Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190529

Area Forecast Discussion
National Weather Service Des Moines IA
1229 AM CDT Sat Jun 19 2021

Issued at 923 PM CDT Fri Jun 18 2021

Small area of elevated showers and weak convection continues to
develop along the US30 corridor southeast toward Grinnell. Have
adjusted PoP to reflect this area. Main show still expected to
begin within the next 1 to 2 hours farther south. CAMS have been
generally consistent on location with most of the activity
expected south of I80 and toward the border. Already storms firing
over southern Nebraska where moisture convergence and low level
jet interacting with upper level front. In our area, looking for
about 4 to 5 hours of storms once they begin with hail still the
main concern. /rev


.DISCUSSION.../Tonight through Friday/
Issued at 304 PM CDT Fri Jun 18 2021

Messaging Highlights:

1) Increasing threat for severe thunderstorms this evening and
overnight. Primary threat large hail and damaging winds.

2) Relatively quiet weather Saturday with seasonal conditions

3) Additional rounds of thunderstorms, possibly strong to severe
on Saturday night and Sunday.

4) Cool down with quiet and pleasant weather early next week

This evening and tonight...Confidence: Medium to High

As of 3pm today...satellite imagery indicated several waves of
mostly mid-level clouds moving from west to east across the region
associated a plume of mid-level moisture and weak lift. Occasional
light, high based showers with an few lightning strokes have been
observed. Big moisture gradient noted today across Iowa, with sfc
dewpoints in the upper 60s south, and in the upper 40s north.
Highs today should top out in the mid 80s to lower 90s.

Later this evening, things should get interesting in a hurry. A
weak upper shortwave will move across the upper Midwest initiating
a modest 30-40 kt LLJ after 00Z. Moderate WAA and moisture
convergence along a west/east frontal boundary across southern
Iowa is expected to initiate deep convection between about 02Z
and 04Z, mainly south of Interstate 80. A potent CAPE/shear
environment is expected in this vicinity, with MUCAPE values
greater than 2500 J/kg, very steep mid level lapse rates of 8C+
per KM, and strong deep layer of 40- 50kts. Model sounding
"analogs" indicate a fairly robust signal for large to very large
hail (golfball or larger). In addition, the various CAMS/HREF
output show a strong updraft helicity (UH) signal in this region,
further supporting the notion of rotating storms and severe
weather. The primary threat appears to be large/very large hail
and damaging wind, The limiting factor for tornadoes is higher
LCLs /cloud bases/. SPC upgraded to an Enhanced Risk in this
region which looks appears justified. Finally, while areas of
heavy rainfall are possible, due to the ambient dry soils and
drought conditions, that area can take a lot of rainfall so hydro
issues are not currently anticipated.

Saturday: Confidence: Medium to High

Any lingering showers/tstms should be south and east of the CWA by
Saturday morning.  Weak high pressure building across the region
should result in quiet weather the remainder of the day. A
northerly wind flow with usher in a drier airmass with dewpoints
statewide in the upper 40s to low 60s. Highs will still be warm in
the mid 80s to lower 90s.

Saturday Night and Sunday: Confidence: Medium

The quiet weather will be short lived as another upper level
shortwave quickly ejects from the 4-Corners region on Saturday night
into the Midwest on Sunday. In response, a moderate ~40 kt LLJ will
develop on Saturday night with Iowa in the exit region. Strong
moisture advection and isentropic ascent is expected to initiate
cluster of showers and thunderstorms late Sat night into Sunday
morning. Elevated hailers would be the primary threat with this
first round of precipitation. This initial wave should push east,
with partial clearing developing by Sunday afternoon ahead of an
approaching cold front. Depending on the magnitude of moisture
return and instability, another round of severe weather is
possible Sunday afternoon along and ahead of the cold front.
Additional details on this severe weather threat should become
more evident over the next few days.

Extended Forecast Monday thru Thursday...Confidence: Medium

A respite from the recent heat will arrive next week as much cooler
and drier conditions are expected behind the cold front. Expect
highs only in the 60s and 70s and Monday with 70s on Tuesday. There
will be a few additional chances for precipitation next week, namely
Tuesday night and again Thursday thru Friday.


.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1229 AM CDT Sat Jun 19 2021

Strong to severe storms over the southeast the main story through
09z with additional convection still possible at KOTM. The
remainder of the forecast area will remain VFR with some mixing
aft 15z, then diminishing aft 00z. /rev




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