444
FXUS63 KDMX 182337
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
637 PM CDT Tue Jun 18 2024

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Storms and Locally Heavy Rains into Wednesday
- Much Cooler Wednesday
- Stalling front & continued wet into Thursday
- Heating up into the weekend/Storms PM Saturday

&&

.DISCUSSION...
Issued at 215 PM CDT Tue Jun 18 2024

.Short Term /Tonight through Wednesday Night/...

Confidence Short Term:  Medium to High

Low pressure trough just west of Iowa this morning, situated in
eastern South Dakota/northern Nebraska with a trailing cool front
and adjacent warm fronts over southern MN and north central
Minnesota. Dew points have pooled into the lower 70s over much of
Iowa and eastern Nebraska with a large pool of mid to upper 60s from
Wisconsin south to the Southern Plains.  Aloft, a strong LLJ of 45
to 50kts continues to pull a large and continuous ribbon of +15C dew
points into northwest Wisconsin. The sharp low pressure trough lies
across western Nebraska with much cooler air over the Northern
Rockies and Pac Northwest. Though we remain capped at H700 with +12
to +14C temperatures over the region now, by late afternoon and
evening there will be enough lift/sfc moisture convergence to break
the cap along the approaching cool front in eastern Nebraska, storms
will begin to develop quickly into the evening hours. The main
concern tonight will be damaging wind gusts and locally heavy
rainfall. The environment will be less supportive of large hail and
tornadoes, but with the instability there will be a potential for a
brief funnel or tornado. As for upper level wind fields in this
event, the bulk of any collocated winds through the column will
briefly be over northern/northwest Iowa between 00 and 06z. That
said, any severe into the evening will more likely be pulse reports
than anything organized to more long lived. Tonight as the boundary
sags into the area, PWATs will be ramping up to 2 to 2.25 inches
along and just behind the boundary from around 02z through 12z. Warm
cloud depths are over 4000m which is more than enough to promote
efficient rainfall. At this time, the sfc boundary will drift into
southern Iowa while the H850 boundary remains a little farther north
toward the I80 corridor. This will likely promote additional showers
and thunderstorms for much of the day tomorrow. With the mid to
upper level moisture still pooling along the upper level boundary,
there will be a risk of repeated moderate rainfall across central to
southern Iowa. Tonight the main risk appears to be across the north
and maybe limited to a small section of northern/northeast Iowa
where there is a risk of localized 2 to +4 inches due to the
boundary becoming a bit more east west oriented there for a slightly
longer period of time overnight. With the boundary farther south
Wednesday, the risk will shift toward central to southern
Iowa/northern MO during the day and afternoon hours. Tomorrow will
have little chance for any severe weather. Tomorrow the higher PWATs
and more efficient warm cloud depths will set up south of I80 into
MO through Wednesday night. At this time there is not much dynamics
to sustain any long lived thunderstorms or training storms for
tomorrow. For the moment given the spread of uncertainty as to
if/where any persistent storms occur, we will not be issuing any
headlines for flash flooding. Rather, if it looks more likely over a
region of the forecast area due to less frontal movement or storm
propagation tonight or tomorrow, then one will be considered.
However, the front over southern Iowa/northern Missouri is expected
to lift north once again and reach northern Iowa by Thursday
morning. This will likely add to our several day rain totals over
the region. With the passage of the front overnight, mins will
settle into the upper 50s to lower 60s northwest, while the central
to southeast holds onto upper 60s to lower 70s.  Highs tomorrow will
be considerably cooler; afternoon readings in the lower to mid 70s
most areas with lower 80s possible in the south. Tomorrow night mins
will be similar to tonight.

.Long Term /Thursday through Tuesday/...

Confidence: Medium

As the boundary retreats north into Thursday morning, another round
of showers/storms can be expected. With the better forcing over
western/northwest and northern areas, the general focus of any
heavier rainfall will shift back to northern Iowa and areas north.
At this time, low pressure is expected to deepen over the High
Plains and this will likely lift the front farther north into
Minnesota and out of the region by later Thursday night. As warmer
air moves back into the region at H700, it will be more difficult to
see storms building over our area, but late Friday night into
Saturday there may be a tendency for storms over southern MN to
build south into the cap. Highs Friday and Saturday will return to
the 90s for Friday and Saturday before a brief cool down Sunday back
into the 80s. Both operational medium range models show the system
that tracked north into MN/southern Canada will drag a cool front
southeast through Iowa by later Saturday evening. This will lead to
another round of storms Saturday afternoon/evening across the
region. At first glance, there appears to be a bit more support for
organized severe weather with increasing wind fields through the
column Saturday afternoon and evening. Though currently not
outlooked, the east and northeast may see an uptick in strength
during the period. Once the front pulls east we will see a few days
of much warmer and quieter weather as the H700 cap returns across
the region. Highs will build back into the lower to mid 90s by
Monday and Tuesday with additional storms possible Tuesday.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 633 PM CDT Tue Jun 18 2024

Non-VFR conditions will be experienced across the TAF sites
during this period due to combination of TSRA/SHRA, visibility
reductions, and falling ceilings. While there is some degree of
uncertainty with the variables mentioned, especially as you work
further east, confidence continues to be high enough to warrant
prevailing periods of IFR and MVFR conditions at all sites.
Trends will be monitored for necessary updates as current
convection continues to evolve and move into the area.

&&

.HYDROLOGY...
Issued at 215 PM CDT Tue Jun 18 2024

Rainfall, locally heavy in some areas over the next few days will
lead to rises on area rivers again. The question is how much the
rivers will rise. The existing official river forecasts take into
account only 24 hrs of QPF. Our probabilistic guidance, which
includes QPF out to as much as 10 days is showing higher river
stages in many locations and much higher in some locations. One
location (the West Fork Des Moines River at Estherville) is already
above flood stage.

Based on the present probabilistic guidance such as the QPF ensemble
hydrographs, the Hydrologic Ensemble Forecast System (HEFS) and the
National Water Model (NWM), the most likely scenario is many sites
reaching action to flood stage (minor flooding) within the next
week. That being said, even moderate or major flooding is not out of
the question at some locations (but not the most likely scenario).
Right now the locations to be the most impacted look to be in the
Des Moines River basin and its tributaries above Saylorville Lake,
the Raccoon River basin and the Cedar/Iowa/Winnebago River basin. Of
course the actual placement, amount and duration of the rainfall
will be big factors in determining the actual hydrologic
response.

&&

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

&&

$$

DISCUSSION...REV
AVIATION...Curtis
HYDROLOGY...Zogg