Area Forecast Discussion
Issued by NWS Riverton, WY

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408
FXUS65 KRIW 220506
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1106 PM MDT Tue Mar 21 2017

.SHORT TERM...(This evening through Friday) Issued at 305 PM MDT
Tue Mar 21 2017

Frontal boundary made a nice push to the divide overnight as
expected with low clouds, areas of fog and cooler temps behind it.
The srn boundary will likely mix enough for a late day temperature
surge but the north will remain quite cool. Shortwave in the flow
over central UT will move across our ern zones into early this
evening with a band of showers and isold tstms, most numerous when
they interact with our frontal boundary in Natrona County. Daytime
heating will get showers going in the far west and then another
shortwave will moving up from the sw will help organize those later
this evening for another round of light mountain snow and mainly
rain showers on the valley floor (with some mix possible). All eyes
then turn towards our ern Pacific system that moves on the West
Coast Wednesday and then tracks across the Desert SW and srn Great
Basin Wednesday night and Thursday. Diffluent/divergent flow ahead
of it Wednesday night along with two active jet streaks will aid in
significant upper divergence in the west/sw moving into cntrl Wyo
overnight and continuing Thursday to produce a significant swath of
showers/isold tstms going which lift ne overnight. Even though the
main upper low is moving towards the 4-corners most of the guidance
is showing a nrn extension of this low into swrn/cntrl Wyoming
during the day Thursday. GFS is more in the Upper Green with its
H5/H7 circulation while the NAM has the swrn center and a weak ern
Fremont circulation. End result is an increasing threat of a
significant pcpn event for parts of Central and Southwest Wyoming.
It`s also starting to look like the rain that develops across the
Lander Foothills and at least higher spots in the Wind River Basin
(if not all) will change over to snow after sunrise or so Thursday
morning and continue at least through midday or may all day. Further
east in Natrona County, they will be warmer and maybe dry slotted
initially until the low shifts east Thursday night. Have lowered
temps to get some snow accumulation from the Upper Green to the
Lander Foothills and Wind River Basin. Later shifts will have to
watch this closely for highlights. As the low strengthens and the
energy translates sewd into sern CO, our system will wind down from
nw to se later Thursday night. Friday will be dry with rebounding
temps.


.LONG TERM...(Friday night through Tuesday) Issued at 305 PM MDT
Tue Mar 21 2017

The transitory ridge behind the exiting low will be leaving the
area Friday night. A moist southwest flow will usher in the
onslaught of another round of precipitation for the far west ahead
of the next Pacific trough late Friday night which is now out at
165W. On Saturday, significant valley rain and mountain snow will
move into Western Mountains. Some areas out west could potentially
approach a half inch of precip with this trough with the western
mtns potentially seeing a fair amount of snowfall accumulation.
This trough will split just like its predecessor this coming
Thursday, with the blocking mean ridge still over the Great
Plains. Again, the southern stream energy will focus way down in
NM and Arizona Sunday morning. Then A ridge will approach the area
Sunday for a respite in the precip out west. Then on Monday, yet
another trough approaches Wyoming delivering more precip to the
west Monday and Tuesday. This system will also split, with the
southern stream energy over the Four Corners Region acdg to the
GFS, but this one will likely close off sending some precip back
around the low center to some of our zones east of the divide. The
Canadian model is the slowest in handling these two troughs with
the Euro much faster and less amplified with the GFS the best
compromise. The Euro never closes off the second trough, while the
Canadian shunts the southern stream energy way south to the
Mexican border. The timing and placement of these features were
primarily guided by the GFS. No big swings in temperatures are
anticipated. With the split flow in place, not alot of wind is
expected during the extended period.

&&

.AVIATION...(For the 06Z TAFS)
Issued at 1106 PM MDT Tue Mar 21 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Scattered showers will occur over the west through 12Z. Local MVFR
conditions may occur in NW WY. Otherwise, VFR conditions to prevail
through 06Z Thursday. Scattered showers will reform over southwest
WY Wednesday afternoon into the evening. An isolated thunderstorm
cannot be ruled out. Snow will fall in the higher mountains with
some mountain obscuration. Please see terminal forecasts for more
detailed information.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail over the region through the period.
Scattered showers and isolated storms will develop over southern
and central WY Wednesday afternoon into the evening. Then showers
will become more numerous over this area by 06Z Thursday.

&&

.FIRE WEATHER...
Issued AT 113 PM MDT Tue Mar 21 2017

Fire danger will be low through the weekend. The exception to this
rule will occur on Wednesday which now shows signs of having
elevated fire danger across portions of central WY as gusty
southwest winds combine with lowering relative humidities and a few
chances for gusty isolated thunderstorms in the afternoon/evening.
Thursday also shows signs for embedded thunderstorms, however the
day on the whole will have increasing moisture spreading across WY
along with increasing chances for precipitation...both liquid and
frozen.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Flood Watch through late Wednesday night for WYZ013-023-025>027.

&&

$$

SHORT TERM...Skrbac
LONG TERM...Lipson
AVIATION...AR
FIRE WEATHER...Skrbac



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