667
FXUS63 KFSD 191938
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
238 PM CDT Sun May 19 2024

.KEY MESSAGES...

- Scattered showers and a few thunderstorms continue through
  the afternoon and early evening.

- Greater risks for convection arrive by mid-late evening as a
  linear cluster of storms moves in from the west and additional
  storms form over the heart of the Tri-State area. Severe
  weather risks remain low, but not impossible.

- Most of the daytime hours of Monday will stay dry in the
  region, with temperatures warming into the 70s.

- Confidence remains high for widespread heavy rain potential
  Monday night through Tuesday. Probabilities for 1" of rain top
  90% in most locations, with a more narrowed area of potential
  for 2+" of rain over 50% in the 24 hour period ending early
  Wednesday. We`ll need to monitor for potential flood impacts.

- Slightly cooler and continued unsettled conditions into next
  weekend, with only a low end risk for a strong storm around
  Friday.

&&

.DISCUSSION...
Issued at 234 PM CDT Sun May 19 2024

THIS AFTERNOON: Two distinct vorticity centers continue to move
through the Tri-State area at mid-afternoon. One is moving into
southern Minnesota with a second crossing eastern Nebraska.
Fairly widespread upper cloud cover continues to cool
temperatures east of I-29. Further west, VWP continue to show a
modest warm advection profile in the low levels, with scattered
showers and isolated thunderstorms continuing on the LLJ west
of I-29. Short range models have struggled picking this activity
up, but with the waves traveling eastward into the late
afternoon, I could see this activity also continuing to slide
east with the low-lvl wind field.

TONIGHT:  Convection already forming over western South Dakota and
Nebraska this afternoon within a corridor of 2000 J/KG MLCAPE within
a narrow tongue of mid-50 dew points.  This convection will roll
eastward initially and then northeast, potentially reaching the
Missouri River by mid evening. Instability will be growing in the
far western zones this afternoon, but a rapid dropoff in dew points
further east, the extend of surface based instability is only 800-
1000 J/KG near our border. This activity is likely to become
elevated as it moves into the CWA along a northeast track.  Some
smaller hail and perhaps marginal strong wind gusts may be possible
up to the James River, but feel this risk is lower. Further east,
once the LLJ begins to increase towards 03Z, we should begin to see
a ramp up in elevated showers and scattered thunderstorms throughout
the Tri_State areas. Again, any severe weather risk would be
marginal hail, but lifting off the 700 mb layer only results in 500-
700 J/KG MUCAPE. Eventually all of this activity slides east after
midnight, with a variable wind potentially leading to stratus/fog
development.

MONDAY: A surface front slides southeast through the CWA early
Monday morning settling into portions of eastern Nebraska and
Northwest Iowa in the afternoon. We should be dealing with quite a
bit of subsidence through the day that would hold off any
convection, and soundings show a stout EML over-top the front in
Iowa and Nebraska through the afternoon.  But, given pesky southwest
flow can never rule out some elevated convection forming in the
afternoon.  Highs are likely to climb into the 70s, but will be
dependent on cloud cover.

MONDAY NIGHT:  Heights will begin falling quickly Monday afternoon
in response to deeper troughing entering the Rockies. Strong
elevated moisture transport will begin to arrive through the
evening, pushing PWAT values towards 200% of normal by Tuesday
morning.  There remains a bit of uncertainty as far as how quickly
deeper synoptic lift will begin to slide over the area.  Faster
solutions suggest the potential for scattered strong to severe
convection along the surface frontal boundary in Nebraska and Iowa
in the evening, within a pool of 1000-2000 J/KG MLCAPE. Slower
solutions however hold off on developing convection until nearly
midnight or later still bringing a risk for elevated strong to
severe convection with hail as the primary threat along a line
from Yankton to Marshall and southeast. Some CAMS are suggesting
a stronger MCS may form over western Nebraska and Kansas and
slide northeast into northwest Iowa by daybreak Tuesday. If the
boundary layer remains loosely stable, then some wind gusts
could translate northward.

TUESDAY: More concerning however will be the deepening of a surface
and upper low early Tuesday with MSLP charts suggesting the 993mb
low over eastern Nebraska may be sitting below the 10th percentile
of climatology for mid-May.  The deep synoptic lift and westward
advection of moisture over the elevated front on the western side of
the upper trough should be sufficient to produce extensive
rainfall. Rainfall development and deepening of the upper trough
will be even further supported by the potential double jet
structure forming Tuesday afternoon. A mixture of moderate
stratiform rain with embedded thunderstorms could produce rapid
rainfall of 2-4" from south central South Dakota into western
Minnesota by Tuesday evening. Timing of all these features will
be critical to the forecast, as any slowdown could place
portions of the southeastern CWA into a higher risk for surface
based convection Tuesday afternoon

Ensembles have made only slight changes to the locations of the
heaviest potential rainfall through Tuesday. Still suggesting
widespread 1"+ amounts, but a stronger likelihood of a narrow
corridor of 2-4" from south central South Dakota into West Central
Minnesota.   While a good portion of the CWA can handle this
rainfall, a glance at river forecast QPF ensemble projections
suggests that 2-3" of rainfall will lead to river rises nearly
everywhere, but basins such as the Big Sioux (mainly south of Sioux
Falls) could see very minor flooding, with the Little Sioux and
Ocheyedan River more heavily impacted.

WEDNESDAY-SATURDAY:  We`ll have at least one or two "drier" days for
Wednesday and Thursday.  Some wrap around light showers may be
possible Wednesday within the cooler airmass regime that slides into
the area.  However by Thursday we should have enough mid-lvl ridging
to completely dry out.  This will be a short dry spell, as medium
range guidance provide moderate confidence in another trough moving
through the Plains late in the week.  There is a bit more
uncertainty on if moisture (especially at the surface) can return in
time for any severe weather threat.  However rain risks have trended
upwards further with at least a 50% probability of 0.50" of rain in
the ensemble world. Temperatures should return to near to
slightly above normal levels.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1152 AM CDT Sun May 19 2024

Scattered showers continue to move through portions of the Tri-
state area this morning, one area tied to a departing shortwave,
and a second area developed on a low level jet. A few showers
may continue to move eastward this afternoon as the jet veers in
direction more to the southwest. For most, VFR conditions will
continue with a gusty southeast wind up to 30 mph.

More uncertainty develops later this evening as a cluster of
convection may slide eastward out of western South Dakota.
Additional scattered activity may form later this evening closer
to the Tri-state area as the LLJ increases once again.

However, most guidance supports all of this activity moving east
of I-29 by 6Z and out of the CWA by 06z. Behind this convection,
a period of lower stratus may be possible into Monday morning.

&&

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

&&

$$

DISCUSSION...Dux
AVIATION...Dux