Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 200552 AAA

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1252 AM CDT Thu Jul 20 2017

Issued at 1252 AM CDT Thu Jul 20 2017

06z aviation discussion updated below.

UPDATE Issued at 757 PM CDT Wed Jul 19 2017

Allowed the heat advisory to expire as temperatures have fallen
into the 70s and 80s across the region this evening.

UPDATE Issued at 635 PM CDT Wed Jul 19 2017

Amended pops for the rest of the night. Removed pops from James
valley west except for south of I-90. Still have some slight
chances for showers and thunderstorms for the southeastern cwa
later this evening, including Watertown, because of the decent mid
to upper level dynamics.


.SHORT TERM...(This Evening through Thursday Afternoon)
Issued at 307 PM CDT Wed Jul 19 2017

Forecast challenges include when/where any convection may initiate
late this afternoon/early evening along with temperatures.

Currently, rain-cooled stable boundary layer air resides across the
northeastern two-thirds of the cwa, with readings still only in the
60s this afternoon over the Prairie Coteau over into west central
MN. Further south and west towards the Pierre area, temperatures are
beginning to nudge their way closer to the lower 90s this afternoon.
As far as heat indices go, the rain-cooled boundary layer hasn`t
depleted the low level moisture any. Dewpoints are still in the
upper 60s to low 70s across the region, but where temps are no
longer expected to reach up into the upper 90s to low 100s, heat
indices in the advisory area will only reach the marginal range at
best, except for an area south of a line from Mission Ridge to Gann
Valley where the combination of hot temperatures and high dewpoints
should be able to push heat index readings up to 100 or better for a
couple hours late this afternoon/early this evening. Also, just
about all the convection from earlier today has pushed off into

Rest of this afternoon through late this evening: given the degree
of boundary layer stabilization that has occurred across northeast
South Dakota and west central MN today, not really expecting much in
the way of surface-based convective activity there over the next 6
to 12 hours. If convection does re-develop across that area, it
would be elevated in nature and forced mainly from a combination of
strong mid-level waa/moisture advection and upper level jet
enhancement. Can`t rule out a strong storm or two there (large hail)
from super-cellular elevated cells. The more probable region over
this cwa for convection rooted/nearly rooted in the boundary layer
is the far southwestern zones (I-90 corridor) where less influence
from stabilizing effects of morning outflow exist. Capping inversion
is being eroded and instability/shear off the Rap Refresh model has
an instability gradient of 1000 to 4000J/kg of CAPE positioned over
Lyman County where 0-6km bulk shear is 30 to 55 knots. If storms do
initiate or move into far southwestern zones, very large hail and
strong/damaging winds would be the primary severe weather threats.

Late tonight through Thursday: much of the rest of tonight and
Thursday should be quiet, dry and warm while the cwa is in between
mid-level shortwaves and under the influence of surface high
pressure and northeasterly boundary layer winds.

.LONG TERM...(Thursday Evening through Wednesday)
Issued at 307 PM CDT Wed Jul 19 2017

The meandering frontal boundary will return back north across our
forecast area Thursday night. This will be while 2 areas of low
pressure organize across western ND and western NE by daybreak
Friday. While still divided, much of the precipitation will likely
slide to our north or south. For Friday night-Saturday, expect much
of the activity to our north, closer to the 500mb low crossing south
central Canada. The only time with confidence to put chance pops in
the forecast was overnight Thursday night-Friday morning, and again
Friday evening.

Expect dry weather to return to the region Saturday through Monday
morning. The 500mb ridge looks to slowly build back overhead towards
the end of the weekend into early next week. The main fly in the
ointment is another 500mb low crossing central Canada midweek. The
12Z GFS is the farthest south with the track, suppressing the ridge
Tuesday-Wednesday. Still, confidence is high that we`ll still
experience temperatures in the 80s and 90s.


.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
Issued at 1252 AM CDT Thu Jul 20 2017

Some fog is expected at ABR and ATY tonight. Otherwise, VFR
conditions are expected through Thursday night.




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