Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 210530 AAA

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1230 AM CDT Fri Jul 21 2017

Issued at 708 PM CDT Thu Jul 20 2017

Monitoring the current line of storms in northwest SD and
southwest ND as the push to the east towards the CWA (Corson
County). The storms are currently in an area of favorable
environmental conditions: steep mid level lapse rates, MLCAPE
values exceeding 1000-1500 J/kg, and effective shear values
around 60kts. In addition, dry low levels and a favorable
inverted-v RAP soundings have led to DCAPE values to exceed 1600
J/kg west of the Missouri River. Latest trends in the storms show
the southern cores weakening some what and the outflow on Bowman
radar out running the main storms to the east of Hettinger, so may
see some weakening before they reach the CWA. That being said, do
expect at least strong storms and continued the mention of a
severe threat in the grids. Still some uncertainty in how far east
things will track, as models strengthen the low-level jet and in
turn increase instability across the area through the night.
Followed existing forecast thought for slight to low chance PoPs
spreading east through the night.


.SHORT TERM...(This Evening through Friday Afternoon)
Issued at 319 PM CDT Thu Jul 20 2017

Forecast challenges surround convective potential tonight and then
again Friday afternoon.

Currently, temperatures are warming through the 80s and 90s on light
southeast winds under a mostly sunny sky.

The synoptic scale setup tonight favors convective development out
west (across the northern high plains). The HRRR today begin to
latch on to a solution that bring convection all the way east into
portions of north central South Dakota late this evening (at or
after 10 PM CDT) and then similar to the other day begins to dig the
southern-most end of convective cells southward into portions of
central and south central South Dakota during the late night hours
tonight. Although, it has become consistent in producing a solution
like this, it does not have much support from any of the other CAM
solutions or 12Z non-CAM solutions today. Left in a small pop across
the southwestern zones late tonight transitioning over into
northeast South Dakota/wc MN toward Friday morning where all the
best forcing/lift seems to re-focus some potential for elevated
convection. Temperatures tonight on southerly winds, advecting
moisture northward into the cwa, and potentially maintaining a
partly to mostly cloudy sky should remain up, with readings
generally in the 60s and 70s.

On Friday, a cold frontal boundary will begin a slow push east
through the region, and by early afternoon there could be some
isolated to scattered thunderstorms developing along that boundary
(and any leftover OFB`s from overnight convection). Mid-level
thermal capping temps are not overly strong in today`s 12Z
solutions, so it`s seems reasonable to expect at least isolated to
scattered thunderstorms developing by mid to late afternoon on
Friday. The concern here is that deep layer shear and mixed layer
CAPE Friday afternoon over the James River valley eastward into wc
MN favor severe convective development. LCL`s may be a little bit on
the high side (2500ft to 3500ft agl) for tornado potential, but for
sure in the ball park. High temperatures tomorrow are going to warm
into the upper 80s to low 90s from the James River valley eastward
into MN, while readings climb nearly to 100 degrees over into the
Missouri River valley. Hot temperatures and high dewpoints will
result in Heat Indices ranging from 99F to 104F across portions of
central, south central and northeast South Dakota. A Heat Advisory
has been issued as such.

.LONG TERM...(Friday Evening through Thursday)
Issued at 319 PM CDT Thu Jul 20 2017

Thunderstorms may be ongoing across eastern SD Friday evening. A
trough will be overhead, extending from the parent lows across
southern Saskatchewan and southeastern SD to eastern NE. The trough
will exit east by 06Z (possibly lingering over our west central MN
counties for another couple of hours). Otherwise, dry high pressure
will slowly build into the region. Dry weather should be the rule
through at least Monday evening. At 500mb, the low over southern
Saskatchewan at 00Z Saturday will just shift east into Ontario by
Saturday night, keeping a trough overhead into Monday.

Fire weather concerns increase for Saturday, as temperatures rise
into the 90s for most locations. The main difference will the the
loss of moisture. Instead of dewpoints hovering near 70F like Friday
afternoon, they will be in the low 40s as dry high pressure builds
in from the west. RH values fall to 14 to 20 percent across most
areas west of the James River Valley. Winds our of the northwest may
end up being stronger then forecast, so will need to continue to
keep an eye on Saturday afternoon.

The sfc high will be across ND Sunday afternoon, and slowly move
across Minnesota Sunday night into Monday, with a ridge remaining
across the forecast area. The next lee trough will near SD Tuesday
morning, with unsettled weather making a return. Low pressure will
likely remain across the Central Plains Tuesday night through at
least Thursday, with the best chance of precipitation for now
looking like late Wednesday afternoon through Wednesday night. Lot
of time for change though between now and then.


.AVIATION...(For the 06Z TAFS through 06Z Friday Night)
Issued at 1228 AM CDT Fri Jul 21 2017

Generally VFR conditions are expected through the time period, but
with IFR possible during any thunderstorms. Showers and storms
will have the most affect on MBG/ABR through the early morning


SD...Heat Advisory from noon CDT /11 AM MDT/ today to 8 PM CDT /7 PM
     MDT/ this evening for SDZ016>018-033>037-045-048-051.



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