Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201136 AAB

Area Forecast Discussion...UPDATED
National Weather Service Des Moines IA
636 AM CDT Tue Aug 20 2019

...Updated 12z Aviation...

.DISCUSSION.../Today through Monday/
Issued at 348 AM CDT Tue Aug 20 2019

Synoptic Overview:

High pressure centered near the Four Corners region continues to
keep mid to upper-level flow amplified over the western CONUS for
much of the this week, while closed low remains in place over Hudson
Bay. Thus the upper-Midwest will remain in moderate northwesterly
flow, with a few short-waves and jet streaks and associated surface
features to bring periods of rain showers and thunderstorm activity.
Main focus with this forecast package will be on the severe weather
threat overnight into Tuesday Morning, and then rain/thunderstorm
chances again Wednesday morning through afternoon.


Current Severe Weather Threat (Through 14z Tuesday): Convection
Initiation took place in portions of northwest and west-central Iowa
late this evening. It has not taken long for these storms to produce
severe hail, especially the deviant left-movers. Greatest axis of
instability still exists from Lamoni northwest to Sioux City with
values of SBCAPE between 3000 to 4000 J/kg. Still looking at plenty
of effective bulk shear (greater than 30 kts) throughout central
Iowa to get updrafts rotating on new initiation. Still looking at
thunderstorms to congeal into an expansive MCS that can still be
capable of producing damaging winds into the morning hours. A close
eye will need to kept on the 0-3 km shear vector and its orientation
to any thunderstorm cluster, as a brief spin-up cannot be ruled out
in this highly sheared environment. DCAPE values across central and
west-central Iowa remain above 1000 J/kg, and model soundings still
favoring dry air from 925 mb through 650 mb making this environment
primed for a damaging wind event. Severe Thunderstorm Watch 601 is
currently out for most of the area until 12z. If the congealing of
individual storms takes longer and the cold pool remains strong,
local extension in time may be necessary early to mid-morning.

Rest of the Afternoon:

Main challenge for this afternoon will be maximum temperatures. With
the realization of an expansive MCS developing, is looking more like
the area will be experiencing widespread cloud cover that may take
longer to clear out than guidance is otherwise indicating. Thus for
Tuesday afternoon, have bumped down temperatures a few degrees but
have not made any drastic changes. With added moisture from
rainfall, dewpoints will climb into the mid 70s this afternoon. Even
with the high dewpoints, right now looking at heat indices in the
upper 90s. Currently not expecting many locations to push heat
indices much above 100. However, if there is decent clearing prior
to peak insolation by late-morning to early afternoon, this may
change the temperature story. In terms of precipitation, not
expecting much activity during this afternoon. A few light sprinkles
or brief shower may pop up, but no substantial forcing returns until
Wednesday morning.


The next H5 short-wave and vorticity maximum is set to swing through
Iowa and looks to sweep a cold front through beginning late morning
and continuing into the afternoon. 00z runs of GFS/NAM pinpoint best
convergence along a line from Omaha to Lamoni, and then eastward
over to the Highway 63 corridor. So far the Wednesday activity is
not looking like it will have what it needs for any appreciable
severe threat. MUCAPAE during the time of favorable forcing is not
pushing much above 1000 J/kg, with 0-6 km bulk shear only around 20
kts. The better shear environment is kept behind the boundary in
most of the 00z runs through Wednesday late afternoon. Will have to
remain aware of rainfall potential, especially if the convection
early Tuesday morning produces very robust rainfall. For now, will
keep POPs concentrated along the Missouri River Valley and along
Iowa-Missouri state line.


Thursday and beyond large-scale flow will primarily be north-
northwesterly and will also weaken as H5 closed low over Hudson Bay
is pushed eastward. This will keep temperatures on the cooler side.
Next appreciable chance for precipitation will come Sunday with
short-wave swinging down across the Northern Plains into the Upper-
Midwest. However there is some disagreement between GFS and ECWMF on
exact timing, with the GFS trending a little faster with this


.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 634 AM CDT Tue Aug 20 2019

Rain showers and low CIGs through late Tuesday morning. Weather
conditions will slowly improve VFR this afternoon. More active
weather returns Wednesday, toward the end of the TAF period.
Biggest concern is the next hour at OTM.




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