Area Forecast Discussion
Issued by NWS Rapid City, SD

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532
FXUS63 KUNR 261551
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
951 AM MDT Fri May 26 2017

.UPDATE...
Issued at 947 AM MDT Fri May 26 2017

15z surface analysis had weak ridge over the CWA with low over ID,
which was under upper low/shortwave spinning over southern ID.
This wave will rotate east today bringing increasing pops this
afternoon into the evening from west to east. Latest RAP/HRRR
showing forcing/QPF further north than going model consensus. Will
have to watch satellite/radar trends, but will continue with
current forecast given surrounding office collaboration and rest
of model guidance. Southern 1/3 of CWA still best target for nice
QPF totals by Saturday morning.

&&

.DISCUSSION...(Today Through Thursday)
Issued at 208 AM MDT Fri May 26 2017

Mean troughing will continue over the region through the weekend
as a closed upper low spins over southern Canada. Associated split
vort max over the NW CONUS will eject toward the region today and
support increasing chances for showers late this afternoon (far
west) and esp tonight across the southern half. This system will
be fighting a lot of dry air and extent or precip/QPF amounts will
be limited, esp outside of the southern third. However, strong
forcing per a jet streak interaction and decent pos theta-e adv
will support a good chance rain southern areas and have trended
pops up there, mainly tonight. Weak convergence over the BH later
this afternoon could support a few high based shra, although dry
profiles will be highly prohibitive. The next system will rotate
around the main upper vortex and support afternoon diurnal shra
Sat over much of the fa. However, precip amounts will be fairly
light and isolated with any shra activity. Lingering slight
chances for shra remain for Sunday with cyclonic flow supporting
some weak diurnal instability over the western third. Cool conds
will continue through the weekend with 60s-70s on the plains, 50s
in the Black Hills.

Upper low will shift east Mon of next week as the western CONUS
ridge trends east. This will support a warming and drying trend
mid next week, although warmer temps and fair conds will be short-
lived as the next deep upper trough enters the Pac NW. This
trough will support breakdown of the ridge with perturbed flow
aloft spreading east into the Northern Plains late in the week.
Precip chances will hinge on whether deeper GOMEX moisture can
advect into the region. The ECMWF and GFS do indicate this
happening by next weekend, which would support better chances for
rain (as well as rainfall amounts) in addition to an uptick in svr
wx chances.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Saturday Morning)
Issued At 441 AM MDT Fri May 26 2017

VFR conditions are expected today. Late this afternoon
and evening, scattered showers and thunderstorms will move into
northeastern WY and spread across mainly the southern half of
western SD. MVFR/local IFR conditions will be possible in
showers/storms.


&&

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

$$

Update...Helgeson
DISCUSSION...JC
AVIATION...Pojorlie



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