FXUS63 KDMX 221059

Area Forecast Discussion
National Weather Service Des Moines IA
559 AM CDT Sun Jul 22 2018

.DISCUSSION.../Today through Saturday/
Issued at 323 AM CDT Sun Jul 22 2018

Expectations have not changed much from this time yesterday with
only a few windows for precipitation and at best normal
temperatures. The weekend will end with fair weather outside of
fog late tonight, and Iowa between a quite anomalous southwest
upper level ridge and corresponding Ohio Valley long wave trough.
A weak short wave will drop through the northern Plains Monday
resulting in a decent H85/H7 thermodynamic forcing couplet.
Surface flow and associated convergence will be weak however so
while the the aforementioned lift may help generate some elevated
convection into the afternoon, surface based convection may
struggle to develop. If something can form it should generally be
sub-severe with 1000-1500 j/kg MLCAPE and marginal deep shear.

Surface high pressure will follow behind into Tuesday with
another pleasant day. The Canadian upper level trough will mature
into midweek however with heights lowering into Iowa. The short
wave ushering in this trend will have decent QG forcing with depth
and some weak return flow will induce stronger instability into
peak heating with MLCAPEs to 2K j/kg or more. Surface flow is weak
again however so adequate convergence will remain questionable.
Models are quite sporadic in generating precip because of this so
PoPs are currently limited, but would not be surprised to see
things increase with time considering the degree of forcing. If
anything can form into the early evening hours brief diabatically
driven severe storms are possible due to stronger deep shear as
seasonally strong mid level flow sags southward into Iowa. Have
slight PoPs Wed afternoon, but timing confidence is low with the
ECMWF slower than the GFS. The chances could very well end up
further into the night.

Looking ahead into next weekend, Iowa weather will be influenced
by the southern end of broad Ontario troughing with little
instability or forcing and temps just below normal. There are some
sporadic PoPs through this period as weak waves will ride the
flow from time to time, but confidence in their strength and
timing is quite low. Anything that would occur would likely be
elevated and insignificant.


.AVIATION.../For the 12Z TAFS through 12Z Monday morning/
Issued at 558 AM CDT Sun Jul 22 2018

Isolated MVFR visibilities in fog are not expected to impact TAF
sites early this morning with high confidence in VFR conditions
and nothing more than high cloudiness through the day. Radiation
fog development is anticipated early Monday to some degree however
so have included MVFR mention at all sites except KDSM late in
the period until confidence increases. Current thinking is that
KDSM heat island will preclude radiation fog from forming.






NWS DMX Office Area Forecast Discussion