164
FXUS63 KDMX 281737
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1237 PM CDT Fri Jun 28 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Non-severe thunderstorms moving out this morning.

- Storm redevelopment later this afternoon into this evening as
  a cold front moves through the state. There is uncertainty on
  the amount of storm coverage and severe hazards, but most
  likely hazards would be large hail and damaging wind gusts.
  Flash flooding continues to look less likely.

- Wonderful weekend weather for late June - low humidity, no rain,
  cooler

- Storm chances return later Monday into Tuesday. Severe and locally
  heavy rainfall may result with details in the coming days.

- Hydrologic discussion below main discussion details more on ongoing
  river flooding and future locally heavy rainfall potential

&&

.DISCUSSION...
Issued at 408 AM CDT Fri Jun 28 2024

Showers and thunderstorms were prevalent in the evening into the
early hours of this morning as the warm air advection kicked in in
earnest with an 850mb 55 knot low level jet along the Missouri
River. This has resulted in light rainfall amounts with rates
generally up to around 0.1 to 0.15 inches per hour. Farther to
the northwest, a mesoscale convective system was trucking across
South Dakota with the HRRR, RAP, WRF-ARW, and RRFS having a
generally good handle on timing and placement. GOES-East clean
IR imagery has shown cloud tops warming with a southern flank
trying, but failing to sustain itself. FSD WSR-88D imagery shows
that storms are struggling to develop south of I-90, likely due
to the 14C 700mb temperature cap in this area. At the same time
over northeastern Kansas and southeastern Nebraska, clouds tops
have been cooling as new storms develop. Both of these systems
may scrape our border areas this morning, but all expected to
remain below severe limits. This along with our current activity
over central Iowa should be moving off into eastern Iowa by
mid-morning.

The question for the next round of storms later this afternoon into
this evening will be the amount of recovery for possible
destabilization ahead of an incoming cold front as well as spatial
extent of storm coverage. It is less of a concern of dewpoints as
those will be back into the upper 60s to low 70s by this afternoon
as breezy southerly winds prevail. However, the amount of solar
insolation is in question with the 0z SPC HREF mean cloud cover
showing quite a bit of low clouds. NAM forecast soundings show
more low level saturation suggesting clouds and this is
reflected in a lack of surface warming and surface based
instability availability over much of Iowa. In contrast, the GFS
and HRRR forecast soundings at DSM and CSQ shows more recovery
potential and surface heating to allow it to overcome any warm
capping aloft by late afternoon. By that time, SBCAPE and
entrainment CAPE will have built to around 1500 (GFS) to 2000
to 2500 J/kg (HRRR) with favorable 0-500m and 0-1km storm
relative helicity for rotating storms. However, streamwiseness
is not all that great for vorticity ingest unless the storm
deviates to the Bunkers right motion vector or the surface
winds back, which is possible given potential outflow boundary
from early day storms. If this were to occur, a tornado or two
would be possible, primarily over southern Iowa where it is
believed that destabilization is most likely. Otherwise across
central Iowa, low level lapse rates improve to around 6C/km with
similar mid-level lapse rates based on forecast soundings. The
cold front should help provide needed convergence and lift for
storm development, which may contain large hail up to around
golf ball sized and damaging wind gusts up to around 60 mph.
Bottom line, the amount of recovery will be key in what kind of
hazards may evolve and storm coverage in this period. As for
heavy rainfall, additional rainfall up to 1 to 1.5 inches is
shown from deterministic models over parts of central and
southern Iowa with the 0z HREF localized probability matched
mean showing a few areas with 1.5 to 2.5 inches.

With the cold front moving through the state tonight, storms will
end by Saturday morning and we`re looking at a wonderful late June
weekend weatherwise. Winds from the northwest will deliver drier,
lower humidity air and lower highs back into the middle 70s to
middle 80s Saturday, lows in the 50s Sunday morning, and highs in
the middle 70s on Sunday. Much needed dry conditions for those water
logged Iowans will prevail Saturday into Sunday as high pressure
moves across the state.

