Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 172310
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
510 PM MDT Fri Mar 17 2017

.SHORT TERM...(This Evening Through Saturday Night)
Issued at 225 PM MDT Fri Mar 17 2017

20z surface analysis had low just north of MN and high over MT
with modest pressure gradient over the CWA resulting in breezy
northwest winds on the SD plains. Lots of cirrus-filtered skies
this afternoon.

Tonight, surface high slides through CWA as upper ridge builds into
the northern plains. Return flow behind high will be pronounced
overnight with 2-4mb/3hr pressure falls in the west as well as
robust warm air advection from 850-700mb. Should be recipe for
gusty southerly winds over portions of northeast WY. Lows will be
near guidance, but will likely be steady or slowly rising late over
the higher terrain.

Saturday, upper and thermal ridge peak over the CWA. Temperatures
will reach well into the 70s in the west and 60s from K2WX to KICR
due to easterly component to low level winds. Breezy spots expected
given gradient and deep boundary layer mixing potential. Rather warm
temperatures expected Saturday night with only partial decoupling
over the higher terrain. Temperatures will be near guidance for
highs and generally above guidance for lows.

.LONG TERM...(Sunday Through Friday)
Issued at 225 PM MDT Fri Mar 17 2017

Sunday, upper ridge breaks down as shortwave trough zips through
the Rockies into south central Canada. It will push a cold front
through the CWA. Trends have shown front coming a bit faster each
run and thus, have confined warmest temperatures to the southern
third of the CWA. Weak instability along frontal boundary may be
enough for isolated afternoon convection with chances for mixed
RA/SN showers Sunday night. Gusty northwest winds expected behind
the front.

Monday, frontal boundary stalls across southern WY into NE. Upper
ridge builds back over the rockies with isentropic lift concentrated
just north of the frontal boundary under right entrance region of
departing 120kt jet streak across the Midwest. Models showed
increased PoPs with this system along with chance for accumulating
snow over the higher terrain. Guidance temperatures look ok.

Tuesday and Wednesday, upper ridge speeds into the Midwest leaving
southwest flow aloft over the CWA and the expected shortwave-fest
and low confidence PoPs. Temperatures will be seasonal Tuesday and 5-
10F above normal Wednesday.

Thursday, models hinting at split trough as next system moves
through suggesting good chances of precipitation, but no dynamic
system to pull a lot of cold air into the CWA for substantial wintry
weather.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 509 PM MDT Fri Mar 17 2017

VFR conditions will continue through the period. Gusty southerly
winds will develop over the northeast Wyoming overnight and
spread into western South Dakota Saturday.

&&

.FIRE WEATHER...
Issued At 225 PM MDT Fri Mar 17 2017

Warm and dry westerly flow will support critical to near critical
fire weather conditions Saturday afternoon over portions of
northeast WY. RH is expected to drop to around 15 percent. Winds
will easily blow at 15-25 MPH in the morning, but may tail off in
the afternoon as a lee trough approaches. If enough mixing occurs,
higher winds aloft will push wind speeds higher with likely lower
RHes. Fire Weather Watch will continue given uncertainty where
the strongest winds and lowest relative humidities will be.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...Fire Weather Watch Saturday afternoon for WYZ259-297.

&&

$$

SHORT TERM...Helgeson
LONG TERM...Helgeson
AVIATION...7
FIRE WEATHER...Helgeson



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