642
FXUS63 KABR 211737 AAC
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1237 PM CDT Tue May 21 2024

.KEY MESSAGES...

- System today will produce strong winds and heavy rainfall. The
  focus for the highest winds and greatest moisture totals is far
  northeast South Dakota and western Minnesota. Winds there will
  top 45-55mph and rainfall amounts could exceed 2-3 inches.

- An unsettled weather pattern continues with another shot of
  moisture possible Thursday into Friday and again Sunday into Monday.

- Except for a brief potential warm up on Thursday, temperature
  guidance remains near to below normal Friday through Monday.

&&

.UPDATE...
Issued at 1230 PM CDT Tue May 21 2024

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

UPDATE Issued at 1030 AM CDT Tue May 21 2024

No notable changes planned to the today period forecast. Seeing
additional shortwave energy in water vapor imagery moving toward
this region behind the current upper circulation bringing moderate
to heavy rain to far northeast South Dakota and west central
Minnesota. This is making it difficult to lower the "categorical"
(80+ %) PoPs in place across the James River valley, as guidance
still wants to bring additional measurable rainfall up into
eastern South Dakota and western Minnesota this afternoon into
this evening. However, given that continuous moderate rainfall is
not occurring right now across the James River valley, did go
ahead and nudge high temperatures up 3 or 4 degrees throughout
Brown/Spink and Hand Counties for today. Updates are out.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY AFTERNOON/...
Issued at 307 AM CDT Tue May 21 2024

Colorado low associated precipitation is already worked into the
forecast area and the effects of the system will pretty much
dominate the short term period.

The main issues are winds and rainfall totals. For winds, on the
backside of the system there is a very tight pressure gradient in
response to the nearly 3-4 standard deviation below climo surface
pressure (below 990mb). GFS 1/2km winds top 50kts by 18Z today and
peak across our western Minnesota counties at over 55kts at 00Z.
Probability of exceeding advisory criteria is nearly 100% for
portions of the northern Coteau, while the probability of exceeding
55mph is in the 30-50% range up around Hillhead for a 3 hour window.
As such, will leave headlines in place, as upgrading doesn`t appear
to be justifiable for such an isolated portion for such a short
duration.

As for precipitation totals, the far east portion of South Dakota and
western Minnesota will definitely be under the deformation
zone/TROWAL rainfall, however the western extent of this
moderate/heavy rain band is still in question. Places farther west,
Mobridge for example, will just see scattered coverage of showers
and much lower QPF if any measurable moisture. Between the Missouri
valley and Coteau is where the tight gradient in QPF is still
uncertain, with GEFS plumes showing a range of 0.12 to 3.69 for
Aberdeen, with each member spread across this distribution...or
another way to say it is that there is no clumping towards a
definite solution. As should be expected, the distributions for
Mobridge are a more centered towards a tenth of an inch or less,
while Watertown total is also still somewhat uncertain but confined
to a higher QPF range of 1 to 2.5 inches. CAM ensembles QPF
probabilities for the far northeast include the chance of exceeding
2" at 50-70% to 3" at a 20-40% chance. Should also note that with
freezing levels stuck around 8-9kft there could be some
brightbanding around the edges of the CWA that could bump up radar
estimates - which just happens to fall in the portion of the CWA
that is expected to see the heaviest rainfall totals.

&&

.LONG TERM /WEDNESDAY EVENING THROUGH MONDAY/...
Issued at 307 AM CDT Tue May 21 2024

The main story for the long term will be on the ongoing unsettled
weather pattern. Clusters agree on the longwave trough pattern
continuing over the western CONUS as the upper level low is
forecasted to be positioned over Ontario at 00Z Thursday. Another
low will swing south then southeastward from western Canada and over
the Pacific Northwest during this time as well. Friday, this
longwave trough continues to deepen and becomes more positive tilted
as this northern low/shortwave will then push east/northeast over
the northern portions of the Rockies and Northern Plains with
Clusters in overall agreement on timing and intensity. This wave
pushes off towards the northeast Friday night into early Saturday we
"rinse and repeat" as another shortwave swings in from the northwest
and over the Northern Plains Sunday into early Monday. A +PNA
pattern sets up towards the beginning of next week. Clusters agree
on this overall setup but there are differences in timing and
intensity for the weekend into early next week.

With this upper level pattern, this first low is forecasted to be
fairly stacked as the system pushes east with the surface low
occluding along the way and mid low becoming an open wave. Latest
NBM has pops (30-60%) Thursday afternoon with pops increasing to 60-
75% Thursday evening/night. Dp`s raise into the 50s Thursday
afternoon/evening with GFS indicating mid level lapse rates 7-8C,
highest over central to southern SD. However, the main forcing looks
to stay more south of the CWA at this time, so it`s a wait and see
type of scenario. CSU does show a 5% prob of severe weather with
CIPS keeping any severe threat further south. Chance of rain (20-
50%) continues Friday with the highest pops over the eastern CWA
before moisture ends west to east Friday afternoon/evening as a high
moves in. Maybe some slight pops Saturday around and west of the Mo
as a surface trough sets up ahead of the next system. This system
(and its surface low) brings more chances of rain Sunday/Monday with
the highest pops around 40-45%, at this time, and decreasing west to
east early Tuesday. Confidence is low on any severe chances this far
out with CIPS/CSU not showing any probabilities just yet.

Highs will be around average Thursday falling to below average on
Friday behind the cold front, with highs in the upper 50s to lower
60s. Temps look to rebound back into the upper 60s to the 70s for
the weekend into Monday and a little warmer on Tuesday.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1230 PM CDT Tue May 21 2024

Terminals KABR,KATY,KPIR,KMBG

KATY will be in and out of IFR conditions through late this
afternoon before conditions improve to prevailing VFR. But, VFR
conditions are forecast at KABR/KPIR and KMBG through the TAF
valid period. North winds for KATY have come up and will range
from 20 to 30 knots sustained, with gusts ranging from 35 to 45
knots through late this evening, before diminishing overnight.
Rain is expected to persist at KABR and KATY this afternoon. But,
after 00Z, TAFS for all four terminals should be dry.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...Wind Advisory until 11 PM CDT this evening for SDZ007-008-011-
     019>023.

MN...Wind Advisory until 11 PM CDT this evening for MNZ039-046.

&&

$$

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