605
FXUS63 KFSD 201123
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
623 AM CDT Sun Apr 20 2025

.KEY MESSAGES...

- Rain returns to locations east of I-29 today. A quarter to a
  half an inch of rain is expected, highest along the
  highway-71 corridor.

- Temperatures look to be near seasonable to a bit above average
  for most of next week. Chances for rain continue throughout
  the week but details are uncertain at this time.

&&

.DISCUSSION...
Issued at 301 AM CDT Sun Apr 20 2025

Quiet conditions begin today early this morning. These conditions
will be short lived though as the next chance for rain returns to
the area during the morning hours. A shortwave trough is just
beginning to eject into the Southern Plains and is resulting is
severe storms in this area. The wave will quickly push
northeastwards throughout the day today, bringing rain chances to
locations east of I-29. The dendritic growth zone (DGZ) will be
saturated along with sufficient omega (upward motion) in the DGZ
with value`s up to 10 to 15 ubar/s. Medium range guidance does show
the potential for some banding within the greater precipitation
shield as there is weaker frontogenesis (FGEN) at 600 mb. There
remains two scenarios that look to play out today however. The
first, and most likely scenario, is that rain pushes into locations
east of I-29 and delivers a quarter to a half an inch of rain with
isolated higher amounts possible. The second, and less likely
scenario, is that the hi-res models are correct and this system
tracks just a little more southeast. The further southeast track
along with convection taking up some of the moisture that would get
wrapped into the backside of the low will result in lower rainfall
amounts, down to a tenth to a quarter of an inch. Though the
location for the highest rainfall in both scenarios remains across
northwest Iowa, especially along the highway-71 corridor. Will be
keeping an eye on trends throughout the day today. Outside of rain
chances, today will be a cloudy day with high temperatures warming
to the low 50s across northwest Iowa to the low to mid 60s across
central South Dakota. Low temperatures will fall to the 30s
overnight.

Any remaining light rain will push east of the area by early Monday
morning. This will allow for shortwave ridging to push through the
Northern Plains, bringing a brief period of dry conditions. 850 mb
temperatures will warm to about +5C to +10C aloft. Mixing these
temperatures to the surface will result in near to above average
high temperatures in the mid 60s to low to mid 70s. Winds will be
light to marginally breezy at best as surface ridging slides through
the area as well. The light winds will keep fire danger at bay
despite humidity values falling to critical levels along and west of
I-29. The break in rain chances will be short lived though as
heights begin to fall aloft Monday evening. This will bring a new
chance for rain to the area Monday evening and night. Moisture
return does not look all that impressive so only minor amounts of
instability is expected, on the order of 300 J/kg or less. While
thundershowers are possible, severe weather is not expected.

Any chance for rain will come to an end during the morning hours on
Tuesday as subsidence takes over on the back side of the departing
upper level wave. Although a weak cold front will pass through the
forecast area, high temperatures are still expected to warm to the
60s and 70s. Dew points will be lowering throughout the day within
the post frontal airmass. This will lead to lower humidity values,
potentially falling to critical thresholds along with breezy
northwest winds. This looks to result in elevated to potentially
reaching critical fire danger. Latest LREF probabilities only show a
20-30% chance for humidity levels to fall below 25%. As of now,
areas along and west of the James River will need to be monitored
for potential fire danger. Any fire danger potential should come to
an end by Tuesday evening as temperatures begin to cool. Low
temperatures will fall to 40s overnight.

A more active zonal upper level pattern sets up for the middle and
end of the week. This looks to keep chances for rain in the forecast
for this entire timeframe. The ensembles support this as they show a
a broad 30-80% chance for exceeding a tenth of an inch of rain for
this timeframe. Though a few periods have higher probabilities than
others. One of those periods is Wednesday evening through Thursday.
Medium range guidance shows a shortwave trough pushing into the area
beneath the right entrance region of an upper level jet. This could
result in showers and thunderstorms but instability remains
questionable at this time. The ensembles show just about a 0% chance
for CAPE values to exceed 500J/kg. Thus, severe storms are unlikely
at this time.

The other period of time with higher probabilities for rain is on
Sunday. Medium range guidance shows an upper level trough in the
western CONUS ejecting into the Plains but there is considerable
variance in the evolution of this wave. Although the ensembles
remain more excited for this period of time, their presentation of
the upper level wave still supports the uncertainty in the waves
evolution. Have left model blended PoPs in place but trends will be
monitored.

Outside of rain chances, high temperatures look to remain near to
above average in the upper 50s, 60s, and 70s. Low temperatures look
to fall to the 30s and 40s.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 622 AM CDT Sun Apr 20 2025

VFR conditions begin the TAF period this morning with light
southeasterly winds in place. Rain is expected to begin to push into
the highway-20 corridor by late morning/early afternoon. The rain
will further expand northwards, encompassing most locations east of
I-29 this afternoon. Ceilings and visibilities are expected to lower
to MVFR/IFR levels in the rain and around where rain falls. Winds
will become marginally breezy with gusts up to around 20 knots
across northwest Iowa this afternoon through the rest of the TAF
period. The rain and MVFR/IFR ceilings and visibilities will persist
for the bulk of the TAF period as well though most of the rain looks
to be done falling around midnight. The MVFR/IFR ceilings and
visibilities look to remain, though on an improving trend, to finish
out the TAF period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.

&&

$$

DISCUSSION...Meyers
AVIATION...Meyers