341
FXUS63 KABR 131126
AFDABR

Area Forecast Discussion
National Weather Service Aberdeen SD
626 AM CDT Tue Aug 13 2024

.KEY MESSAGES...

- Dry today but with increasing coverage of storms
  tonight/Wednesday (50-70%). Severe weather risk is limited (Marginal
  Risk) with heavy rain the main concern.

- Lingering showers and thunderstorms early Thursday (30-50%) will
  taper off from west to east through the day with a general 20-35% of
  additional precipitation Thursday afternoon and evening.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY AFTERNOON/...
Issued at 242 AM CDT Tue Aug 13 2024

Mid level warm advection responsible for some mid level clouds in
south central South Dakota with the potential for weak elevated
convection as the core of the elevated mixed layer moves out of
western SD/NE and WY. 700mb temperatures are a bit more subdued
compared to previous forecasts, closer to +11C meaning a much less
robust CAP overhead during our progression though Tuesday/Wednesday.
Today, we will see increasing heights as a ridge axis sets up
directly overhead. Southwest flow waves migrate into western South
Dakota and northern North Dakota, but these are out of position to
provide any lift across the CWA today. Thus a dry forecast late
morning/afternoon. The surface gradient is on the increase however,
and while only about 10mb, the increased winds will also draw up
some much more humid air. This actually will translate into the
convective potential tonight as 850mb winds increase to 30kts and
PWATS increase to almost 2 inches in the NAM with nearly a 100%
probability of exceeding 1.75 inches in the HREF which is 2-3
standard deviations above climo for mid August. Its not just the low
level moisture on the increase, but much of the PWAT increase is
owed to a plume of moisture in the mid levels coming in from the
Desert Southwest monsoon.  Heights drop as the western most
southwest flow shortwave moves into the CWA Tuesday night/Wednesday
morning, providing enough lift for convection. Severe weather threat
is marginalized by moist adiabatic profiles and weak wind shear.
NAM/HREF MLCAPE is only around 1000j/kg across the western CWA with
values dropping to just a few hundred j/kg into the James valley
with only 30-35kts 0-6km shear in the NAM and 25kts in the HREF.
Thus, the more probable outcome is slow moving, heavy rain producing
thunderstorms.

&&

.LONG TERM /WEDNESDAY EVENING THROUGH MONDAY/...
Issued at 242 AM CDT Tue Aug 13 2024

Clusters overall agree on the synoptic setup for the end of the week
with a shortwave over the Northern Plains and a low amplitude ridge
just to the west. A shortwave will also be positioned over the
Pacific Northwest. By Friday, everything shifts a bit east as the
shortwave departs the area and the axis of the ridge is positioned
over the Northern Rockies/western Dakotas. Minor differences on the
amplitude as Clusters 1 and 2 (made up of 40-50% EC members) have it
a little more amplified than Clusters 3/4 (both 37% GEFS) which will
determine how warm we get. This shortwave over the Pacific Northwest
looks to diminish or get absorbed as a stronger trough sets up over
the Pacific Ocean. With the wave deepening/mid level low forming,
this will amplify the ridge over much of the central CONUS for
Saturday. Day 6 and onward we have an omega block pattern with the
shortwave trough over the Ohio River Valley/Northeast and the now
closed low off the Pacific coast as the ridge sits in the middle
with really not much movement. There are differences between the
Cluster ensembles on the intensity and exact position of low and
amplitude of ridge as this is over a week out.

Steering flow aloft Wednesday evening looks to be zonal and pretty
light (15-25 kts) through Thursday morning as the surface/850mb low
will move northwest to southeast over SD. It is also packing lots of
moisture per monsoonal flow being pulled north and east from the
shortwave. NAEFS indicates 2-3 standard deviations above climo for
for specific humidity at 850-925mb Wednesday evening through
Thursday. PWAT is about 2.5 standard deviations above climo as well,
ranging from 1.50-1.75" east of the Mo River. EC EFI QPF is around
0.5-0.6 over the northern CWA with a shift of tails of zero.
Ensembles have a good handle on showing this moisture moving
southeastward over the CWA, but there is still slight timing issues
on when it departs. Latest NBM from 00-06Z Thursday shows this with
30-70% pops, highest over the eastern CWA. Pops of 20-50% continue
around and east of the Mo River between 06-12Z Thursday. NBM
probability 24hr QPF>0.25", ending Thursday evening (additional
rain) is 40-60% around and east of the James River.

Lingering precip (20-35%) looks to exit this area by the afternoon
with 20-35% pops scattered over the CWA behind this system as
elevated moisture is still possible from the wave. So with slow
movement of the system along with the excess moisture is why there
there is marginal risk (5%) for heavy rainfall/flash flooding per
WPC. CAPE values are marginal with values of 1000-1500 j/kg over
central SD through Wednesday evening (with lesser values to the east
of here) and shear between 20-30kts out of the west. So with
shortwave energy aloft (although weak), warm/moist air mass ahead
of the cold front(dewpoints in the 60s), and some shear leads to an
isolated threat of severe weather Wednesday evening with the SPC
having the entire CWA in a marginal risk.

Otherwise high pressure system moves in for Friday into early next
week. It is possible for several shortwaves to traverse the top of
the ridge and travel downwind and into the Northern Plains along
with surface lee troughs setting up. NBM indicates slight pops of 15-
25%, mainly from north central to south central SD. Low confidence
exits on exact timing and location as this variation is seen between
the QPF on Clusters. High temperatures range in the 70s and 80s
across the CWA through the long term period which is average to
slightly below average. However, with differences in the ridge`s
amplitude the NBM 25-75th spread is 4 to 7 degrees. We really see
that spread increase early next week with a spread of 10-12 degrees.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 622 AM CDT Tue Aug 13 2024

Terminals KABR,KATY,KPIR,KMBG

Primarily VFR conditions at KABR/KATY, with some low stratus that
could potentially back into the KABR terminal this morning before
mixing out, and some fog development for an hour or two at KATY.
MVFR CIGS at KPIR will persist much of the morning but eventually
mix out. Potential for thunderstorms will increase during the
morning hours Wednesday but at this point the chance is too low
for inclusion in this round of TAFS.


&&

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

$$

SHORT TERM...Connelly
LONG TERM...MMM
AVIATION...Connelly