Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181740
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1140 AM MDT Thu May 18 2017

.SHORT TERM...(Today through Saturday night)
Issued at 115 AM MDT Thu May 18 2017

...Variety of Advisories and Warning in Place Today...

Imagery shows developing very complex upper level pattern with
longwave trof across most of the CONUS and embedded strong lows over
the upper midwest and another more vigorous cold core low moving
through the intermountain west centered over nrn UT and headed
towards Colorado. SFC has developing strong cyclogenesis occurring
over sern CO with modest frontogenesis taking place over srn MT into
nw/nrn WY. Increasing low to mid-lvl moisture and precipitation now
rapping around the upper and SFC lows into/through sern/ern/nrn WY.

Today, strong lee side cyclogenesis takes place once again from the
High Plains of sern CO into the cntrl/srn Plains of KS/OK in
response to the development and approach of the upper cold core low.
Loads of severe wx will take place over portions of the cntrl and
srn Plains later today...while WY gets cold rain and snow. Dry
slotting experienced earlier today is beginning to end with low to
mid-lvl moisture pumping towards/into the FA from the south and east
around said low(s). Additionally, beginning later this morning and
continuing through Friday, H7 temps drop from the -1 to -3 range to
-4 to -8 east of the Divide...with colder temps across the the
ern/sern portion of WY closer to the upper low. Surface temps will
respond accordingly, dropping into the 30s this morning...rebounding
later in the day to the upper 30s and lower 40s for most of cntrl
and srn WY. Warmer up north- perhaps making it into the lower 50
across the Big Horn Basin.

With relatively large amounts of available moisture making its way
into the region east of the Divide and across srn WY, in addition to
the unseasonably cool temperatures behind a swd plunging cold front,
and being May with high sun angles, an extremely tricky snow forecast
comes into play...especially below 7000 feet. Previous warm weather,
cloud cover and the vigorous nature of moisture return back into the
FA will also play against the snow making process especially if/when
things saturate and if dew points remain a few degrees or more above
the freezing mark. If this were March it would be another story and
the state would likely be shut down. But it`s not. So, erring on the
side of caution, a variety of Advisories and Warnings are now in
place across the central and northern FA...some impact based more
than actual quantitative values. Even the "storm track" of the upper
low has been progressively drifting southward...which in effect has
taken away some of the moisture that looked originally available to
the Big Horns a day or two ago...so now somewhat of a lower end
warning than before...but still a warning nevertheless.

This wx maker finally exits Friday, only areas of lingering and
decreasing rain/snow showers will remain through Friday evening as.
the upper levels become n/nw oriented. An overall nice spring day
will be on tap for Saturday...albeit on the cool side but with
little wind. Precip chances will return to northwest WY late
Saturday afternoon and overnight as an upper level shortwave moves
through the flow and increases instability...with light showers a
possibility.

.LONG TERM...(Sunday through Thursday)
Issued at 115 AM MDT Thu May 18 2017

In wake of the major spring storm in the short-term, a north to
northwest upper flow is expected over the Northern Rockies for
much of the extended. Global models show one distinct shortwave
pushing southeast from northern Intermountain West across the
region Sunday, and then another one dropping south out of Canada
into the northern High Plains on Monday. The system on Monday will
be the stronger of the two with a decent cold front pushing south
across the region. There will be decent chances of convection
both Sunday and Monday with an emphasis across the east on Monday.
The 00Z ECMWF continues to be much faster in sweeping out the
moisture by Tuesday with dry conditions while the Canadian/GFS
show enough moisture and instability for some convection. For now
will continue to have slight chances over the east on Tuesday,
but will have the west dry for now.

Global models then show another shortwave trough of varying
intensities in the northwest flow possibly impacting the area in
the Wednesday/Thursday time frame. Will broad brush mainly slight
chance to some low chance mountain PoPs for now.

Temperatures in the extended will be seasonal to slightly below
average for mid-late May, but nothing as cold as what occurs in
the short-term.

&&

.AVIATION.../18Z ISSUANCE/
Issued at 1140 AM MDT Thu May 18 2017

Moisture continues to stream north and east around an upper low
situated over Colorado late this morning. Recent IR imagery showing
more significant downslope drying west of the Continental Divide,
while cooler cloud tops remain east of the Divide and across the
south. Overall, a messy day across the eastern terminals with
conditions IFR/LIFR with not much hope of MVFR at the central
terminals at KRKS until after 06Z/Friday. The one exception will be
KWRL where downsloping will allow for VFR or occasional MVFR
conditions. To the west, should see gradually improving conditions
this afternoon and evening. Occasional MVFR until mid-afternoon with
VFR anticipated after that time at KBPI, KPNA, and KJAC. North to
northeast wind 15-25kts will increase this afternoon from KRIW to
KCPR and south to KRKS. Expectation is for those winds to decrease
between 01Z-04Z/Friday. Mountain obscurations will be widespread
across the region today and this evening. There will be gradual
improvement across the west late this afternoon and evening. Trend
late tonight and Friday morning will be for gradual improvement to
at least low end VFR between 15Z and 18Z/Friday. The holdout will
be KCPR where conditions may not become MVFR until just before
18Z/Friday as low slowly spins onto the Plains.

&&

.FIRE WEATHER...
Issued AT 115 AM MDT Thu May 18 2017

Fire danger low across the forecast area through the rest of the
week as all fuels are currently in green-up and a late season winter
storm pushes through the forecast area...continuing into the Friday.
Precip will be increasing east of the Divide this morning...with
snow levels reaching valley/basin floor levels at times. More
significantly, copious amounts of precipitation is expected to fall
with this developing system. Additionally, expect winds to become
quite gusty today around the region east of the Divide and across
southern WY. Smoke dispersion fair to good afternoons through the
rest of this week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM MDT Friday for WYZ007-010-011-
017-018-020-030.

Winter Storm Warning until 6 AM MDT Friday for WYZ015-019-022.

Flood Watch through late tonight for WYZ010-011.

Winter Storm Warning until 6 PM MDT this evening for WYZ008-009.

Winter Weather Advisory until noon MDT today for WYZ002.

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...CNJ
FIRE WEATHER...Braun


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