Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 191920

Area Forecast Discussion
National Weather Service Aberdeen SD
220 PM CDT Sun May 19 2024


- Showers and thunderstorms are forecast to move over central and
  north central South Dakota late this afternoon through late this
  evening (30-65%), with a slight risk (2/5) of severe weather
  attached to them. The main threats include 1.00-1.50in hail and
  wind gusts to 60-70mph. A marginal risk (1/5) extends off into
  northeast South Dakota and west central Minnesota.

- An active pattern continues next week with additional rounds of
  showers and thunderstorms for Tuesday-Friday. Near to slightly
  below normal temperatures are expected Tuesday-Friday as well.


Issued at 210 PM CDT Sun May 19 2024

At 2 PM CDT, under a mix of sun and clouds, temperatures were noted
warming into the 70s to low 80s throughout and west of the Missouri
River valley, while mainly cloudy skies today have resulted in a
slower warm up throughout and east of the James River valley where
readings have only warmed into the 60s to low 70s. Southeast winds
of 15 to 30 mph with gusts to 45 mph across western and central
South Dakota is helping to draw moisture rich air up into the region
(surface dewpoints increasing into the low to mid 50s over said
area; +9C to +11C dewpoint air progged at 0.5km and 850hpa). These
stronger southeast winds have yet to materialize out over the
northeast corner of South Dakota and west central Minnesota.

The set-up for late afternoon through late evening convection
remains in place and, for the most part, relatively unchanged. The
inverted surface trof/deepening surface low in the lee of the Black
Hills in response to both daytime heating and the shortwave noted in
water vapor moving over western South Dakota should provide the
needed forcing and lift for convection. There is also a 90+knot
upper level jet max in play over the region to lend support to
strong convection potential. The instability component (higher
octane air in the form of 50`s dewpoints) is in place as well. With
deep layer shear of 40 to 50+knots and CAPE in exceedance of
1000J/kg, still thinking storm mode should come around to
supercellular, with large hail and damaging winds the primary
threat. Not sure the RAP13 is treating the boundary layer correctly.
It is progging an increase in low level (0-1km) CAPE at Bowdle, SD
this evening from <5J/kg at 00Z to ~110J/kg at 02Z, while increasing
0-1km shear from <10knots to ~20-25knots. This is also a position
east or northeast of a surface low forecast to be in the vicinity.
Perhaps the increase in low level shear is conflated by developing
low level jet winds at 02Z. Could there end up being a localized and
small time window risk area for tornado? Perhaps, but the pickens is
awfully slim. Much higher threat potential exists for large
hail/damaging straightline winds. With a general removal from needed
unstable air the further east convection goes tonight, CAM guidance
is still suggesting convection could eventually evolve into some
sort of line or line segments that race off to the east into MN,
decreasing in intensity as it does so.

Most of Monday should be dry, but as the next upper low begins to
take aim on the central plains, expecting rain chances to ramp up by
Monday evening, as deformation-zone forcing starts to take hold of
Nebraska and portions of South Dakota. The southwestern portion of
the CWA (the area of central and south central South Dakota
basically south of U.S. Highway 212) appears to be in a position to
receive a goodly amount of deformation-supportive rainfall. This is
noted in some of the ensemble-based 12 and 24 hour probabilities of
0.50in and 1.00in rainfall, respectively. Temp-wise, low level
thermal progs and the ensemble-based 850hpa standardized anomalies
tables indicate that the next 36 to 48 hours should hold
temperatures generally near normal for this time of year.


Issued at 210 PM CDT Sun May 19 2024

The period will start of wet, windy and cool...essentially a
continuation of the active pattern that`s been in place across the
Northern Plains for several days and weeks. Lead s/w activity will
kick off some showers, perhaps a thunderstorm or two Tuesday
morning. This will be out ahead of the main upper trough that is
progged to shift northeast out of the Central Plains into the Upper
Midwest later Tuesday afternoon into Tuesday night. This system will
aid in developing a Colorado sfc low that guidance progs to track
northeast out of the Central Plains toward western IA during the
daytime Tuesday before heading toward southern MN and western WI
Tuesday night into early Wednesday. This system is going to spread a
swath of precipitation northward across our forecast area on
Tuesday. As our area will remain on the northwest side of the low
pressure system, this will largely be a deformation zone or TROWAL
type rainfall event...some of which is going to provide a decent
dose of rainfall to parts of our CWA.

The latest QPF forecast ranges still remain consistent from the
previous fcst package in that they show mainly very little if
nothing for rainfall across north central SD to more than 2 inches
of potential rainfall for parts of northeast SD and west central MN.
NBM ProbQPF values of reaching or exceeding 1 inch in 24 hrs ending
at 1AM CDT Wednesday has highest values across those eastern zones
of ours. Essentially from the James Valley east into west central MN
has a 50 percent chance or greater of receiving an inch of rain.
Highest probs are in the Watertown area with chances at 70-75
percent. Confidence if modestly high at this point that our eastern
zones are in line for the greatest rainfall from this event. Backing
this up is EC`s EFI output indicating a value of 0.8-0.9 across east
central SD and and the Shift of Tails value of 1-2 across northeast
SD and west central MN. This is basically telling us information
about how extreme an event this potentially be in terms of rainfall
and these values indicate that rainfall could be a high end event
for those sections of our forecast area. It will be interesting to
see if trends in these outputs chance over the next 24 hrs.

This system will wind down toward Wednesday morning. Weak sfc high
pressure will take over briefly giving us a break from the rainfall
Wednesday into a portion of Thursday, although a follow-on upper
trough working in behind the first, more robust one could maintain
some shower activity to parts of the area on Wednesday. The next
system is progged by guidance to shift out of the PacNW into the
Northern High Plains and Northern Plains later Thursday through
Friday. This will be our next good shot at more precip for our CWA.
At this point in time, that system is progged to move out and leave
us with a more drier conditions late in the period. Temperatures
through the period will again maintain seasonal normals at times to
slightly below normal values at other times between Tuesday-Sunday.
The presence of clouds and rainfall will largely drive the days we
see below normal temperatures.


Issued at 106 PM CDT Sun May 19 2024


VFR conditions for all terminals through at least 03Z before some
MVFR/IFR CIGS move in and persist into Monday morning before
dissipating. Otherwise, the main issue for TAFS is the potential
for strong storms to develop and move through KPIR/KMBG around
23-01Z before moving east and weakening. If storms move through
KPIR/KMBG terminal space, they could produce large hail and
damaging winds. Thunderstorms could be reaching KABR/KATY around
03-05Z, with a decreasing risk of being severe. Thunderstorm gusts
up to 50 mph and some small hail would be possible at KABR/KATY.





LONG TERM...Vipond