696
FXUS63 KFSD 181931
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
231 PM CDT Sat May 18 2024

.KEY MESSAGES...

-  A dry overnight period gives way to scattered showers
   developing Sunday morning. Some gusty winds are possible
   Sunday morning at times.

-  Confidence is lower, but scattered showers and thunderstorms
   that develop late Sunday morning continue Sunday afternoon
   and evening, with low potential for strong convection with
   hail arriving late in the evening and into the overnight
   hours.

-  Confidence increasing for potential of strong convection
   Monday evening and early overnight along and southeast of a
   Yankton to Marshall line. Most likely elevated hail risks,
   but portions of Iowa could see all hazards.

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

&&

.DISCUSSION...
Issued at 228 PM CDT Sat May 18 2024

THIS AFTERNOON: Another sunny and warm day across the region, but we
continue to see occasional gusts over 30 mph at times.  Weak cold
air advection has actually reversed this afternoon, which should
allow us to warm a bit further into the 70s, with 80s on north
central Iowa.

TONIGHT: Confidence remains high that we`ll see dry conditions
through at least 4am.  Models still support a cluster of convection
developing over SW Nebraska and NW Kansas later this evening. Mean
westerly to slightly northwest flow should keep this activity south
of the forecast area as it reaches us shortly after daybreak.  A
second area of scattered elevated convection may form over
central and north central SD on the nose of the LLJ. This
activity should focus along or north of Highway 14. Between
these areas, substancial dry air in the column should limit any
northward advancement of precipitation with ceilings remaining
quite high.

SUNDAY-SUNDAY NIGHT: Pesky southwest flow will make for more
forecast challenges for Sunday, as small perturbations in the flow
are poorly handled by model data.  There seems to be some consensus
on a subtle wave arriving by mid-day out of central Nebraska which
may begin to drive the development of scattered showers and
thunderstorms.  This activity will be non-severe as it moves
through, but could produce scattered rainfall totals of 0.25-0.50".
With a split branch of the LLJ trailing the departing wave, a SW to
NE orientated band of showers may continue into the evening
from SW Minnesota through NW Iowa. The ECMWF is by far the
deepest solution with the passing shortwave Sunday afternoon,
with higher QPF than most other models. There is still some
potential for late afternoon evening development West River as a
surface low and front enter the Dakotas. MLCAPE upwards of
1500-2000 J/KG does exist towards the James River, but effective
may keep this activity very multi- cellular if it crosses the
Dakotas late in the evening/overnight. The increasing meridional
LLJ after dark may also allow additional convection to form
further east, which could bring a marginal hail threat towards
I-29 and western Minnesota into Monday morning.

MONDAY-TUESDAY:  A very brief period of shortwave ridging is
possible Monday, and this may lead to mostly dry conditions into the
evening hours.  Temperatures may warm up into the 70s once again.
The deeper southwesterly mid-lvl flow will send at least 1 or 2
additional waves into the Upper Plains Monday night into
Tuesday. At the surface however, a warm front is likely to
develop across central Nebraska into northwest Iowa late in the
day. A fairly unstable airmass will develop either side of this
boundary, with upwards of 2000-2500 J/KG MLCAPE by Monday
evening. A sharp increase in mid-lvl flow late in the day may
make the potential for supercells slightly more possible along
the boundary, especially in eastern Nebraska into northwestern
and north central Iowa. The instability gradient of most
consequence seems to be focused along and southeast of a line
from Yankton to Marshall, but the area for the potential of
surface based activity may just be within the SE portions of the
CWA. While most models are following this line of thinking, the
ECMWF is a bit deeper with the wave ejecting northeast,
suggesting a slower and more delayed onset to strong accent
towards the overnight hours and moreso into Tuesday.

Perhaps the risk with the highest confidence will be for heavy
rainfall.  Forecast PWAT values breaking the 1.50" mark by Tuesday
morning are nearly 200 percent of normal and surpass the 95
percentile across the Tri-State area. However in this case it`s
certainly possible for two areas of heavy rainfall to develop, one
over Iowa/Minnesota driven by deeper convection, and a second area
from south central South Dakota into northeastern South Dakota
focused by stronger 850:700 convergence.  ECMWF ensembles continue
to support 100% probabilities of >1" over much of the area,
with nearly 70% probabilities of >2". The GEFS/GEM are slightly
lower with probabilities, but both suggest high probabilities
for 1-3" of rain in many locations by late Tuesday. What`s
noticeable is that there are some hints at a more
climatologically significant rainfall within ECMWF guidance as
the 24 hour forecast QPF ending 06Z Wed from central to
northeast SD and western Minnesota is approaching 75% of the 100
ARI. This carries through to an EFI greater than 0.7 over a
large portion of the CWA, which also suggests this could be a
climatologically significant rainfall event if things come
together a certain way. This bears some watching as it could
send rivers upwards by Wednesday or at least prime most of the
area for flooding if additional rain arrives later in the week.


WEDNESDAY-FRIDAY:  The deepening upper low moves into the western
Great Lakes by late Wednesday, pushing the area into gusty northwest
winds and much cooler temperatures.  Highs likely stay in the 60s
Wednesday.  Ridging moves into the Plains to provide at least a
second day of dry weather for Thursday, but that will be short-
lived. Deterministic and ensemble guidance both indicates yet
another trough enters the Plains late in the week.  This trough
provides a brief jump in temperatures, but also another risk for
rainfall by Friday into Saturday.  Depending on the track of this
wave, we`ll have to monitor for severe weather risks.


&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1202 PM CDT Sat May 18 2024

VFR conditions and gusty northwest winds will continue through
the daytime hours of Saturday. Overnight, winds may weaken, but
high clouds gradually spread northward. However a mid-lvl cloud
deck will approach from the southwest towards daybreak as some
rain tries to moves through Nebraska. This rain may struggle to
reach northward as dry air continues to be an issue. What is
likely at this point would be a thickening mid-lvl cloud field
with a PROB30 group for SHRA developing by mid-morning.

One thing to watch will be winds Sunday morning, as some
stronger gusts may start to mix downward quickly in the morning.
Additionally the edges of any stratiform rain may provide some
focus for evaporative cooling enhanced winds that could push 35
to 40 knots.

&&

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

&&

$$

DISCUSSION...Dux
AVIATION...Dux