435
FXUS63 KDMX 301823
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
123 PM CDT Tue Apr 30 2024

...Updated to include Hydrology Discussion...

.KEY MESSAGES...

- Enhanced Risk This Afternoon/Evening for Very Large Hail and
  Damaging Thunderstorm Winds
- Warm and breezy today || Next System arrives Wed Night
- Thursday small risk of severe and moderate/locally heavy
  rainfall possible

&&

.DISCUSSION...
Issued at 307 AM CDT Tue Apr 30 2024

.Short Term /Today through Wednesday/...

Confidence Short Term:  Medium to High

Quick response in pattern to an approaching significant H500/H300
jet max now entering the Rockies, but extending back to the Pacific
Northwest to a large scale H500 low over British Columbia. Early in
the day we will see a gradual increase in south southwest winds as
warm air advection begins to rapidly increase to our west. Lee side
troughing at H850 is already beginning with a low over eastern
Colorado and another more northern stream low over southeast
Montana. Though the northern stream low will be dominant in this
event, a remnant weak surface low is expected to maintain some
integrity along the approaching cool front as the system swings east
this afternoon and evening. 0-6km BWD will already be around 45kt
ahead of the system by 18z, and increase thereafter to 55 to 65kt
around 00z from southeast back to west central Iowa. Soundings
continue to show strong turning in the 0-3km layers with significant
westerly winds from H500 and above. Though a warm front is expected
to lift northeast today allowing for the area to warm into the 70s
to lower 80s, a second trough/warm frontal boundary will lift
northeast to southern Iowa by early evening and make it just
over the IA/MO border between 23-01z. The HRRR and HREF are
painting 2 favorable areas for stronger supercells/meso/helicity
tracks: one nearer the US30 corridor and the other near to
south of I80. Looking at the timing of the peak of the event;
many of the strongest storms will reach the west between 4-5pm;
nearer the I35 corridor by 5-6pm and in the south between 6-7pm;
farther southeast between 7pm and 8pm. We continue to see a
strong signature for very large hail greater than 2 to 3" in
diameter along with the potential for higher end damaging
thunderstorm wind gusts of 60 to +70 mph. Soundings continue to
show appreciable drying in the lowest layers with DCAPE 800 to
1000 j/kg as well. The combination of both may result in wind
driven hail in some of the stronger storms. At this time, the
LCLs are not all that favorable for tornadoes; most of the
soundings around 1200 to 1300m near the peak. The one concern is
in the southeast where the 0-1km SRH increases as the weak
southern stream low approaches around 23-00z. We could see a few
tornadoes early in the event over the west, but this area will
also have to be monitored as the event unfolds. The eventual
line of storms/embedded supercells will move east rapidly and
exit the area around 03-04z. Rainfall amounts this afternoon and
tonight with the storms should bring up to 1 to 1.5 inches in
the stronger cells. Lows tonight behind the system will cool
slightly; with mins northwest in the mid 40s to the lower 50s
southeast. Wednesday will begin rather calm, but already another
H500/H300 speed max will be moving across the Rockies. This
will develop another lee side trough with increasing H850/H700
flow already by 18z Wednesday. This means a rapid return of
moisture is expected over the area by Wednesday night into
Thursday. Highs Wednesday ahead of the next system will recover
to the mid 60s northwest to the lower 70s southeast. Some
showers and thunderstorms may enter the far west/southwest by
late afternoon as the warm air advection aloft/moisture begin to
arrive and lift isentropically into the region.

.Long Term /Wednesday Night through Monday/...

Confidence: Medium

As we move along to Wednesday night into Thursday, the main wave and
upper level jet max will line up a bit farther west/southwest than
our event today. This will mitigate the risk of any higher end
severe weather over our area. The main concern right now would be an
MCS with some minor severe threat overnight Wednesday night into
Thursday morning. A broad H850/H700 LLJ of 40-50kts will move
quickly northeast into Iowa by 12z. The stronger H500/H300 winds
lags the lower level features and this is more conducive to locally
heavy rainfall. With the strongest push of warm air northeast of
Iowa by 21-00z Thursday, we will be mainly influenced by the wave
lifting northeast, just to our west. A line of showers and storms
will move across the state Thursday. With the winds generally
parallel to the boundary, locally heavy rainfall and a chance of a
few stronger storms will remain possible. With a few rounds of
storms later Wednesday night and Thursday, much of the state will
see additional moderate rainfall. The potential MCS Wednesday night
will have 1.25 to 1.5 PWATs available for rainfall. Generally, a
factor of 2 can be used in faster moving situations; so expecting
about 2 to 3 inches in the heavier storms; lesser amounts elsewhere.
Thursday 1.5 inch PWATs line up along the boundary over the
southeast/central during the afternoon. Given the progressive nature
of the boundary, a 1 to 2 inch rainfall max amount will be expected
in some of the heavier cells. Though widespread flooding is not
anticipated, localized urban challenges may arise due to rainfall
rates or repeated rainfall in the same areas and mainly in the form
of ponding or rises in streams/creeks or urban travel issues. Lows
Wednesday night will remain in the mid to upper 50s south with 40s
far north. Thursday will be warm and humid, with highs in the mid
60s far northwest to the mid 70s southeast. The remainder of the
extended may see a chance of showers or thunderstorms later Friday
night and early Saturday (30%) and possibly again Monday. Mild
weather is expected with highs generally in the 60s/70s and lows in
the 40s/50s.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1244 PM CDT Tue Apr 30 2024

VFR cigs with rain and some thunderstorms ahead of a line of
severe thunderstorms near the 00z hour for most terminals.
MVFR/IFR cigs and vsbys to accompany these storms. Severe
weather possible in the TEMPO timeframe and include hail and
damaging winds. Skies quickly clear back to VFR behind with
winds easing through the overnight hours. LLWS possible at
northern terminals overnight.

&&

.HYDROLOGY...
Issued at 116 PM CDT Tue Apr 30 2024

How quickly things can change, going from drought conditions to
potentially wet conditions. Potentially heavy rainfall over the next
few days may lead to a significant hydrologic response in the short-
and long-term.

In terms of short-term response (flash flooding potential and urban
flooding potential), the main concern would be Wednesday night into
Thursday. QPF values as high as 1.5 to 2 inches are prevalent across
parts of the CWA with locally higher amounts possible. Antecedent
conditions show the soil moisture is highest across the southern
2/3rds of the CWA with the highest values across the southern third.
Runoff would be increased where soil moisture is highest, however
higher rates may lead to local issues regardless.

Of greater concern is longer term (rivers). Probabilistic and QPF
ensemble guidance is showing significant within-bank rises are
likely on many rivers with several locations in the Des Moines,
Raccoon, Skunk, Cedar and Iowa river basins reaching action stage. A
few locations may reach flood stage especially if the rainfall comes
in on the high side mainly in the Des Moines River basin above Des
Moines. The peaks would occur later this week into this weekend.
Conditions will have to continue to be monitored.

&&

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

&&

$$

DISCUSSION...REV
AVIATION...Jimenez
HYDROLOGY...Zogg