Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 170920
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
320 AM MDT Thu May 17 2018

.DISCUSSION...(Today Through Wednesday)
Issued at 320 AM MDT Thu May 17 2018

Current surface analysis shows low pressure over north central
MT, with a nearly stationary warm front stretching eastward
through ND into northern MN. A surface trough stretches south from
the low into central WY at this time. Upper level analysis shows
low pressure now over western NV, with southwest flow across the
Rockies and into the high Plains. Ridge axis has shifted east of
the area. Skies are clear to partly cloudy across the area. Low
level southerly jet and weak mid-level theta-e advection is
helping to produce some showers and storms from southeast MT to
far northwest SD, as well as south central SD, early this morning.
This activity should move out of the area and start to dissipate
around daybreak. Temps are mild this morning, 59s and 60s, with
southerly winds breezy in spots from southern SD to northeast WY.

Ingredients are coming together across a good part of the region for
some severe thunderstorms to develop later today into this evening.
Frontal boundary to the north and northwest of the region will slide
slowly south today, approaching the ND/SD border late today and
easing into far northern and western portions of the forecast area
tonight. Low pressure along the front will move into southeast MT
this afternoon. Surface trough/dryline to the south of the low will
move into and gradually cross northeast WY later this morning and
afternoon. Southerly winds will continue to bring moisture into the
area today, with surface based CAPE values of 1500 to 3000 j/kg
progged later this afternoon across a good portion of the area,
especially from the Black Hills eastward. CIN will erode this
afternoon as temperatures rise into the 80s across the plains and
70s over the Black Hills. The Black Hills and northeast WY should be
the first places to see storms develop early this afternoon as
fairly strong shortwave approaches this afternoon. Activity will
continue to develop and expand east through the rest of the
afternoon and into the evening. Forecast soundings show shear to be
adequate for supercell development this afternoon, but likely not
long-lived as flow aloft is not that favorable. Activity over the
Black Hills and western SD late in the day will likely merge into a
MCS early this evening and track east into central SD, weakening
with time. The best threat for severe storms with large hail and
some strong wind gusts will be from far northeast WY through much of
western SD in the 2 PM to 9 PM time range. Some heavy rainfall is
also likely, especially from the Black Hills through west central
into parts of northwest SD, as the general track of the activity
will be to the northeast.

As the upper low moves east across the Great Basin, Central Rockies,
and into the Central PLains Friday through Saturday, disturbances
will continue to bring good chances for rainfall through the period,
especially across the Black Hills and southern SD. The cold front
will move very slowly southeast across the forecast area late
tonight through Friday evening, with areas south and east of the
Black Hills staying mild on Friday. This will also allow for the
potential of a few strong to marginally severe storms across
southwest into central SD Friday afternoon, with the better severe
weather potential further south across the Central Plains. Cool and
showery weather is then expected on Saturday, especially across the
Black Hills and southern SD, with highs only in the 50s and lower
60s.

The weakening upper low will pass south and east of the area
Saturday night and Sunday morning, with slowly improving conditions
late in the weekend. Shortwave ridge passing over the region late
Sunday into early next week will bring some drier and eventually
warmer conditions. Another upper low is progged to move into the
western US early next week and eventually move far enough east to
bring some better shower and thunderstorm chances toward late
Tuesday and midweek. Temperatures should be back around seasonal
averages for Monday through Wednesday.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Thursday Night)
Issued At 1013 PM MDT Wed May 16 2018

Scattered showers and thunderstorms are expected to develop
after 19Z on Thursday afternoon and will become numerous over
western South Dakota on Thursday evening. Some strong to severe
thunderstorms are expected, with large hail and damaging wind
gusts. Outside of the showers and thunderstorms, VFR conditions
are expected.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION...26
AVIATION...10



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