Area Forecast Discussion
Issued by NWS Rapid City, SD

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589
FXUS63 KUNR 201946
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
146 PM MDT Wed Sep 20 2017

.DISCUSSION...(This Evening Through Wednesday)
Issued at 141 PM MDT Wed Sep 20 2017

19z surface analysis had cold front from MN to KS with high
pressure over the central plains behind it. Developing lee trough
over the Rockies combined with high producing gusty west/southwest
winds over much of the CWA this afternoon.

Tonight, surface high slides southeast as lee trough strengthens
with rather strong warm air advection under increasing southwest
flow aloft in response to upper trough digging over the western
US. Strong low level jet develops over western SD overnight with
35-60kt 850mb winds. Moisture looks scant, so mid/high clouds
will be main effect along with the usual Black Hills/foothills
downslope warming and gusty winds over the higher terrain of the
Black Hills and over the southern SD plains. Temperatures will be
near guidance on the plains and above guidance for the Black
Hills.

Thursday, warm air advection mostly on the SD plains continues under
southwest flow aloft. Dry boundary layer will preclude pops,
although a few showers may develop off the Big Horns late and
approach the CWA as a cold front starts to sink southeast toward
CWA. Temperatures will vary greatly with readings in the 60s west
and near 90F east, which is pretty close to guidance.

Friday through Sunday, well-defined southwest flow aloft slides
slowly east as upper trough approaches the plains. Low level cold
advection expected Friday. Several shortwaves will move through the
CWA with increasingly saturated forecast sounding profiles, mainly
due to cooling temperatures, although deeper moisture return is
present east of the Black Hills. Region will be under right entrance
region of jet and expect bouts of showers, especially Saturday
afternoon through Sunday morning. Wet bulb heights suggest snow
will mix in over the higher terrain of northeast WY and the Black
Hills. Temperatures will be in transition Friday with 15-25F
below normal highs for the weekend.

Monday through Wednesday, upper trough splits with main energy
shifting northeast and pseudo-cyclonic flow over the CWA. This may
lend itself to a few showers, but temperatures recover with some
60s by Wednesday.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1058 AM MDT Wed Sep 20 2017

VFR conditions are expected through the period. However, LLWS is
expected to be prevalent tonight, especially from 06z-12z, over
southwestern into west-central SD as a strong (50-60 kt) low-
level jet stream develops.

&&

.FIRE WEATHER...
Issued At 141 PM MDT Wed Sep 20 2017

Gusty west/southwest winds will continue through early this
evening. MinRH`s near 15% will occur over the Red Flag Warning
areas, so will continue with headlines.

Very warm weather is expected Thursday over western SD, but RH`s
will be higher than today. Much cooler and wetter weather moves
in for Friday through the weekend.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Red Flag Warning until 7 PM MDT this evening for SDZ262>264.

WY...Red Flag Warning until 7 PM MDT this evening for WYZ298-299.

&&

$$

DISCUSSION...Helgeson
AVIATION...Bunkers
FIRE WEATHER...Helgeson



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