Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201718

National Weather Service Des Moines IA
1218 PM CDT Tue Sep 20 2016

...Aviation Update...

.SHORT TERM.../Today/
Issued at 408 AM CDT Tue Sep 20 2016

High pressure has settled into Iowa overnight and has brought
temperatures and dewpoints in the low to mid 50s across the
north. A broad area of stratus and fog has developed across much
of Wisconsin and has moved into far northeast Iowa. Some of this
fog/stratus will likely clip into parts of the east forecast area
including the Waterloo vicinity in the next few hours.

Mid-level theta-e advection is already in progress through the
Siouxland with clouds developing in response. A few sprinkles may
occur with this mainly north of the state. Strong theta-e
advection in the lower levels will commence this afternoon as
the high pressure moves to the east. A bit of a moisture
discontinuity though considering that by the time better
saturation occurs in the lower levels drying will be occurring
aloft. Therefore not expecting any precipitation through the day
today other than localized sprinkles far north. Temperatures will
be slightly cooler than Monday with highs in the lower to mid 80s.

.LONG TERM.../Tonight through Monday/
Issued at 408 AM CDT Tue Sep 20 2016

Active weather pattern was the main focus this week and into the
weekend. Leaned more towards the GFS and NAM solutions through
mid-week and then closer to the GFS this weekend into early next

Tonight into Thursday...A couple of big changes WRT to pops during
this period as delayed the onset of precip much later to roughly
between 03-06z Wednesday and shifted the higher pops further
northeast overnight. Both the 20.00z GFS and NAM are in decent
agreement with the convection developing along or north of the
Minnesota border past 03z tonight. Upper level ridge remains a
stronghold over the south-central portion of the country with Iowa
placed between the aforementioned upper ridge and the zonal flow
aloft to the north. Warm front turned stationary looks to orient
itself across southern Minnesota into northeast Iowa and
southern Wisconsin by Wednesday morning. Storms look to develop
closer to 06z Wednesday or even afterwards with the LLJ increasing
overnight. Good moisture convergence over far northern Iowa into
southern Minnesota with decent moisture transport, plenty of PWATs
available as they range around 1.5 inches, and the Corfidi vectors
and storm motion oriented in similar direction (east-southeast).
Thinking the main threat late tonight into tomorrow will be heavy
rain. However, the severe threat still exists as there remains
enough CAPE available and strong enough shear to support a decent
updraft to produce some hail. Especially with the unstable mid-
level lapse rates over far northern Iowa. Plus, if any cold pool
develops, the wind threat would increase as well tonight.

Storms are likely to linger in the northeast and then redevelop
later in the day Wednesday as a strong 500mb vort max ejects out
of the Central Rockies. Focus of storm development remains over
western to northern Iowa with again heavy rain and some isolated
severe threat possible into Wednesday evening. Timing of this next
shortwave looks to be delayed until 00z Thursday in western to
northern Iowa and have highest pops over the aforementioned
location Wednesday night. The severe weather risk is dependent on
how quickly the first wave of storms exits or diminishes during
the morning to allow for destabilization prior to the next
shortwave. Strong QG forcing with the second shortwave, so if
does have some instability to work with, the deep layer shear
present might be enough to support supercells initially late
Wednesday afternoon. Low confidence with any tornadic potential
with weak low level shear and helicity.

Lowered pops over the southern portions of forecast area during
the day Wednesday. Thus, with going a bit drier across the
forecast area, strong WAA will push temperatures well into the
upper 80s to lower 90s over the southern two-thirds of the CWA.
Plus, strong mixing looks to push the wind gusts over 20-25 knots
during the afternoon hours. By Thursday, storms will be slow to
exit the forecast area as the shortwave is likely to be centered
over Iowa by around 12z Thursday.

Friday through Monday...upper level ridging builds into the region
and provides at least a 24-36 dry period Friday into Saturday. The
next main cold front looks to push through the state by Saturday
night into Sunday with the front slow to exit the region towards
Monday morning. Large surface high pressure builds into the state
by Monday and provides some welcomed dry weather into Tuesday.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1216 PM CDT Tue Sep 20 2016

VFR expected through TAF period with a fair shot at some MVFR
windows at KMCW/KALO with VCTS in the TAFs for expected convection
north of the area tonight creeping southward. Outside shot KFOD
sees some as well. Otherwise, winds become concern tomorrow with
sustained in excess of 12 kts gusting to around 20kts or more.




SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Curtis is the U.S. government's official web portal to all federal, state and local government web resources and services.