Heavy Rain in Iowa, August 19 to 21, 2018

Area Forecast Discussion
National Weather Service Des Moines IA
447 AM CDT Sun Aug 19 2018

.SHORT TERM.../Today through Tonight/
Issued at 446 AM CDT Sun Aug 19 2018

All focus in the short-term is on the upper trough/sfc low slated to
blast through the Midwest, impacting the DMX CWA late this morning
through Tuesday morning. Models have had this system the past
several days, so confidence is very high. Forecast updates will
probably have 100 POPs in them, especially tonight.

Synoptically, 08z water vapor imagery clearly picks up on the upper
low spinning over NE Wyoming. All models in agreement with
propagating this upper low into western Iowa by 12z Mon. Ahead of
the upper low, demonstrating solid NW tilt with height, the
accompanying sfc low should be into western Iowa from 06z - 12z
Mon... pushing northeast into NE Iowa by 21z Mon. The 00z Sun ECMWF
has been a slight slow, southern outlier with its track, so have
leaned towards a non-ECMWF model consensus blend. Interestingly,
this system resembles one that has winter-like features (except for
temperatures, obviously)... a dry slot is being suggested, heavier
precip is focused on a deformation zone in the NE quadrant, and
there will likely be strong NW winds behind this low.

Drilling down into details, height falls are impressive as the
height field is -2 to -3 std dev... almost off the charts on the
NAEFS return interval. A 30kt LLj is well-phased with the core of
the low to transport PWATs nearing 2 inches into central to eastern
Iowa. MUCAPE values average around 1000 J/kg, hodograph analysis
pegs storm motions around 10 mph, warm cloud depths easily blast
4km, and 0-6 km mixing ratios easily reach 8 g/kg. A warm, moist
conveyor belt seems to be activated in a textbook location. Etc.
Etc. Long story short, high confidence in notable precipitation
across Iowa. QPF may need to be notably amplified across the
northern/NE CWA for Monday afternoon and evening as strong ascent
processes suggested.

Ongoing drought conditions across southern Iowa will help mitigate
flood potential. Northern Iowa, however, much more moist. Growing
season on the downswing, so vegetation will not absorb as much
rainfall as say a month ago. With heavy rain occurring overnight
tonight, especially remember to turn around, don't drown.

.LONG TERM.../Monday through Saturday/
Issued at 446 AM CDT Sun Aug 19 2018

Tuesday through Thursday...
The above-described system should be east of Iowa by Tuesday
morning, with strong NW winds expected to fill in. At this time,
do not think wind headlines will be needed. However, this will
need to be monitored. As of now, the best wind parameters look to
be in eastern Iowa, outside the DMX CWA on Tuesday morning. By
Tuesday afternoon, the system is far enough east of Iowa to relax
pressure gradients to 3mb/100 miles, likely not enough to support
hazard- level winds. However, wind gusts of 30 to 35 mph for
Tuesday afternoon seems plausible... potentially causing travel
impacts on east-west oriented roads.

Temperatures have been showing excellent run-to-run consistency with
being much cooler for Monday through Wednesday. 850mb temps look to
be around +10C to +12C. The ECMWF has been the most consistent model
with this system, so leaned towards it. Overall, feel pretty good
about current temperatures forecast through Friday.

Next Weekend and Beyond...
Summer, however, is not over. This weekend will remind us of
that. The past 3 nights, there has been a strong signal for a
death ridge setup across the south/central-southern CONUS. 850mb
temps have been hinting at +25C, which is certainly possible.
Current forecast temperatures from Saturday through at least next
Tuesday are probably about 5 degrees too low. Several consecutive
days of highs in the low to mid 90s across Iowa, with lows in the
70s, and dewpoints in the upper 60s to low 70s seems very
plausible. If this scenario plays out like it may, heat- related
headlines will need to be watched closely for Saturday/Sunday
through at least next Tuesday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1208 AM CDT Sun Aug 19 2018

Quiet conditions overnight. There is a chance for patchy fog
across northern Iowa, which may result in brief MVFR conditions
at MCW and ALO. IFR may be possible at times, however with decent
mixing have elected to keep MVFR in TAFs. Toward the end of the
period, wide spread showers will impact most of the terminals with
thunderstorms possible.

&&

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

SHORT TERM...Kotenberg
LONG TERM...Kotenberg
AVIATION...Krull

Hazardous Weather Outlook
National Weather Service Des Moines IA
414 AM CDT Sun Aug 19 2018

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-200915-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene-
Boone-Story-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper-
Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
414 AM CDT Sun Aug 19 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Showers and thunderstorms will enter central and western Iowa late
this afternoon and overspread the area overnight. The threat for
severe weather is low. Locally heavy rain is possible tonight.

.DAYS TWO THROUGH SEVEN...Monday through Saturday

Additional showers and storms are possible into Monday with the
threat of severe weather remaining low. Again there is the
potential for locally heavy rainfall.

Another chance for thunderstorms arrive Thursday night and Friday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated at this time.