Monday looks to be a breezy day as the high pressure moves away from
the region and a cold front approaches from the west. Storm
chances will be increasing most likely later on Monday as
theta-e advection ramps back up over the region ahead of a
longwave trough advancing from the Pacific Northwest over the
north central states. Deep layer shear looks favorable for storm
organization and a severe threat, but any appreciable
instability doesn`t arrive until Monday night and more so on
Tuesday ahead of the cold front. Tuesday also is looking like a
potential hot day with NBM and the 100 member ensemble showing
a 40 to 70% chance of reaching or exceeding 90 degrees roughly
south of Highway 30. Of course, this will depend on the extent
on any cloud cover with the current forecast right near 90
degrees in these areas. Storms ahead of the cold front later
Tuesday afternoon into Tuesday evening could be strong to
perhaps severe. Locally heavy rainfall will also be possible as
precipitable water values of 1.75 to 2 inches Monday night
increase to above 2 inches with favorable, deep warm cloud
depths for efficient warm rain processes on the day Tuesday.
850-300mb winds are a bit on the fast side, though could
potentially parallel the cold front. Therefore, will need to
monitor the flash flood threat in the coming days, but the WPC
excessive rainfall outlooks on Monday and Tuesday seem fair at
this time given antecedent conditions and uncertainty. It seems
largely agreed upon that the cold front will push south of the
state by Wednesday, but the GFS pushes it farther south while
the ECMWF keeps it closer and thus lingers storm chances over at
least southern Iowa. Both of these models suggest that a
shortwave trough moves down late in the week the possibility of
more widespread chances. Details will become more clear in
timing and location as we approach the latter half of next week
and Independence Day.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1225 PM CDT Fri Jun 28 2024

Messy aviation forecast over the next 12 to 18 hours as the
area deals with variable cig restrictions, rounds of shra/tsra,
then some patchy fog tonight into early Saturday morning. As of
1730z, cigs over much of the area are MVFR with pockets of IFR.
Radar imagery shows a complex of showers and storms moving into
southeast Iowa. This will continue to push east, with additional
development already noted over northwest Iowa. Expecting
further development by later this afternoon and evening,
impacting terminals from west to east into the evening hours.
Timing of tsra and extent of categorical impacts to individual
terminals remains uncertain at this time, although sporadic
strong wind gusts and heavy rain reducing visby below 2sm is
possible at times.

Conditions should improve as storms push east by late evening into
the early overnight hours. Model guidance suggests patchy fog may
develop east of I-35, and have included mention of MVFR visby at
KALO and KOTM. VFR conditions prevail area-wide by mid-morning
Saturday and through the rest of the TAF period.

&&

.HYDROLOGY...
Issued at 408 AM CDT Fri Jun 28 2024

With the exception of the Des Moines River at Stratford, rivers have
now crested and are in recession. There is still moderate to major
river flooding on the West Fork Des Moines with record flooding
still ongoing in Humboldt. This morning, we also ended the flood
warnings at Shell Rock and Waterloo as they have dropped below flood
stage. Cedar Falls is tracking below the evening forecast and will
likely fall below flood stage later today.

Rainfall early this morning is focused more over the northern half
of the state, which has less infiltration capacity based on NASA
SPoRT relative soil moisture. Fortunately, the rates are on the
lower side and not consistent with some breaks between rain
episodes. Looking at worst case flash flooding guidance, it
still shows that flash flooding is about 1 to 1.5" away from a
concern at the 1 hour and 3 hour time windows. Additional
rainfall of 1 to 1.5 inches with a few spots up to around 2.5
inches is expected today, though this is expected to be more
focused over central and southern Iowa.

On river flooding, we will continue to use the traditional 24 hours
of rainfall. This rainfall may cause some re-rises over northern
Iowa with an example that Shell Rock River at Shell Rock may near
flood stage on Sunday. Any re-rises from this rainfall will be
captured by this morning forecasts and can be addressed in a hydro
update later today. Looking at a longer time horizon using the
experimental Hydrologic Ensemble Forecasting Service (HEFS) and the
GEFS 10 day QPF, it continues to look like below flood stage flows
will be the most likely outcome over this period at forecast points
that are not currently experiencing above flood stage flows. For
rivers with currently above action stage conditions or higher (Des
Moines above Saylorville, Cedar, Shell Rock, and Winnebago), the
ongoing rainfall now through today as well as next week will prolong
the above action stage flows or slow the recession and in some
instance may cause a secondary, lower crest. Of course, will need to
monitor the rainfall for next week given the favorable heavy
rainfall parameters Monday night into Tuesday for any flash
flooding potential and impact on rivers.

Make sure to stay up to date on the latest forecasts and river
information, avoid flooded areas and rivers, and never drive around
barricades or through flooded areas.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Ansorge
AVIATION...Martin
HYDROLOGY...Ansorge