Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 270919
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
319 AM MDT Mon Mar 27 2017

.SHORT TERM...Monday through Wednesday night

Quite a challenging forecast in the humble Riverton weather abode
which is causing many headaches and mashing of teeth, maybe I should
see the dentist. One of those situations of 1 or 2 degrees could
make a big difference in the forecast. I will explain below.

Things are mainly quiet this morning with only some light snow
across the western mountains. However, things will turn active this
afternoon. Some moisture will move north and interact with the left
front quadrant of a jet streak that will develop some precipitation
across southern Wyoming this morning and spread north through the
afternoon. Meanwhile, areas East of the Divide should be mainly dry
and mild today although clouds will increase, thicken and lower
through the day. In the meantime, a cold front will be sliding south
into the state. The moisture and front will then meet and develop
some overrunning across central Wyoming, and this is where the
fun, or lack of fun, begins.

The question is whether or not it will snow and if it does, how
much, and how much will stick. The factors for will be 700
millibar temperatures dropping to sufficient levels to support
snow. In addition, a bit of a jet max, although not a very strong
one, will move across the area and enhance lift and could increase
evaporational cooling. In addition, a 700 millibar circulation
will pass the south and turn flow into a favorable northeast
direction to enhance upslope flow. However, there are factors
against it. One, will be starting off very mild so it will take a
while to cool down so this could fall much of the precipitation to
fall as rain. In addition, with the recent long stretch of mild
weather, ground temperatures are warm so much of the snow that
falls could melt unless the rate is heavy. The models also have
differences with the placement of the heaviest QPF and hence the
heaviest possible snow.

The trailing trough axis does look a bit slower as well so
precipitation will persist on Tuesday. As a result, we lowered
high temperatures East of the Divide several degrees. With
northeasterly flow established, the heaviest precipitation will
fall in the areas favored by this pattern, a.k.a. the east slopes
of the Wind Rivers to Casper as well as near Thermopolis and east
slopes of the Big Horns. However, we will also be fighting the
strong late March sun. So still some uncertainty even this close.
At this time, the most likely areas to see advisories would be the
Green and Rattlesnake Range, portions of Sweetwater County and
the Big Horn Range. It is still questionable in the lower
elevations since temperatures will be borderline. Wind could also
be a story for Tuesday even if precipitation does stay mainly rain
in some areas. Areas that are favored by strong northerly flow,
like Buffalo and the northern Big Horn Basin could see some strong
winds. At this time, it does not quite look like high wind, but
gusts over 40 mph look like a good possibility. As for highlights,
with the borderline temperatures, we will allow the day shift to
make the final decision for the most part. We will hoist one for
the Big Horns however, since precipitation would be mainly snow
there.

Quieter weather should move back in for Wednesday as ridging builds
in. High temperatures could be tricky in areas that get snow
however. The next wave, this one possibly a bit more potent will
move back into the west Wednesday night but the worst effects would
hold off until Thursday.

.LONG TERM...Thursday through Monday

Attention is drawn to the next storm system that moves into Nevada
on Thursday and then into southern Utah Friday morning. A good chance
of rain and snow in the west Thursday with isolated to scattered
rain showers east of the divide Thurs afternoon and Thurs night.
The storm moves slowly toward the four corners region during the
day Friday and into northwest New Mexico Fri night. Models show a
wet day for Friday into Friday night east of the divide and over
the southern zones. GFS model much higher with forecast precip
amounts than the ECMWF model at this time. The other issue is when
or if precip turns to snow at the lower elevations east of the
divide and in the south Friday. Models show 700MB temps get close
toward Friday morning in parts of the region. This will have to be
watched with time and adjustments made one way or the other.
Precip in NW WY will be decreasing Friday and Fri night. Precip
may linger into Saturday morning in the south and east zones
before ending Saturday afternoon. The rest of the region looks dry
all day Saturday. Dry weather Sunday. Models differ for next
Monday as the GFS is dry with its next weather system in western
Oregon. The ECMWF is wet as it has another weather system moving
into the Great Basin Monday. The pattern supports the ECMWF with a
two or three day gap between systems. Have split the difference
for next Monday keeping low chance of precip in the west and
southwest and dry weather elsewhere. Plenty of time to watch this
pattern evolve. Temps will be mild Thurs, then cooler Fri and Sat,
warming back up Sun and next Mon.

&&

.AVIATION...12Z Issuance

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

Despite lowering mid-cloud deck invading from the west, VFR
conditions are anticipated through about 15Z/Mon. KJAC has
the best chance of low end VFR or MVFR conditions through early
Monday afternoon, mainly between 15Z-21Z/Mon. Moisture then
begins to spread across southwest Wyoming and left exit region
of 90kt jet will aid lift during the late afternoon in the
vicinity of KRKS. Precipitation will shift to this area during
the late afternoon and evening with gradually lowering ceilings
and visibility. Heavier precipitation and dynamic cooling will
likely enable the rain to change to snow at KRKS between
04Z/Tue and 06Z/Tue. KBPI and KPNA may remain VFR, but have at
least added MVFR ceilings during the late afternoon and evening
Monday. KBPI, KPNA, and KJAC should all be low end VFR after
06Z/Tue as dynamics move east and deeper moisture slowly slides
east of the Continental Divide. Loss of dynamics should also
allow for decreasing snow and some visibility improvement at KRKS
between 09Z-12Z/Tue. Mountain top obscurations will be
widespread throughout the period, but especially after 00Z/Tue.
Developing surface low in southeast Wyoming will also allow for
a tightening pressure gradient and increasing northwest 12-25kt
wind at KBPI, KPNA, and KRKS after 06Z/Tue.

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

Mid and high clouds will drift east across the region through
the morning and afternoon hours as VFR conditions prevail through
00Z/Tue. Southward moving cold front should reach the central basins
between 00Z-03Z/Tue. Moisture spreading across southwest Wyoming will
begin to overrun the deepening upslope across central Wyoming by
midnight. This should bring light rain and MVFR conditions to KCPR,
KLND, and KRIW by 06Z/Tue. Depending upon the precipitation intensity,
the rain may mix with or change to snow at all three terminals by
12Z/Tue. Mountain obscurations will be widespread in central Wyoming
after 03Z/Tue with mainly mountain top obscurations to the north.
Light rain should be less intense at KCOD and KWRL and have only gone
to low end VFR late Monday night. Developing surface low in southeast
Wyoming will also allow for a tightening pressure gradient and therefore
increasing northerly wind of 12-25kts late in the period.

&&

.FIRE WEATHER...

A system moving up from the south and a cold front moving in from
the north will spread rain from south to north through the day
today. Expect mixing and smoke dispersal to be mainly fair to poor
but good from eastern Sweetwater County northward through Natrona
and Johnson Counties. Cooler air will move in from the north tonight
and may change some rain to snow although pinpointing the heaviest
snow is still very difficult. Unsettled and cool weather with rain
and snow showers will continue on Tuesday before a mainly dry day
Wednesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Tuesday for WYZ008-009.

&&

$$

SHORT TERM...Hattings
LONG TERM...AR
AVIATION...Jones
FIRE WEATHER...Hattings



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