Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212103

Area Forecast Discussion
National Weather Service Riverton WY
303 PM MDT Wed Jun 21 2017

.SHORT TERM...(This evening through Thursday)
Issued at 216 PM MDT Wed Jun 21 2017

Thunderstorms have developed along the Utah/CO/WY state line and
are affecting Sweetwater County south of interstate 80. We expect
isold activity to form along the northern tier of Wyoming later
this afternoon and evening ahead of Thursdays shortwave with the
asct cold front already having pushed across Northeast Wyoming
where it is only 75 degrees in BYG. The best chance of a wetting
rain will be in the northwest through this evening. In fact, today
appears to be the day with the most overall instability through
Saturday of this week. Any cell that forms through this evening
has the potential to produce strong gusty winds, especially in
Sweetwater County where inverted V soundings are more pronounced.
In addition, 700mb 30 knot winds will superimpose on any of these
wind gusts asct with these cells as the pressure gradient
increases ahead of Thursdays shortwave. In fact, synoptic scale
winds have increased in and of themselves in places like KRIW
where a gust of 42 mph has occurred this afternoon. The
aforementioned shortwave will swing across the area Thursday as it
pinwheels around a prominent circulation over Canada. Cooler air
will continue to filter in behind the cold front Thursday morning
making for a cooler day Thursday with high temperatures around 10
degrees cooler east of the divide than today. Most of the sharply
cooler air will filter in behind the front east of the divide at
the low and mid levels. By Friday morning h7 temps will be down to
-1.5C in far Northern WY east of the divide but still 11.5c in
 Southwest WY. Then another smaller closed low will rotate around
 the main Canadian low and send another vort lobe and asct
 reinforcing back door cold front across areas east of the divide
 Friday night and Saturday with h7 temps falling all the way down
 to -3C in Johnson County by Saturday which would translate into
 any showers that develop with the next front falling as snow all
 the way down to 7000 feet in some local areas in the northern Big
 Horn Mtns. Snow levels at 9k are expected across the rest of the
 mtns east of the divide, granted precip coverage will be
 relatively sparse. In contrast, h7 temps Saturday in SW WY will
 be 9.5C where little overall fluctuation in temperature is
 expected. Highs east of the divide Saturday will be 20 to 25
 degrees cooler in portions of central Wyoming like Casper and
 Riverton compared with the low 90s currently being experienced
 today. Even though moisture will increase somewhat at the
 500-700mb level through Saturday, the instability will decrease
 with each fropa now through Saturday so only isolated thunder is
 expected with any showers that develop Thursday through
 Saturday. The strong thermal ribbon along the divide Saturday
 will likely act as a focus for convection.

.LONG TERM...(Thursday night through Wednesday)
Issued at 216 PM MDT Wed Jun 21 2017

The period will begin with northwest flow aloft and the forecast
area still under the influence of cyclonic flow associated with a
trough over the Upper Midwest. There could be some lingering
showers and thunderstorms Saturday night across the northeast
zones, before the ridge builds to our west and the trough in the
Midwest slides east. This will help to keep conditions dry for
Sunday and Sunday night. Warmer air will also begin to push into
the forecast area from the Great Basin, so temperatures will climb
several degrees from the cooler temperatures of Saturday. 12Z/Wed
run of the ECMWF has come more in line with the 12Z/Wed GFS run
with regard to this ridge being dirty and not that amplified.
Therefore, Monday afternoon and evening could see convection
across northwest Wyoming, while most areas remain dry. 700mb
temperatures will rebound to between 12C and 16C Monday, so expect
more seasonal surface temperatures. Model solutions offered for
Tuesday and Wednesday depict more troughing across the
Intermountain West and Northern Rockies. A series of shortwaves in
zonal flow aloft will flatten the ridge and periodically traverse
the forecast area. This will produce a chance of showers and
thunderstorms across mainly the northern- third of the state both
Tuesday and Wednesday. After another warm day on Tuesday,
temperatures will begin to back off Wednesday, with potentially
even cooler air arriving Thursday.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 216 PM MDT Wed Jun 21 2017

VFR will prevail at all terminals through 00Z/Fri. The far south,
including the area near KRKS, remains the most likely section of
the forecast area for afternoon and evening convection to
continue. There could still be isolated activity near the higher
terrain. Dry sub-cloud layer with temperature-dew point spreads
again nearing 50 degrees will lead to outflow wind gusts to 45kts.
Convection will mainly be diurnally driven with any lingering
showers and storms ending by 04Z/Thu. Cold front will bring north
wind of 10-12kts into the Big Horn Basin late this evening and
across central sections east of the Continental Divide between
06Z-09Z/Thu. This wind will increase to 14-24kts Thursday morning
at KCOD and during the afternoon at other terminals east of the


Issued AT 216 PM MDT Wed Jun 21 2017

A cold front will advance toward the area during the rest of today
and cross the state tonight and tomorrow morning. As a result, a
gusty wind will continue across many areas during the rest of this
afternoon and evening. Most areas should remain rain free with
any showers or thunderstorms confined to the northern mountains
and portions of Sweetwater County. Strong and erratic winds will
be the main threat from any storm. Temperatures will remain warm
to hot for the rest of this afternoon with relative humidity in
the teens in many lower elevations East of the Divide and in
Sweetwater County. Mixing and smoke dispersal will range from
good in the northern basins to very good to excellent in the west
and south. A cooling trend will take place after today through




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