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Area forecast discussion
National Weather Service Birmingham Alabama
543 PM CST Fri Nov 22 2019

for 00z aviation.


Short term...
/updated at 0300 PM CST Fri Nov 22 2019/

Through Saturday night.

A period of wet and unsettled weather is encroaching upon central
Alabama this afternoon. In response to low-amplitude upper-level
diffluence downstream of trough across the plains, surface
cyclogenesis is occurring to our southwest near southern Louisiana
& a Gulf Coast baroclinic zone. A quasi-stationary frontal
boundary is draped from the weak surface low northeastward into
northern Mississippi & the Tennessee River valley. South of the
boundary, dewpoints have risen to the upper 50s and lower 60s,
with temperatures in the mid 60s to upper 70s (mgm has risen to 78
as of 230 PM cst). Moist isentropic ascent and confluent flow in
the 925-850 mb layer have fostered areas of rain showers from the
Louisiana Gulf Coast northeastward along the I-20 corridor in
central Alabama. Resultant periods of rainfall are forecast to
continue for the next 24-30 hours with embedded thunderstorms
possible tonight and into tomorrow as an axis MUCAPE ~250-500 j/kg
collocates with the northeastward moving system, generally
propagating along the temperature gradient associated with the
surface front.

With the low progged to be near Columbus, MS around midnight,
warm sector conditions will help keep temperatures relatively mild
overnight, generally at or just above respective dewpoints in the
upper 50s/lower 60s. Warm advection may allow temperatures to
increase in some areas overnight, which would trend above
guidance in some cases, though has been adjusted to best forecast
expectations with this update. Nonetheless, cooler air will hold
off until the cold advection regime takes over after frontal
passage tomorrow, which should begin across our northwest counties
by around sunrise as the surface low ejects to the northeast. As
such, the trailing cold front will be the primary feature
associated with a line of showers and storms moving from west to
east. The bulk of the rainfall should be south of the I-85
corridor by the late afternoon, signaling the end of rainfall
area-wide by around midnight (outside of some patchy light
drizzle). Overcast may linger overnight, generally clearing
throughout Sunday morning.


Long term...
/updated at 0300 PM CST Fri Nov 22 2019/

Long-term update.

Other than a few tweaks to the temperatures in the extended
forecast period, no wholesale changes were made. Guidance
continues to struggle somewhat with the evolution of the next
trough passing to our north in the Tue/Wed timeframe. Until
better consistency is achieved, a conservative approach will
continue to be used with pops. The farther north this system
trends, which has been the case with a more open-wave/transient
shortwave as compared to previous days, the drier it will likely
be across central Alabama. Thanksgiving day is forecast to feature
60s/70s with a 30-40% chance of rain showers along a stalled
frontal boundary from the exiting Tue/Wed system. The previous
long-term discussion covers this below.


Sunday through Friday.

The trough and associated cold front are well to our east Sunday
morning. High pressure builds in across the Gulf states leading to
drier weather Sunday and Monday for central Alabama. A weak trough moves
through the plains Tuesday and a surface low lifts northeastward
through the mid-MS valley into the Ohio River valley. A cold front
stretching southward from this surface low is expected to move into
our area Tuesday evening through early Wednesday morning. However,
as it moves into central al, much of the forcing is lifted well to
the north. Therefore, any coverage of rain along the front will be
decreasing as we go into Wednesday afternoon.

Guidance is in general agreement on the overall synoptic setup
for the end of next week. A deep trough will be over the West
Coast, a ridge will be positioned over much of central US with the
center of the high in the Gulf of Mexico, and another trough will
slide just off the East Coast. A weak upper level shortwave is
expected to slide through the base of the trough to our west, and
lift along the north side of the high pressure Thursday and
Friday. There's still uncertainty on where this axis of lift will
setup, so i'm keeping pops in the 30-40% range along and north of
I-85. At this time, any rain Thursday and Friday looks to remain



00z taf discussion.

A weak cool front will stall over north Alabama tonight and
provide a Focal Point for sct to nmrs showers, impacting the
northern taf sites. Vsbys will generally be in the 3-5 mile range
overnight, due to fog and rain. Cigs were mostly aoa 5k feet agl
initially, but should quickly lower to MVFR conds by 03z, and then
below 1000 feet agl by 06z. At kmgm and ktoi, conds will remain
rain free with cigs aoa 5k ft thru 05z, with MVFR cigs developing
by 06z, with LCL IFR cigs possible. A strong cold front will push
eastward across central Alabama during the day Saturday. South
winds will increase overnight to 5-8 kts, then veer to a southwest
direction as the front approaches. The cold front should be near
ktcl by 15z, and the I-65 corridor by 17z. Heavier showers will
accompany the front with vsbys falling below 3 mies at times. Low
cigs and light rain/drizzle will continue after fropa.



Fire weather...

Periods of rainfall will continue this afternoon as rain showers
and embedded thunderstorms move across central Alabama tonight and
into Saturday morning. Rain chances generally decrease from west
to east along a cold front passing across the area throughout
tomorrow afternoon. As a result, minimum relative humidity values
will be elevated through the weekend. Cooler and drier weather
arrives by Sunday morning and should remain in place through early
next week.


Preliminary point temps/pops...
Gadsden 58 64 38 57 33 / 80 100 30 0 0
Anniston 59 65 40 57 34 / 70 100 30 0 0
Birmingham 62 64 40 57 36 / 80 100 20 0 0
Tuscaloosa 63 64 39 58 36 / 90 90 20 0 0
Calera 61 64 39 57 35 / 70 90 20 0 0
Auburn 58 67 43 58 38 / 30 100 30 0 0
Montgomery 61 70 43 61 36 / 30 90 20 0 0
Troy 60 72 43 60 37 / 20 100 20 0 0


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