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fxus64 kbmx 212353 cca 
afdbmx 


Area forecast discussion 
National Weather Service Birmingham Alabama 
545 PM CST Sat Nov 21 2009 


Update...for aviation. 


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Discussion... 


High pressure and a very dry airmass are keeping most of the rain 
away from central Alabama. Slowly but surely though...the atmosphere 
will begin to saturate and rain will eventually begin to reach the 
ground. Given the conditions...total rain amounts for this weekend 
should remain below 1 inch. 


Continue to base the forecast on the sref solution. It has 
performed much better than the GFS for this event as it had a 
better handle on the high pressure and the amount of dry air in 
the area. With the delay of this system...had no choice but to 
increase rain chances for Sunday. Rain will be light...but it will 
be enough to measure. 


Low pressure will exit the area on Monday...but clouds will be 
slow to retreat. The next cold front will move through the area 
late Tuesday into Wednesday. Moisture will be limited...so kept 
rain chances out of the forecast. 


A second cold front is set to move through late next week. Airmass 
behind this front is expected to be much colder which will help 
bring temperatures back down to normal. 


10/arm 


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Aviation...00z taf discussion. 


Clouds and rain continue to slow as the airmass remains very dry. In 
fact preliminary data from the upper air radiosonde shows a dry slot 
..above eet...Between 850 and 800 mb with relative humidity values as low as 5 
percent. So this means that we are really going to have to saturate 
from top down in order for the precipitation to make it to the 
surface. The more moderate rain that will be capable of doing this 
is finally beginning to work closer to the taf sites in the west. 
Expect the rain to finally make it to the surface at tcl by 1z as a 
heavier band of rain works in. The band will work into eet and bhm 
between 130 and 3z. This is not really a matter of if it will rain 
tonight...but actually when will it start. Once it does...showers 
will be possible for the next 12 to 24 hours along with low ceiling 
and restricted visibilities. IFR ceilings will likely develop at kmgm 
and ktoi by 09z...and there is chance IFR ceilings could spread 
northward into all taf sites after 12z...but drier easterly low 
level flow may keep ceilings above 1000 feet above ground level. 


Overall trend will be for deteriorating conds during the overnight 
hours. 


16 


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Bmx watches/warnings/advisories...none. 


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