Severe Thunderstorm Warning 242, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
329 PM CDT Fri Sep 27 2019

.DISCUSSION.../Tonight through Friday/
Issued at 328 PM CDT Fri Sep 27 2019

Surface boundary has arrived in central Iowa and is currently
situated near Vinton to Knoxville to Lamoni at 3 pm. The boundary
will continue to settle south and will be nearly out of the
forecast area by 00z this evening. The surface boundary is
relatively shallow and only around 2 kft deep with the 850 mb
boundary lagging more into central Iowa through early to mid
evening. Strong instability has developed ahead of the surface
boundary with MLCAPE in excess of 2500 J/kg over southeast Iowa.
This instability will remain in the forecast area ahead of the
boundary through 23z. A few strong storms may develop prior to the
surface based instability exiting the area and will have the
threat for large hail and possibly a tornado or two given the good
low level turning and shear. Elevated storms on the cool side of
the surface boundary are expected ahead of the 850 front with
overrunning of moisture and instability occurring. A few storms
with large hail are possible in this region as well. Rainfall
amount of one to two inches are possible over southern Iowa before
the precipitation ends late tonight.

The boundary will surge back north on Saturday with showers and
thunderstorms returning to the area from south to north
especially later in the afternoon and overnight. Locally heavy
rainfall will be possible Saturday night as PWAT values increase
to 1.75 inches and warm cloud depth increase to near 12 kft. PWATs
and warm cloud depths will continue to increase on Sunday and
will keep the locally heavy rain threat going for areas north of
the lifting warm front.

The warm sector will be over Iowa for much of Monday with high
temperatures well into the 80s. The cold front will begin
descending into northwest Iowa by late afternoon and will then
settle very slowly south through Iowa Monday night through Tuesday
night. Severe weather will be possible especially near the
boundary late Monday in conjunction of when the greatest
instability is in place. The greatest threat through this period
will be heavy rainfall. PWAT values over 2 inches and warm cloud
depths over 13.5 kft are will be conducive to very efficient
rainfall producing storms. Training of storms is also likely with
storm motions parallel to the surface boundary. A band of very
heavy rainfall will be possible with the greatest threat currently
projected over the northern half of the state. Will continue to
monitor this period for the heavy rain threat and better define
the threat area.

Much cooler air will filter into Iowa as the boundary drops south
through the state.  Some raw model temperatures for Wednesday only
have high temperatures in the 40s over portions of northern Iowa. A
large Canadian high pressure will arrive and encompass much of the
Upper Midwest and into Ontario by Thursday and Friday. This system
will bring cool fall like weather to the state.  Overnight
temperatures Thursday night in particular will be interesting as
overnight lows in the 30s may be possible over the north if good
radiational cooling conditions do occur. Some upper level warm
advection may already be occurring Thursday night though as the
high pressure begin to shift east therefore mid level clouds may
be arriving and shut down the radiational cooling.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1227 PM CDT Fri Sep 27 2019

Boundary over northwest Iowa will settle southeast through the
state today. Areas of fog and drizzle possible immediately ahead
of the boundary along with MVFR to local IFR cigs. Thunderstorms
are possible mid afternoon into the evening over southeast Iowa
mainly impacting KOTM though a few storms are possible early near
KDSM/KALO. A break in the precipitation is expected late tonight
into Saturday morning over the south. The wind will become
northerly with fropa.

&&

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

DISCUSSION...Donavon
AVIATION...Donavon

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