048 
fxus63 kdvn 052021 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
315 PM CDT Sat Jul 5 2008 


Synopsis... 
typical Summer regime with main westerlies across southern Canada 
where middle level flow is semi zonal. Couple of disturbances in the 
flow with one strong middle level wave passing across Saskatchewan 
province with trailing vorticity lobe ejecting from northern rockies into 
the northern plains early this afternoon. Second and more vigorous wave 
was seen churning off the British Columbia coast. At the surface...high 
pressure was over the Ohio Valley with return flow becoming 
established across the County Warning Area. Deeper moisture axis surface-850 mb was 
positioned roughly from the lower Mississippi Valley to the northern 
plains. Mixing and light flow making for another comfortable day 
across most of the County Warning Area with dewpoints generally around 50 to l50s with 
few M/u50s over portions of southeast Iowa... while temperatures u70s to 
l80s. Few-scattered cumulus ... but overall copious amounts of sunshine with 
water vapor showing lots of drying across area in subsident region 
behind departing middle level shortwave from portions of the Ohio Valley 
into the middle Atlantic. ..05.. 


&& 


Short term...tonight and Sunday... 
main concern is precipitation chances. Models overall in reasonable agreement on 
main features. Middle level shortwave over Saskatchewan is expected 
to track eastward reaching Hudson Bay on Sunday...with upper ridge 
building into the plains... result will be middle level flow becoming 
increasingly diffluent across County Warning Area and with moisture increasing... 
more favorable regime for precipitation... with main challenge timing and 
location. For tonight... watching the Dakotas for convective 
development. Presently much or the northern plains is strongly 
capped... with +14c 700 mb temperature at kbis at 12z per radiosonde observation. However... deep 
moisture axis (sfc dewpoints M/u 60s and 850 mb dewpoints 12-15c) in place and 
large scale ascent increasing ahead of approaching cold front /wind 
shift/ and with trailing lobe of vorticity to sweep across region... 
should be enough to weaken cap and allow convection to develop over 
eastern Dakotas into portions of NE and western Minnesota ahead of trailing 
cold front this evening. Best shear and potential for organized 
convection or mesoscale convective system/S appears to reside over the Dakotas into western 
Minnesota. Corfidi forward propagating vectors transition from west to 
northwest across this area... thus with organized convection with 
cold pool the potential exists to eventually move S/southeast with time... 
with brunt of convection focusing just west of the County Warning Area into central 
Iowa in line with key thresholds/techniques including cool side of 850 mb 
of moist axis... and 700 mb temperature gradient of +9 to +13. Close enough 
though that western sections could get brushed by some precipitation late 
tonight and Sunday morning... and have added probability of precipitation for much of the 
area west of the Mississippi. Maintained chance probability of precipitation most of the area 
Sun afternoon with heating and residual boundaries. 


As for min temperatures... expect gradient from east (coolest) to west 
(warmest). Went generally a Cat below coolest guidance east with 
light winds and mainly clear skies much of the night... while went 
with blend in the west where clouds expected to arrive sooner and 
winds up near 10 kts later tonight. Maximum temperatures on Sunday a bit 
challenging... but with the anticipation of clouds and some 
convection played it a bit conservative especially west of the 
Mississippi and north of I 80... generally l/M 80s. ..05.. 




