298
FXUS63 KFSD 181755
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1255 PM CDT Tue Jun 18 2024

.KEY MESSAGES...

- A passing front will bring new risks for strong to severe
  convection to areas east of I-29 by mid-late afternoon.
  Primary risks will be 1.5" hail and brief 60 mph wind gusts,
  though isolated tornadoes and flash flooding are possible.

- Additional rainfall expected through the Little Sioux River
  basis later this afternoon and evening may push current
  flooding higher.

- A brief dry day on Wednesday will give way to yet more rain
  into Thursday. Non-severe thunderstorms will again bring
  widespread rain. Probabilities of >0.50" totals are at 80%
  with 1" probabilities around 40%. Several high resolution
  models do suggest some potential for greater than 2" totals.

- Thunderstorms risks continue into Friday, with potential for
  heavy rainfall Thursday night, and then lower potential for
  strong to severe storms Friday afternoon and evening.

- Cooler temperatures and likely dry conditions this weekend
  into early next week, though rain chances return by Tuesday.

&&

.DISCUSSION...
Issued at 218 AM CDT Tue Jun 18 2024

THIS MORNING: Widespread convection continues over western and
north central South Dakota this morning. With the responsible
wave moving northeast into North Dakota, and mean steering flow
also pushing northeast, this strong activity should avoid the
CWA outside of a bit of an outflow/gravity wave sent eastward by
daybreak. Further southeast, gusty southeast winds up to 40 mph
continue through the higher elevation areas. The increased
mixing is holding temperatures in the 70s in most locations,
providing a warm start to Tuesday.

TODAY:  Rising surface pressure over the Western High Plains will
begin to push a cold front southeast into the Tri-State area today.
This front will begin to approach the I-29 corridor early this
afternoon, with some indications of high based elevated showers
possible ahead of the  front.  Soundings also show a fairly stout
warm later around 750:700 mb ahead of this boundary.  We may need a
few of these elevated showers to help cool the warm nose enough to
allow for stronger surface based convection to form east of I-29 by
3-4PM.  Most of the surface based convection will reside within a
corridor of 1800-2500 J/KG MLCAPE, but also on the edge of
stronger deep layer shear AOA 25-30 knots. So while the
predominate nature of activity will be multi-cellular, an
isolated embedded supercell could be possible. Primary hazards
will be hail up to 1.5", and given low-lvl flow running mostly
parallel to the convection, some modest bowing segments could
be possible. A narrow corridor of heavy rain is also possible
once deeper convection develops over northwest Iowa. HREF PMM
data suggests totals between 0.5-2", focused mostly through the
Little Sioux River basin which may force currently flooding
higher.

TONIGHT: High pressure moving southeast through the Dakotas may
force the front southeast overnight, but some continued overrunning
of the low-lvl baroclinic zone could lead to additional shower
activity along the Missouri River and into northwest and western
Iowa overnight. Further north, temperatures will fall into the upper
50s to lower 60s with lighter winds.

WEDNESDAY-THURSDAY:  Mid-lvl ridging over the eastern seaboard will
retrograde back westward on Wednesday and Thursday, leading to a
northward retreat of both the surface and elevated front late
Wednesday into Thursday.  Most of Wednesday is likely to be cooler
and dry, but shower and isolated thunderstorm risks spread northward
quickly Wednesday night into Thursday.  Broad warm advection in
combination with a weak mid-lvl perturbation moving northeast
through the Dakotas into Thursday will bring high rain chances.
GEFS/ECE both bring up to an 80% probability of 0.50" of rain with
nearly a 40% probability of greater than 1" of rain by Thursday
afternoon.  The surface warm front lifts northward into Friday
morning, but with an increasing LLJ approaching 40-50 knots through
the Plains, the concern of deepening moisture return and renewed
convection north of the warm front could pose another heavy
rain risk into Friday.

FRIDAY:  Most medium range guidance pushes the warm front north of
the area, planting the CWA into the warm sector. Both shear and
instability look marginal into Friday night, but slow storm motion
in any storm that develops may create locally heavy rainfall into
Saturday morning.

SATURDAY-MONDAY:  A deeper upper trough will swing eastward along
the Canadian border and into the Great lakes over the weekend. This
will bring a period of cooler more pleasant conditions to the region
this weekend.  This cooler period will end quickly as heights rise,
warm advection returns, and both temperatures and rain risks
increase late Monday into Tuesday.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1255 PM CDT Tue Jun 18 2024

Surface cold front nearing I-29 as of 1730Z will push east
across southwest MN and northwest IA through early this evening.
As the front moves into a more unstable airmass, scattered to
numerous showers and thunderstorms are expected to develop.
Better chance for thunder looks to be east of KFSD but will
likely impact KSUX at times through 18/23Z-19/00Z. Variable
gusty winds, IFR or lower visibility in pockets of heavy rain,
and possibly hail may accompany the strongest storms. Threat for
these stronger storms moves east of our forecast area by 19/01Z
with showers and perhaps isolated thunderstorms lingering near
and south of US Highway 18 into Wednesday morning.

Away from any thunderstorms, MVFR ceilings have become more
widespread west of the cold front, but improving conditions are
expected through this evening as deeper dry air builds in behind
the front. Could see additional MVFR stratus sag southward into
areas mainly north of I-90 late in the period, but confidence is
low in areal coverage/timing, so kept KHON at SCT below 3kft AGL
for now.

&&

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

&&

$$

DISCUSSION...Dux
AVIATION...JH