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401
FXUS63 KABR 191813 AAC
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
113 PM CDT Sun May 19 2024

.KEY MESSAGES...

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- Rain showers and a few thunderstorms (20-45%) will continue over
  the area this morning, moving east/northeast. A second wave of
  showers and thunderstorms are forecasted to form over north
  central to south central SD late this afternoon through this
  evening (30-65%).

- There is a slight risk (2/5) for some of these storms to become
  strong to severe storms this evening from north central to south
  central SD. Main threats include up to quarter size hail and
  wind gusts between 60-70mph. A marginal risk (1/5) extends
  eastward into northeastern SD.

- 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.
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&&

.UPDATE...
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Issued at 106 PM CDT Sun May 19 2024

See below for an aviation forecast discussion for the 18Z TAFs.

UPDATE Issued at 1100 AM CDT Sun May 19 2024

The only change made to the today period was to clean up the PoPs
grids (which weren`t all that bad to begin with) for the current
WAA-forced areas of rain shower activity, based off current radar
and HRRR/ECAM radar trends over the next 3-4 hours. The rest of
the today period has been left unchanged.

Still expecting surface-based convection to develop later this
afternoon over western South Dakota, where it`s been sunny and is
expected to continue to be so through this afternoon. The
shortwave expected to aide in convective initiation is moving up
through northeast Wyoming right now. Supercell convection that
develops this afternoon will move east-northeast, unless it should
split and become right-mover dominant. Right-movers will be
straight out of the west or a little north of west and a bit
slower than mean storm motion. Given that storm-mode is
supercellular, at least initially, and CAPE should be between
1000J/kg and 2000J/kg, expecting adequate updraft helicity/updraft
strength for some decent sized hail this afternoon/evening
(at least 1.00-1.50in diameter), and given the strength of mid-
level winds in the 45 to 55 knot range, can`t rule out
thunderstorm wind gust potential on the order of 55 to 70 mph.
Right now, the primary threat area is considered to be a box that
includes Corson, Dewey, Campbell, Walworth, Potter, northern Sully
and Hyde, as well as western McPherson, Edmunds and Faulk
counties mainly between 6 PM CDT and 10 PM CDT.
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&&

.SHORT TERM /TODAY THROUGH MONDAY AFTERNOON/...
Issued at 319 AM CDT Sun May 19 2024

As of 3AM, scattered rain showers and isolated thunderstorms (15-
25%) continue to push northeast over the CWA. Short term models
agree on a surface trough positioned over the western parts of SD
early this morning as the main trough deepens over the Pacific
Northwest into the Northern Rockies. On the PVA side of this trough,
shortwave energy will be pushing northeast over the area with a LLJ
forming over south central SD (30-40kts) and pushing northeastward
into the afternoon (30-35kts). CAMs indicates showers/few storms
becoming more widely scattered, east of the Mo River, late morning
through midday (20-45%). Precip is forecasted to move out of the
eastern CWA by late this afternoon/evening.

Our attention then turns to later on in the day as we expect a
second round of thunderstorms. An embedded negative tilted shortwave
moves over the Northern Plains, with southwesterly winds increasing.
At 850mb to the surface through the day, a low pressure system will
form over western SD as low level winds remain out of the south
ahead of it. The warm front lifts north through 18Z, leaving the CWA
in the warm sector and on the northern edge of a dryline. By this
evening, the center of the low is forecasted to be over northern SD
with much of the low level moisture from north central to south
central SD between the fronts. Through the overnight, the low will
track east/southeast with the cold front trailing and forecasted to
be out of the CWA by Monday morning. RAP indicates a cap setting up
over central and western SD through the morning into the midday and
over northeastern SD by the evening. This cap looks to erode from
north central to south central SD late this afternoon as HREF
indicates a narrow area of CAPE increasing to 1000-2000 j/kg in this
area. Mid level lapse rates will increase to 7-8C over the CWA and
bulk shear between 40-50kts out of the west then southwest. CAPE
does weaken with sunset with a cap setting back in over central SD
and continuing over northeastern SD.

With dp`s rising into the 50s, instability, and lift, it`s all there
for convective initiation (assuming the cap breaks). HREF 4hr max
2- 5 UH>75m2s2, indicates organized convection, as cells start
firing up over north central to south central SD between 22Z-00Z
and spreading eastward. UH>150m2s2 indicates a few discrete cells
could form in this area of higher instability, where shear is
perpendicular to the front. CAMS show cells merging and becoming
more of a messy linear system (as bulk shear becomes more
parallel) ahead of the cold front, pushing east across the CWA.
HRRR, HIRES ARW/FV3, and NSSL-WRF show this transition where NAM
NEST and Rap show more of a messy scattered threat and no linear
transition. So confidence remains lower on the exact outcome and
far south the line could get. Depending on the LLJ, storms may
stay strong as they march east into MN, otherwise less instability
will be over the area giving way to just general showers/storms
as it pushes into MN. The SPC has upgraded to a slight risk (2/5)
from parts of north central to south central SD with a marginal
risk (1/5) spreading eastward over much of the CWA. Main threats
is up to quarter sized hail and wind gusts (60-70mph).

Behind the system, the CWA looks to remain dry most of the morning
but that will be short lived as a surface trough forms overhead and
a low pressure system to the south. With shortwave energy aloft, we
could see lift for more rain/storms. Latest NBM has a 15-30%,
highest west of the Mo River Monday afternoon.

&&

.LONG TERM /MONDAY EVENING THROUGH SATURDAY/...
Issued at 319 AM CDT Sun May 19 2024

No break from the active pattern. The first system in the extended
is a subtle southwest flow shortwave coming in from the southwest of
the state Monday night. Just some mid level moisture/ascent with
this one, with stable north/northeast low level flow out of weak
high pressure to the north. Really not much to write home about, but
it does precede a stronger system that will definitely overshadow any
moisture that falls Monday night. By Tuesday morning this Colorado
Low will deepen around Sioux City before tracking northeast to
Minneapolis. 990mb is a little out of place for May, as the surface
pressure falls to 2 standard deviations below climo.
Thankfully no Arctic airmass gets pulled into the region so the
primary p-type is rain, with the bulk of that in the eastern CWA
associated with a deformation zone/wrap around. Forecast QPF ranges
from essentially nothing for north central South Dakota, to +2"
potential for eastern SD/western MN. Our numbers are a little shy of
the NBM 48-hour 75th percentile. The GFS shows just a slightly
farther east bullseye for QPF and GEFS plumes run lower for the most
part though a few members do have as much QPF as the official
forecast. The point is that there is still some wiggle room, in
addition to what will be a sharp contrast with the western gradient
in moisture over the CWA. Not only will it be wet, it will also be
cool and windy on the back side of the system with 850mb
temperatures down to as low as +3 to +6C. The gradient and low cloud
potential should keep temperatures from bottoming out Wednesday
morning, with lows in the low/mid 40s.

Then we have a system for Thursday/Friday timeframe. Deterministic
models still have a hard time with timing/intensity of the system
which is associated with shortwave that takes a more typical late
spring trajectory across the northern Rockies. Clusters are pretty
spread out on this one, so will continue to advertise low confidence
on what this system will bring. As we have brought up across several
discussions, this overall pattern will continue to keep us below
normal with respect to temperatures throughout the extended.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
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Issued at 106 PM CDT Sun May 19 2024

Terminals KABR,KATY,KPIR,KMBG

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.

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
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&&

$$

UPDATE...Dorn
SHORT TERM...MMM
LONG TERM...Connelly
AVIATION...Dorn