FXUS65 KRIW 260843 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Riverton WY
243 AM MDT Wed Sep 26 2018

.SHORT TERM...(Today through Friday night)
Issued at 202 AM MDT Wed Sep 26 2018

After our one day reprieve from critical fire weather conditions,
red flag conditions will once again return as the gradient increases
as a result of the high within the rex block off the Pacific Coast
builds east and weak disturbance in the fast west northwest flow
coming out of Canada increase the pressure gradient. The fire
weather watch has been upgraded for fire zones across Southwest,
Central, and East Central Wyoming for today and early this
evening. A cold front will backdoor into Johnson County Thursday
resulting in high temps almost 20 degrees cooler there. This
front will have less of an affect on other areas east of the
divide Thursday but will be enough to end the critical fire
weather conditions east of the divide. In the meantime, Southwest
wyoming will continue to be caught in the riptide of dry westerly
flow drawing in more dry air to continue the critical fire weather
conditions out there. The red flag warning now in affect will
continue in the southwest zones through Thursday and later shifts
can extend these warnings if warranted.

Behind this cold front, snowfall will occur over the Big Horn
Mountains with mostly rain at the adjacent lower elevations of
Johnson County. Elevations above 8000 feet in the Big Horn Mtns
will pick up 3 to 6 inches of snowfall with isolated heavier
amounts at the higher elevations as h7 temps fall to -4C. Snow
will fall at and above 6500 feet. Lesser amounts of precip will
fall east of the divide along and north of the Owl Creek Mountains
and east of the Casper Arch.

Will let the freeze warning for the lower elevations of Fremont,
Sweetwater, and Natrona Counties through 9 am this morning. The
temperature is already 33 degrees at KRIW as of 2:30 am.

.LONG TERM...(Saturday through Wednesday)
Issued at 202 AM MDT Wed Sep 26 2018

A weakening transitory ridge is expected to push into the region
Saturday ahead of an upper low pushing into N. California, and a
trough digging southeast across the Canadian Prairies. This ridge
will result in mainly above average temperatures and dry
conditions. A weak cold front over N. Wyoming on Saturday could
keep temperatures more seasonal in that area. The main concern on
Saturday will be elevated to to critical fire weather over the
central and southern areas with an emphasis over the southwest.

The upper low over N. California Saturday is expected to open up and
push across the northern Rockies in the Sunday/Monday time frame
with the ECMWF about 12 hours faster than the GFS. A cold front
associated with the northern stream trough in Canada is expected to
push south, east of the Divide Saturday night and Sunday. The best
chance of precipitation looks to be across the north, but doesn't
look to be that significant or widespread at the moment. Snow levels
across the north look to lower to around 9 kft. Elevated to critical
fire weather looks to continue over the southwest on Sunday.

On Tuesday and Wednesday, models differ on the evolution of Tropical
Storm Rosa, and another trough/upper low approaching the West Coast
on Monday. The 00Z GFS swings the relatively low-amplitude trough
into the Great Basin Monday night, and across the central/southern
Rockies Tuesday. On the other hand, the ECWMF and Canadian models
show the system across the eastern Pacific becoming a cut-off upper
low off the California Coast Tuesday/Wednesday. The ECMWF solution
draws up moisture from Rosa northward across the Desert Southwest
Monday spreading into the Rockies late Monday night/Tuesday. Both
the ECMWF and GFS solutions give the area a chance of precipitation
Tuesday, but for different reasons. Also, both the GFS and ECMWF
show those features on Tuesday pulling away from the area Tuesday
night, but unsettled weather could continue into the day Wednesday.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 202 AM MDT Wed Sep 26 2018

West of the Continental Divide...(KJAC, KRKS, KBPI, KPNA)

VFR conditions will prevail through the period. Local MVFR
conditions due to reduced visibility in smoke may occur along
southeast of the fires near Bondurant. This may include KBPI and
KPNA airports at times today and tonight. A gusty west to northwest
wind of 15 to 25 knots will develop after 18Z in most locations with
local gusts to 30 knots. The wind will decrease around 02Z in most

East of the Continental Divide...(KCPR, KCOD, KRIW, KLND, KWRL)

VFR conditions will occur through the forecast period. Areas of west
wind of 15 to 25 knots will occur in east central WY this afternoon
and diminish around 01Z. This would include the KCPR airport.


Issued AT 202 AM MDT Wed Sep 26 2018

Critical RH values will continue for many areas west of the
Divide this week. Gusty westerly winds with gusts of 30 to 35 mph
return today. These winds combined with low RH will mainly affect
western and southern portions, but will also affect Natrona and
the higher terrain of Southern Fremont Counties today. Thus,
have issued red flag warnings to cover these areas today and
continued the red flag warnings for areas west of the divide
through Thursday. Critical fire weather conditions will likely
continue through the rest of the week over Southwest Wyoming.
Cooler and more moist weather will return to northern zones late
in the week.


Red Flag Warning from noon today to 8 PM MDT Thursday for

Freeze Warning until 9 AM MDT this morning for WYZ017-018-020-

Red Flag Warning from noon today to 8 PM MDT this evening for



LONG TERM...Murrell
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