390
FXUS63 KDMX 020837
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
337 AM CDT Sun Jun 2 2024

.KEY MESSAGES...

- Fog potential remains this morning mainly east of a Forest
  City to near Waterloo to near Ottumwa line

- Today`s highest concern for severe weather is over northern
  Iowa late this afternoon/evening and then western half of the
  state late this evening and overnight. Damaging wind gusts
  and possibly localized flash flooding the main concerns.

- Storms later Tuesday into Tuesday evening bring a higher chance of
  severe storms and possible locally heavy rainfall

- River rises are likely through the end of next week due to rainfall

&&

.DISCUSSION...
Issued at 337 AM CDT Sun Jun 2 2024

Of first concern this morning is the fog potential, which so far has
remained largely just east of the area with just Forest
City/FXY reporting reduced visibility as low as a quarter at
times. GOES-East Nighttime Microphysics RGB shows fog signal
over portions of far eastern Iowa that are then masked by high
clouds moving eastward across the state. High resolution
deterministic guidance and 0z HREF continues to place the fog up
against our forecast area along Highway 63 from Ottumwa to
Waterloo. There may be a higher chance over northern Iowa
looking at current satellite as the high clouds have not been as
prevalent there and may allow for the best radiational cooling
setup. Guidance is showing this as well and will see if other
sites join and if severity warrants, a dense fog advisory is
possible yet this morning.

Any fog will dissipate a few hours after sunrise and at the same
time we will be watching convection arrive from Nebraska. Models
have not done outstanding with initializing overnight, but the
latest RAP and to a lesser degree the HRRR and RRFS seem to have
the best handle. These would support convection diminishing in
intensity and spatial extent as it moves into our forecast area
along roughly Highway 71 a little after sunrise. There is
greater uncertainty in what happens through the remainder of the
daylight hours with run to run inconsistency in the high
resolution models including the HRRR not lending itself to any
confidence building. Most high resolution guidance suggests
isolated festering of showers or storms as they move slowly
eastward through early afternoon with this most probable over
northern Iowa. There still remains a signal for severe
convection late this afternoon into this evening over northern
Iowa with the 6z HRRR coming in rather aggressive, which has a
companion in the FV3 with also an uptick in the 0z HREF
paintball reflectivity. Both models show wind gusts to around
50 knots and the environment would be supportive of large hail
as well. These models also paint a concerning picture for
isolated flash flooding as an initial round moves through
followed by a west to east line of storms over the same area
before the overnight convection arrives. More on this in a
moment.

As just alluded to, attention will also be west over Nebraska as
storms develop later this afternoon and grow upscale into a
mesoscale convective system that will be cooking with severe,
possibly significant, wind gusts. While it does look like these
storms lose steam as they approach the Missouri River, the
severe risk may not fully subside as it enters western Iowa and
our western forecast area. High resolution guidance has shown
wind gusts to near 50 knots, though latest is keeping that
between 40 and under 50 knots. 0-3km bulk shear will be around
30 knots and oriented favorably to the line and any surges may
allow for possible QLCS tornadogenesis.

Back to the flash flooding concerns tonight. The 1/12z and
2/00z HREF localized probability matched mean has a bulls-area
between Highways 20 and 3 around Storm Lake with 5+ inches in 24
hours. This is being driven by the FV3, which shows 3 hour
rainfall of 5+ inches and that is 2.5 times the 3 hour flash
flood guidance (!). Of course, the FV3 tends to be a more
aggressive models, but seeing a signal in the HRRR of 3-5 inches
in roughly the same area with the GFS showing 4-6 inches along
and north of Highway 3. Since this is the first run of the
extended HRRR to show such and there is quite a bit of
uncertainty given whether or not the late afternoon/evening
convection plays out and if the evolution described above would
come to fruition, will not jump into a flash flood watch just
yet.

The bulk of showers and storms will be pushing out Monday morning as
the subtle shortwave moves into the Great Lakes region. There may be
a short period of dry conditions across much of central Iowa;
however, guidance does show redevelopment of showers and festering
storms in the afternoon most prevalent over southern Iowa where weak
low level thermal lift will remain. These may maintain into the
night in a more isolated fashion as a lead shortwave trough moves
through Missouri and may help to provide some nearby forcing.

The main longwave trough will be advancing quickly from the
British Columbia province and Northwestern States Monday night
as it dips into the Dakotas and negatively tilts on Tuesday.
Surface low pressure will stay over the Canadian prairies of
Saskatchewan and Manitoba with a cold front extending southward
to Iowa. Ahead of this front, instability will grow with strong
MUCAPE over 2500 J/kg and there may be a little better area
overlap of this instability with favorable deep layer shear for
severe storms. SPC highlights the area with a marginal on their
day 3 outlook with the Colorado State`s machine learning random
forest outlook showing probabilities over the state as well. As
for the flash flooding threat, the environment will have
precipitable water values over 1.75 inches and may be pushing to
near 2 inches with very favorable warm cloud depths of 3500 to
near 4000m for efficient warm rainfall processes. Soundings
continue to support locally heavy rainfall being moisture laden
through much of the column with NAEFS specific humidity at 925mb
at the 99.5th percentile over portions of the state. Any storms
earlier in the day will be more favorable for heavy rainfall
from the standpoint of weaker 850-300mb flow while later in the
day the flow will be more progressive. Rainfall that occurs
between now (Sunday) and Tuesday may also prime an area and be
more susceptible to flash flooding. For now, WPC has highlighted
our forecast area with a slight risk (2 out of 4) of excessive
rainfall. Rainfall totals from coarser global models in 6 hours
may be 1 to 2 inches, though likely will see some higher amounts
as we enter the high resolution guidance window. This is also
true in the ensemble data world with 25th percentile between a
third and a half an inch to 75th percentile of 1 to 1.5 inches.
The rainfall from this weekend through Tuesday night will lead
to river rises. The 1/12z experimental Hydrologic Ensemble
Forecasting Service (HEFS) data shows a few locations (HBTI4,
CEDI4) with a 50% chance of exceeding flood stage by mid to late
week with a 30% chance of several points within the Des Moines,
Raccoon, and Cedar Basins exceeding flood stage.

Behind the cold front that moves through Tuesday night, drier
air will arrive along with seasonal temperatures in the 70s to
around 80 degrees for the remainder of the week. The longwave
trough will wrap up into a sprawling closed low over the Great
Lakes region from midweek into next weekend. This will keep the
flow over the region northwesterly. Forecast for now is dry
through the end of the week, but do note in the ensemble data
some light QPF from various members.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1104 PM CDT Sat Jun 1 2024

Fog is expected to develop over eastern Iowa tonight, and might
affect some of the eastern terminals, MCW/ALO/OTM, before or
around sunrise Sunday. However, confidence in dense fog is low
given these terminals will be on the western periphery of the
higher threat area, and high clouds will be moving in from the
west concurrently. For now have maintained 1SM BR groups at
those three terminals, but amendments may be needed overnight.
Aside from the possible fog, VFR conditions will prevail through
the TAF period, with isolated SHRA/TSRA possible mainly Sunday
afternoon and evening, then again just after the end of the 06Z
TAF period.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Lee