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afdmob 


Area forecast discussion...mesoscale update... 
National Weather Service Mobile Alabama 
1018 PM CDT Sat Mar 20 2010 


Mesoscale update...latest upper air and GOES water vapour imagery 
shows well-defined deep closed circulation moving from the Southern 
Plains into the lower MS valley. Downstream...central Gulf Coast region 
came under influence of a region of differential divergence...aided 
by left exit region of upper level jet streak. Rain area was 
enhanced by the associated deep layer lift. Lower tropospheric 
pattern was somewhat more complicated as two areas of surface low 
pressure (central Arkansas and in vicinity of southeast la) were observed along surface 
front. The surface cold front was located from central Arkansas southward to 
SW la at 21/00z. 


Overnight...surface low to our north deepens and intensifies just prior to 
occlusion which is forecast by midday Sunday. This will whip and 
accelerating surface cold front through region early Sunday and the 
heaviest rains will end from the west...only to be replaced by low 
clouds and sprinkles and maybe even drizzle by late Sunday as 
stratus decks advects into and over in association with lower 
tropospheric layer lifting and an increasing deep-layer cyclonic 
circulation. 


Regarding the formation of a heavier rainband later tonight... 
atmospheric instability but 21.21 mob WRF shows a narrow tongue 
of instability (100-200 j/kg) advecting northward ahead of heavier 
rainband which develops right along the surface cold front. Will stick 
with isolated thunder confined to the coast. Over the marine 
waters...some updrafts may tap surface-based cape and gust to 50 knots 
along the line. Stay tuned in case a special marine warnings is 
issued. 




Regarding rainfall...rain will continue spreading into the region 
this evening and will overspread the entire area by 1 am CDT. 
Observations have shown moderate to heavy for past two hours. 
This will deposit one to two inches by sunrise Sunday... mainly west 
of the I-65 corridor from Choctaw County Alabama southward to 
coastal Alabama. As the line of heavier showers moves through the 
region just ahead of a cold front during the pre-dawn 
hours...additional moderate to occasionally heavier rainfall will 
shift east of I-65 after sunrise. A new set of grids were sent about 
an hour ago to put tweaks on timing of pop12...weather and quantitative precipitation forecast. 


Marine...update out by 1030 PM CDT with no major changes save minor 
tweaks. 


Mob watches/warnings/advisories... 
Alabama...none. 
Florida...none. 
MS...none. 
GM...Small Craft Advisory from 10 PM Saturday to 1 PM Monday for the 
following zones: coastal waters from Destin to Pensacola Florida 
out 20 nm...coastal waters from Pensacola Florida to Pascagoula 
MS out 20 nm...Mobile Bay...waters from Destin to Pensacola 
Florida from 20 to 60 nm...and waters from Pensacola Florida to 
Pascagoula MS from 20 to 60 nm. 


&& 


$$ 


/23 medlin