Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212011

Area Forecast Discussion
National Weather Service Riverton WY
211 PM MDT Sun May 21 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 200 PM MDT Sun May 21 2017

Two weather events appear to be overlapping at the present time.
The older showers from earlier this morning are moving northwest
to southeast with this morning`s shortwave and are overriding the
newer thunderstorm/convective cells tracking more nnw to sse
associated with the difluent flow out ahead of the next northern
stream shortwave. The trend will be for the older shower activity
to be replaced by more of these fresh convective cells for the
rest of the afternoon and early evening as instability continues
to increase. The actual southbound northern stream shortwave axis
pinwheeling around the prominent circulation over western Lake
Superior will be pushing south through Buffalo around 18Z Monday
and then clearing the CWA by 00Z Tuesday. The associated cold
front will be pushing through northern Wyoming around 15Z and
then Lander to Riverton to Casper around 18Z and then Rock Springs
around 21Z. The strongest frontal and post frontal winds will
likely occur over northern Johnson County Monday as this is a
favorable pattern for this area to blow post-frontal being
closest to the main cyclonic flow. Winds in this area should be
just below high wind criteria with gusts reaching 50 mph. The
second windiest area will be over the northern Big Horn Basin. An
extensive area of rain, along with snow above 6500 to 7000ft
over the Bighorn Mountains, will arrive into northeast Wyoming
Monday morning. This area of precip will slide south as a
northwest- southeast oriented band of precip which will be
covering an area from northwest Wyoming including Cody, down
through central Wyoming including Riverton and Lander, and then
southeast into Casper. This band of precip should keep the airmass
stable enough through the best daylight heating hours to keep the
conditions relatively stable so will keep thunder out of this
area for Monday. However, areas to the south and west of this area
as well as northeast Wyoming could still see showers along with
isolated thunderstorms producing small hail (given the low wet-
bulb zero values), with areas like Buffalo having enough time to
reheat and destabilize behind the front under the -4.5C h7 temps.
Snow levels across most of the mountain ranges Monday should hover
around 10k in the Wind River Mtns and 7k in the Bighorn and
Absaroka mtns by midday. Snow amounts should range from 2 to 4
inches in the Bighorn Mtns Monday to one to two inches over the
higher Northern Absaroka Mtns. The combination of cool temps and
strong north winds should give the Bighorns some unseasonably cold
wind chills in the upper teens with this wx event, not to mention
a chilly driving rain in Buffalo. The Upper Green River Basin,
Sweetwater County, and the area in the vcnty of the mouth of the
Wind River Canyon should also witness blustery north winds but to
a lesser to degree.

Tuesday will be an interim day with a north wind still around
keeping temps on the cool side but milder than Monday as we will
still be partially within the grips of the large trough in the
center of the country. The airmass will be rather stable Tuesday.
Then by Wednesday we will see a dramatic warm up in temperatures
as a ridge struggles to hold together and flatten as it heads our
way with the mean ridge still along or just off the Pacific Coast.
However, H7 temps will dramatically warm up to 11C. We will see
highs in the upper 70s to low 80s at the lower elevations east of
the divide and the low to mid 70s west. With the ridge pinching
off ahead of the next shortwave, a westerly difluent flow will
quickly become established aiding in lift to get some isolated
thunderstorms to occur mainly in the far north on Wednesday.

.LONG TERM...(Wednesday night through Sunday)
Issued at 200 PM MDT Sun May 21 2017

