253 
fxus63 kdmx 201725 aaa 
afddmx 


Area forecast discussion...updated 
National Weather Service Des Moines Iowa 
1130 am CST Thursday Nov 20 2008 


Updated for the 18z aviation discussion... 


Discussion... 
stratus continues to overcast the area behind yesterdays cold 
front...and looks to continue through most of today. A shortwave 
moving quickly through northwesterly flow aloft will push a weak 
reinforcing shot of colder surface air through the state. Weak lift 
associated with this shortwave and high relative humidity near the low-level 
inversion as indicated by forecast soundings are supportive of 
scattered flurries this morning...except in the southwest where lift 
will be weaker...relative humidity lower...and satellite imagery indicates possible 
breaking up of the stratus deck. Surface observation already show flurries in ND 
moving this way. However...forecast soundings also indicate 
significant low-level drying with mixing during the afternoon so 
have removed mention of flurries after 18z. Otherwise few changes 
made to todays forecast...it will be cold and blustery across the 
area. 


Little of concern outside of temperatures tonight and into much of Friday 
as surface high settles into MO valley reaching MS river by late 
afternoon. Have gone just above guidance tonight as...although not 
as emphatic...forecast sounding continue to show mixing through much 
of the night keeping temperatures up somewhat. Only exception was far 
western sites...especially in the nish valley. Into Friday have 
continued to go toward low end of guidance based on forecast 
soundings which are in good agreement. MOS likely being tugged 
upward by climatology with unseasonably cold airmass in place. 


GFS/European model (ecmwf) remain favored models with late Friday/Sat system and remain 
similar to what was depicted last night. Middle level thermodynamic 
and kinematic forcing sweeps through Iowa with weak static stabilities 
on southwestern flank. Uvm in 700 mb/h6 dendritic layer...or 290-295k...is 
quite strong and saturated and will result in plenty of snowfall 
production aloft. Only drawback is overcoming dry air in lowest 1km 
which will take awhile and probably not occur until forcing wanes 
somewhat. Phasing of dendritic uvm and adequate low level moisture 
seems to occur near fsd/spw shortly after 00z...progressing across 
northern Iowa to ebs/ifa/miw area toward daybreak. This dry low level air 
will likely keep quantitative precipitation forecast amounts fairly low. Recent GFS Cobb ratios 
have been 10-13:1 and staying in that line suggests snows from a 
dusting to around an inch with precipitation lingering into Sat morning 
before ending. Would not be surprised to see very isolated higher 
amounts if saturation occurs sooner considering strength of uvm and 
potential instability...but confidence not enough to put in general 
forecast. Still curious that MOS probability of precipitation have not responded more...but 
have continued to go high end chances and low likelies in favored 
areas. 


Warm advection and increasing clouds look to keep temperatures nearly 
steady Friday night...with mins toward high end of guidance. Climatology may 
again be tugging at MOS Sat vs soundings so have again gone toward 
low end with little change from previous forecast Sat night. Early 
next week basically unchanged with closed low and associated forcing 
staying to our north. Could see some light snow showers/flurries as 
weak short waves traverse deep cyclonic flow Sun night and Monday...but 
that is about it. Remainder of forecast dry as ridging crosses 
central Continental U.S. Midweek. 


&& 


Aviation...20/18z 
taf sites continue to be impacted by MVFR/borderline VFR ceilings...with 
drier air expected to continue to overtake County Warning Area as high pressure 
builds southeastward. Therefore expect bkn-ovc025/035 deck to slowly 
break up through the late afternoon hours into the evening 
hours...with clear skies by late evening. Gusty northerly winds to 
diminish this evening into tonight...and back to the west/northwest 
into Friday morning. 


&& 


Dmx watches/warnings/advisories...none. 


&& 


$$ 


Jlw/bss/mja