Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171732
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1232 PM CDT Mon Jul 17 2017

.SHORT TERM.../Today through Tonight/
Issued at 347 AM CDT Mon Jul 17 2017

Highs today and convective trends into early Tuesday morning will be
the primary concerns this period. The weak boundary that produced
isolated storms last evening was along or just north of the MO River
at 08z. The airmass was still fairly unstable nearby with MUCAPEs
2000-3000 j/g but weak flow and the lack of convergence has
precluded any nocturnal warm advection precip. This boundary is
expected to wash out even further into today, and while the
aforementioned instability axis will lift through IA into MN during
peak heating the lack of convergence should keep anything from
developing even though there is a coincident CINH reduction. Nearly
all convection allowing and parameterized models are dry through
this evening.

Confidence is not the greatest regarding highs today. The thermal
ridge will ooze eastward with the axis just ahead of a Dakotas/MN
front. This would suggest going above persistence in most locations
but unlikely yesterday a SW-NE band of patchy stratus at varied
levels has developed from Des Moines south and west. This may result
in less aggressive mixing through the day with unfavorable wind
direction for that everywhere but brisker winds NW. Thus have
generally gone above persistence in most spots but didn`t go quite
as warm from Des Moines to Ottumwa, who were in the mid 90s Sunday,
as the low level moisture may temper warming a bit. Even so, heat
indicies should be well through 90s by afternoon across the western
2/3rds of the forecast area, and likely even reaching 100F in spots.

Late tonight attention will turn to the Siouxland area. The northern
Plains jet segment will eventually lower heights and combine with
frontal convergence to develop storms during peak heating from SD
into MN. Warm/theta-e advection and 30-40kts of inflow into
convergence along the 305K isentropic surface should sag slowly
south and east into the night sustaining convection and may just
allow a weak MCS to reach far NW sections toward the end of the
period.

.LONG TERM.../Tuesday through Sunday/
Issued at 347 AM CDT Mon Jul 17 2017

Little change to the overall pattern today but there are a few
forecast considerations. First temps...The southern third to half of
Iowa will be under the influence of an upper ridge and be scorching
hot by mid week.  I have raised max temps 3 to 5 degrees above model
blends and guidance.  The hottest days appear to be Wednesday and
Thursday now but temps above 90 will be with southern Iowa into the
weekend. Even with beefing up temps and dewpoints, central and
southern Iowa remain below heat advisory criteria through Tuesday.
Wednesday and Thursday we do appear to reach criteria but this is a
couple days out at this point and the question of frontal position
and where storms will fire is still in question.

Regarding storms...We start Tuesday with a shortwave riding the top
of a stout upper ridge and this appears to just brush northern Iowa.
At the surface a cold front will extend from south central MN back
towards Sioux Falls.  This is expected to be the focal point for
storms however there is decent convergence that develops across NW
Iowa so there may be storms into NW Iowa late in the night.  This
front will slowly sag into Iowa Tuesday and Tuesday night.
Thunderstorms will likely develop/shift south through the day
Tuesday and just how far south storms develop is still in question.
Definitely the northern third to half of the state has the best
chance for storms however we will be very unstable and there is a big
push of forcing from SW IA into central MN moving across so
widespread storms may be occurring in the afternoon and evening. The
forcing weakens and moves off to the southeast Tuesday evening so
storms should diminish in the late evening/overnight.  Current model
runs are split on this scenario with a couple hanging on to storms
overnight.  This looks unlikely as we do stabilize quite a bit
overnight.

Wednesday through Friday...the upper ridge tries to rebuild pushing
the frontal boundary back north in time and really pumping up the
temps and humidity.  It will be hot during this period but. just how
far the front makes it north is questionable.  In addition, a low
develops over Canada sharpening the ridge more to our west and
giving us a more northwest flow over the eastern portion of the
state.  During this time several shortwaves moves through the flow
and down across northeast IA so storm chances continue but models
diverge and when and where with the Euro now keeping precip more
north and east.  The GFS wants to develop an MCS in NW IA just about
every night and drop it either east or southeast, depending on the
night.  At this point it`s the odd model out.  By this time the
frontal boundary sags into southern Iowa then shifts back north
Friday night into Saturday as that upper low over Canada works south
bringing the next better chance for storms.

Given the instability and steep low level lapse rates, storms
Tuesday/Wednesday could become severe with damaging wind and hail
the main threats though initially there could be a brief window for
an isolated tornado.  This is however such a slim possibility that
for now it will not be mentioned, though we will continue to assess
once frontal position and track of shortwaves becomes better
defined.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1232 PM CDT Mon Jul 17 2017

Main challenge will be the extent of convection aft 12z Tuesday.
Models generally trimming chances over the area...though some
hires members slow lingering convection over the west between
15-18z. Otherwise...not anticipating another narrow ribbon of
IFR/MVFR cigs Tuesday though the ARW hints again at potential
along I35 corridor nearer KDSM/KMCW. Given uncertainty...will not
add mention yet. Weak wind shift lingers into 12z Tuesday. Light
br fog may develop...have added 5sm at KMCW between 09-13z. /rev

&&

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

$$

SHORT TERM...Small
LONG TERM...FAB
AVIATION...REV


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