Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 302106
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
306 PM MDT Thu Mar 30 2017

.SHORT TERM...(This evening through Sunday)
Issued at 105 PM MDT Thu Mar 30 2017

Major upper low is heading towards the nw part of the Great Basin
attm and is still forecast by all guidance to dig/strengthen as it
heads towards the 4-corners area on Friday. With the warm advection
pumping up the upstream ridge and strengthening the backside jet
into this system, we should see continued strengthening tonight into
Friday. This system remains one of the more challenging forecasts in
recent memory with regards to the rain snow line around Fremont
County especially. What makes it challenging is the amount of
precipitation forecast by most of the models in Fremont County. Most
have around 3 inches in Lander and generally 1.5 to 2.5 in the Wind
River Basin with less towards Shoshoni. Depending on whether that is
rain or snow and you see why it makes a huge difference. No real
clarity on temps today as some guidance has trended a little warmer
again on the 12z runs compared to last night`s runs. H7 temps are
generally forecast to be in the -4.5 to -5.5c range in LND at 12z
(new 18z NAM12 run is considerably warmer now through 12z then
colder again) Friday with classic warm advection pattern leading
to warmer temps further north in the Wind River Basin. Colder air
generally filters in during the day so areas that change to snow
early enough could see very heavy totals. The lowest spots of the
basin could see more rain initially and then potentially change to
snow at times during periods of heavier pcpn and then change back
during lulls. The coldest air arrives Friday evening when most
areas will see light snow before ending late. Will upgrade the
South Pass area to a blizzard warning due to the snow and very
intense ne wind (50-70 mph). Will also upgrade the Lander
Foothills and Beaver Rim zone to warnings. Will also add in the
Bighorns, Owl Creeks, Bridgers and Absarokas (mainly south half)
to a winter weather advisory from midnight tonight to midnight
Friday night for generally 5 to 10 inches. And finally will throw
in the Wind River Basin to a winter weather advisory and indicate
how elevation dependent it will be. Fort Washakie and other areas
above 5500 feet will see the longest duration and thus the most
snow while the lowest spots like Shoshoni and Boysen Reservoir get
the least with more rain than snow Friday. Riverton town will be
extremely tough with potential for a rain snow mixture or
changeover at times on Friday depending on intensity followed by
some lighter snow Friday night before ending late. Casper looks to
be mostly rain Friday changing to snow around 00z or so. Will
also have to watch western Natrona above 6k feet on Friday and
also southern Johnson County where colder air sits during the day
Friday. Possibly advisories or short notice warnings depending on
how Friday shapes up. Sweetwater County looks very windy but with
limited precipitation. Improvement then Saturday with dry
conditions and temps recovering. Next shortwave spreads a chance
of higher elevation snow to mainly the nrn mountains with a few
light rain showers across the nrn half lower elevations Sunday.

.LONG TERM...(Sunday night through Thursday)
Issued at 105 PM MDT Thu Mar 30 2017

The transitory ridge will be exiting off to the east, and the next
series of northern stream troughs will begin to affect our wx later
on Sunday. The first trough will zoom by to the north over Montana
Sunday and produce some light showers in Northwest Wyoming with the
resultant instability. Then the main trough will approach from the
northwest Sunday night with the troughpa Monday. Although the center
of the weak low will track southeast across Northeast Wyoming
Monday, most of the vort energy will track southeast from central
Idaho to Southwest Wyoming, which is a somewhat favorable track for
a Lander/East Slope of the Wind River Mtns type precip event. The
above scenario is the GFS soln. The GFS also tracks the h7 low east
across Northern Sweetwater County with qpf amounts around an inch not
far from this circulation and over the Southern Wind Rivers. The
Canadian model still keeps the energy south over the 4 Corners
region, with the ECMWF keeping it more of an open trough. For the
first time, all three models agree on the timing of the troughpa
around 18z Monday. Will side with the GFS for now as h7 lows have
been tracking in this fashion as of late. Indicated about half of
the QPF for Monday of what the GFS is advertising for Monday, given
the uncertainty from the different model solns. For now have
indicated up to 7 inches of snow in the mtns with an inch or less in
the valleys across the CWA. H7 temps will be around -10c and sfc
temps well above freezing, hence the airmass will be rather
unstable. With the h7 low in the picture and the given instability,
it appears that this could be a typical early spring snow event
where total snow amounts will vary quite a bit over a short
distance, given the variation in precip amounts and differences in
temps, depending on elevation. This event is still long way off.
This will add another layer of moisture and snow pack on top of what
occurs tomorrow, Friday, so potential flooding could again be a
possibility. On Monday night and Tuesday morning, precip will be
exiting to the east. A slow warm up is expected Tuesday through
Thursday. Only some light showers expected in the far Western Mtns
Thursday with a weak vort lobe in the nw flow.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 105 PM MDT Thu Mar 30 2017

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

Scattered rain and mountain snow through the day. Areas of MVFR
conditions will occur along with some mountain obscuration. KRKS
will stay VFR through 00Z. In southwest WY around 00Z, rain will be
increasing in coverage along with MVFR conditions and mtn
obscn prevailing through 12Z Fri. Cigs and vsby will slowly improve
through the day but MVFR conditions will still be fairly widespread.
Winds will increasing from the north except from the northwest at
KPNA and northeast at KRKS.

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

Isolated showers will occur for the rest of this afternoon. After
00Z, areas of MVFR cigs will develop along with an increase in
rain coverage. Around 06Z Fri, widespread rain will occur along
with MVFR to IFR conditions and mtn obscn. Widespread MVFR with
lcl IFR conditions will prevail throughout the day Friday except
widespread IFR conditions can be expected from KCPR through KLND.
Please see the terminal forecasts for more details.


&&

.FIRE WEATHER...
Issued AT 105 PM MDT Thu Mar 30 2017

A major west spring storm will impact the region tonight through
Friday night with cold rain and heavy wet snow for much of the area.
The highest impacted areas will be along and east of the divide.
Fire danger will generally remain low for the next couple days.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to midnight MDT
Friday night for WYZ002-007>009.

Winter Storm Warning from midnight tonight to 6 AM MDT Saturday
for WYZ019.

Winter Storm Warning from 3 AM Friday to 6 AM MDT Saturday for
WYZ018.

Blizzard Warning from 9 PM this evening to 6 AM MDT Saturday for
WYZ014-015.

Winter Weather Advisory from 3 AM Friday to 3 AM MDT Saturday
for WYZ017.

&&

$$

SHORT TERM...Skrbac
LONG TERM...Lipson
AVIATION...Lipson
FIRE WEATHER...Skrbac


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