Area Forecast Discussion
Issued by NWS Riverton, WY

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507
FXUS65 KRIW 251721
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1120 AM MDT Tue Apr 25 2017

.SHORT TERM...(This afternoon through Wednesday)
Issued at 636 AM MDT Tue Apr 25 2017

Updated to upgrade the Buffalo area and nrn Johnson to a winter
storm warning. Snow continues to pile up with heavy snow and
strong wind continuing. More heavy snow this morning before
tapering off.

With the contrast in the weather pattern...we will again divide the
forecast into West and East of the Divide this morning.

West of the Divide...The low that is bringing the precipitation
continues to drift east-northeastward across the area now, moving
across Natrona County as I write this. The best lift has shifted
to the east of the area, however the Pocatello radar continues to
show more precipitation moving into the area. The models continue
to show showers moving through the area through the morning so we
will continue the advisories and special weather statements ride
for now. The showers should decrease in intensity later in the day
with a lull, not really a break but a lull in the activity from
this afternoon through Wednesday morning. Following that the next
in the seemingly never ending series of fronts and troughs
approaches and will bring the next chance of rain and snow to the
mountains. With this system, timing is still a bit uncertain with
the NAM a bit faster than the GFS with the best QPF. We made few
changes for now and should be able to fine tune it more as we move
closer to the event. These showers will likely continue through
Thursday as well with some waning in the activity on Thursday
night. At this time, it looks fairly likely that more highlights
may be needed starting later on Wednesday. Temperatures, as
expected, will continue to average below normal.

East of the Divide...Once again we have that difficult decision of a
few degrees or a few hundred feet of elevation making a big
difference in the forecast. Things look OK for now in around Buffalo
with snow changing over right on schedule. Quite a bit of QPF,
already around eight tenths at the Buffalo airport at this time.
That being said, a lot of the QPF was lost to rain already. However,
temperatures are getting rather close to that freezing mark. Warm
ground temperatures are keeping pavement wet right now, but that may
change. This could be a situation where a few hundred feet of
elevation could make a difference between 6 to 8 inches of snow and
only 1 or 2 in Johnson County. For now, we will let the advisories
ride but we could make a game time decision to upgrade. Another area
of concern is the Cody Foothills, a place we have been burned on a
few times this year. Right now, precipitation is still mainly rain
but this could change. We will stick to the SPS for now. Elsewhere,
any snow amounts should remain below advisory levels. This will
especially be the case in places where changeover occurs later where
the late April sun will keep paved surfaces mainly wet.

Most of the models show the steadiest precipitation ending by noon
as low pressure moves away into the Plains states. Almost all
precipitation should end this evening with almost all areas mainly
dry after midnight. At this point, Wednesday looks like the break
between two systems. The NAM especially shows a mainly dry day. Even
the GFS has mainly dry conditions. The day should turn out like
Monday, with a few afternoon showers around but most areas rain free
most of the time. Temperatures will be warmer than today, but still
average below normal.

The next system will then move in for Thursday with another chance
of precipitation. And yes, this could mean snow as well for many
areas. 700 millibar temperatures are expected to drop through the
day, reaching minus 6 to minus 8 by 00Z Friday. If there is one
saving grace, a bulk of the possible snow may fall during the day
when the sun may prevent much of a problem on the roads. Low
pressure developing over Colorado could bring more snow, but more
likely on Friday. My colleague will have more on that.

.LONG TERM...(Friday through Monday)
Issued at 636 AM MDT Tue Apr 25 2017

Cold longwave trough will be well entrenched across the area to
start this period with potential for significant snow in parts of
Central Wyoming. Models continue to struggle with the details and
most of the guidance has a rather unorganized upper low but there`s
still a period in there where it`s quite favorable based on mean
trough over the area. Bottom line: Still a decent shot of some
accumulating snow which will likely start at the end of the short
term period and carry over into the start of this period. Conditions
improve quickly later Friday night or Saturday as the upper low
drops well to the south and east somewhere between the TX panhandle
and the KS area. Still some cold air aloft for snow showers in and
near the mountains on Saturday but otherwise, most areas will see
limited pcpn by Saturday but it`ll be chilly for late April.
Shortwave ridging moves over head Sunday but next shortwave
approaches the far west late Sunday for an increasing chance of mtn
snow showers and valley rain showers in the afternoon. The next
digging trough moves in Monday possibly closing off over nrn UT or
swrn WY on Tuesday. Decent potential for more widespread snow east
of the divide. Lowered temps some but probably not enough based on
current thinking.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday noon)
Issued at 636 AM MDT Tue Apr 25 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Isolated to Scattered MVFR/IFR conditions in lower CIGs with
occasional VIS restrictions in light snow showers will remain
vicinity and north of a KAFO-KPNA-KDUB line through 20z, with a
drier westerly flow then prevailing across SW Wyoming. A cold front
will continue to keep some light snow or rain mixed with snow
showers pushing south of KPNA-KBPI later this afternoon. A ridge of
high pressure will build in this evening, with decreasing snow
showers...but a few lingering vicinity KJAC and surrounding
mountains by 12z Tuesday with improving conditions elsewhere.


EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

An upper level low pressure system will continue to spin over
eastern Wyoming early this afternoon before sinking to the south
along with with the large band of heavier precipitation and
widespread MFVR/IFR conditions vicinity and north of a KDUB-KLND-
KCPR line. A relatively cold northerly flow will push south behind
the low pressure system with snow levels near valley floor level.
Snow may become mixed with rain through this afternoon with drier
air and improving conditions gradually taking over from the north
and west by this evening.

&&

.FIRE WEATHER...
Issued AT 636 AM MDT Tue Apr 25 2017

A cool and wet pattern looks to persist for much of the week. Low
pressure will bring rain and snow to many areas East of the Divide
today with lingering showers across many areas West of the Divide. A
gusty north to northwest wind will occur through the day across the
northern half of the state. Mixing and smoke dispersal will be
generally fair to poor. Tomorrow will be drier and milder East of
the Divide but another front will bring another chance of rain and
snow to the western valleys and mountains Wednesday afternoon and
night.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MDT today for WYZ001-002-012-
014-015-024.

Winter Weather Advisory until 6 PM MDT this evening for WYZ008-
009.

Winter Storm Warning until 6 PM MDT this evening for WYZ010.

&&

$$

SHORT TERM...Hattings
LONG TERM...Skrbac
AVIATION...Braun
FIRE WEATHER...Hattings



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