Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 222026

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
226 PM MDT Fri Sep 22 2017

.DISCUSSION...(This Evening Through Friday)
Issued at 225 PM MDT Fri Sep 22 2017

The latest satellite images had a large upper low spinning over
the western CONUS with deep southwest flow over the CWA. A surface
cold front had pushed completely through the CWA with gusty
northwest winds and cooler air in its wake. Low clouds persisted
across much of the northern/eastern areas, and a few showers were
starting to develop over eastern WY.

Models are in very good agreement on the overall pattern through
Monday as the large upper trough over the western CONUS slowly moves
eastward and ejects several short-wave troughs across the northern
plains. The three main waves will track across the CWA the next
three nights, the weakest of which will be tonight, with the
strongest on Saturday night. The upper low should be to Wyoming on
Monday as it takes on a positive tilt. The result will be periods of
showers tonight through Monday with much cooler weather (highs 10-20
degrees below average).

Warm-air advection and positive theta-E advection from 850-700mb
will accompany each wave, with the most pronounced advection
Saturday afternoon and evening. This also is when the right-rear
quadrant of the upper-level jet streak will move across western SD,
showing a decent divergence signature. On Sunday night the jet will
be a little farther east with the best upper-level divergence over
south-central SD. Nevertheless, modest Q-G forcing is progged during
the period with attendant 700-mb frontogenesis (although the better
frontogenesis, at 850 mb, will be southeast of the CWA). The models
also point to two main swaths of heavier precipitation -- one over
the WY/SD border and another over south-central SD -- which is
consistent with the positions of the upper-level jet and midlevel
frontogenesis. Thus, would expect widespread 0.25-0.50 inches of
rain with a band of 1 inch or so somewhere across far western SD and
another with 1-3 inches across south-central SD.

Although surface-based CAPE will be zero, modest steep midlevel
lapse rates will give rise to some elevated CAPE of 100-500 J/kg
that will be supportive of thunderstorms over southern parts of the
CWA tonight and Saturday. Regarding snowfall, models are still
fairly warm, with the SREF, NAM, and GFS not indicating any snow
over the Black Hills until Sunday night/Monday morning. Forecast
soundings also support this, but still would not be surprised to see
some snow Saturday night/Sunday morning if heavier precipitation
sets up over the Black Hills. Widespread low clouds, and possibly
fog and drizzle, will prevail in-between the waves, especially on

The upper low will move across the area Monday with lingering
showers, with possibly another short-wave trough in northwest flow
moving across the area Tuesday. Although some ridging should return
to the area for the last half of next week, confidence is somewhat
low on how much and fast of a warm-up will occur given the potential
for short-wave troughs to move south out of Canada on the backside
of the departing upper low. However, the operational GFS appears to
be an outlier relative to the ensembles by keeping heights too low
across the northern plains on Thursday.


.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 1126 AM MDT Fri Sep 22 2017

Areas of MVFR conditions can be expected this afternoon from
northeast WY to northwest SD, with VFR conditions generally to the
south and southeast of the Black Hills. Scattered showers and a
few thunderstorms will begin to develop over eastern WY, southwest
SD, and western NE by late afternoon, then track northeast across
most of the forecast area tonight. MVFR to IFR conditions will
develop across much of the area tonight, with at least local LIFR
conditions expected by Saturday morning. Scattered to numerous
showers will continue across the area on Saturday, with MVFR to
IFR conditions persisting.




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