Scientific Forecaster Discussion
fxus64 kbmx 251721
Area forecast discussion
National Weather Service Birmingham Alabama
1221 PM CDT sun Aug 25 2019
for 18z aviation.
/updated at 0342 am CDT sun Aug 25 2019/
Today and tonight.
The active weather pattern continues today and tonight across
central Alabama. A weak frontal boundary is currently stretched through
our south-central counties and has been helping generate some
isolated to scattered shower activity early this morning. I would
expect this to continue through much of the morning hours.
To our northeast, we have a backdoor cold front moving through
northern Georgia, denoted by the low 60s dewpoints and mid 60s
temps. It's not much of a frontal boundary, but is expected to move
westward into eastern and northeastern central Alabama by the early
afternoon. To our northwest, we have a trough deepening into the mid-
MS valley, which will lead to height falls across our area. This
should help slightly enhance convection along the stalled frontal
boundary, and along with daytime heating and the backdoor front,
will generate more widespread showers and thunderstorms. The best
chances will be generally west of I-65 where there will be more
abundant moisture. The drier, easterly flow could help limit
thunderstorm coverage in our far eastern counties this afternoon,
but I still think there's a chance for some activity, so i'll leave
roughly 50% pops in the forecast for those areas. Pwats remain on
the upper end of climatology (97.5th percentile for this time of
year), so we should see very efficient rainfall with these storms
again today. If any develop and remain stationary over urban areas,
or locations with poor drainage, we could once again see flooding
impacts. I'll hold off mentioning any flooding concerns in the severe weather potential statement
for now because of the low confidence in whether the storms will set
up over urban areas or over rural areas, where vegetation would
mitigate the flood impacts.
Meanwhile, an area of convection in the western portions of the Gulf
of Mexico has developed a weak surface low. As the trough dips into
the mid-MS valley this afternoon, the surface low gets picked up and
lifted northeastward. It is expected to move through MS and approach
our western counties late this evening into the overnight hours.
Precipitable waters begin to increase from the 2.00" that we've seen the past
couple of days to 2.30"+, which would be near the Max climatology
for this time of year. Therefore, rain chances likely won't diminish
much after sunset like we typically see in a Summer pattern.
Instead, showers and thunderstorms should continue for much of the
area through the overnight hours.
Instability increases quite a bit during the afternoon today,
especially west of I-65, where values could 2000-3000+ j/kg. This
could produce some stronger storms capable of gusty winds that could
bring down trees. With the dry weather we've had so far this Summer,
we could see some weakened trees being brought down with sub-severe
/updated at 0342 am CDT sun Aug 25 2019/
Monday through Saturday.
On Monday, widespread showers and storms continue as an upper-level
trough moves across the forecast area. At the surface, an area of
low surface pressure is expected to track northeast through southern
Mississippi and ultimately make its way through portions of western
Alabama. This disturbance, arriving from the Gulf Coast, will drive
southerly winds across the region which will advect a very tropical-
like airmass into central Alabama. Pwats may rise to as high as 2.2
to 2.3 inches such that heavy downpours may lead to continued
flooding issues in poor drainage and urban areas. Severe weather is
currently not expected on Monday since instability will have a
difficult time increasing within a rain-cooled airmass, but we will
need to monitor for any enhancements in low-level helicity as the
surface low moves across. More thunderstorms are expected on Tuesday
as a cold front approaches from the northwest, but should see a bit
of dry air work in behind the shortwave trough from Monday with
shortwave ridging also noted in the low-levels which will help
reduce the overall rain coverage. Temperatures are forecast to
remain below 90f on Monday with highs in the low/mid 80s and upper
80s on Tuesday as widespread rain and cloud cover keeps US a bit
cooler although humid.
A northeast-southwest oriented cold front, associated with broad
deep-layer troughing over Ontario, will pass through central Alabama
late Tuesday afternoon into Wednesday. As noted above, the presence
of drier mid-level air from 700-500mb ahead of the front may hinder
the spatial development of rain during the overnight hours. However,
latest guidance diverges a bit on the timing of the frontal passage
and amount of dry mid-level air. Therefore, decided to keep pops
around 20-40 percent for Wednesday due to the lower confidence with
highest chances along and southeast of I-20. A much drier airmass
sets in behind the front for Thursday through Saturday with ridging
amplifying back over the southeast. May see a small increase in rain
chances for our southeastern counties on Saturday as a low-level
inverted trough develops over the Gulf Coast, but confidence on this
solution remains low at this time. Therefore, plentiful sunshine is
expected for the end of the work week with highs in the upper
80s/low 90s and overnight lows in the mid/upper 60s.
18z taf discussion.
Persistent cloud cover will continue over most sites with
exception at tcl/mgm/toi. Thunderstorms and rain potential will be greatest at
tcl/eet/mgm/toi with best timing from 25/21z through 26/01z. Non-
zero risk elsewhere but confidence too low to include in this
issuance. Easterly low-level flow is well established across the
eastern sites with more of a southeast influence across central
and western terminals. Rain showers potential will persist after sunset
through much of the evening hours.
Shower and thunderstorm chances remain elevated through Tuesday as
we remain in a moist tropical airmass. With the increased
moisture, minimum relative humidity values will be elevated as
well. Gusty winds will be possible in close proximity to any
thunderstorms that do develop. A surface frontal boundary is
expected to move through Wednesday into Thursday. Although drier
air will arrive, minimum relative humidity values for the end of
the week should still remain above critical levels.
Preliminary point temps/pops...
Gadsden 68 81 69 87 69 / 70 80 60 60 40
Anniston 69 82 70 86 71 / 60 80 60 60 40
Birmingham 71 84 72 89 72 / 70 80 60 70 40
Tuscaloosa 71 84 73 90 73 / 70 80 40 70 40
Calera 69 83 71 89 71 / 70 80 60 60 40
Auburn 70 82 71 86 72 / 60 80 60 70 30
Montgomery 71 85 73 90 73 / 70 80 50 70 30
Troy 70 84 71 88 71 / 70 80 50 70 30