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Area Forecast Discussion
National Weather Service Riverton WY
1056 AM MDT FRI SEP 30 2016


Showers are lingering in the Big Horn Basin as the weak upper
level trough exits to the northeast. There will be just enough
lingering instability and moisture around behind the mid level
trough to follow to initiate some isold convection this afternoon
and early evening across the higher mountains. On Saturday, the
closed low swinging around the base of the prominent long wave
trough off the Pacific Northwest coast. The southwest flow out ahead
of this large weather feature will become increasingly difluent over
western Wyoming where a good chance of showers and thunderstorms are
expected. This closed low will eject a shortwave northeast across
the area enhancing lift. However, the instability will be marginal
so the convection should be relatively weak for the most part. Then
on Sunday, the upper low will track to western Oregon per ECMWF and
Northern California per GFS. In any event, moisture and instability
ahead of this system will continue to stream northeast to areas east
of the divide with showers and a few thunderstorms spreading into
areas east of the divide. Temperatures will begin a downward trend
Sunday as cooler air begins to filter in behind Saturdays trough.  
One side note, added fog to the far western valleys for early this


Strong upper low is still forecast to be moving towards the Nrn UT
area on Monday and then most likely track across Central Wyoming
Monday night into Tuesday. This favorable track will likely result
in significant lower elevation rain and higher elevation snow. The
new Euro lifts the low further north than the GFS/GEM so we'll have
to watch that closely but leaning towards the slightly further south
solutions still. On Monday, we should see some surface cyclogenesis
developing over sern WY/nern CO with the cold front edging into the
west and north. Might see some stronger tstms along and north of the
front up north and in the west with increase with increasing qg and
jet streak forcing. Widespread pcpn in the west and spreading to
most of the north on Monday. As the upper low moves ewd...pcpn
should become more widespread in the central/ern sections with the
exact track key. On Monday night with a track over the south
half, the area to the north should see widespread rain/mtn snow
with more showery weather south. wraparound pcpn will prevail over
most areas east of the divide on Tuesday with snow above 6-7K
(roughly). Might see a quick swath of snow along the I80 corridor
also later Monday night and Tuesday. Upper low exits the region
later Tuesday night into Wednesday with some wraparound in the
east continuing. Another quick hitting shortwave may race through
the cool, cyclonic nw flow around Wednesday night before shortwave
riding moves in on Thursday/Thursday night. Another shortwave may
approach at the end of the period with a chance of showers and
thunderstorms in the west.



VFR conditions to prevail through the forecast period...except for
KJAC early this period with BR/LIFR/IFR CIGS slowly ending. Isolated
showers or storms will then develop and move across portions of the
area west of the Divide this afternoon and continue through early
evening. Occasional/partial mountain obscuration may occur at times.
After 07Z Saturday and through 15Z Saturday, VCFG may be found near
a few terminals mainly west of the Divide...especially near KJAC
which could get to IFR conditions once again by 10Z Saturday. 



South southwest flow moving up from the great basin will bring
increased moisture and mild air over western and central Wyoming.
This will result in increased shower and thunderstorm activity
to the area through Friday. The best chance for precipitation
will be in the mountains. Relative humidity will be elevated
through the weekend with some cooling. More widespread
thunderstorm activity can be expected across western and central
Wyoming by sunday. Expect cool showery weather through early next
week with descending snow levels.


.RIW Watches/Warnings/Advisories...None. 




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