Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 172331

Area Forecast Discussion
National Weather Service Des Moines IA
631 PM CDT Tue Jul 17 2018

...Updated for 18/00Z Aviation Discussion...

.SHORT TERM.../Tonight through Wednesday/
Issued at 301 PM CDT Tue Jul 17 2018

Convection in southwest Iowa following along moisture/instability
gradient aligning from NW-SE through the region. This activity
will clip only the far southwest counties of the forecast area
late this afternoon before exiting the area by evening. Otherwise,
ridge axis will be just east of the forecast area through
Wednesday morning with relatively drier air remaining across
central Iowa. The main focus of low level moisture transport and
bulk forcing will be across the Plains where convection is
expected later today into tonight. Temperatures overnight are
forecast to cool nicely under generally clear skies and lighter
winds with values a little cooler than normal.

On Wednesday, will see increasing southeast flow and eventually
increasing moisture transport. Q-convergence/mid level forcing is
expected to move into northwest Iowa by later in the afternoon
with convection in the Dakotas and Nebraska moving east into Iowa
by afternoon and have timed POPs into the west accordingly.
Farther east, little precipitation is expected along and east of
the I35 corridor through much of the day although a stray storm
will be possible in the increasing moisture advection.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 301 PM CDT Tue Jul 17 2018

Most concerns in the longer term will be from Wednesday night into
early Friday as upper low passes through the region. Moisture
transport across the Plains will shift into Iowa on Wednesday
night as low level jet axis veers overnight. Convection will
spread through central Iowa overnight. Localized heavy rainfall is
anticipated given the PWATs of 1.5-2.0" and generally slow system
movement. Many locations, however have seen under an inch of rain
since the beginning of the month and given the intense growth of
vegetation, there is some tolerance built into the system.
Therefore, widespread water issues are not foreseen at this time
although some isolated flash flooding will be possible, especially
in urban settings. The overall severe threat should be somewhat
muted as well given weak shear.

The activity lifts north and east on Thursday with the main push
of moisture as the upper low intensifies across Minnesota. Models
continue to have a difficult time discerning subsequent
precipitation across Iowa during this time. Consensus indicates
additional precipitation in far northern Iowa through the day
nearer the forcing associated with the upper low and left exit
region of the upper jet. Most models also have the surface front
through the state with additional convection farther south sparse
if not altogether finished. However, the GFS maintains the surface
boundary farther west and develops robust convection across
central Iowa Thursday afternoon/evening. This solution is the odd
duck at this time and somewhat discounted, but should it come to
fruition, shear profiles would be much more supportive of severe
convection by later Thursday afternoon into the evening with a
forward propagating convective system. Will continue to monitor
closely with upcoming solutions.

Thereafter, relatively quiet weather is expected with broad
northwest flow aloft and seasonably cool weather into early next
weekend. Any precipitation threat at this point is relatively
minor and associated with another frontal passage sometime next
Monday into Tuesday timeframe.


.AVIATION.../For the 00Z TAFS through 18Z Wednesday evening/
Issued at 631 PM CDT Tue Jul 17 2018

Widespread VFR conditions are expected throughout central Iowa for
duration of the forecast. Surface winds will be light through
tonight and increase slightly from the east southeast into Wednesday





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