946
FXUS63 KDMX 201749
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1249 PM CDT Thu Jun 20 2024

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

.KEY MESSAGES...

- Rain focusing primarily northeast through this morning. Non-
  zero chance of redevelopment this afternoon. Main round of
  rain arrives in the north tonight.

- Rain lingers in the north through the day Friday and becomes
  more intense for the overnight. Some storms strong to severe
  with gusty winds and hail early on. Heaviest rain looks to
  remain in MN at this time.

- Another push of rain and storms Saturday with some storms
  becoming strong to severe. Dry period expected at least
  through Monday with temperatures returning to near 90 degrees.

&&

.DISCUSSION...
Issued at 308 AM CDT Thu Jun 20 2024

A stalled out front remained over southeast Iowa overnight. With
high pressure to the north, LLJ moisture was redirected into the
Northern Plains and away from the front. Despite the weak
kinematics, plenty of moisture and some instability allowed for some
showers and storms to fire off the front overnight. With 2 inch
PWATS, thunderstorms along this line could produce localized amounts
up to a half inch as they lift northeast through the morning.
Moisture transport will remain focused west of the state today which
will work to keep most of the area dry today. Cannot completely rule
out some precipitation along the boundary in central Iowa today as
diurnal destabilization takes place, but confidence is low (<20%)
due to the lack of synoptic forcing nearby. At the very least, cloud
cover will linger along the front today and make high temperatures
underperform. Have lowered high temperatures to reflect this
possibility with cool sector sites in the upper 70s to low 80s. The
southeast will have the least residence time of the front today and
will be allowed to rise into the mid 80s this afternoon.

For tonight, the LLJ strengthens and generates an MCS across
Nebraska. This MCS will follow along the instability gradient
which will be somewhere near the IA-MN border, per model
consensus. The northern extent will depend on how far north the
boundary gets today. The LLJ will also veer eastward overnight
tonight, adding to the heavy rain potential through the day
Friday. Right now, model consensus keeps the heaviest rain in
southern Minnesota, but local convective elements can alter the
final location of boundaries, meaning a heavy rain scenario in
northern Iowa cannot be completely discarded. In what feels like
a broken record this week, the cold pool will drop the boundary
into Iowa with rain chances lingering in northern Iowa
throughout Friday. Have also lowered highs Friday in response,
but expecting rain cooled areas in the north to have highs lower
than currently forecasted. The LLJ will angle across the state
overnight Friday with the highest precipitation occurring along
the aforementioned front. At this point, synoptic forcing from
an approaching lee side cyclone will increase coverage. Shear
will also increase beneath the influence of the jet, so some
storms could become severe with primarily damaging winds and
hail early in the evening. Primary threat will transition to
primarily heavy rain overnight. The LLJ from the southwest will
shove the boundary north through the night, with current model
output keeping its residence time in Iowa fairly short. Far
northern counties could see 1-2 inches of rain with locally
higher amounts with higher amounts remaining in MN. Trends will
continue to be monitored.

The upper level ridge will begin to pivot westward this weekend,
allowing the low to trek northeast Saturday. Its trailing cold front
will move across Iowa. Instability will build ahead of the front.
Deep layer shear of around 30kts will aid in organization and make
some strong to severe storms possible. As for the threat of heavy
rain, this round will be more progressive in nature, keeping
rainfall amounts under an inch (some higher amounts from
thunderstorms). We will be afforded a dry period to into early next
week until the ridge nestles over the desert southwest and opens up
the Gulf to the Plains next week. The southwest flow during this
time, barring rain chances, will push highs back to near 90
degrees.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1241 PM CDT Thu Jun 20 2024

A moist airmass remains over the area, leading to a difficult
aviation forecast through the morning. MVFR conditions are
expected to stick around through the next few hours, but
uncertainty remains on if/when improvement to VFR will be seen.
This time of year, ceilings generally improve quickly through
the day as the surface warms and mixes. However, moist air and
instability aloft has hindered this improvement, which has led
to prolonged periods of low ceilings lasting into the
afternoon. As of right now, not expecting MVFR ceilings to go
anywhere real soon, but still anticipating some improvement
later this afternoon as mixing occurs and cloud cover lessens.

Beyond ceilings, expect light and variable winds through the
night, with the potential for more MVFR to IFR ceilings
overnight and into the morning hours at northern sites (KMCW,
KALO and KFOD). Further south, conditions will be warmer and
dry out some, keeping ceilings in VFR. A few isolated showers
and thunderstorms are possible through the afternoon and
overnight, but confidence is too low to include in TAFs so will
amend or add in future issuances once impacts appear more
likely.

&&

.HYDROLOGY...
Issued at 213 PM CDT Wed Jun 19 2024

Little change in thinking from previous discussion. Heavy rain still
poised to fall over basins in the north and west over the next few
days. Main concern remains the West Fork Des Moines River Basin with
heavy rain possible in its reaches in northwest Iowa as well as into
Minnesota. Ensemble output brings sites nearest the state line into
minor flood stage from a mix of local rain and upstream runoff.

&&

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

&&

$$

DISCUSSION...Jimenez
AVIATION...Dodson
HYDROLOGY...Jimenez