisture and rain chances return for Thursday, with
widespread rain expected Thursday night, exiting Friday morning.


Gadsden     47  33  46  42  53 /   0  10  70 100  60
Anniston    50  34  47  43  55 /   0  10  70 100  60
Birmingham  49  36  48  45  54 /   0  10  80 100  40
Tuscaloosa  49  36  49  45  55 /   0  10  90 100  20
Calera      50  36  49  45  54 /   0  10  80 100  30
Auburn      50  35  48  45  57 /   0  10  60 100  50
Montgomery  52  36  52  47  59 /   0  10  70 100  30
Troy        52  37  54  48  60 /   0  10  60  90  30




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