Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 171909

Area Forecast Discussion
National Weather Service Aberdeen SD
209 PM CDT Sun Sep 17 2017

.SHORT TERM...(This Evening through Monday Night)
Issued at 208 PM CDT Sun Sep 17 2017

High pressure will continue to slowly move away from the area to the
southeast. There is a weak wave upstream, and we are already seeing
a mix of mid/high cloud upstream moving into the CWA on return flow.
Majority of the CAM solutions bring this feature across the southern
tier of the CWA during the overnight/early morning hours, with
mainly a mid level deck across our forecast area. Will maintain the
20-30 percent chance for moisture - just expected amounts should be
just a few hundredths of an inch.

A stronger low level jet develops for Monday night, around 50kts.
Profiles are generally unsupportive of elevated convection with deep
warm/mixed layers up to 12kft. Pressure falls are also light, so a
nocturnal boundary layer should keep winds from mixing to the
surface although this may be an issue for Aviation.

.LONG TERM...(Tuesday through Sunday)
Issued at 208 PM CDT Sun Sep 17 2017

It will be the tale of two halves of the CONUS in the long term,
with an upper level trough over the western CONUS and a ridge over
the eastern CONUS. That will place the Northern Plains in a
favorable location for unsettled weather, with several rounds of
showers and thunderstorms.

The first wave of showers/thunderstorms will occur on Tuesday, as
a negatively tilted upper trough is exiting the Northern Rockies
and moving into the Northern Plains. At the surface, a low in the
Canadian Prairies will stretch a cold front south and through the
plains states. This will lead to a favorable setup for
thunderstorms and possibly severe weather. Comparing the forecast
to severe weather outbreak conceptual models, it has a lot of
things going for it. Strong negatively tilted upper trough
leading to an abundance of shear (0-6km and effective bulk shear
of 50kts and 35-40kts respectively) and a surface boundary to aid
convection. Ahead of the front and in the warm sector, models are
showing ML CAPE values rising into the 1500 J/kg range due to
700-500mb lapse rates exceeding 7.5 C/km and strong southeasterly
flow pushing dewpoints into the lower 60s. But, models are
consistent in the warm sector being capped much of the day due to
a decent EML starting around or just below 850mb. Thus, thinking
the warm sector won`t see much convection for much of the day and
the main focus for convection and severe weather will be right
along the front. That is where the front will overcome the EML and
lead to convection moving into or developing in the eastern
Dakotas (and at this time being most likely in the James River
Valley). Where that convection develops, the steep low level lapse
rates and abundant shear should lead to severe storms although
they will likely be elevated to start. With the elevated nature
initially, expect some severe hail (NCAPE values peak around .15)
before transitioning to a linear system moving to the
east-northeast (front moving east but storm motions indicating
northeast movement). Think the greatest threat at this point will
be over northeast SD and into west-central Minnesota late Tuesday
afternoon and into Tuesday night. One other item to mention for
Tuesday will be the gusty winds over central SD. With better
mixing during the morning (EML weakening much earlier), may see
winds gust into the 30-40mph range out of the southwest near PIR
(and areas east of the Missouri River). But it will all depend on
the timing of the front and amount of mixing, as NAM/GFS are still
varying at this point. If the GFS ends up being correct, there
would be some fire wx concerns for our SW CWA on Tuesday. Then
behind the front, a strong pressure rise (7-10mb in 6hr) will
occur through the evening hours. Low level winds aren`t too
strong, but that will likely be enough to counteract the reduced
mixing and still produce a brief period of gusts to 30-35 mph
right behind the front.

Should see a quieter period for Wednesday as the upper trough
departs and a weak surface ridge moves into the area from a high
in the central Mississippi Valley. Late in the day, expect winds
to be on the increase again out of a southerly direction as the
next low develops in far NE WY and SE MT. Have trended up
winds/gusts from the general model consensus for Wed
afternoon/evening. Meanwhile, an upper trough will become
reestablished over the western CONUS Wednesday night and into
Thursday and start ejecting waves towards the area late Thursday
afternoon and continuing into the weekend. There will also be a
surface low/trough near the CWA during this period, so will be
showing chance PoPs for the combination of the two features during
that period. Temps late this upcoming week and into the weekend
will be highly depending on the position of the surface trough.
Looking at ECMWF/GEFS ensemble members, there are a variety of
solutions (for example, EC ensembles range from the low 80s to low
60s for highs Fri/Sat at KABR), so don`t have any confidence on
highs and will follow a general consensus for now.


.AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon)
Issued at 1216 PM CDT Sun Sep 17 2017

VFR conditions for all terminals. Winds will become predominantly
from the south.




SHORT TERM...Connelly
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