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Area Forecast Discussion (AFD)

Issued by Des Moines, IA (DMX)

                            
000
FXUS63 KDMX 171738
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1238 PM CDT Tue Sep 17 2019

.DISCUSSION.../Today through Monday/
Issued at 416 AM CDT Tue Sep 17 2019

*/ Highlights... 

 -Several rounds of showers and thunderstorms possible over the next 
several days.
 -Heaviest rainfall may be in central to northern Iowa, with a non-
zero chance of flooding issues
 -Well above-normal temperatures and humid conditions to continue 
through this week

*/ Summary...

Another hot and humid day is in store for central 
Iowa. Expect temperatures for today to be similar to where they 
were yesterday, topping out well into the 80s. With dewpoints 
expected to remain in the low 70s, heat index values will likely 
push the low 90s once again. Spotty showers and thunderstorms may 
develop in east-central Iowa. Though these storms are not expected
to be long-lasting, a few of these storms may be strong.

For tonight, moderate rainfall is possible primarily across 
north- central to northwestern Iowa- where rainfall amounts may be
in the one-quarter to one-half an inch range...with locally 
heavier amounts possible. 

Another shot of moderate to heavy rainfall is possible Wednesday 
night into Thursday morning. The potential rainfall amounts in this 
round of precip is worth monitoring closely, as central to northern 
Iowa may be impacted. Should the moderate rain forecast for tomorrow 
night pan out, the grounds may be well-saturated in advance of this 
next round of rainfall... introducing the non-zero chance of 
flooding. 

Yet another slug of showers and storms are forecast sometime near 
Saturday that could bring widespread moderate rainfall totals. 
Above- normal temperatures and humid conditions should continue 
through Saturday, before a cool-down temporarily arrives for 
Sunday.

*/ Technical Discussion... 

Today... 

With 12z Tue 850mb temps similar to 12z Mon 850mb temps, plus 
scattered clouds expected, temperatures should be similar for 
Tuesday to where they were yesterday. Introduced spotty showers and 
storms to east-central Iowa. Instability will be sufficient for 
convection. Weak focusing mechanism presented in low-levels with 
slight sfc convergence, 925-950mb frontogenetical forcing, and 
850mb moisture flux convergence. This should be sufficient to 
punch through cap... trouble is the 0-6km shear profile is not the
most impressive as indicated by a tight hodograph profile. 
Ultimately, expecting any storms to be more pulse-like in nature 
versus the sustained strong/severe category.

Tonight... 

08z Tue water vapor imagery shows a well-defined upper low spinning 
near the Washington coastline. Models in good agreement showing this 
upper low racing across the Rockies, making it into North Dakota by 
12z Wed. In advance of this upper low, a weak impulse is picked up 
from western Kansas through New Mexico. This impulse will be 
dragged up into NW Iowa overnight. Low-level support is very 
impressive as shown by an elongated 50kt LLJ in the NW corner of 
the state. 0-6km mixing ratios shatter 8 g/kg as PWAT values reach
1.5 to 1.75 inches. 0-6km cross-section across the northern/NW 
portion of Iowa shows RH values around 80 percent, with warm cloud
depths reaching 4km. So expecting mildly decent precip 
efficiency. 

Tomorrow night...

Another shortwave blasts through the Midwest and into NW Iowa 
between 06 and 12z Thursday. Similar to tonight's setup, decent 
moisture available with overall mesoscale features similar to 
tonight. Orientation of this precip round is much more extended in 
areal coverage, stretching from Mason City through Creston. 
Models, especially the 00z Tue ECMWF, may be too far north with 
the core of this precip... so have begun to nudge higher QPF 
amounts further south/SW. Big thing to watch with this round is 
the potential for heavy precip to fall over already saturated 
areas. Over the past 7 days, northern to north-central Iowa has 
received around 600 percent of their normal precip. If rainfall 
tonight exceeds going forecast, ground may not be able to take too
much more rain. So will monitor for flood potential.

Friday through Saturday...

A rather potent-looking upper low is being progged to come onshore 
near the Oregon coastline and dip down through the Four corners 
Region. Meanwhile, another shortwave is being progged to propagate 
eastward across the northern CONUS. These two features more-or-less 
Fujiwara over the upper Plains and eject towards Minnesota. The 
impact of this would be yet another...potentially stronger... surge 
of moisture being advected into Iowa. Long-range models are pushing 
PWATs of 2 inches into Iowa. So will have to monitor this for 
another moderate to heavy rainfall potential. Behind this, high 
pressure should come in for Sunday and cool us down for a day or two.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1238 PM CDT Tue Sep 17 2019

Some gusty southerly winds this afternoon, with increasing high
clouds late tonight. A line of showers/storms is expected to
impact the northern TAF sites late tonight/early Wednesday and
could make its way to KDSM and KALO toward mid/late morning before
dissipating toward mid-day/early afternoon. Expect lower VFR CIGS
with the line of precipitation. 

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

DISCUSSION...Kotenberg
AVIATION...Beerends