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Area forecast discussion 
National Weather Service Birmingham Alabama 
1213 am CDT Fri may 25 2018 

for 06z aviation. 


Short term... 

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 
through Wednesday. 

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. 


Long term... 
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 
tropical pattern. 

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. 



Fire weather... 

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 


Bmx watches/warnings/advisories/... 

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