Area Forecast Discussion
Issued by NWS Rapid City, SD

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567
FXUS63 KUNR 211756
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1156 AM MDT Mon May 21 2018

.DISCUSSION...(Today Through Sunday)
Issued at 155 AM MDT Mon May 21 2018

07z surface analysis had high over the Midwest with southeast
return flow over the CWA. Water vapour had upper low over south
central SD with the main effect some high clouds over western SD
and lower clouds/-SHRA over eastern SD. Upper trough over the
western CONUS spinning an upper low over CA. This feature will be
the main forecast concern.

Today/tonight, SD upper low moves east and dissipates in response to
upper low deepening over the west and subsequent upper level ridge
building over the Plains states. Rising heights and implied
subsidence will try to suppress attempts at convection today.
Expected weak MLCAPE values by this evening near lee trough could
spark isolated TSRA, but chances low given latest MLCIN projections
and CAMS. Temperatures warm nicely with sustained warm air advection.

Tuesday/Wednesday, upper low moves from southern NV to eastern MT
by 12z Thursday. Upper ridge over the CWA shifts east Tuesday
with southwest flow aloft. A couple of decent shortwaves will move
through the CWA with the first one later Tuesday and another one
Wednesday. Surface low will develop over eastern WY, enhancing low
level southeasterly flow. At least 1-2KJ/kg MLCAPE develops
Tuesday afternoon with 10-20m/s 0-6km bulk shear. Isolated severe
storms will occur later Tuesday afternoon/evening and then likely
evolve into some sort of MCS as 30-40kt low level jet fuels
activity east of the Black Hills Tuesday night. Jumbled atmosphere
Wednesday morning should recover decently by Wednesday afternoon
with upper low/surface low passing mainly to our west allowing
increased solar insolation. Sufficient instability/shear for a
marginal risk of severe thunderstorms Wednesday afternoon/evening.
Temperatures mid-week will be 5-10F above normal.

Thursday through Sunday, upper ridge builds into the central CONUS
with a distinctly drier trend as well as continued above normal
temperatures. Can`t completely rule out TSRA during the period with
the best chance Sunday night into early next week as another upper
level trough approaches.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Tuesday)
Issued At 1155 AM MDT Mon May 21 2018

Isolated thunderstorms will be possible over northeast WY and far
southwest SD late this afternoon and evening, with brief IFR vsbys
possible. Otherwise, VFR conditions are expected through the TAF
period.

&&

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

$$

DISCUSSION...Helgeson
AVIATION...Johnson



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