618
FXUS63 KFSD 172329
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
629 PM CDT Fri May 17 2024

.KEY MESSAGES...

- Isolated thunderstorms may develop late Friday afternoon and
  evening over SW Minnesota and adjacent areas of Iowa and South
  Dakota. Non-severe storms could bring a few reports of hail
  or gusty winds.

- Thunderstorm chances return late Sunday into Monday, with a
  continued uncertain potential for severe weather.

- The highest focus for convection arrives Monday night into
  Tuesday bringing a risk for severe storms to the Tri-State
  area along with widespread potential for 1-2+ inches of
  rainfall. This rainfall could bring some rivers back into
  flooding levels.

- Cooler temperatures but continued unsettled weather remains
  through the rest of next week.

&&

.DISCUSSION...
Issued at 236 PM CDT Fri May 17 2024

THIS AFTERNOON:  A very warm day across the Tri-State area as
temperatures have risen into the  middle and upper 80s in most
areas.  Some areas will likely see a late day surge to 90, as mixing
intensifies.  There remains a narrow tongue of surface dew points in
the lower to middle 50s in the CWA.  With the increased surface
convergence this afternoon, it`s not out of the realm of
possibility to see a few high based thunderstorms form along a
line from near Yankton to Sioux Falls to Marshall/Windom later
today. MLCAPE may approach 500 to 900 J/KG along this line, and
while the severe weather risk is low, given sharpening mid-lvl
lapse rates some small hail or localized wind gusts could be
possible.

TONIGHT:  Lingering isolated thunderstorms may persist through mid-
evening before dissipating.  Additional convection is likely over
the western Dakotas later tonight along an area of low pressure and
surface front.  This front will push eastward overnight with an area
of elevated moisture likely developing into ACCAS overnight as it
moves eastward.  A few high based showers/virga are possible across
most of the Tri-state area through 9am Saturday.   Behind this
front, CAMs all suggest potential for 35 to 45 mph winds moving into
central South Dakota by 3-4am, and the James River valley by 7am.
Winds are expected to weaken as they travel east and the brevity of
higher winds may be too short to issue an advisory for ATTM.

SATURDAY: Breezy west to northwest winds will prevail through
Saturday, though wind gusts will fall into the 20 to 25 mph range by
the afternoon.  Fairly dry air is expected to both advect southeast
and then mix down in the afternoon, and have also favored
temperatures slightly above the NBM in the afternoon.

SUNDAY: High pressure slides through the Tri-State area Saturday
night into Sunday, however broad warm advection and a subtle
shortwave is likely to initiate convection over the Nebraska
Panhandle and western Kansas overnight. Some sort of MCS may
develop and track into eastern Nebraska and northwest Iowa
early Sunday. The GFS/NAM/GEM all remain focused over the
southern counties for heavier rain, while the ECMWF is shifted a
few counties further north. If a larger scale MCS does develop,
it could interrupt moisture and potential for any further north
isolated development across the Dakotas. At this point morning
activity will likely not be severe. We will however had to watch
for some increased potential later in the evening and overnight
though this remains highly uncertain. If further south tracks
of morning activity verify, then more focus towards afternoon
and evening activity develop along a boundary over the western
Dakotas late in the afternoon. This activity would then track
eastward overnight, it would likely continue eastward though a
MUCAPE pool of 1500-2000 J/KG until weakening near the James
River. Could see potential eastward expansion of SPC outlooks
towards the James River Valley and further into the Dakotas.

MONDAY-TUESDAY:  Broad southwesterly flow leads to a very complex
and then potentially wet forecast for a large portion of next week.
By Monday morning a surface front will try to settle into Northwest
Iowa.  A cooler day is likely Monday with lingering clouds, and this
will keep temperatures into the upper 60s to lower 70s.  However,
the pattern setting up would be favorable for sharply increasing
PoPs by Monday evening into Tuesday.  A deeper influx of moisture is
expected to arrive late Monday, pushing PW values towards 1.5".
Wherever the surface boundary falls, only modest inhibition may
exist by the evening, increasing the potential for a few
stronger surface based storms. MLCAPE between 1500-2000 J/KG
along with a modest increase in effective shear towards 35
knots, could produce an increased severe weather risk in the
evening. Greater convection risks develop after dark however as
the LLJ and low-lvl frontal circulation increases, mid-lvl dPVA
develops, and broad upper diffluence from an approaching upper
jet moves into the area. This will produce the potential for a
fairly large area of heavy rainfall potential. GEFS/ECE
probabilities for >1" rain break the 70% range, with
probabilities for >2" of rain already at 20%. Some initial
severe weather risk may continue into the early overnight hours,
mainly due to an elevated hail risk. Both the CSU Machine-
learning probabilities along with the CIPS analogs both support
the potential for further north expansions of the severe weather
outlooks from SPC.

WEDNESDAY-FRIDAY: We`re going to remain quite active through most of
next week as a large trough remains centered over southern Canada.
This trough is going to keep a risk for showers and thunderstorms
nearly every other day into next weekend, along with a drop in
temperatures towards or even slightly below seasonal normals.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 629 PM CDT Fri May 17 2024

Mostly VFR conditions begin the TAF period. Showers and storms that
developed earlier in the afternoon continue across parts of
southwest Minnesota and northwest Iowa. Good news is that all
showers and storms area away from all TAF sites and are not moving
towards a terminal. An outflow boundary produced from the storms
looks to creep into FSD so have added a quick TEMPO group for
east/southeasterly winds. Otherwise, the showers and storms will
dissipate this evening.

The low level jet (LLJ) will ramp up this evening and bring low
level wind shear (LLWS) to all TAF sites beginning this evening and
continuing through most of the night. A cold front will push
through the area during the overnight hours and will turn winds at
the surface to out of the northwest in its wake. The winds will be
gusty as the front passes through with gusts up to 30-35+ knots,
strongest west of I-29. There could be some sprinkles to very light
showers as the front passes through but confidence is not high
enough to include in a TAF at this time.

Breezy northwesterly winds will persist for the rest of the TAF
period with gusts up to 20-30 knots.

&&

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

&&

$$

DISCUSSION...Dux
AVIATION...Meyers