573
FXUS63 KDMX 271745
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1245 PM CDT Thu Jun 27 2024

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

.KEY MESSAGES...

- Increasing clouds with storm chances returning late today
  into Friday morning with another round of storms later Friday
  afternoon and evening. Flash flooding is looking less likely
  given current rainfall, but severe storms are possible Friday
  afternoon and evening.

- B-E-A-utiful late June weekend with more seasonal temperatures,
  low humidity, and dry conditions.

- Storm chances could return as early as later Sunday night, but more
  likely later Monday into Tuesday. Severe and locally heavy
  rainfall may result, but will be more defined in the coming
  days.

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

&&

.DISCUSSION...
Issued at 333 AM CDT Thu Jun 27 2024

This morning, Iowa resides on the northeastern periphery of the
southwestern US mid-level ridge with a longwave trough over the
Eastern US and another one on the other side of the ridge over the
Pacific Northwest. A shortwave trough topping the ridge over western
Nebraska this morning is sending mid and and high level clouds
eastward over portions of western Iowa this morning as viewed
on GOES-East Nighttime Microphysics RGB imagery. The shortwave
along with strong theta-e advection will bring a continued
increase in cloud cover and eventually a return of showers and
thunderstorms. While some convective allowing models (CAMs) are
aggressive with their arrival time of these chances, have
tempered them with forecast soundings showing residual low level
southeasterly flow and some remaining low and mid-level dry air
that will need to saturate. So while there may be a few showers
over the north later today, the main round of showers and
storms will arrive later this evening into the overnight into
Friday. MUCAPE values will be generally under 500 J/kg and while
shear is more appreciable given the 35 to 50 knot 850mb low
level jet, the severe risk will be quite limited during this
period. The 0z HRRR surface wind gusts did show near severe
gusts arriving into western Iowa, but the simulated reflectivity
is unimpressive. HRRR forecast sounding at CRL show no eye
popping downdraft CAPE values around this time, but there is
some mid-level dry air that grows with time, which may be why
there is this signal in the HRRR. This may be what SPC is keying
off of for bringing their marginal risk into our western areas,
but their discussion notes uncertainty in eastward extent and
bolsters our confidence in a very limited severe risk.
Precipitable water values will be on the increase as well
nearing or topping 2 inches by Friday morning with deep warm
cloud depths for favorable warm rain processes. However, the
latest National Blend of Models (NBM) and the 100 member
ensemble have both shown a decrease in probability of one inch
lessening concerns for flash flooding. Rainfall amounts from
deterministic models through midday Friday are at most 2 inches
(e.g. ECMWF, WRF-ARW) with the 0z HREF localized probability
matched mean showing just a bullseye or two of up to 1.25
inches.

While models show that the showers and storms will be moving into
eastern Iowa later Friday morning, it remains to be seen what
kind of break in cloud cover will occur on Friday. Moisture
recovery will be going well with dewpoints back towards 70
degrees into the afternoon hours as strong southerly breezes
prevail. Forecast soundings show plenty of deep layer shear for
storm organization at 35 knots plus, but there is a warm capping
layer. Soundings also show that the NAM and GFS struggle to
warm at the surface with lingering cloud cover debris and thus
do not realize the surface instability and have more meager
instability overall. In contrast, the HRRR shows more potential
of realizing the surface instability from I-80 southward with
the CSQ sounding having nearly 2000 J/kg of SBCAPE by late
Friday afternoon. So, it`s possible storms may struggle more
over northern and central Iowa with weaker low level lapse
rates, though the cold front and shortwave may provide the
necessary forcing for strong to severe storms capable of large
hail and gusty winds later in the afternoon/evening. Over
southern Iowa, looking at the same HRRR CSQ sounding, it shows
high values of 0-1km storm relative helicity so a tornado or two
may be possible, especially in the vicinity of a surface low
and warm front that may move into this portion of the state.
This severe potential will hinge on more clearing to allow for
more instability by mid to late afternoon. Rainfall amounts,
despite an otherwise favorable heavy rainfall setup, look to
largely stay below 1.5 inches, which will be spotty. Total
rainfall from Thursday night through Friday night will be half
an inch to an inch in most areas with a few locations between 2
to 3 inches. Thus, the concern for heavy rainfall and flash
flooding looks quite limited.

