509
FXUS63 KDMX 010829
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
329 AM CDT Mon Jul 1 2024

.KEY MESSAGES...

- Seasonal temperatures through Friday with more active boundary
  near Iowa/Missouri through Tuesday and back into the area by
  Thursday.
- Two periods of locally heavy rainfall possible late tonight
  and again Tuesday afternoon; another possible period southeast
  Wednesday night into Thursday evening.
- More spotty showers/storms next weekend

&&

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

.Short Term /Today through Tuesday/...

Confidence Short Term:  Medium to High

Little overall change in the forecast in the near term. Typical
uncertainties with convective rainfall location will persist after
24 hours, though today is coming into better view for location.
Tonight most of any storms continue over the panhandle of Nebraska
and eastern Colorado. High pressure remains anchored this evening
from Minnesota south to the Gulf of Mexico. This will help the cause
briefly yet this morning. The high is beginning a slow slide east
but the ridge axis will be able to keep enough dry air in the region
this morning to weaken any approaching storms after 12z. The main
moisture channel at H850, still very robust with 15C dewpoints,
stretches from the Gulf back northwest into the eastern Rockies.
Within the next 18 hours, this will be shifted back into Iowa as
expected the past few days. Both synoptic and CAMS have developed an
MCS over Kansas this morning and will lift that north northeast into
southwest/western Iowa after 12z. This should weaken rapidly by 9 to
10 am as it encounters drier air north and east of the system. As
the system over the Rockies lifts east northeast this evening, the
moisture channel and better forcing will arrive this evening with a
line of showers/storms developing over northwest Iowa. This will
generally ride east northeast along the better thetae gradient,
bringing most of the rain from the first round into the north half
of the state. There has been no real change from the previous
forecasts for the amount of available moisture and warm cloud depths
tomorrow night and again Tuesday/Tuesday night.  We continue to see
PWATs of 2 to 2.25 inches feeding the area of convection tomorrow
night over northern Iowa along with warm cloud depths over 13kft.
The good news, if we can call it that, is that the trough appears
progressive enough to alleviate a higher risk of significant
rainfall in any one location. Despite that, we may still be looking
at isolated 1 to +1.5 inch totals in bands of storms that fire
generally in an east west fashion over western to northeast Iowa.
The greater instability later today will be over eastern Nebraska
into far western Iowa. This should quickly wane into the evening
with only a rather low chance for any severe storms in our area.
Tuesday will see the boundary sag farther east and south with a more
typical and higher levels of instability in the afternoon/evening
over the forecast area. The greatest shear will be along and behind
the front, but 0 to 6km bwd of 40 to 50kts ahead of the front will
be more than adequate to promote storms. The main upper level wave
will pass across the region tomorrow between roughly 23z and 06z
with storm coverage beginning to ramp up aft 18z from southwest
areas northeast to around Waterloo. The line should quickly fill in
south of I80 between 20 and 22z tracking east southeast with time.
The severe threats tomorrow afternoon into the evening will include
some potential for a brief spin up as well as large hail and wind
with the higher risk south of I80 aft 20z. We are now in an enhanced
risk in that area. Even with storms being progressive, the same
efficient rainfall processes will result in some rainfall totals
exceeding 1 to +2 inches over the south as the storms ramp up in the
afternoon. The highest totals should be with the more organized
storms south of I80 at that time. As clouds return to the region
this morning with a small chance of lingering showers in the
afternoon, highs will be held back in the upper 60s to lower 70s.
Tonight will be much milder; especially over the south/southwest
where mins will hold in the mid to upper 60s. Tuesday will likely
see periods of showers and storms over the north with partly sunny
skies at best in the south where passing showers and storms will
increase in coverage after noon. Generally, highs will reach to near
80 north while the south will see the mid 80s.

.Long Term /Tuesday Night through Sunday/...

