122
FXUS63 KFSD 180724 CCA
AFDFSD

Area Forecast Discussion...CORRECTED
National Weather Service Sioux Falls SD
224 AM 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.

- 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%.

- 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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1039 PM CDT Mon Jun 17 2024

Continued development of isolated showers/storms possible
through the overnight hours between I-90 and Hwy 14. Watching
development at the time of this discussion across western SD and
NE as this may slowly propagate eastward overnight as well.
Confidence is low if convection will reach areas near the James
Valley, so have left KHON dry for now. Additional showers and
storms are expected through the day tomorrow, with the best
chances in the afternoon along/east of I-29 as storms develop
near the cold front. Storms may linger east of I-29 into late
evening. Severe risk looks low tonight, but strong to severe
storms are possible Tuesday. Locally heavy rainfall is possible.

Stratus and lower visibilities with storms are expected, with
lower end VFR to patchy IFR stratus around already tonight.
Expect conditions to deteriorate as convection expands.
Improvement in conditions anticipated behind the cold front
later in the period.

Finally, LLWS prevails across the entire area overnight as the
LLJ strengthens to 45+ knots. There is both a directional and
speed component to this. Wind gusts 20-35 knots through the
Tuesday afternoon/evening. Direction will shift to more
northwesterly by the end of the period behind the cold front.

&&

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

&&

$$

DISCUSSION...DUX
AVIATION...SG