Spencer, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

761
FXUS63 KFSD 201634
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1134 AM CDT Sun Jul 20 2025

.KEY MESSAGES...

- Strong to severe storms are possible this evening and night.
  Storms look to track through locations along and north of
  I-90. Damaging winds to 60 mph, large hail up to the size of
  golf balls, and a tornado are all possible.

- An active pattern persists aloft for the entirety of next week
  which looks to bring daily chances for showers and storms. A
  few of the days could see strong to severe storms though
  specific details are uncertain.

- Seasonally warm conditions are expected next week with
  afternoon heat indices into the 90s each day. Monday and
  Tuesday could see heat indices into the 100s.

&&

.UPDATE...
Issued at 808 AM CDT Sun Jul 20 2025

A few minor modifications to the morning forecast. Typical
increase in daybreak lift along 850:700 mb layer on the LLJ
(combined with weak mid-lvl PV passage) has resulted in
scattered showers and isolated lightning through the James River
valley and narrow portions of northwest Iowa. Will increase PoPS
through mid-late morning.

 &&

.DISCUSSION...
Issued at 331 AM CDT Sun Jul 20 2025

Mainly quiet conditions begin the day today as a weak low level jet
continues to veer off to the east. Some patchy fog is possible
across parts of the Missouri River Valley early this morning as
winds go light and variable while dew points remain relatively
higher in this area. Any fog that does develop will burn off during
the morning hours as diurnal mixing strengthens. While the morning
hours will be quiet, things will change for the afternoon, evening,
and overnight hours. Warm air advection (WAA) will strengthen as 850
mb flow begins to turn more southerly. At the same time, weak
shortwave troughs will be pushing through the the zonal flow aloft.
This could result in additional chances for showers and storms to
develop though this remains somewhat uncertain given the weak
forcing in place. The main consensus from the latest hi-res guidance
is that most of the afternoon should remain dry as capping will be
in place. At the same time, another shortwave trough will be pushing
into eastern Montana/North Dakota and will develop convection ahead
of it. Looks like this convection could develop along a surface
trough across western South Dakota or the elevated 850 mb
boundary/CAPE gradient across parts of North Dakota. As of now, most
of the CAMS have convection developing on this gradient and sliding
southwards into locations along and north of I-90. The storm mode
will favor a mesoscale convective system (MCS) as it pushes into the
area during the later evening and overnight hours. The environment
will consist of CAPE values on the order of 1,500 to 2,000 J/kg and
0-10 km shear values of 40-50 knots. 0-7 km theta-e difference shows
somewhat weaker values around -10K though this should still support
a damaging wind threat. Think damaging winds to 60 mph will be the
primary threat with some potential for an embedded tornado given 0-3
km shear vectors are perpendicular to the MCS. Confidence in this
tornado potential is on the lower side though given 0-3 km shear
magnitudes of 20-25 knots. Large hail up to the size of golf balls
is possible as well though this will be contingent upon whether a
discrete cell can fire ahead of the MCS. This MCS will push through
the area during the overnight hours before finally exiting the area.
Low temperatures will fall to the 60s to perhaps only about 70F
overnight.

Monday looks to be a rinse and repeat of the same daily storm
chances as an active pattern persists aloft. High temperatures will
warm a bit more to the 80s to 90s across the area, though the
warmest temperatures will reside along and west of the James River.
This heat along with dew points moistening to the 70s will make for
a hot and humid day across the area, especially along the Missouri
River. A Heat Advisory may be needed in this area as heat indices
push into the low 100s. An elevated mixed layer (EML) will be in
place, resulting in large capping across the forecast area. With
weak forcing in place, this could keep conditions mainly dry across
the area. However, the CAMS do show the possibility of convection
developing just west of the Missouri River Monday evening in another
surface trough. This looks to be dependent on whether surface
temperatures can heat out enough to overcome the cap. Will keep an
eye on this potential but should convection develop, it has the
potential to be strong to severe. Mean flow does parallel the
surface trough which could potentially keep convection just west of
the area, though again, will need to watch this potential.
Otherwise, low temperatures will fall to the upper 60s and 70s
Monday night.

The rest of the week look to continue rain chances as the Northern
Plains remains on the gradient of an upper level ridge. While this
will allow for weak shortwave troughs to pass through in an unstable
environment, latest medium range guidance does show the ridge
strengthening a little bit more, keeping the strongest winds aloft
north of the forecast area. This will also keep the strongest
vertical shear north of the area as well. Sufficient instability
looks to be in place but this displacement in shear and instability
could potentially hamper severe weather chances a little bit.
Machine learning guidance continues to support this as this guidance
shows daily 5% to 15% chances for severe weather Tuesday through
next weekend. Specific details remain uncertain given the weaker
forcing in place but Wednesday through Thursday could be the next
period to watch as the right entrance region of an upper level jet
pushes through the Northern Plains. Aside from rain and storm
chances, high temperatures will remain near to above average in the
80s and 90s. With sufficient moisture in place, these temperatures
will coincide with humid conditions, making for classic summer heat
and humidity for the foreseeable future. Tuesday may be another day
that could need heat headlines as heat indices again warm to the
100s but trends will be monitored.


&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1131 AM CDT Sun Jul 20 2025

MVFR to IFR ceilings continue to focus in areas along and south
of I-90 at mid-day. Meanwhile, a potent shortwave crossing the
ND/SD border will continue to push convection eastward into the
afternoon. Given the progression of low-lvl northeast, it`s not
impossible that this convection may try to build southeast into
the Highway 14 corridor later this afternoon. Additional
convection may develop over central SD on the trailing outflow
from this boundary later this afternoon and evening, but overall
forecast confidence remains low.

Overnight, we`ll see a return of MVFR to IFR ceilings moving
north which are likely to stay in place deep into Monday
morning. Convection risks overnight are so uncertain in both
development and coverage to preclude any mention overnight above
a prob30.

&&

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

&&

$$

UPDATE...Dux
DISCUSSION...Meyers
AVIATION...Dux