Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231658

Area Forecast Discussion
National Weather Service Riverton WY
1058 AM MDT Sun Apr 23 2017

.SHORT TERM...Sunday through Tuesday night

All is quiet right now across western and central Wyoming. However,
we will be going into a more active pattern with a series of fronts,
troughs, lows and other things that make meteorologists crazy
crossing the area. What we don`t know, especially further out, are
the details of each one.

We are fairly sure about today though. The first in a series of
troughs and fronts will approach the west. However, this one does
not have a ton of moisture to work with. In addition, the models
show almost all initial upper level forcing going into Montana. So,
although there will be some rain and snow showers in the
northwestern mountains, they do not look heavy at this point. A bit
more energy may move into the south late today and touch off a few
thunderstorms. These do not look really strong though. There could
be some gusty winds or small hail, but this pretty much describes
almost every thunderstorm in Wyoming. Areas East of the Divide
should be dry and mild for a vast majority of the day, although
with more high clouds that yesterday. In addition, a gusty breeze
will develop across the southwestern wind corridor. But again,
nothing unusual for Wyoming. Both the NAM and GFS do show some
convection moving from Sweetwater to Natrona County so this looks
to have the best chance of thunderstorms late today. Most
convection should end by midnight with only a few leftover showers
for later tonight.

Another, more potent system will then move in for Monday. This will
also be a colder system with snow levels dropping down to around
7000 feet or so and possibly lower. Future shifts may want to think
about winter weather highlights for Monday, although the best
precipitation should hold off until Monday afternoon. Meanwhile,
there is some disagreement East of the Divide for Monday. The GFS
spreads showers across much of the area in the afternoon. Meanwhile.
the NAM is drier and keeps things largely dry until the evening
hours. My gut feeling is something is possible but again a majority
of the time should be rain free. With the cooler weather and lower
wet bulb zero levels, we will again have the chance of small hail
with any thunderstorm that pops up.

Tuesday at this points looks to be the 3C`s...cloudy, cool and
cruddy. The NAM especially shows this with a 700 millibar low
developing and causing some upslope flow that could bring a dreary
day with some rain and mountain snow. There is still uncertainty
where the heaviest QPF will occur however. So for now, we kept POPS
in the chance category for the most part until we can get better
agreement with guidance. Temperatures will average well below normal

.LONG TERM...Wednesday through Sunday
Issued at 300 AM MDT Sun Apr 23 2017

Wednesday through Friday will continue with the cool, cloudy and
crummy weather pattern. Periods of rain and mountain snow in the
west with scattered rain east of the divide Wed and Thurs. Now in
the 00Z runs of the GFS and ECMWF models, there exists the
potential of a snow event in central WY Thursday night and Friday.
If the models continue this trend, there will be ample cold air
aloft and a storm track that would favor a decent snow event in
central WY. Have not gone all in on this storm yet, as each night
the models seem to change the timing and track of the system. Have
increased the precip chance Thursday afternoon through Friday
morning and have lowered the high temps in the central zones to
reflect a snow scenario. Certainly there is time to watch this
potential event over the coming days. Saturday looks drier with
only scattered rain and mountain snow showers. Next Sunday looks
dry for most places for now with a few rain or snow showers in the
northern and northwest WY mountains. Temps will start to moderate
some Saturday and next Sunday once this weather pattern breaks
and more sunshine occurs.


Issued at 300 AM MDT Sun Apr 23 2017


Generally VFR to prevail west of the Divide through 12z Monday, with
increasing chances for MVFR in lower CIGs (and possibly reductions
in VIS) as an another upper level disturbance moves into/across the
forecast area. In the meantime, an upper level shortwave will also
move across the region later this afternoon and evening with
scattered mountain snow showers, isolated thunderstorms and
occasional mountain obscurations a possibility. Most of this
activity will begin after 21z... diminishing later this evening,
with lingering isolated to widely scattered rain and snow showers
still possible across far west Wyoming - vicinity KAFO-KJAC through
12z Monday.


VFR conditions will prevail through the forecast period at the
terminal sites. An upper level disturbance and associated cold front
will spread isolated to widely scattered showers and a few
thunderstorms across the area after 22z continuing through 06z
Monday. A more organized line of showers and thunderstorms is
expected to develop along a vicinity KRKS-KCPR line this evening
with possible brief MVFR/IFR conditions in heavier showers. This
activity should diminish after 06z Monday.


Issued at 300 AM MDT Sun Apr 23 2017

A front and upper level disturbance will approach today and bring
some rain and snow showers to the west today, mainly to the north.
However, wetting precipitation is not expected for most areas. In
areas East of the Divide, most of the day will be dry and warmer
with a gusty breeze developing from Rock Springs through Casper.
Isolated thunderstorms may develop in some locations late today and
this evening. Mixing and smoke dispersal will range from poor in the
northwest to excellent in Sweetwater County and Natrona County. At
this point, much of next week looks cool with a few chances for rain
and snow.





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