240 
fxus63 kdvn 212104 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
300 PM CST Sat Nov 21 2009 


Synopsis... 
ridge of high pressure is sliding off to the east today...allowing 
pressure to fall across the area...winds to pick up out of the 
southeast...and temperatures to warm into the low to middle 50s for 
many locations across the County Warning Area today. Stratus and fog from this 
morning has mixed out to some extent...though it is nearly intact 
across portions of northern Iowa and southwest Wisconsin...and we 
have patched of it extending down into our northeast Iowa counties. 
Aloft there is an amazing pocket of warm air ahead of our next upper 
level trough...which is moving out of The Rockies at this time. At 
850mb we have an area of +12 to +14 air over the northern High 
Plains...and they are into the 50s and even low 60s at the surface 
today as a result. There is also a fairly decent trough in the Lee 
of The Rockies today...and while there is no temperature difference 
across it now...that is partially due to downsloping behind it. The 
Gulf is already open...as indicated by a narrow plume of moisture 
with 850mb dewpoints of +5 or higher extending up into Oklahoma. 
This was generated by the cut off southern stream upper low in that 
area. ..le.. 


&& 


Short term...tonight and Sunday... 
main forecast is for clouds tonight and Sunday...with stratus and 
fog of particular concern. Surface dewpoints advecting into the 
area in the middle and upper 40s indicates that our dewpoints should 
rise for the next 6-12 hours...but then drop off again towards 
morning and Sunday as the slightly drier pocket over southern MO 
moves in. We are also seeing convergence and pooling to our west 
ahead of the trough that is should develop into a cold front 
tonight...and this should also act to dry our our dewpoints late 
tonight and Sunday. These dewpoint issues will have an effect on 
our area in temperature and fog development. Expect that a new area 
of stratus...already in evidence over eastern Kansas and Nebraska 
will advect towards the area tonight...and ahead of it we should get 
some fog development later tonight as temperatures drop in the clear 
air with the dewpoints still relatively high. Tough to say...as 
models are not handling this well right now...but I currently expect 
the stratus back in here somewhere around midnight or shortly 
thereafter. We may get some drizzle where the stratus moves 
in...especially if we already had some fog development before it 
moved in. Our northeast is at risk of dense fog again tonight as 
they should stay clear the longest and get the coldest. Sunday we 
should gradually mix out the stratus like we have today...leaving US 
with some scattered-broken stratocu by afternoon. Warm air advection should 
bring US up into the middle-upper 50s if not near 60 as is currently 
happening over southern MO. ..le.. 




Long term... 
Sunday night through Monday night...light southeasterly low level flow 
back from ridge center should keep low stratus and much of any chance 
of light precipitation off to the west of the dvn County Warning Area Sunday night...with 
just varying layers of ac and cirrus. Any breaks in these higher decks 
along with a decrease in the surface winds could mean areas of fog again 
but will leave out mention for now. Expect widespread lows in the 
upper 30s to lower 40s as long as most of the high clouds maintain. 
12z run short range models generally agree that as the next large 
piece of Pacific wave energy digs inland across The Rockies and 
northern plains this period...sprawling southwest mean steering flow 
will engulf much of the middle and upper level MS river valleys into Monday 
evening. Monday may remain mostly cloudy with continuing bouts of ac 
and cirrus...but low level southeasterly return flow will ensure another above 
normal temperature day with some areas locally possibly striving up into the 
upper 50s with at least some temporary insolation. As low to middle 
level cyclogenesis revs up across the Central Plains Monday night in 
response to the approaching strong wave...the initial arrival of 
the first wave of lift and associated precipitation into the western County Warning Area 
dependent of course on adequate low level saturation. The 12z run GFS 
continues to be the most bullish on low level moisture return and column 
saturation enough for a quarter inch of rain into the western 
third of the County Warning Area by 12z Tuesday...while the NAM and UKMET are mainly 
dry through the period. The new 12z run European model (ecmwf) has become more 
progressive with the approach of the developing cut-off low complex 
with the feature into southwest Iowa by Tuesday morning which is curious as 
it still advertises a near vertically stacked system which should be 
slower like its earlier 00z run. Will keep chance probability of precipitation across the western 
2/3s especially after 06z Tuesday but think with the increasing east- 
southeast low level flow of a drier airmass source...the UKMET and NAM are 
on to something with their delay of the onset of precipitation at the surface 
especially in areas along and east of the MS river. If surface dewpoints 
increase Monday night low temperatures will have a tough time dipping below the 
lower 40s. 




In the extended/Tue-Sat/...main challenge in the long term continues 
to be how the cut-off low pressure complex impacts the middle week and 
Holiday period. Interesting how that in general the 12z medium 
range model runs have trended more progressive with a closed and 
near vertically stacked low pressure system through the region Tuesday 
into Wednesday. This would suggest that likely probability of precipitation warranted County Warning Area-wide on 
Tuesday for the main surge of pre-system lift utilizing moist conveyor... 
but a wane in the action from the southwest-to-northeast Tuesday 
afternoon with the arrival of in-wrapping low to middle level dry slot 
and cold conveyor. Depending on the model...the cold conveyor even 
switches the thermal profile into a rain/snow mix or even all snow 
from the same direction/SW to NE/ during the afternoon and Tuesday 
evening. It/S just how much precipitation is left in the County Warning Area for this 
cooling to interact with especially south of i80...if indeed 
this new quicker timing is even correct. For now will up the probability of precipitation to 
likely County Warning Area-wide Tuesday but keep precipitation all liquid through 00z Wednesday. 
Will go with a mix across the western third after 00z Wednesday more for a 
wrap around precipitation scenario into the cold core like the Canadian 
Gem is suggesting...while lingering light rain or drizzle exits out 
of the far east. Further shifts may have to end precipitation temporarily 
later Tuesday afternoon and then walk in straight snow showers or 
flurries Tuesday night and Wednesday in wrap around into colder air. 


As the main cyclone continues to bomb over the Great Lakes Wednesday into 
Thursday...the new 12z European model (ecmwf) has sided with the the 12z GFS in suggesting 
a secondary low to drop down to the west of the main system and 
interact with it some almost fujiwara-like. This system would bring 
banded swaths of mainly snow under it Wednesday night across at least the 
northern 2/3s of the dvn County Warning Area with some light accumulations possible. 
The UKMET and Canadian Gem do not have this secondary system and just 
advertise tight cyclonic flow with embedded flurries or light snow 
showers of no consequence Wednesday night into Turkey day. Right now do not 
trust the GFS and Euro with this secondary system and will just 
advertise light snow showers or flurries Wednesday night into Thursday but 
trend down temperatures. Thursday could be a cold blustery raw day with 
highs only in the 30s...flurries and winds gusting over 30 miles per hour. 


Ridging and thermal moderation look to follow in behind the exiting 
Great Lakes cold core cyclone for the start of next weekend. ..12.. 


&& 


Aviation... 
main question is for fog and stratus development tonight. Expect 
stratus to move in and lower to around 15-25 hft late this evening 
and overnight. Ceilings should be lowest west...and highest east. Kmli 
and kbrl will be near the edges of this stratus...but should see at 
least a short period of it. Fog development near the edges of the 
stratus and to the east may again be dense...but we should see 
visibilities 1-2sm under the stratus like we had this morning. 
Burn-off in the morning should occur sometime between 16-20z...with 
some low ceilings remaining at kcid and kdbq a few more hours. 
.Le.. 


&& 


Dvn watches/warnings/advisories... 
Iowa...none. 
Illinois...none. 
MO...none. 


&& 


$$ 


Le/12 

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