Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 220450

Area Forecast Discussion
National Weather Service Des Moines IA
1150 PM CDT Mon May 21 2018

...Updated for 06z Aviation Discussion...

Issued at 935 PM CDT Mon May 21 2018

Confidence remains high that areas of fog will develop later
tonight over central Iowa with plenty of low level moisture and
generally light winds prevailing. However, uncertainty remains in
how low and where the lowest visibilities will be so will hold
off on dense fog advisory at this point. Many of the latest high
resolution models show visibilities under a mile 50 to 60 miles
either side of a line roughly from Spencer to Webster City to
Ottumwa toward daybreak Tuesday. However, 18z HREF favors
southwest of this line and while the 21z SREF probabilities did
come up to between 40 to 50% for visibilities under 3 miles and 1
mile compared to 30 to 40% in the 15z run, they appear displaced
too far to the northeast given current cloud cover. Will continue
to interrogate the arriving high resolution models and monitor
trends from GOES-East Nighttime Microphysics and aviation
probabilities for possible future changes. Otherwise, going
forecast remains on track.


.SHORT TERM.../Tonight through Tuesday/
Issued at 344 PM CDT Mon May 21 2018

The primary concerns initially will be low cloud and fog trends.
A short wave shows up nicely in the water vapor imagery along the
southern SD/MN border. There is some forcing with this feature
still lingering over northeast Iowa, but it is rather weak
resulting in just deeper moisture with drizzle or sprinkles more
than anything and this should depart by sunset if not earlier.
Most models suggest this low cloudiness will exit this evening,
but with light winds and temp/dewpoint depressions this will make
favorable conditions for fog development. Have added considerable fog
wording overnight and would not be surprised to see an advisory
eventually issued.

By tomorrow, weak short waves will continue to eject out of the
current CA/NV upper low increasing kinematic and especially
thermodynamic forcing into the MO Valley as the upper ridge slides
east. This could dribble weak convection into western IA during
the day, with better potential into the next period. Highs should
be warmer than today, but admittedly confidence is not great. Most
models do erode the fog and stratus by Tue afternoon, but this is
somewhat suspect due to continued light winds from an unfavorable
direction for scouring things out much.

.LONG TERM.../Tuesday night through Monday/
Issued at 344 PM CDT Mon May 21 2018

The pattern through this period will have two minor transitions,
but overall be rather summer-like with prolonged warm and humid
conditions. At onset, upper level ridging with a corresponding
increase in warm/theta-e advection will be building into the MO
Valley. This will likely result in an MCS developing through the
Siouxland area, slowly moving across northern IA into Wed morning.
The severe potential does not look too great with weak mid level
winds and little deep shear as the ridge builds eastward, but a
few short lived strong to severe storms are possible on the nose
of 1-2K j/g MUCAPE. Although they will diminish the severe
potential, these light winds aloft will also help the locally
heavy rain prospects. H85/H3 mean winds will only be around 15kts
by Wed morning with precipitable water values 1.5 or higher,
around 200% of normal and pushing climatological extremes
northwest for mid/late May. Warm cloud depths will also be to

This theta-e advection surge should push north of IA into
Wednesday with a rather homogeneous airmass, little baroclinicity
in place and heights pushing climatological extremes. Some low
PoPs will linger north with MLCAPEs reaching around 2K J/kg, but
there may be weak CIN and it will be unfocused with weak
convergence. These conditions will continue into Thu and Fri, and
even build day to day, with MLCAPEs 2-2.5k j/kg by Thu and over 3K
by Fri.

The pattern will change somewhat by this time however with minor
height falls moving through the northern Plains and western Great
Lakes in response to a short wave along the US/Canadian border.
This will bring another peak in PoPs for peak heating Fri into Fri
Night. Even thought the deep shear is negligible, there will be
some severe hail and wind potential considering the degree of
instability and moisture weighted to the low levels. The heavy
rain parameters mentioned earlier will remain in place so locally
heavy rain concerns will persist as well. Anything that develops
should wane quickly by late evening with the loss of insolation
and weak inflow.

The pattern will flip back to upper level ridging into the
weekend and early next week. This will be just far enough west to
keep us from getting too hot however with little return flow and
low level winds remaining weak and not better than easterly toward
the end of the period. The weekend into Mon will mainly be dry
with little in the way of convergence or appreciable warm

Temp wise, any air conditioners will start getting a workout with
multiple days of highs in the 80s and dewpoints in the 60s
starting Wed. There will be little break even after the Friday
system with the front essentially washing out. Heat indices
through the entire period should remain below 90 however.


.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1151 PM CDT Mon May 21 2018

Fog has begun to form over parts of north central Iowa with
visibilities already dropping into IFR at times. Expect fog to
become more widespread with IFR or lower restrictions at all but
ALO before sunrise. ALO should have more cloud cover and mitigate
the lower visibilities. DSM was lowered into LIFR with this
issuance, but will need to be monitored for possible amendment.
Conditions will improve by mid-morning with VFR returning. While
not in the official TAFs, scattered showers and storms will be
possible over northwestern Iowa Tuesday evening.




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