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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KDMX 141950
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
250 PM CDT Sun Apr 14 2024

.KEY MESSAGES...

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- Continued rather warm through tomorrow with a potential for
  elevated fire weather conditions in northern/northwest Iowa

- Ramping up severe into Tuesday with potential for decent round
  of rainfall as well with this upcoming event; 1 to +2 inches.
  Rainfall rates may create periods of impactful travel or minor
  street flooding in urban areas Tuesday.

- Colder late week into weekend - frost/freeze potential for
  mainly the north half of the region.
-- End Changed Discussion --

&&

.DISCUSSION...
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Issued at 250 PM CDT Sun Apr 14 2024

.Short Term /Tonight through Monday Night/...

Confidence Short Term:  Medium to High

Still quiet and pleasant for most of the short term period. The
anticipated weak trough/cool front has dropped south of DSM at 14z
with light north winds through the overnight hours. The center of
high pressure over North Dakota is expected to move east overnight
and carry the core of the ridge into northeast Iowa by 12z. With
today still being quite warm across the region, the slow advection
of cooler air tonight will bring overnight mins to the lower 50s
from the I80 corridor south to the MO border while the north will
likely see lower to mid 40s overnight tonight. Our significant storm
is currently sitting above San Francisco at H500 and will begin to
move east over the next 24 hours, bringing it to Utah where it will
further progress to Colorado by 00z Tuesday. Lower at H850, the
system at that level will be just slightly east of the H500 low at
00z Tuesday. At this time the increasing southeast flow Monday will
inhibit much of the expected moisture transport into our area until
after 00z. The first real slug of warm air advection accompanied by
an embedded wave will begin the onset of precipitation in our
south/southwest and that should occur near or shortly after 00z
Tuesday. Tomorrow will be warm again, but with the expected slightly
cooler airmass, we should remain in the lower to mid 70s north to
the upper 70s to lower 80s in the south/southwest. The afternoon
will see relative humidity dropping to 20 to 25% northwest where
wind gusts will be topping 30 mph at times. This area will see
slightly elevated fire weather concerns tomorrow, but should remain
shy of any watch/warning criteria. Will pass along to incoming shift
for more evaluation.

As storms begin to ramp up in the mid to late evening hours over the
south/southwest and move nearly north overnight we will have some
risk for severe storms; namely an elevated hail or wind threat in
the early evening south and perhaps later in the evening west. Most
of the storms should weaken as they move into central to northern
areas overnight as a more stable airmass is encountered. We will
also be monitoring the region for heavy downpours and decent rain
producing storms during the period. Available moisture is pretty
meager early today. The 12z subjective H850 analysis showed a weak
return of moisture so far with under +5C dewpoints south to south
Texas. It will take until Monday morning/afternoon for a better
response as the strong low begins to track east. Eventually we are
looking for a ribbon of moisture with the approaching first wave
Monday night with GFS forecast PWATs up to 1.5 inches from 00z to
12z. During this time, we will just exceed the minimum threshold for
warm cloud depth precipitation processes and this is expected to
last through Tuesday over most of our area. So any storms will be
favorable to drop rainfall quickly as they track over our area.

This storm still poses challenges with regard to severe weather on
Tuesday. First, the amplification period of the upper level and
surface low will be from 00z Tuesday through 18z Tuesday and then
begin to weaken slowly through 00z. The peak strength of the jet max
arriving over our area, rightly so, mirrors this process. This has
been a consistently forecast feature of the system and will modulate
the intensity and location of the strongest storms on Tuesday. Lows
Monday night will hold in the upper 40s to lower 50s north while the
south will see upper 50s to lower 60s.

.Long Term /Tuesday through Sunday/...

Confidence: Medium

Still a lot to mull over for Tuesday. We are outlooked for Enhanced
in the south/southeast with a surrounding slight risk. With the
system tracking into eastern Nebraska by 12 to 18z Tuesday, a warm
front will edge into southern to central Iowa between 18z and 00z
Wednesday. The system will also begin the occlusion/slow filling
process during this time, which is what is shifting the stronger jet
winds more quickly east than the more typical synoptic setup in
spring. None the less, 0 to 6km bulk shear is still 45 to 55kts;
MUCAPE well over 1000 J/kg and sufficient directional shear along
the boundary as well. The introduction of a dry slot should focus
most of the convection on the leading edge as this all pivots east
northeast through 00z. The one limiting factor during the morning to
early afternoon will be heating. There is a decent likelihood of
overcast conditions through 18z prior to the warm front/occlusion
lifting northeast from 18z to 00z when most of the severe weather is
expected. This may slightly delay the onset of the strongest
convection and shorten the time the storms are in our area. Will
need to continue to evaluate that into Tuesday. Supercells with
hail, wind and some tornadoes are possible. Again Tuesday with any
of the stronger storms, the amount of available moisture should
result in rainfall of 1 to 2 inches in a short period of time. A
first look at rainfall potential, the probability of getting an inch
or more is over 50 to 60% from west central to northern Iowa for
this event. Ensemble means support event rainfall totals of 1 to 2
inches with the EPS a bit higher than the GEFs and is leaning toward
2 to 3 inches and also through central Iowa. We continue dry enough
to have little worry about any widespread flooding, though any urban
runoff could result in ponding on highways/interstates and minor
street flooding during the event. Motorists will need to pay
attention to rainfall rates and visibility while traveling Tuesday.
Highs Tuesday will be in the 70s with stronger southeast to south
winds during the day. As the system begins to track through the
region Tuesday night, we will quickly cool back to the 40s and 50s
Wednesday morning. Showers and isolated storms will linger into
Wednesday morning with a brief break in the precipitation prior to a
weaker northern stream wave bringing more clouds and some showers
back to the region Wednesday night into Thursday morning.

As we round out the week and get into the weekend, much cooler
Canadian high pressure will drop south into the Central Plains.
Though clouds are expected from time to time as the cooler air aloft
will result in steeper lapse rates and recent rainfall will allow
for some boundary layer moisture recycling, we will remain rather
cool compared to this current stretch of 80s.  In fact there is a
good chance that Friday through Sunday highs will range from the 40s
to the 50s over the area with continued northwest flow. Overnight
mins will dip to the lower 30s already by Friday morning and into
the weekend as well. Any early blooming plants and vegetation could
be affected by the cold temperatures and precautions taken as
needed.
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&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1233 PM CDT Sun Apr 14 2024

Few concerns next 24 hours. Mixing through 00z has increased
gusts to near 20kt at times today and will subside near 00z.
Otherwise, VFR conditions through period with increasing east
southeast winds/gusts aft 15z. Overnight will remain light/var
becoming more east by 12z.

&&

.FIRE WEATHER...
-- Changed Discussion --
Issued at 250 PM CDT Sun Apr 14 2024

We once again overachieved on heating and mixing today, despite
the slightly cooler air moving in aloft over the region. With
that the relative humidity lowered just enough with the stronger
mixing to warrant an SPS for the northern areas this afternoon
until 6 pm. Tomorrow may show similar trends over the northwest
or portions of the north as winds increase and relative humidity
lowers during the afternoon hours. Though warning criteria will
likely not be met, elevated fire weather conditions may again
occur and some form of special messaging would then be issued.
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...KCM
AVIATION...REV
FIRE WEATHER...REV