Long term... 
Sunday night through Monday night...flattening out steering flow and 
low level boundary getting shunted down into the northern County Warning Area or just to 
the north will make this period a potentially stormy one under the 
mesoscale convective system gun. Many model solutions out there and middle level cap just to 
the southwest being a factor...probably no model solution has a 
handle. But looking at low level jet and potential thta-east advection and 
convergent fields...as well as forcing from any vorts rolling along 
southern edge of freshly established northern Continental U.S. Westerlies... 
like the way the 12z UKMET handles the general scenario and will 
follow. This model suggests convection to take off Sunday evening 
from eastern Nebraska...to east central Minnesota with this activity propagating 
southeastward across much of the dvn County Warning Area later Sun night into Monday 
morning. Highest probability of precipitation already across the northwest third to half 
of the local forecast area and will only make minor tweaks. For Monday... 
of course what happens Sun night and lingers into Monday am to be a big 
player...but the UKMET/and the new hires European model (ecmwf)/ allow the main 
boundary to migrate to just north of the dvn County Warning Area along the Iowa/Minnesota 
border and into central WI by 00z Wednesday after any morning boundary 
or lingering surface cool pool mixes out. Then refocusing low level jet with thta-east 
rich airmass feed could produce a large half-ring of fire on this 
feature and lager overall boundary complex...with several mesoscale convective system/S 
taking off Monday night from the west Central Plains...across much of the 
north half of Iowa...and then into the Great Lakes/much of lower Michigan. Will 
go with likely probability of precipitation across the northern third for Monday night...to 
moderate chance across the south. Low to middle level shear parameters 
marginal at best these periods to support widespread severe storms...but 
convective available potential energy on Monday may grow to 2-3k j/kg depending on heating/airmass 
recovery from morning convective debris. Thus still a threat 
for some damaging wind mix Downs. High precipitable water/S of 1.5 to 2 inches and 
marginal middle level steering flow on forecast soundings for both Sunday 
night and Monday into Monday night...suggest any storm complex to be a 
heavy rain producer and that may be eventually the more widespread 
threat. The north half or northwest third of the local forecast area may 
be susceptible for flash flooding by Monday night...as it appears now 
that these area will have the best chance to experience repeated 
mesoscale convective system activity. Many variables though to hammer out at this point and 
each period will affect the following period...but all signals 
suggest a stormy start to the week for portions of or even most of 
the forecast area. 


In the extended/Tue-Sat/...latest available suite of medium range 
models continue to suggest Tuesday to be a transition day as digging 
northern stream wave energy will look to carve out eventual northwest 
flow across the western Great Lakes by middle week. This process will also 
mean a low level boundary/front sweeping east-southeast through the area 
and associated potential surface low of yet unknown strength...making 
for another storm/mesoscale convective system chance with frontal passage. The new 12z GFS seems too 
strong and south with passing surface low and my be feedback driven. 
Will lean on general UKMET and European model (ecmwf) solutions which still kick up 
storms on Tuesday and possibly lingering into the evening especially 
south and east. Depending on what happens in the earlier periods/sun- 
Monday night/...may have to have some type of flash flood headline 
issued or extended for portions or all of the dvn County Warning Area. Will up the 
probability of precipitation on Tuesday to 40 percent. This process and resultant northwest 
steering flow will then usher down a seasonably strong 1020 mb 
Canadian high down the upper Midwest for a cooler than normal 
midweek period...and longer range signals of upper ridging sliding 
eastward and strengthening over the region may keep a lid on any 
potential convection through Friday. The Euro is the coldest with it/S 
thermal profiles on Wednesday and would suggest highs in most of the dvn 
County Warning Area having trouble getting out of the 70s. Will lean toward the mex 
which seems to be a Happy medium with upper 70s to low 80s at this 
point. Wednesday night into early Thursday morning could have widespread lows 
in the middle to upper 50s...with even some lower 50s possible which 
would be near record territory for a few climate sites such as dbq. 
Will also trend down Thursday highs some. 


Low level return flow will look to become more pronounced next Friday into 
Sat with thermal moderation and return flow tapping into warmer 
profiles for highs possibly hitting 90 next Sat. Will keep Friday and 
Sat dry as the latest medium range guidance suggests just sporadic 
and widely sctrd showers/storms possible these days...with best 
chance for organized convection closer to main forcing in the 
northern plains and across Minnesota. Middle level warm wedge may look to 
become an issue for this area as well next weekend. ..12.. 


&& 


Aviation... 
VFR conditions expected to generally persist through the taf cycle. 
Only fly in ointment will be potential for some rain showers/thunderstorms and rain activity 
moving into portions of eastern Iowa late tonight and Sun morning... as 
thunderstorms and rain develop this evening over portions of the Dakotas into Minnesota and move 
east/southeast overnight. Kcid looks to have best chance at seeing precipitation and 
have gone with thunderstorms in the vicinity late tonight with prob30 group into middle morning 
on Sunday... while went with vcsh at kdbq and kmli. Any precipitation late 
tonight into Sun morning should be weakening and thus generally VFR 
conditions though cant rule out brief period of MVFR visibilities with 
thunder. ..05.. 


&& 


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


&& 


$$ 


05/12 

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