Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 201721
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1121 AM MDT Thu Jul 20 2017

.DISCUSSION...(Today Through Wednesday)
Issued at 243 AM MDT Thu Jul 20 2017

08z surface analysis had frontal boundary from southwest MT into
east central WY, snaking into northwest IA. MCS continues along
the boundary from far southern SD into northern NE per latest
IR/regional radar loop. System supported by moist easterly low
level flow, shortwave over southeast WY, and right entrance region
of 130kt jet across ND/MN. Guidance in good agreement taking bulk
of complex east of CWA by 12z, but there could be lingering
precipitation through 15z.

Today and tonight, weak shortwave ridge builds into the CWA behind
departing shortwave. There will be another shortwave moving across
the northern Rockies into the northern Plains tonight. This
shortwave will take developing lee trough and turn it into a weak
cold front, sliding into the CWA late tonight. Ahead of the front, 1-
2KJ/kg MLCAPE develops with 25-50J/kg MLCIN by afternoon. Some CAMS
show convection developing by 20z across the higher terrain of WY/MT
and near the Black Hills, but some hold off until 00z with cap
holding until upper support arrives. In any case, activity would
move east through the CWA. 0-6km bulk shear 15-25m/s from south to
north, which would be sufficient for a few severe storms. SPC paints
slight risk to account. Temperatures will be several degrees above
normal.

Friday, aforementioned cold front moves east with leftover
instability potentially created a few -TSRA over eastern portions of
the CWA mainly in the afternoon. A few breezy spots possible, but
guidance in poor agreement. Temperatures won`t change much behind
front, although dew points will be noticeably drier.

Saturday through Thursday, flat upper ridge Saturday begins to
amplify Sunday through early next week. Isolated -TSRA may occur,
but looking a fairly dry period for most folks. Temperatures will be
5-10F above normal on average with perhaps Monday looking to be the
warmest day.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 1121 AM MDT Thu Jul 20 2017

Thunderstorms will redevelop late this afternoon and evening across
the Black Hills and northeastern Wyoming. Activity will then push
east into central SD in the late evening. Outside of storms, VFR
conditions are expected. Breezy SE winds will continue over much
of the area this afternoon.

&&

.FIRE WEATHER...
Issued At 243 AM MDT Thu Jul 20 2017

Drier air will push into the area Friday and Saturday with a few
breezy spots possible. Minimum RH will dip to 15-25%. Conflicting
guidance on how strong the winds will be as well as how dry the
air will be precludes the issuance of a fire weather watch this
morning, but future shifts will need to monitor.

&&

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

$$

DISCUSSION...Helgeson
AVIATION...13
FIRE WEATHER...Helgeson



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