909
FXUS63 KABR 031521 AAB
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1021 AM CDT Thu Jul 3 2025

.KEY MESSAGES...

- Heat and humidity will remain with us today and Friday. A heat
  advisory is in effect for portions of north central and
  northeast South Dakota/western Minnesota this afternoon. Heat
  indices around 100 degrees.

- A system Friday/early Saturday will bring relief from the heat
  and humidity in the form of thunderstorms with the highest
  chances (60 to 80%) along and east of the Missouri River. The
  main concern is the potential for heavy rainfall.

- Weather pattern stays active into early next week with
  disturbances Saturday night/Sunday and again Monday night
  bringing opportunities for rainfall.

&&

.UPDATE...
Issued at 1014 AM CDT Thu Jul 3 2025

Satellite indicates scattered clouds associated with the remnants
of a MCV moving over the northern portions of SD into ND. Radar
indicates ongoing light showers associated with this over north
central SD and currently pushing eastward. Some of the CAMs
indicate these showers will continue eastward while others show
them dissipating. I have adjusted the pop grids to show this.
Otherwise, the heat and humidity will be the main concern. As of
10am, temps are already in the upper 70s to the lower 80s and
dewpoints in the upper 60s to the lower 70s, so quite muggy out
there. The Heat Advisory for portions of central, north central,
northeastern SD, into west central MN will go into effect at 3pm
CDT and continue until 8PM CDT as heat indices around 100 are
expected. Otherwise the forecast remains on track.

UPDATE Issued at 630 AM CDT Thu Jul 3 2025

Aviation discussion updated below for the 12Z TAFs.

&&

.SHORT TERM /TODAY THROUGH FRIDAY AFTERNOON/...
Issued at 329 AM CDT Thu Jul 3 2025

For today, the main concern are temperatures and humidity. HREF
probabilities of exceeding 100 degrees are focused mainly across
central South Dakota (60-80%), while the probabilities for dewpoints
exceeding 70 are focused mainly up through the James valley and
western Minnesota (60-80%) with a gap created by the Coteau. HREF
deterministic peak heat index of about 100F is focused mainly in the
James valley, and the probabilities of exceeding 100 heat index
values is a smattering for the Missouri valley of 30-70%, upwards
of 80% across the James valley, and in the 40-50% range for
western Minnesota and far northeast SD. NBM is more broadly 50%
probabilities of exceeding a high of 100 outside of the eastern
Coteau, with the mean dewpoint of 70 a little farther east than
the HREF.

The main wrinkle is the surface flow. We have a warm front migrating
northwards this morning, with an easterly component across the
northeast that transitions to south southeast, with a shift to a
more south southwest component happening earlier than originally
anticipated a few days ago (this was expected to happen Friday).
Looking at individual CAMS, they all share this trend, which would
suggest better mixing during peak heating - though slight timing
differences affects how far north this transition occurs.
Additionally, there is a layer of dry air showing up in central
Nebraska that migrates north northeast, although its uncertain how
well this dry air will mix out the high humidity at the surface.
CAMS universally cause dewpoints to dip into the 60s west of the
James river prior to the conclusion of peak heating, possibly
scouring out the highest humidity air and thus upending heat
advisory level heat index values. Thus, low to poor confidence in
comparison to the above probabilistic information above.

In addition to all the mesoscale minutia, we also have the potential
for high clouds migrating out of Wyoming and up over the ridge
across the northern tier of South Dakota which would impact
insolation, though it could just as easily go into North Dakota and
not impact the forecast.

Heat continues into Friday. NBM temperatures are not quite as
extreme, though with a surface boundary, we could see a little more
south southwest component meaning NBM would underperform but with
better mixing comes a tick lower humidity. For the most part, will
continue with straight NBM which doesn`t quite get us into heat
advisory territory.

Additionally, with a frontal feature and southwest flow shortwave,
convection is expected to develop during the mid/late afternoon
hours. CAMS are mostly in agreement with this premise. Thinking is
that a southwest northeast boundary with weak flow aloft, middling
lapse rates and moisture up through 500mb, we wouldn`t have strong
cold pools and/or rapid storm motion but CAMS are migrating
convection well enough to somewhat subvert expectations. Regardless,
HREF is giving us a 40% probability of exceeding 2 inch PWATS along
the boundary and as such would expect high precipitation efficiency.
Freezing levels in the NAM are also up around 14kft meaning deep
warm cloud process and whatever hail pulse convection can generate
melting before reaching the surface.

&&

.LONG TERM /FRIDAY EVENING THROUGH WEDNESDAY/...
Issued at 329 AM CDT Thu Jul 3 2025

Surface boundary will still be tracking eastward across the eastern
CWA Friday evening, with good agreement amongst the GEPS/GEPS/ENS in
showing the position from southeast ND, southwest into the James
River valley and onward to south central SD at 06Z Saturday. By 18Z
Saturday it appears the boundary is far enough east into MN to let
PoPs reduce to 30-50% across the far east, much lower than the high
chances (70-90%) in the forecast for earlier Friday evening. PWATs
are still rather high Friday night into Saturday morning, generally
a tad either side of 1.75in shown by the Grand Ensemble. It`s not
until Saturday evening that these values lower down to around an
inch or less. There would appear to remain at least a marginal
threat for heavy rain and strong wind gusts and small hail between
00-06Z Saturday. MUCAPE values off the Grand Ensemble still
generally between 1000-1500 J/KG, although shear remains weak.

Cooler air follows in the post-frontal atmosphere for Saturday and
Sunday. 850mb temps in the +20s C on Friday gradually cool to
between +13C and +16C by 18Z Sunday. Highs cool down to the mid 70s
and low 80s on Sunday. Good agreement in warming conditions again by
mid-week, with highs likely getting back into the 80s and low 90s.

As for precipitation, the overall pattern seems to stay somewhat
active. We see a wave already impacting the region Saturday night
into Sunday, and will have 40-60% chances for showers and
thunderstorms along and west of the Missouri River Saturday night.
NBM PoPs also pinpoint a time frame Monday night with decent
potential (40-50% chances) for rainfall. Mid/upper level flow
pattern looks to set up over the Northern Plains, likely bringing
disturbances across the region. Of course, models having a tough
time with timing of these waves, so rainfall chances are somewhat
broadbrushed through the period.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 630 AM CDT Thu Jul 3 2025

Terminals KABR,KATY,KPIR,KMBG

VFR conditions are forecast through the TAF period. Southerly
winds will become gusty today and range from 20-27 knots (gusts).
There may be some low-level wind shear (LLWS) overnight and will
take a closer look at that in the 18Z/00Z TAFs.


&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory from 3 PM this afternoon to 8 PM CDT this evening
     for SDZ004>006-008>010-016>018-021-037.

MN...Heat Advisory from 3 PM this afternoon to 8 PM CDT this evening
     for MNZ046.

&&

$$

UPDATE...MMM
SHORT TERM...07
LONG TERM...TMT
AVIATION...TMT