491 
fxus63 kdmx 091136 aaa 
afddmx 


Area forecast discussion...updated 
National Weather Service Des Moines Iowa 
536 am CST Friday Jan 9 2009 


..updated aviation discussion... 


Short term...today 
warm advection precipitation quickly ending over north central Iowa 
this morning with dry air beginning to advect into the area as can 
be seen with ceilings rising to above 9 kft. Final surge of warm 
advection now moving though central Iowa along with a fair amount of 
associated lift...however dry air will preclude and further 
development in the near term. With dry air moving in...expect mainly 
dry conditions to maintain through late morning with the exception 
of some light precipitation over the very far northeast early this 
morning. 


A strong short wave currently moving across western North 
Dakota will drop southeast this morning and will begin to affect the 
northern and central Iowa by this afternoon. Have increased probability of precipitation 
this afternoon as the short wave and middle level frontogenesis 
arrives. Despite the dry air in place...it will likely be overcome 
by middle to late afternoon over the northwest and west central 
counties...given that primary lift will be in dendritic 
zone...allowing for bigger flakes to fall into the dry layer and 
lead to more rapid saturation. A brief period of freezing rain is 
possible over the west central and central sections as melting 
snowflakes fall through a warm layer with some of the precipitation 
surviving the dry layer and reaching the surface as freezing rain. The 
warm layer will gradually cool due to evaporative cooling then 
quickly dissolve once saturation occurs and wet bulb temperatures 
fall to below zero c. At this time...the event does not appear to 
warrant headlines with the period of freezing rain expected to be 
brief if it does occur and new snowfall amounts generally an inch or 
less during the period. Isolated 2 inch amounts are possible. 


Long term...tonight through Thursday 
fairly active long term still on track...with initial concerns 
centering on tonight as upper level wave drops southeast from eastern 
Montana/western ND today sliding across western/scntrl Iowa tonight. Surface low to 
drop south of the area as well...and become centered near southeast MO by 
late tonight. Colder air to push southward...with def zone snow 
expected to develop and slide south across the area this evening 
into the overnight hours. Highest quantitative precipitation forecast to remain across the east 
central County Warning Area...with somewhat higher snow ratios expected there as 
well. Therefore with around a tenth or less of quantitative precipitation forecast...could see 
around 2 inches of snow...with lighter amounts as you move westward. 
Snow to taper off late tonight as the system slides east and high 
pressure builds into the County Warning Area for Saturday. Another weak wave...with 
weak warm air advection to slide across the north Saturday night. GFS stronger with 
this system than the ec/NAM...but all models advertising some quantitative precipitation forecast. 
Lift looks OK...with moisture questionable...so opted to insert only 
a slight chance for snow across the few northern tiers of counties 
for Saturday night. Better chance for precipitation comes Sunday night into 
Monday as a stronger system dives southeastward in fast-moving 
northwest flow pattern aloft. This system will bring the first in 
many pushes of much colder air to the state as 850 mb temperatures drop 
into the -10c to -15c range by 12z Tuesday. Second surge of colder 
air to drop in behind next wave dropping southeastward Tuesday into 
Tuesday night. This system will bring another chance for some light 
snow...with 850 mb temperatures dropping to around -20c across the County Warning Area 
by 12z Wednesday. Area of Arctic high pressure to then dominate 
Wednesday into Thursday with 850 mb temperatures remaining in the -20sc across 
the area. Therefore after near normal temperatures through the 
weekend...all sites trend toward well below normal by middle to late 
next week. Right now highs in the single digits to middle teens by middle 
week...with lows hovering within a few degrees both above and below 
zero. However with this much cold air sinking southward...and 
possibility for new snow cover...temperatures may need to be trended 
downward a few more degrees or so. 


&& 


Aviation...09/12z 
dry air has moved into central Iowa this morning and will yield VFR 
conditions initially. A system will move out of the Dakotas late 
this morning and into the afternoon...bringing lowering ceilings and 
increasing precipitation chances. Widespread MVFR ceilings with 
local IFR can be expected by middle to late afternoon then continue 
through much of the overnight hours. Snow will develop most 
locations and bring reduced visibilities for a period. The 
precipitation may be mixed with freezing rain for brief periods this 
afternoon and evening. 


&& 


Dmx watches/warnings/advisories...none. 


&& 


$$ 


Short term/aviation...donavon 
long term...albrecht