Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 220844

National Weather Service Des Moines IA
344 AM CDT Thu Sep 22 2016

.SHORT TERM.../Today/
Issued at 342 AM CDT Thu Sep 22 2016

Ongoing convection continues in axis of low level jet inflow
across northern iowa and additionally supported by weak wave
entering western Iowa. Most intense convection is occurring behind
a slowly advancing outflow the only recently became observable on
radar. Rainfall rates in strongest areas remain 2-3 inches per
hour and this concern will persist through sunrise although the
slow southward progress should help any one area from getting as
dramatic amounts as seen last evening near Charles City. That
being said, the threat still exists for flash flooding across
northern Iowa through the day and the headline will continue
unchanged at this point.

Still difficult to see evolution of convection later today as
current convection gradually diminishes with the passage of the
weak wave this morning. Where the surface and outflow end up and
eventually provide focus for additional development is still being
considered. It will likely remain somewhere across northern half
of Iowa by later this afternoon with additional threat of heavy
rain and strong storms. Temperatures today will remain cooler
across the north where outflow will be south of area with a
modified airmass in place. Farther south, will continue to see an
unseasonably warm and humid airmass in southern Iowa with
temperatures remaining nearly 15 degrees above normal.

.LONG TERM.../Tonight through Wednesday/
Issued at 342 AM CDT Thu Sep 22 2016

The main forecast concern was focused on additional heavy rain
chances tonight and again Saturday night into Sunday. Models have
become into better overall agreement throughout the forecast
period with the ECMWF following the GFS on its respective solution
next week. Leaned closer to the GFS

Tonight into Friday...With the boundary remaining planted across
northern Iowa and another shortwave to eject across the Central
Plains tonight combined with the increasing LLJ, another round of
storms, possibly heavy rain, looks probable for northern portions
of Iowa. Fairly strong QG forcing moves across northern Iowa
this evening with good isentropic upglide along the 300-310K
surface. Decent moisture transport overrunning the surface
boundary providing some moisture convergence across northern Iowa.
PWATs remain well above normal for late September as they range
near 2 inches by 06z Friday. Warm layer cloud depths over 4000
meters and with BUFR soundings at MCW to FRM remaining deeply
saturated, additional heavy rain is possible over portions that
have already received significant rainfall. Thus, flash flooding
as well as river flooding could be exacerbated even further by
Friday morning. Storms look to quickly diminish past 12z Friday
with the LLJ diminishing.

Saturday through Monday...cold front and associated upper level
low pressure will transition across the region Saturday night
into Sunday. The best potential for rain looks to be from 00-12z
Sunday and unfortunately another long duration round of rainfall
possible. Instability is weak as well as the deep layer shear
ranging less than 20 knots Saturday night. Low confidence with any
widespread severe storms. Thus, the main concern is the heavy rain
potential with similar PWATs and warm layer cloud depths compared
to tonight. The GFS/ECWMF 00z runs have dried out Monday but with
some lingering low stratus in the CAA and cannot rule out some
scattered showers during the day Monday into Monday night.

Tuesday through Thursday...The remainder of the week remains
uncertain with the models continue to inconsistent with their
respective solutions. However, confidence is increasing toward a
dry and cool than wet and cool solution.


.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1152 PM CDT Wed Sep 21 2016

Additional thunderstorms are expected to spread across northern
Iowa late tonight and linger into Thursday. Have included VCTS at
the northern sites where thunder is more likely, compared to the
VCSH at KDSM during the morning hours Thursday. Southerly winds
expected to transition to more southeasterly to easterly toward
Thursday evening.


Issued at 342 AM CDT Thu Sep 22 2016

While the overall threat in southern portions of the watch seem to
be waning, recent storm motion should carry convection into this
portions of the watch this morning with some limited threat.
Therefore will maintain the watch for now. As mentioned in the
body of the discussion above, eventually placement of
outflow/boundary by this afternoon will determine where eventual
redevelopment will take place later today. It still appears this
should be across the northern half of the state and therefore will
leave headline alone. May need to be extended into tonight if the
threat is deemed sufficient later today.


Flash Flood Watch through this evening for IAZ004>007-015>017-



LONG TERM...Podrazik
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