599
FXUS63 KFSD 171950
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
250 PM CDT Thu Jul 17 2025

.KEY MESSAGES...

- Dry through tonight, with small storm chances (25-35%) Friday
  morning mostly along and north of Highway 14. Storm chances
  increase (35-55% chance) Friday evening with the passage of a
  front, with storms capable of all modes of severe weather.

- Forecast trends support lowering severe weather risks for the
  upcoming weekend in most areas, focused mostly along or south
  of Highway 20 on Saturday.

- Highly unsettled pattern develops next week, with nearly
  daily risks for convection along with rising temperatures.

- Machine learning models suggest increased severe weather
  probabilities focused Monday through Wednesday, especially
  during the late evening and overnight hours.

- As muggy conditions arrive, several days of increased heat
  risk may develop from Tuesday through Thursday.

&&

.DISCUSSION...
Issued at 244 PM CDT Thu Jul 17 2025

NOW THROUGH TOMORROW AFTERNOON: It`s a mild and pleasant afternoon
across the region, with highs today mainly in the low-70s. A cumulus
field across the region will dissipate this evening. The quiet
conditions are due to high pressure settling over the region. As
this high shifts east of the area into tonight, look for the return
of southerly flow and thus warmer and more humid conditions heading
into tomorrow. Also, as moisture increases with this southerly flow,
look for clouds to increase once again tonight and into tomorrow
morning.

A small mid-level disturbance moves near the North/South
Dakota state line and could provide some forcing for very isolated
showers/storms Friday morning (25-35% PoPs mostly north of I-90).
There looks to be around 1000 J/kg of MUCAPE or less Friday morning,
so enough for thunder, but severe weather looks unlikely with any
morning activity. Instability looks to increase to around 2000-3000
J/kg in the afternoon; however model soundings indicate a strong cap
in place through much of the afternoon hours. If an elevated storm
can get going and eventually tap into the boundary layer, it
will have plenty of instability as mentioned, and mid-level
lapse rates upwards of 8 C/km (potentially as high as 8.5 C/km)
combined with bulk shear around 40 kts. This would lead to large
hail up to ping pong ball in size being the main concern in any
activity that is able to develop Friday afternoon. Other than
the cap, another potential limiting factor to afternoon storm
development will be the aforementioned morning rain chances.
This could potentially lead to lower afternoon instability
should coverage of morning storms be more than expected, as the
clouds associated with this activity could limit warming.
Speaking of warming, highs will be closer to average Friday, in
the upper-70s to mid-80s. It will be a humid afternoon, with dew
points into the upper-60s to low-70s, highest south of a warm
front which will set- up around the I-90 corridor Friday
afternoon. The warm front will provide an extra source of lift
heading into the evening, increasing the chances of storms
breaking the cap (30-50% PoPs in the evening).

FRIDAY EVENING and NIGHT:  Isolated to scattered thunderstorms along
the approaching frontal boundary may continue to drift southeast
through the evening.  The ramp up of low-lvl flow in the evening
along with modest mid-lvl cooling from the approaching shortwave may
lead to some increase in storm coverage late in the evening and into
the early overnight hours. While initial updrafts will support
continuing hail risks, some cell mergers may promote localized wind
focus into the early overnight hours as storms move into NW Iowa/NE
Nebraska/S Minnesota.   We`ll also have to watch for the potential
of any backing surface winds, which could significantly enhance low-
lvl helicity (some models indicate upward of 150-200 M2/S2) and
increase the tornadic risk into the late evening in any
discrete storm immediately ahead of the boundary. Storms
eventually push south and east away from the area after
midnight, with high pressure moving into the region for Saturday
morning. One item of note, should the cap look weaker, or
forcing stronger, could see upgrades to severe weather outlooks
especially along or south of I-90.

SATURDAY: Latest model guidance continues to support a
greater influence of high pressure and broad low-lvl isentropic
downglide through the day on Saturday. This would tend to favor a
dry forecast in most locations, as the effective baroclinic zone
gets shoved southward into Nebraska and central Iowa.  However, the
presence of mid-lvl vorticity and lingering moisture between 700:500
mb would indicate the continued potential for showers or isolated
thunderstorms late in the day, with stronger storm potential
shifting south of the MO river.

SUNDAY: Quasi-zonal to slightly southwest mid-lvl flow develops by
Sunday forcing return flow into the western High Plains on Sunday.
Still under the influence of Great Lakes high pressure, we may see a
bit more stratus early Sunday with continued cooler temperatures
near or just slightly below seasonal normals in the afternoon.  By
the overnight hours a lead shortwave ejects into the Plains,
shifting the low-lvl jet eastward and introducing a risk for
elevated convection over the CWA.

MONDAY-THURSDAY:  Broad mid-lvl ridging  builds over the Central
CONUS for much of next week, leaving the CWA on the edge of a belt
of stronger westerlies at the apex of the ridge through the week.
With longwave troughing stalled over the West Coast, this will leave
the Northern Plains susceptible to a myriad of smaller scale
vorticity maxima that will travel over the ridge.  It is fairly
likely that one or more MCS will develop through the Plains next
week, though considerable  uncertainty remains on the location and
track of these features.  Given the rising temperature and
instability, the increase in the machine learning severe weather
probabilities is justified from Monday through Wednesday.

As far as temperatures are concerned, synoptic patterns support
rising temperatures back above seasonal normals into the mid 80s to
lower 90s.  What will be more noticeable will be the rise in dew
points through the week, with the southerly to southeasterly surface
flow bringing those typical mid to upper 70 dewpoints northward and
peak evapotranspiration adding a bit more moisture to the mix.
Daily heat risks will begin to rise Monday, peak Tuesday, and
continue to remain elevated through the week. LREF histograms show a
significant right hand skew, supporting high confidence in dewpoints
approaching 75-80 degrees each afternoon in the mid-week. Of
course any convective MCS could have a profound impact on each day,
which makes confidence on high temperatures average at best.


&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1231 PM CDT Thu Jul 17 2025

A fairly large scattered to broken CU field is noted across the
region to start the period. Any lingering MVFR ceilings should lift
to VFR through the rest of the early afternoon as daytime heating
continues. A high pressure system is keeping things dry with fairly
light east-southeasterly winds across the area this afternoon. As
this shifts east through tonight, expect southerly flow to increase
and thus moisture will increase as well. Thicker clouds will move in
from west to east through the night as this moisture returns, but
should remain VFR through tonight. With these clouds moving into
areas along and east of I-29 later in the night, can`t rule out some
patchy light fog in these areas as temperatures there have a better
chance of cooling to saturation. However, any fog is expected to
remain fairly thin, and chances were not high enough to include in
any of the TAF sites at this time.

Heading into the daytime hours on Friday, an additional CU field
will develop and lead to a brief period of MVFR conditions during
the mid-morning hours, though these clouds will likely lift back to
VFR just after the period ends. As far as storm chances go, the
better chances will be after the period into Friday evening;
however, can`t rule out a few thunderstorms mainly along the Highway-
14 corridor in South Dakota. For this reason, put a couple of PROB30
groups in for KHON from 13Z to 18Z Friday. Southeasterly winds
will strengthen through the morning on Friday, gusting between
15-20 kts at times.

&&

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

&&

$$

DISCUSSION...Dux/Samet
AVIATION...Samet