402
FXUS63 KDMX 031711
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1211 PM CDT Wed Jul 3 2024

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

.KEY MESSAGES...

- Break from wet and stormy weather today
- More spotty showers/storms Thursday through Monday with a
  marginal severe threat Thursday afternoon.
- Heavier rainfall possible over far northern Iowa, but
  remainder of area not expected to be significant.
- Holiday  activities may be impacted from time to time with
  the most challenges tomorrow north of US20.

&&

.DISCUSSION...
Issued at 332 AM CDT Wed Jul 3 2024

.Short Term /Today through Thursday/...

Confidence Short Term:  Medium to High

05z subjective sfc analysis shows a weak low over south central with
another weak low over southeast Wisconsin.  The main boundary and
upper level support has been progressively moving east tonight,
taking the heavy rainfall and severe threat away from the region. In
its wake is a rather rain cooled airmass with significant boundary
layer moisture. Patchy fog, dense at times, can be expected through
the early morning hours; especially in valley areas. Today would be
a great day to celebrate the holiday, as tomorrow will not be quite
as nice. A weak wind shift will pass through the region early this
morning, then weak high pressure fills in for the balance of the
day. With ample heating, some fair weather cu and mild afternoon
highs can be expected. Highs today reach the lower to mid 80s north
and the mid to upper 80s in the south. Clouds will be on the
increase this evening, ahead of the next system. Disorganized low
pressure over the southwest will deepen by this afternoon in the lee
of the Rockies as a northern stream short wave approaches. This will
begin to draw the deeper moisture back into the Western Plains late
today and tonight. A combination of return warm air advection west
and a warm front into northern Missouri will spread areas of storms
north with time, reaching the Iowa borders nearer 12z tomorrow.
Tomorrow the main trough will track into Iowa and slowly edge east
northeast through 00z. This will bring increasing shower and
thunderstorm coverage during the morning into early afternoon, with
another round late afternoon and early evening mainly east of I35.
With current timing, areas along and west of I35 and south of US20
may dry enough by mid evening to keep holiday activities intact
during the evening. North of US20 may not have the same benefit as
the main upper level system will bring a longer period of showers
and storms to the north tomorrow during the afternoon and evening.
Wet conditions may last into the night as the heavier showers
diminish late evening. Tomorrow we are outlooked for a marginal risk
of severe weather during the afternoon over the area. A fairly
strong mid level jet max will cross the area from 18 to 00z with
H500 to H300 winds of 50 to 80kts. This will lead to BWD of 50kts
over most of the region in the afternoon. Some organization of
storms is anticipated with a small risk of all modes tomorrow. PWATs
will not revisit the high levels we have seen today and yesterday,
but by mid to late afternoon an unwelcome 1.5 to 1.75 ribbon
will accompany the afternoon boundary crossing the region. Warm
cloud depths tomorrow will reach about 10kft which will help
warm rain processes, but not reach the insane levels seen
Tuesday afternoon. The 18z run of the GFS showed near 16.5kft in
the afternoon along the boundary that dropped between 5 and 6
inches of rain in our area. Summing all this up for tomorrow, we
may end up with areas of a half inch to 1.5 inches of rainfall,
all unneeded, with the heavier totals closer to the Iowa
Minnesota border and areas north. Another area of a half inch to
inch may fall along the Iowa Missouri border during the period.
Lows tonight will be mild again in the lower 60s north to the
mid to upper 60s south. Highs tomorrow will struggle with the
clouds and showers, but should top out in the mid 70s north to
the lower 80s south.

.Long Term /Thursday Night through Tuesday/...

Confidence: Medium

With the upper level low closing off over southeast MN late
Thursday, clouds and some showers continue north Thursday night.
More typical of spring, this low will swing a secondary trough
southeast into Iowa Friday, keeping showers/iso storms and cooler
temperatures on the way.  Predominantly, the upper level flow will
remain from the west northwest through the end of the period. This
will keep our area in some clouds into the weekend, but some sun is
expected on Saturday. As another trough takes aim at the area
Saturday night and Sunday, more showers and storms are expected.
Without going into much detail, we are likely to be looking at more
showers and scattered storms Monday into Tuesday as weak waves track
southeast from the Northern Plains into Iowa via the northwest flow.
And with ample moisture around, it doesnt take much more than a weak
wave to trigger some showers and storms; such is 2024. There will be
little change in day to day highs from Friday through Tuesday. We
will be range bound from the upper 70s to lower 80s with lows in the
lower to mid 60s region wide.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1211 PM CDT Wed Jul 3 2024

VFR conditions will prevail through the TAF period. Only high
clouds are forecast through tonight, then later on Thursday
morning scattered showers and thunderstorms will move into the
area. Some terminals may be periodically affected right around
or shortly after 18Z Thursday, but will account for that in
subsequent TAF issuances.

&&

.HYDROLOGY...
Issued at 332 AM CDT Wed Jul 3 2024

With additional rainfall Thursday into Friday over northern Iowa and
southern Minnesota, a continued slowing of the fall rate of some of
the rivers can be expected in the West Des Moines basin.  Farther
east, in the Cedar River Basin, we are expected to see a brief
uptick at Mason City and Shell Rock today to near minor or just
above minor flood stage due to yesterdays rainfall. At Cedar Falls,
a rise to near moderate stage and at Waterloo, a rise to minor is
expected both this weekend. The remainder of the river basins are
expected to see only in bank rises to near or above action stage.

Downstream at Saylorville, water will continue to raise the pool
height with the current forecast expected to reach to near 882 ft by
around July 10th. This is based on current water in the system and
additional rainfall may alter that forecast.

While we have had a long, two days in terms of locally heavy
rainfall and areas of flash flooding, today and tonight will provide
a much needed but brief stretch of dry weather. Tomorrows forecast
precipitation, though unneeded, should not bring much concern for
localized flooding. If a quick 1 to 2 inches of rain fell later
tonight or Friday in the Cass County east to Mahaska County corridor
or in the far south near Wayne and Appanoose Counties, there would
be a small risk due to lower flash flood guidance tonight.  At this
time the forecast risk of that much rain falling in an hour or two
is very low. We will continue to monitor the evolution of any
mesoscale convective systems that track near Iowa in the next 72
hours, but at this time there is no immediate concern.

&&

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

&&

$$

DISCUSSION...REV
AVIATION...Lee
HYDROLOGY...REV