Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Birmingham Alabama
1213 am CDT Fri may 25 2018
for 06z aviation.
Well, another quite challenging forecast afternoon across central
Alabama. A broad area of low pressure surface and aloft was still
present over much of Georgia and eastern Alabama. This feature was
under a larger scale ridge in the Tennessee and Ohio valleys. As
the sun rose this morning, numerous boundaries were evident on
several different scales. Showers and thunderstorms developed
along these boundaries central and east before noon, and have
migrated slowly south and west. Instability was present with
sbcapes of 2500-3000, but lapse rates, mean winds, downdraft CAPES
where limited. So overall, the stronger storms the remainder of
the afternoon will be capable of wind gusts to 40 mph and brief
torrential rain. An isolated location may receive a few inches of
rain in a short period of time.
Convection should diminish in coverage through the evening but not
completely go away. Some activity may actually redevelop late
tonight east as a upper vorticity axis moves into the area.
Deep layer winds become much more southerly on Friday as an upper
trough approaches from the northwest and a low was located near
the Yucatan Peninsula. Very high moisture content, instability and
boundary interactions will lead to high rain coverage.
Not much change to the extended forecast. The outlier GFS has
trended much much closer to the European model (ecmwf)/NAM and others consensus as
a system moves northward out of the Gulf. The National Hurricane
Center now has a 90 percent chance of development. Both the
GFS/European model (ecmwf) has lowered pressure and overall speed of the system. We
shall see. There remains a chance of flooding rains each day
All interest in central Alabama should be aware that this system
may affect parts of the Gulf Coast and central Alabama before the
end of the Memorial Day weekend. Stay up to date on the latest
Previous short-term discussion:today and tonight.
The overall pattern has not changed much over the past 24 hours with
a low level circulation over central Georgia and a trof axis
extending westward into South Alabama. Light winds and minimal cloud
cover has produced conditions conducive for fog this morning. A few
places are already below one-half mile across West Alabama, and a
dense fog advisory may become necessary. Precipitable water values
are still above normal with values around 1.75 inches. The majority
of the convection today will be near and south of the low level trof
axis, but thunderstorms possible anywhere this afternoon due to
higher than normal pw's and little or no capping inversion.
Convection will wane after the loss of daytime heating.
Friday through Wednesday.
Trailing vorticity from a shortwave moving through the Midwest
will result in falling 500mb heights on Friday as an upper-level
trough strengthens over the Gulf and adjacent states. A weak
frontal boundary will have dissipated, allowing moisture to return
northward. Cyclonic mid-level flow, daytime heating, and the moist
air mass will result in numerous showers and storms that will be
maximized during the afternoon hours.
Upper-level troughing over the region will be a persistent feature
of the forecast period due to a tilted Omega block pattern at
500mb with ridging over the plains flanked by the Gulf trough and
a closed low over the Great Basin. A developing surface low near
the Yucatan will be lifted northward by the trough. NHC indicates
that this low is likely to develop into a tropical or subtropical
depression. The GFS remains east of the guidance consensus but has
trended westward closer to the other model guidance. Will continue
to rely on the European model (ecmwf) which has support from the NAM and Canadian
as well as its ensembles. The European model (ecmwf) has trended slower in the last
couple of runs. Shear from the trough will prevent significant
strengthening, with the main impacts for the Gulf states
continuing to be heavy rainfall and flooding, as is often the
concern with slow-moving weak tropical systems. Saturday looks to
be the "least wet" day of the Holiday weekend, as the low and
deep tropical moisture remain south of the area, but there will
still be enough forcing from the trough and moisture to warrant
likely pops for diurnally enhanced convection. By Sunday,
southerly flow aloft will over-run strengthening low-level
southeasterly flow likely resulting in more widespread/steady
tropical rains lifting up from the southeast. This will result in
cooler high temperatures, and will only mention a slight chance of
embedded thunder due to limited instability.
Sticking with the European model (ecmwf) solution, the low pressure system will
become vertically stacked and stall out somewhere just southwest
of the forecast area for the first half of next week. Continued
850mb moisture transport east of the low will result in wet
conditions continuing across the area, modulated by any patches of
dry air aloft that wrap around the system. Rainfall will likely
contract closer to the center at night while expanding in coverage
through the day. Greatest potential for flooding will be closest
to where the low stalls, but confidence is low exactly where that
will be. There will likely be some hefty rainfall totals somewhere
across the Gulf states and especially near the coast, but
confidence in placement is low. Will continue to highlight the
flooding potential in the severe weather potential statement. More organized flooding chances
look to hold off until Sunday, but isolated flash flooding remains
possible Friday and Saturday due to the moist air mass and weak
flow through the column.
06z taf discussion.
A few showers and thunderstorms remain on radar at this hour.
Still expected with all of the rain that we have observed across
central Alabama today some patchy fog for later tonight. Will
continue to mention of MVFR cigs/vis then going down to IFR as we
progress through the night. Another round of more widespread rain
showers should start up again by mid morning along with scattered
ts activity during the afternoon as we remain in this moist
Note: be aware that mgm's ceilometer remains out. Techs are aware
of issue. It will hopefully be repaired by sometime Friday. For
now, we will forecast cigs but have amend not schedule in taf as will
have no ground truth for amendments.
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.
Preliminary point temps/pops...
Gadsden 83 66 84 66 80 / 50 50 70 40 70
Anniston 83 66 84 66 79 / 60 50 70 40 80
Birmingham 85 69 85 69 81 / 70 50 70 40 70
Tuscaloosa 86 68 86 68 83 / 70 50 70 40 70
Calera 84 67 84 67 80 / 70 50 70 40 80
Auburn 81 67 81 67 77 / 60 50 70 50 80
Montgomery 86 68 85 69 80 / 70 50 70 50 80
Troy 85 68 84 68 78 / 70 50 70 60 90