Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 222253
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
453 PM MDT Wed Mar 22 2017

.SHORT TERM...(This evening through Thursday)
Issued at 227 PM MDT Wed Mar 22 2017

A line of convection containing some CG lightning and a
likelihood of small hail has formed from Eastern Idaho to
Northeast Utah and is poised for Western Wyoming attm. Another
cluster of thunderstorms is racing northeast over the Southern
Wind River Mountains. This instability is in response to afternoon
heating combined with a jet couplet ahead of the approaching
Pacific trough now about to make landfall. An asct weak, but well
defined mid level low will wobble across Southwest Wyoming with
another somewhat less defined weak mid level low over the Wind
River Basin tonight and Thursday. The combination of the jet
couplet, LFQ of the southern jet, mid level circulations, and QG
forcing will be enough to produce significant precipitaion across
much of the CWA, except the northwest, through Thursday evening,
In spite of the fact that the best energy with this splitting
trough will be shunted well to the south over the Southern Rockies
because of the blocking mean ridge still over the High Plains
Thursday, there will be enough cool air asct with this trough and
enough  cooling to result in a few inches of wet snow
accumulating at the lower elevations across much of the CWA later
tonight through Thursday evening. The onslaught of the main area
of precip arriving in the wee hours Thursday morning will be ideal
for keeping the high temps cool enough for mixed precip in many
areas as temps will not have a chance to recover during the day
Thursday in many areas., An SPS has been sent addressing the light
wet snow accumulations at the lower elevations along with sub
advisory amounts in the surrounding mtns. However, a winter
weather advisory has been issued for the Wind River and Big Horn
Mountains as these mtn ranges will be lie along the axis of the
highest QPF with the advisory valid from 6 pm this evening,
through Thursday until midnight Thursday night. Once cyclogenesis
takes place at all levels later on Thursday, the energy will shift
to our south with the snow (most precip will turn to snow
everywhere Thursday night) will be diminishing and ending. Much of
the precip asct with this weather system will be of a convective
nature until we get on the back side of it by Thursday evening
when we transition to stratiform along with a relatively brief
upslope component for favored areas east of the divide. Many
areas will see from one half to as much as three quarters of an
inch in some areas from this precip event through Thursday
evening so some of the main stem rivers will certainly have to be
watched (including the areas where a flood watch in the far west
is still in effect from now through 12 noon Thursday.) Rain and
snow melt will be a concern.

Friday will be dry.

Then the next Pacific weather system now centered out at 155W will
move in from the west and spread more high elevation snow to the
western mtns and rain to the western valleys on Saturday.


.LONG TERM...(Thursday night through Wednesday)
Issued at 227 PM MDT Wed Mar 22 2017

Another splitting trough moves across the area Saturday into
Saturday night with a period of mountain snow and a period of rain
or mixed rain and snow across the lower elevations. A few rain/snow
showers may linger Sunday mainly in the ern zones with a few
convective snow showers the high country Sunday afternoon. Models
start to diverge early next week with the GEM and ECMWF having a
splitting negatively tilted system swing through on Monday that the
GFS doesn`t show. A stronger low is still expected to dig into the
Great Basin Tuesday into Wednesday. The Euro/GEM are much stronger
and colder with this system as it comes in at a further north
latitude (into the PacNW instead of further south into mainly CA
like the GFS). Hemispheric 5-wave favors the Euro/GEM so will side
with colder temps and a longer duration event Tuesday into
Wednesday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday afternoon)
Issued at 227 PM MDT Wed Mar 22 2017

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

Scattered to numerous showers and isolated thunderstorms will
prevail through this evening (most numerous in the southwest
including KBPI, KPNA and KRKS. Rain showers will turn to a mixture
of rain and snow overnight and all snow by morning for KPNA and KBPI.
KJAC will have showers in the vicinity. MVFR to IFR conds will
become more prevalent for the southwest parts with VFR to MVFR
from KJAC northward. Mountains will be obscured with low clouds
and precipitation at times. Considerable snow or mixed rain and
snow will cover much of the area on Thursday with the highest
probability over the mountains and from KBPI and KPNA southeast to
KRKS. Please see terminal forecasts for more detailed
information.

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

VFR conditions will prevail over the region through 06Z Thursday.
Scattered showers and isolated storms will develop over southern and
central WY this afternoon into the evening. Then showers will become
more numerous over this area by 06Z Thursday. After 06Z Thursday,
areas of MVFR to IFR cigs will occur as rain changes to snow or
mixed rain and snow by morning and continues through the day
Thursday. Please see terminal forecasts for more detailed
information.

&&

.FIRE WEATHER...
Issued AT 227 PM MDT Wed Mar 22 2017

Fire danger will be low through the weekend. The exception to this
rule will occur during the remainder of this afternoon which
shows signs of having elevated fire danger across portions of
central WY as gusty southwest winds combine with lowering relative
humidities and a few chances for gusty isolated thunderstorms in
the afternoon/evening. Thursday also shows signs for embedded
thunderstorms across southern Wyoming, however the day on the
whole will have increasing moisture spreading across WY along with
good chances for significant precipitation at most
locations...both liquid and frozen. Another significant
precipitation event is expected in Western wyoming Saturday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Thursday afternoon for WYZ013-023-025>027.

Winter Weather Advisory from midnight tonight to midnight MDT
Thursday night for WYZ008-009.

Winter Weather Advisory from 6 PM this evening to midnight MDT
Thursday night for WYZ014-015.

&&

$$

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



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