As the cold front pushes through the state Friday night, rain
chances will end by Saturday daybreak with cooler and gradually
drier air filtering into the state as high pressure moves across the
region this weekend. Conditions will be quite nice for late June
with low humidity and highs in the middle 70s to middle 80s on
Saturday and 70s on Sunday with lows in the 50s Sunday morning. With
the high departing later Sunday, moisture return from the Gulf of
Mexico will ensue with the southwestern US mid-level ridge
moving eastward into the southern Plains and lower Mississippi
River Valley with the Pacific Northwest trough advancing
eastward as well. While some showers and storms may return
Sunday night into Monday morning with the start of the low level
thermal lift, the higher chances will come as the trough
approaches and provides additional forcing later Monday into
Monday night. This could be a window for strong to severe storms
with high amounts of deep layer shear and appreciable
instability. WPC`s day 5 excessive rainfall outlook also has a
marginal risk over the state with a slight over northern Iowa
due to their antecedent conditions. The heavy rainfall parameter
space does look favorable, but 850-300mb flow also looks fast
so will need to assess in the coming days further. Additional
strong to severe storms are possible later Tuesday into Tuesday
evening as a cold front moves through the state. Temperatures on
Tuesday have the potential to reach or exceed 90 degrees with
the NBM having a 30 to 50% chance of this south of Highway 20,
but this will ultimately depend on no lingering cloud cover.
While the heat should be short-lived as the cold front settles
south of the state at midweek, it may still be close enough
that additional storm chances will persist as shortwave troughs
move through the northern flow.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1245 PM CDT Thu Jun 27 2024

Mainly dry with VFR conditions this afternoon, as mid-level
clouds continue to stream eastward across Iowa. As low level
saturation increases this evening, expecting to see rain shower
activity increase from west to east and continue into Friday.
Some weak storms may occur later tonight into Friday morning,
though confidence is not high enough to include in the TAFs
specifically and will continue to monitor. Scattered shower and
storm chances continue through Friday, with periods of MVFR
conditions possible by mid to late morning. Winds will increase
out of the south Friday, with gusts upwards of 20-30 knots.

&&

.HYDROLOGY...
Issued at 333 AM CDT Thu Jun 27 2024

Major to record flooding continues on the West Fork of the Des
Moines River early this morning. It looks like both Estherville
and Emmetsburg have reached their final crest and while they may
remain there for a bit, should start to see a slow fall in the
next day or two. Humboldt and Fort Dodge are cresting now or
will be later today. The remainder of the rivers in northern
Iowa, including the East Fork Des Moines, Cedar, Winnebago,
Shell Rock, and Iowa Rivers are all in recession. Both Dakota
City and Shell Rock are forecast to fall below flood stage
within the next 12 hours or so based on current forecasts. The
only rise is on the mainstem Des Moines at Stratford as the
water works its way down towards Saylorville Reservoir, which
should reach its peak around 875 feet around the Fourth of July
and remain there for a period of time.

The next chance for rainfall will be later today through Friday
night. The flash flood risk in this time period is less likely
given lower rainfall amounts as well as lower areal coverage in
higher amounts, which may top out around 2 to 3 inches in a few
locations for the event. As for possible infiltration, northern
Iowa north of Highway 20 has shown a slight improvement with
the roughly top 4 inches of relative soil moisture (RSM) showing
a 2 to 4% drying compared to this time yesterday. The 4 to 16
inch layer RSM continues to show 60% or higher values over this
same area so while the top layer has shown a bit of drying,
these areas will continue to be prone to saturation and more
rapid runoff response. Elsewhere in Iowa, the rainfall from
Tuesday has lowered capacity in southern Iowa with streaks of
60% RSM. Thus, there may be isolated areas that may see more
runoff response. However, in this part of the state USGS
streamflows are generally at the normal percentile at both the
daily and 7 day average. While there is more rainfall forecasted
early next week, there should be some time for soil moisture and
infiltration capacity recovery, but will need to monitor QPF
amounts as the event approaches given the favorable heavy
rainfall parameters.

On the river flooding, the plan is to use just 24 hours of QPF
today. Thus, the first part of this late week rain event
through 7am Friday will make it into today`s river forecasts.
For areas that are not currently experiencing or forecast to go
above flood stage, namely central and southern Iowa, the
experimental Hydrologic Ensemble Forecasting Service (HEFS)
based on GEFS 10 day QPF continues to paint within bank rises at
the 30% chance exceedance level. The 10th exceedance level
would bring a few locations on the Iowa, Raccoon, and Des Moines
into at least flood stage. However, this would require the
higher end rainfall. For northern Iowa and rivers with above
action stage conditions (Des Moines above Saylorville, Cedar,
Shell Rock, and Winnebago), the rainfall Thursday night into
Friday as well as early next week may slow the recession and
prolong above action stage flows.

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...Bury
HYDROLOGY...Ansorge