Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 230522

Area Forecast Discussion
National Weather Service Birmingham AL
1122 PM CST Wed Jan 22 2020

For 06Z Aviation.



Longwave ridging was quickly departing the area to the east while
a broad trough was pushing east over the Plains from Texas north
into the Dakotas. Cloud cover was increasing from the southwest as
a southwest flow aloft was transporting elevated moisture into
the region.

Towards the surface, a residual dry airmass persisted with
surface high pressure extending from near Boston southwest through
the Mid Atlantic region and into northwest Virginia. Lee side
troughing was positioned just to the east of the Front Range of
the Rockies while a cold front was pushing southeast from Montana
extending southwest into Nevada.



/Updated at 154 PM CST Wed Jan 22 2020/
Rest of Today through Thursday.

Expect the mid to upper levels to continue to moisten this
afternoon and evening with thickening high and mid level clouds.
The low and mid levels will remain fairly dry through tonight. The
cloud cover along with east to southeast low-level flow will keep
low temperatures tonight ranging from around freezing far
northeast and in the sheltered colder northeast valleys to
readings in the mid 30`s south and southwest.

Expect further increasing clouds in the low to mid levels
overnight and into the day Thursday. Chances for rain showers will
increase toward midnight southwest with the chances gradually
expanding northeast through the morning hours. Rain showers are
expected to continue to increase from west to east through the
remainder of Thursday.

While some areas will reach freezing tonight northeast, expect
the low-levels to be the driest in these areas overnight through
mid morning Thursday. Temperatures are expected to be well above
freezing before the column becomes saturated enough for
precipitation to reach the surface. Low-level winds will remain
out of the southeast through the day tomorrow with high
temperatures ranging from the upper 40`s north to the mid 50`s


/Updated at 229 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 continues 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.



06Z TAF Discussion.

Current satellite continues to show overcast skies across Central
Alabama thanks to increasing moisture aloft ahead of a digging
upper trough on the lee side of the Rockies moving east into the
Plains through the forecast. As this system moves closer and the
surface ridge pushes east, cigs will lower as the column moistens
and showers move into the area from the west and southwest. During
the day, MVFR and some IFR conditions will be noted as showers
become more widespread by afternoon with lower cigs and some
reduced visibility with heavier showers.




Moisture and rain chances return Thursday, with widespread rain
expected through Thursday night, with the system exiting to the east
Friday morning. Low level winds will increase tonight, from the
southeast to east, but remain below 10kts through tomorrow. Clouds
will continue to increase, with cigs falling below 1500ft tomorrow
afternoon as rainfall arrives areawide. Other than a quick hitting
slight chance of rain Sunday night, drier conditions are expected
through early next week.


Gadsden     35  45  42  53  34 /  10 100 100  60  10
Anniston    36  46  43  56  35 /  20  90 100  60  10
Birmingham  38  46  45  55  36 /  20 100 100  40  10
Tuscaloosa  38  47  44  56  35 /  20 100 100  20   0
Calera      37  47  45  55  34 /  30 100 100  30   0
Auburn      38  49  46  57  36 /  20  60 100  50   0
Montgomery  39  50  48  59  36 /  40  70 100  30   0
Troy        40  53  50  60  36 /  30  50  90  30   0




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