Confidence: Medium

The storms will be moving south of the region by 02z Tuesday evening
with a few areas of lingering showers/iso storms through 06z. We are
still on track for a break Wednesday with a drier west northwest
flow arriving. Temperatures will actually be warmer given decent
H805 temps and mixing during the afternoon. We are still expecting
highs to recover to the mid to upper 80s region wide. The break will
likely be short lived, however. There is better consistency today
for rain chances ramping up again on the 4th, though the
deterministic GFS is likely too far north and convecting too much on
Thursday. Another wave will approach the region Wednesday night with
a renewed push of moisture and isentropic lift across the same
retreating boundary from Tuesday night. An MCS is expected to
develop southwest of Iowa and track into southern/central Iowa by
12z Thursday. The EC is suggesting the low level jet is veering and
remaining farther south/east by Thursday afternoon/evening while the
GFS has a slightly stronger/more northwest wave solution and keeps
the low level jet backed into Iowa Thursday night, creating a large
MCS over the southeast/east. This solution looks to be contaminated
by convective feedback and creates some high qpf; likely not going
to occur to that extent and somewhat farther east into Illinois by
that time. Unfortunately, nearer the upper level low, there is some
consistency of moderate rainfall upstream of the Western Fork of the
Des Moines River over southwest Minnesota. This will need to be
monitored moving forward. Rainfall from late Wednesday night to
Thursday night may add another 1 to 2 inches of rain most focused
over the southeast quarter of the state. This is supported by the
GEFs and it appears that the 00z GFS is quite the outlier in
precipitation amounts. The synoptic low that wraps up late Thursday
night into Friday is likely going to create some lighter showers
over the region for Friday, much cooler temperatures and a fair
amount of cloud cover to round out the week. We should see some
improvement into the weekend, but as bad luck would have it, a
northern stream front is anticipated to arrive into northwest Iowa
Saturday afternoon with a round of showers and possible thunderstorms
both days. Highs Thursday may need to be trimmed a few degrees
with 70s to around 80 or so more likely in our area. Friday will
struggle to reach the mid 70s. By Saturday a break and milder
H850 temps should bring everyone back to the 80 to 85 range.
Sunday will be similar to slightly warmer than Saturday.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1155 PM CDT Sun Jun 30 2024

VFR conditions expected to continue overnight through much of
the daytime hours Monday, though clouds will be on the increase
overnight into Monday morning. Although a few showers remain
possible around midday west to central, confidence in showers
impacting KFOD or KDSM remains lower than 50% needed for
mentions in the TAFs due to dissipating nature of showers and
encountering dry air. Higher confidence in rain or storms moving
in towards the end of the TAF period northwest to southeast
with mentions included in future issuances. Light winds
overnight will become out of the southeast and increase on
Monday morning with gusty conditions expected by Monday
afternoon into Monday night.

&&

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

There will be two periods between now and Tuesday night with another
late Wednesday night through Thursday night when moderate to locally
heavy rainfall may occur.  Details of all three are in the main
discussion and it appears that tonight will be fairly progressive
and what falls will be across the northern basins. Tuesday will see
more storms from north central to south; and especially along and
south of I80 in the mid to late afternoon. The somewhat more progressive
nature of the expected rainfall over the next 2 days will likely
slow the fall of some of the area rivers in the north. Most of
the HEFS guidance suggests most basins will see some slowing in
the fall of already flooded rivers or rises that show no significant
impacts for the others. Depending on the amount/coverage of rainfall
tonight/early Tuesday northeast near the Winnebago and in the
Cedar River basin, the Cedar River may have additional rises and
minor flooding again at Cedar Falls this week. Across the central
and south, the main concern will be localized heavy rainfall
and some urban flooding concerns for any hourly rainfall rates
over 1.5 to 2 inches. We will continue to monitor Saylorville as
well with any added rainfall across the north later today and
tomorrow. The latter week rainfall is generally forecast south
of Saylorville and at this time, is not expected to cause
additional rises there.

&&

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

&&

$$

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