Overall pattern evolution appears to be handled well by medium-range
forecast models with any substantial differences not showing up
until next Sunday. Our Monday shortwave will eventually help to
carve a deeper trough over the Midwest while the next upstream upper
low begins a slow eastward trek through southern Alberta and
Saskatchewan. This upper low will drag a cold front through the
forecast area from northwest-to-southeast Wednesday night. Light
showers will spread accordingly across the forecast area. There
could be some assistance from the right-entrance region of a 90kt
jet which could allow for an increase in coverage, mainly over the
north late Wednesday night. Vorticity axis lingers across the north
and central Thursday with the best potential for more widespread
precipitation over the north. Models agree the upper low will
eventually set-up over Saskatchewan with a series of shortwaves
rippling through west-to-southwest flow aloft. Obviously, timing of
specific waves will be difficult at this range, but the overall
trend means an unsettled and cooler-than-normal pattern for the
latter periods of the forecast. Temperatures are anticipated to be
5F to 10F, to perhaps as much as 15F, below normal by late in the
weekend. This means that our much above normal snowpack won`t be
going anywhere anytime soon. Coolest air is likely to be over
northeast Wyoming, including Johnson County, where the passage of
weak cold fronts could reinforce the cooler air through the weekend.
The upper low does finally move east over the weekend, but the
general troughiness will remain. ECMWF solution hangs back a piece
of energy across western Wyoming on Sunday, whereas the GFS
transitions to northwest flow aloft. The EC solution would bring
0C or cooler 700mb temperatures through most of the forecast area
Sunday, while the GFS keeps that air over northeast Wyoming. The
ECMWF results in cooler highs Sunday, but both patterns would keep
Wyoming susceptible to continued shower activity. Therefore,
early indications are that the Memorial Day Holiday Weekend will
be wetter and cooler-than-normal.


.AVIATION...(For the 00Z TAFS through 00Z Monday)
Issued at 200 PM MDT Sun May 21 2017

West of the Continental Divide...KBPI/KJAC/KPNA/KRKS Terminals

Terminals along and west of the Continental Divide will be VFR
through 00Z/Tue. Ongoing convection is expected to decrease rapidly
this evening as an elongated area of instability sags south and
clears southwest Wyoming. Clearing conditions are expected
overnight before another round of clouds pushes south between 15Z-
18Z/Mon ahead of a more vigorous shortwave. This shortwave will
track north-to-south through eastern Wyoming Monday afternoon. A
west-northwest wind will begin to respond late Monday morning as
the pressure gradient tightens. Wind speeds of 10-15kts will
quickly increase to 15-25kts between 17Z-20Z/Mon. KJAC will be the
exception, with wind speeds remaining 10-15kts. Cold frontal
surge will push south and west into KRKS just prior to 00Z/Tue.
Therefore, have winds becoming more northeast and increasing to
20-30kts at KRKS before the end of the forecast period. Best
instability will be across central and northern Wyoming Monday
afternoon, and have only VCSH in these four terminals. Occasional
mountain top obscurations will be noted through 01Z/Mon and again
after about 20Z/Mon.

East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

Rain showers and a thunderstorm or two continue to track
southeast ahead of a secondary shortwave moving south across the
forecast area. This shortwave will clear most terminals around
01Z/Mon with the showers rapidly decreasing along with the cloud
cover. Conditions will be VFR through the forecast period. Vortex
over Saskatchewan will continue its southward track toward Wyoming
tonight with mid- cloud decks increasing from north-to-south
Monday morning. There is the potential for localized MVFR late
Monday morning and early Monday afternoon as this vortex crosses
eastern Wyoming. Best chance for prevailing light rain will be at
terminals with nearby terrain, namely KCOD around midday and KCPR
early Monday afternoon. Would not be surprised if KLND sees a
brief period of sprinkles and lowered cloud decks early Monday
afternoon. Perhaps more significant will be the increasing wind
in association with this disturbance and its attendant cold front.
The pressure gradient will increase prior to 18Z/Mon with the
cold front set to pass through KCOD, KWRL, and KCPR between 15Z-
18Z/Mon. The front will backdoor into KRIW and KLND between 18Z-
20Z/Mon. North or northwest surface wind of 15-25kts will be
common following frontal passage, with gusts 30-45kts at non-
terminal sites like KBYG and KGEY. Mountain top obscurations will
be frequent along the east slopes of the Wind River Range and
over the higher terrain of northern Wyoming through 01Z/Mon.
Additional obscurations will return across the north between 12Z-
18Z/Monday and remain through the afternoon.


Issued AT 200 PM MDT Sun May 21 2017

Unsettled weather will continue for the next couple of days. An
upper level disturbance will drop south across the area and bring
another chance of isolated showers and thunderstorms, with the
best coverage in southern and eastern areas. Relative humidity
will again remain above critical levels with cool temperatures
continuing. A gusty wind will develop across Johnson County
Monday. Mixing and smoke dispersal will be generally fair to good.
A cold front will bring another chance of showers for Monday with
along with a chance of snow in the mountains before dry weather
returns for Tuesday. A dramatic warm up will occur Wednesday.




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