Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211755

Area Forecast Discussion
National Weather Service Des Moines IA
1255 PM CDT Fri Jul 21 2017

.SHORT TERM.../Today through Tonight/
Issued at 415 AM CDT Fri Jul 21 2017

Stationary boundary remains extended across
Iowa from west-central to east-southeast portions of the state and
will be the focus for additional convection during the morning
hours. Several concerns today with this boundary and how far north-
northeast it will reach by this afternoon and it`s impact on the
maximum temperatures today. The ESRL HRRR and even the 21.06z NAM12
seem to have a decent handle on the current radar trends and have
further thunderstorm development later this morning over central to
western Iowa and leaned toward a blend today. Tonight, leaned closer
a GFS/NAM blend for pop trends.

Surface low over western Nebraska/eastern Colorado looks to
strengthen today and pull the warm front northward through the
morning hours. Upper level ridge continues to dominate over the
south-central CONUS and allowing several ripples of energy to
quickly move across northern Iowa/southern Minnesota late this
afternoon into the evening.  The warm front will likely to be
located near the IA/MN border and extending southeast into northern
Illinois by around 00z-03z Saturday and convection should
initiate/redevelop during this time frame along and northeast of
this boundary. With plenty of instability available and deep
moisture convergence in the aforementioned location...expecting some
fairly widespread convection by tonight. Heavy rain will be a
significant concern with warm layer cloud depths around 13kft to
14kft, precipitable water values over 2.00" and weak Corfidi vectors
oriented southeast along the surface boundary tonight. Thus,
efficient rain producers are highly probable with the training
potential over the northeast sections of the forecast area. May need
to consider a Flash Flood Watch as this are has seen some decent
rainfall this past week. The other concern tonight is with the
severe weather threat...mainly looking a straight-line wind threat,
but certainly cannot rule out some decent sized hail stones across
the north-northeast. The stronger shear is further east-northeast
into far northeast Iowa and southern Wisconsin providing a better
chance for widespread severe storms.

With the warm front surging northward this morning and cloud cover
likely to go with it, high confidence much of the central to
southern Iowa will see another very sultry day today. Nudged up
maximum temperatures slightly as once the front surges northward,
decent mixing develops later this morning into the afternoon hours.
Dew points remain in the lower to middle 70s leading to heat index
values back into the 100-110F range with a few spots possibly
topping 112F. Leaned more towards the NAM12 for temperatures today
as it did fairly well with the maximum temperatures on Thursday in
the headlined area. Des Moines is likely to see another triple digit
high today.

.LONG TERM.../Saturday through Thursday/
Issued at 415 AM CDT Fri Jul 21 2017

The surface boundary will have reached northwest Iowa by Saturday
morning and will continue to settle southeast through the day.
Conditions will remain hot and humid south of the boundary and
temperatures should be back into the 90s south of Highway 30. Have
leaned towards the warmer ECMWF/GFS solutions. Dew points should
be in the lower 70s though some potential for high values pooling
along and ahead of the boundary. Will leave headlines as is for
now but will likely need to reconfigure after today to remove some
of the north counties from the advisory and potentially downgrade
a portion of the excessive heat warning. Thunderstorm chances
associated with the boundary passage will be inhibited much of the
time by warmer air aloft capping vertical ascent. Any thunderstorm
chance looks to be relegated to the far southeast where enough
diurnal heating through the day may erode the cap. Should storms
develop over the south, enough instability and modest deep layer
shear would air the potential for a few severe storms.

A transition to northwest flow will occur Saturday night into
Sunday as a strong upper level system moves across the Great Lakes
region. This will usher in a cooler and more pleasant airmass
into the state. High temperatures may hit 90 once again over the
southern half. A large area of high pressure will settle into the
western Great Lakes on Monday. High temperatures on Monday will
be in the upper 70s to mid 80s and with dew points in the upper
50s to lower 60s, it will be a very nice day compared to the past
few days. The area of high pressure will keep the area dry through
Tuesday then precipitation chances will gradually increase as
return flow develops as the high departs to the east. The best
chances for precipitation will arrive Wednesday night and


.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1250 PM CDT Fri Jul 21 2017

Aviation forecast challenging at KMCW and KALO in terms of timing
thunder arrival and subsequent lowering of vsbys. Seems likely
that both those sites would be impacted by TSRA at some point
during the evening. KDSM may miss out, though cannot rule out need
for VCTS later in the evening as well. Confidence high in VFR for


Excessive Heat Warning until 8 PM CDT Saturday for IAZ057>062-

Heat Advisory until 8 PM CDT Saturday for IAZ023>025-033>038-

Flash Flood Watch through Saturday morning for IAZ027-028.



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