126
FXUS63 KDMX 012355
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
655 PM CDT Mon Jul 1 2024

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Widespread thunderstorms tonight and Tuesday. Heavy rainfall
  possible over parts of central and northern Iowa tonight into
  Tuesday morning.

- A few severe storms and heavy rainfall possible over southern
  iowa Tuesday afternoon and evening.

- Natures own fireworks likely on the 4th of July. A few strong
  storms may occur.

&&

.DISCUSSION...
Issued at 231 PM CDT Mon Jul 1 2024

Another active period coming up in the near term followed by more
storms for the 4th of July holiday. Currently a slowly decaying MCS
is moving into parts of central and west central Iowa with the main
precipitation shield confined to north of Interstate 80 and west of
Interstate 35. All lightning has ceased with this activity and the
precipitation is having difficulty penetrating the dry air still
moving into the area from the large Great Lakes high pressure
system.

A short wave over Colorado this afternoon, will move into the region
tonight. A 50+ kt low level jet (LLJ will crank up quickly this
evening and focus into the northwest half of the state before
tilting more eastward later tonight and into Tuesday morning. Heavy
rainfall is the main threat with the overnight precipitation into
Tuesday morning as tremendous moisture advection fueled by the LLJ.
Deep tropical like moisture is currently in place just to the west
and south of the area and will move east into the area overnight.
The main moisture plume with PWAT values of 2-2.25 inches will be
over the northern half of the the forecast area then slowly settling
south through the day Tuesday. There is the potential for a very
heavy rain band to develop overnight along the nose of the LLJ
feeding into the region with storms training over the same area for
several hours. Storm motions near 40 kts are somewhat of a limiting
factor by very efficient rainfall rates with the available moisture
and with warm cloud depths over 14 kft may offset the shorter
duration period of faster moving storms. A band of 3 to 5 inches
with locally higher amounts could occur which will present a
significant ponding and flash flood threat and may result in new or
renewed flooding on area streams. Therefore, have issued a Flood
Watch for tonight into Tuesday morning.

The surface boundary will be into southern Iowa by Tuesday
afternoon. Despite mostly saturated sounding profiles, descent
instability will develop by the afternoon along and ahead of the
boundary thanks to cooler temperatures aloft. Good deep layer speed
shear will also be present that will aid in the potential for
organized storms to develop. Damaging wind gusts will be possible
with stronger storms. The hail potential will be limited due to the
saturated profiles but a tornado could occur. Heavy rain may also
develop if training of storms can materialize but if
organized/supercell storms do develop, the right mover Bunker`s
motions will move storms off the boundary which may limit the
training potential.

A quiet day will follow on Wednesday. Thursday will have
thunderstorm potential return as an upper low descends into South
Dakota. Climatologically, this upper level setup has been a tornado
producer in July. Much more uncertainty exists at the low levels
with the surface boundary possibly south into northern Missouri
which would shift any surface based convection into that area and
out of central Iowa. The main impact may be on the 4th of July
outdoor holiday activities with showers and storms becoming likely
by the afternoon and evening. Shower and isolated storm chances will
persist into Friday under the cyclonic flow of the upper low to the
north. This could turn into funnel cloud potential pending on how
the low level thermal profiles play out. Another short wave arrives
late Saturday and Saturday night which may more storms to Iowa. Iowa
will be in the long wave trough cool region later Sunday and for
early next week before another active period mid week.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 642 PM CDT Mon Jul 1 2024

Although VFR conditions exist across much of the area early this
evening, rain and storms move into the state late tonight
becoming more widespread over especially central into northern
Iowa through the overnight hours. With this, CIGs drop to MVFR,
or potentially IFR into Tuesday morning north, but visibilities
may drop at times due to heavy rain. As mentioned in the
previous discussion, time windows of precip/no-precip mentions
become difficult on Tuesday morning with some lingering showers
or storms, before additional rounds of showers or storms move
through northwest to southeast in the afternoon to evening.
Adjustments to time windows of RA/TSRA mentions and associated
impacts are likely in the next few issuances. Winds out of the
southeast to south will remain breezy at times through the
period.

&&

.HYDROLOGY...
Issued at 329 AM CDT Mon Jul 1 2024

A Flood Watch is in effect for parts of central and north
central Iowa tonight into Tuesday morning which covers both the
potential for flash flooding and new or renewed river flooding.
The current river forecasts, which includes 48 hrs of forecast
precipitation, do slow the fall along the West Fork Des Moines
river. Rises are forecast across most other basins including the
East Fork Des Moines, Shell Rock, Cedar and Iowa River basins
including a return to Minor Flood stage at Shell Rock, Cedar
Falls and Waterloo. Should a very heavy rain band materialize
overnight into Tuesday morning, additional impacts may occur at
areas including the Saylorville Lake storage area, which is
currently forecast to reach 880 ft by the Corps of Engineers.
Another area of heavy rainfall is possible over south central
Iowa Tuesday afternoon and evening. Note, the QPF utilized by
the RFCs for these forecasts may be on the low end and shifted
to far northwest which may lead to some locations forecast being
underdone. Rivers/streams and soil moisture in that part of the
state are in better shape and can handle heavy rainfall
compared to the recently impacted north. At this time, no
streams are forecast to reach flood stage. For more details
regarding the heavy rain potential, reference the main AFD
section above.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 10 PM CDT this evening through Tuesday
morning for IAZ005>007-015>017-023>026-033>037-044>048-057>060-
070-071.

&&

$$

DISCUSSION...Donavon
AVIATION...KCM
HYDROLOGY...REV