172
FXUS63 KDMX 080733
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
233 AM CDT Wed May 8 2024

.KEY MESSAGES...

- Showers and thunderstorms will redevelop by this afternoon and
  persist into Thursday. There is a Marginal Risk of severe
  weather this afternoon and evening.

- Locally heavy rainfall is likely later today and tonight, and
  could exacerbate ongoing flooding.

- The forecast from Friday through Sunday is mostly dry, with a
  few chances for spotty light rainfall, before rain chances
  slowly increase again early next week.

&&

.DISCUSSION...
Issued at 233 AM CDT Wed May 8 2024

A stacked cyclone near the North Dakota/Montana border early
this morning will gradually fill as it wobbles down into South
Dakota and Nebraska by around midday today, then squeeze and
stretch into Iowa this evening and tonight. As the low
approaches this morning, it will usher in somewhat more brisk
southwesterly flow aloft with isentropic lift and moisture
advection increasing accordingly. A subtle shortwave impulse
currently moving from northern Kansas into southern Nebraska
will cross Iowa later this morning, beginning the trend of cloud
development and slowly increasing POPs through the day. By this
afternoon and evening a warm front will push up into southern
or possibly central Iowa, and the core of the 500 mb low will be
approaching from eastern Nebraska. The broad forcing for ascent
on the eastern flank of this low will thus overlie much of
Iowa, and with gradually increasing moisture will provide the
impetus for festering convection late in the day and through
tonight. Forecast soundings indicate it will take some time for
the column to saturate, but it appears likely showers will be
fairly widespread by this evening at the latest, and there
should be areas of modest instability supporting scattered
thunderstorms with ML CAPE values around 500-1000 J/kg. Wind
profiles show weak low-level flow from the east southeast, but
veering and increasing southwesterly winds above that. Resulting
deep-layer bulk shear is quite strong, and even low-level shear
and helicity will be maximized near and just north of the
surface warm front with 0-1 km SRH likely peaking around 100
m2/s2. In spite of the modest instability, this supports some
potential for organized updrafts capable of hail or gusty winds
and the Marginal Risk outlined by SPC appears appropriate. There
may also be a possibility of weak funnel clouds/brief tornadoes
as the core of the low approaches, however, this potential may
be limited by a lack of low-level instability/0-3 km CAPE,
particularly if cloud cover is pervasive by the late afternoon.
Aside from the potential for severe weather, it should also be
noted that storm motions will be very slow later today and
tonight given the light low-level flow, so pockets of heavy
rainfall will be likely and depending on where these occur, may
exacerbate ongoing river and stream flooding. Any threat of
severe weather, as well as overall coverage of thunderstorms,
will diminish overnight even as scattered to numerous showers
persist into Thursday.

Even as the former High Plains low pressure center encroaches on
Iowa from the west tonight but continues to weaken, another 500
mb trough rounding the Great Lakes will approach Iowa from the
northeast, absorbing the former system and then moving overhead
on Tuesday before clearing to the south and east. The result
will be persistent clouds and showers on Thursday, as well as a
return of modest northerly surface flow. This will make for a
cool, gray, and soggy day and all indications are that
temperatures will have a hard time rising very far. Have
continued trend of lowering forecast highs for Thursday, and it
appears plausible temperatures may not get out of the 50s during
the day. By Thursday evening or Thursday night the clouds and
any lingering showers will finally clear out behind the trough.
Friday will then be fairly pleasant during a brief period of
ridging, but an energetic wave sliding from central Canada down
toward the Great Lakes will then scrape northeastern Iowa around
Friday night, providing a few showers and thunderstorms in that
part of the state but with little impact anticipated. The
forecast for the coming weekend is more uncertain, as Iowa will
reside beneath northwesterly steering flow aloft, with light and
nebulous flow at the surface. Various model solutions spuriously
indicate subtle ripples of forcing aloft leading to renewed
convection, but the timing and placement of these features is
highly variable, and no strong or widespread forcing or
precipitation is anticipated. Thus have maintained a mostly dry
forecast for Saturday and Sunday, except some low POPs late
Sunday when temperatures will be a bit warmer and there may be
sufficient heating for afternoon/evening convection. At the end
of the forecast period, early next week, there are now
indications of broad troughing approaching from the northwest,
resulting in low POPs next Monday and Tuesday but with no real
details to hang your hat on at this range.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1146 PM CDT Tue May 7 2024

The first half of the TAF period appears likely be uneventful
and VFR across the sites. There is a low-end chance for some
light rain to move into the state from the SW overnight/into the
morning, but confidence at this time does not warrant prevailing
or vicinity mentions. More impactful will be TSRA/SHRA and
vicinity mentions that begin to show up around/after 21z within
TAFs. Confidence there is high in convection, but have gone with
vicinity mentions due to initial areal coverage.

&&

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

&&

$$

DISCUSSION...Lee
AVIATION...Curtis