Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 210829
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
229 AM MDT Wed Sep 21 2016

.SHORT TERM...(Today Through Thursday Night)
Issued at 228 AM MDT Wed Sep 21 2016

08z surface analysis had meandering frontal boundaries across WY
into NE and from eastern MT into northern MN. Lift under right
entrance region of 110kt jet streak across south central Canada
supporting showers/storms across ND early this morning, mostly
north of the CWA. Expect that scenario to continue this morning.
East/northeast boundary layer flow increasing with T/Td spreads
closing -- may yet still see some patchy fog this morning on/near
the eastern slopes of the Black Hills. Looking west, water vapour
depicted the main feature of interest for the short term; a
deepening upper trough pushing into southwest Canada/northwest US.

Today through Thursday, upper trough deepens and closes off over
northern CA/OR tonight - moving into UT Thursday. Surface low
persists along meandering frontal boundary from WY into NE as 1025mb
surface high squeezes into south central Canada. Persistent easterly
component to boundary layer flow expected with increasing moisture
given theta-e advection and converging T/Td spreads. Expect
widespread stratus by tonight with areas drizzle and fog at times,
especially late tonight and Thursday morning. Upper forcing
increases ahead of upper low with southwest flow aloft. Shower and
embedded thunderstorm chances increase overnight and especially
Thursday night as first piece of energy rotates into the CWA ahead
of upper low. High pops warranted. Temperatures will be near
guidance today and below guidance Thursday.

.LONG TERM...(Friday Through Tuesday)
Issued at 228 AM MDT Wed Sep 21 2016

By Friday morning, upper low is progged to be over UT, with
ongoing precip across much of the CWA. Models are showing warm air
advection with gusty winds ahead of the 850mb low, resulting in
80s for highs across south central SD, with 60s farther north and
west. The boundary between warm and cool air will depend on where
the low sets up, which the models differ on, as MAV and MET
guidance have a 5-15 degree spread for highs. Shear and weak
instability may allow for some thunderstorms to develop across the
SD plains Friday afternoon/evening, possibly with hail and strong
gusts.

ECMWF and GFS show the upper low briefly becoming an open wave
before it closes off again north of the CWA. Chances for precip will
continue into Saturday, but gradually decrease as the area becomes
dry-slotted. Greatest chance for showers Saturday will be across
northeastern WY, the Black Hills, and northwestern SD as precip
wraps around the low. Behind the low, cooler air will be pushed into
the region, with below average temperatures expected into early next
week. However, as an upper ridge builds over the the western CONUS,
there will be a gradual warming trend, along with the return of dry
weather.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Wednesday Night)
Issued At 228 AM MDT Wed Sep 21 2016

Areas of MVFR to IFR CIGs/VSBY are expected to develop early this
morning with fog and low clouds, mainly around the Black Hills
area. Conditions will gradually improve late this morning, though
MVFR CIGs will likely persist through the day. Late tonight, fog
is expected to redevelop across western SD, possibly with drizzle.
IFR to LIFR conditions will be possible after midnight.

&&

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

$$

SHORT TERM...Helgeson
LONG TERM...Pojorlie
AVIATION...Pojorlie



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