Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 212032
AFDUNR

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

.DISCUSSION...(This Evening Through Monday)
Issued at 214 PM MDT Mon May 21 2018

Latest analysis shows an upper ridge building over the forecast
area. Southwesterly flow aloft exists over the western part of the
forecast area as an upper low remains nearly stationary over
srn California. At the surface, high pressure is in place over the
eastern two-thirds of the CONUS and low pressure over the western
third with the surface pressure gradient resulting in
southeasterly winds over much of the forecast area this afternoon.

Rising heights and implied subsidence may work to suppress
convection today over much of the forecast area today. However
with relatively weak MLCAPE cape values may see some showers and
thunderstorms move into northeastern Wyoming and possibly the
Blkhls and swrn SD during the rest of this afternoon and this
evening.

On Tuesday and Wednesday, the upper low currently over srn
CA will move from southern NV to eastern MT by 12z Thursday.
The upper ridge over the forecast area will shift east on Tuesday,
increasing the southwesterly flow aloft. A couple of upper shortwaves
will cross the region with the first one later Tuesday and
another one Wednesday. A surface low will develop over eastern
WY, enhancing low level southeasterly flow. At least 1-2KJ/kg
MLCAPE is expected to develop on 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 insolation and
diurnal heating. Sufficient instability/shear exist 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 214 PM MDT Mon May 21 2018

Isolated thunderstorms will be possible over northeastern WY and
far southwestern 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...10
AVIATION...Johnson



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