Area Forecast Discussion
Issued by NWS Riverton, WY

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325
FXUS65 KRIW 271746
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1146 AM MDT TUE SEP 27 2016

.SHORT TERM...Today through Thursday night

Imagery shows broad upper ridge over a cut-off low (Baja region)
across the western CONUS. Moisture is located over the southern
tier of states with dry air across most of the north. Surface has
modest high pressure over WY, most of the intermountain west and
spilling out into the Plains. No precipitation over Wyoming or
nearby.

Today through Wednesday: Ridging aloft and high pressure at the
surface continues over WY with modest warming but little else of
significant importance regarding sensible weather except for a few
small areas of morning patchy fog this morning.

Wednesday night through Thursday night: Increasing small chances for
precipitation will begin Wednesday night as a large but slowly
approaching trough from the EPAC finally draws the aforementioned
cut-off low towards the FA. The cut low will then open, weaken and
become absorbed into the leading southwest flow ahead of the main
trof. However, modest sub-tropical moisture associated with the cut-
low will spread into the western FA with lift provided from the
terrain primarily setting off a few showers overnight across
southwestern WY. By Thursday morning the remaining moisture and
energy from what was the cut-off low spreads across the region
mainly west of the Divide. This will increase potential
shower/thunder activity through the day...especially across the
western mountains. A few stronger storms could embed within more
shower activity across the west Thursday as ML CAPE rises with
additional warmth and moisture above 500 j/kg and deep shear
increases to a marginal 15 -25 kts. These storms would generally be
able to produce small hail and gusty winds, but remain well below
severe parameters for the most part. Shower/storm activity will
dissipate into the evening and overnight period. Little impact
expected from any high elevation snow showers as freezing/snow
levels will remain quite high.

.LONG TERM...Friday Through Tuesday

Medium range period will be covered by a strengthening longwave
trough over the west that becomes negatively tilted early next week.
The models continue to struggle with this change but most are
trending stronger/further south again (at least the GFS and GEM)
with our system early next week (the Euro has trended north again
but the Ensembles are still showing a longwave trough over the
west). There are a lot of moving parts with this development as
seen tonight while analyzing the hemispheric maps. It looks the
current system moving into the nwrn part of the Gulf of Alaska
will slowly move/dig se into the PacNW Friday into Saturday. At
the same time, a system near the north end of Kamchatka moves east
and eventually dives swd into this developing mean trough. The
initial trough is kicked out and may impact at least our nwrn
zones around Saturday night. The new trough reloads not only with
this Kamchatka energy but possibly with another system that
travels ewd across the Pacific all the way of the Asian continent
and helps to strengthen our Western U.S. trough early next week.
Hard to tell how all these pieces will come together attm but with
the mean trough likely setting up over the west well into next
week, the odds favor at least the potential of a significant
system impacting our area early next week. New GFS has trended
stronger and further south tonight as expected. It`s still not
closed off but has a fairly significant trough swinging/digging
across the state late Monday night into Tuesday with some fairly
decent cold air (around -25c at h5 and -5c at h7). The GEM is
slower evolving and it doesn`t get interesting until later Tuesday
and into the middle of next week (and quite interesting with a
fairly strong surface high dropping south out of Canada and
dynamics overrunning the upslope frontal boundary). Will generally
have chance mtn pops Mon/Tue with slight chances east but these
will likely need to be increased with time and as the timing and
details become clearer.

&&

.AVIATION.../18Z ISSUANCE/

Widespread VFR conditions expected across western and central
Wyoming through the TAF period. Light wind most expected, though
KCPR may have slightly stronger wind through 21z. A few mountain-
related clouds may form this afternoon, but skies are generally
anticipated to be clear.

&&

.FIRE WEATHER...

Fire danger low all areas today...lasting through the rest of work
week. Upper level ridging and surface high pressure will combine to
provide very stable conditions over the forecast area for the next
several days...with no rainfall, mostly sunny/clear skies and
relatively light winds. The next chance for some increase in
winds,moisture and chances for isolated to scattered precipitation
will arrive late Wednesday night...with modest chances continuing
mainly west of the Divide through Friday.


&&

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

$$

SHORT TERM...Braun
LONG TERM...PS
AVIATION...Branham
FIRE WEATHER...Braun



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