Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 191942

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
142 PM MDT Tue Sep 19 2017

.DISCUSSION...(This Evening Through Tuesday)
Issued at 141 PM MDT Tue Sep 19 2017

19z surface analysis had low near KPIR with pre-frontal trough
into central NE and cold front into east central WY. Lead
shortwave helping convection over northeast SD into eastern ND.
Water vapour had upper trough from southwest Canada into the
central Rockies with main shortwave energy over WY, which is on
the nose of a 150kt jet streak racing through southern ID.
2-3KJ/kg MLCAPE has developed over central SD ahead of pre-frontal
trough with Td/s in the mid-upper 60s. MLCIN appears to be
weakening per SPC mesoanalysis, but no agitated CU evident on
1937z visible satellite imagery. Forcing from WY shortwave evident
by increasing radar returns over WY into northwest SD. This trend
will continue through this evening with expanding areas of
-SHRA/TS. It`ll be a race between the pre-frontal trough and
synoptic lift as to whether strong instability and shear can be
realized over south central SD late this afternoon. If it does,
severe thunderstorms will erupt on eastern end of CWA. Otherwise,
organized severe thunderstorms will be east and northeast of CWA.
Rest of CWA will see -SHRA/TS quickly move across early this
evening with pronounced drying and subsidence overnight.
Temperatures will be near guidance.

Wednesday, subsidence and drying will prevail with breezy/windy
spots as surface high over the central plains combines with
developing lee trough to give modest west/southwest gradient.
Temperatures will be near guidance.

Thursday through Tuesday, southwest flow aloft will persist as upper
trough over western North America dominates. Fairly warm day
expected Thursday with thermal ridge redeveloping over the CWA ahead
of a cold front. This front will drop through Thursday night into
Friday, but then stall over the northern Plains. CWA will be
under/near the right entrance region of active jet stream across the
plains into central Canada. Periodic shortwaves will move through
bringing chances for showers, especially Saturday and Sunday. Could
be a fairly wet period, but confidence not particularly high given
expected sharp moisture gradient under jet.


.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1124 AM MDT Tue Sep 19 2017

Wind shift to the northwest is expected along the cold front which
is now crossing the western SD plains. Showers and isolated
thunderstorms are expected this afternoon and evening. MVFR
conditions are expected in heavier showers/storms. VFR conditions
expected otherwise.


Issued At 141 PM MDT Tue Sep 19 2017

For Wednesday, strong dry and subsidence is expected behind this
afternoon`s/evening`s cold front. Minimum RH`s will be at least as
low as 15-20% - perhaps lower depending on the extent of boundary
layer mixing. Gradient will be fairly tight with downslope southwest
flow resulting in gusty winds at times. Will continue Fire
Weather Watch given uncertainty of RH`s.


SD...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for SDZ262>264.

WY...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for WYZ259-297>299.



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