Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 161750 AAA
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1150 AM MDT Sun Apr 16 2017

.SHORT TERM...Sunday through Tuesday night

Well you certainly do not need a rabbit`s foot to forecast the
weather for today. All in all, todays weather looks as nice and
sweet as a mini egg with mainly dry conditions persisting through
the day. For most areas this morning, at sunrise it should be mainly
clear except for a few high clouds from time to time. Nevertheless,
for most areas the sun should be as bright as a marshmallow peep for
any sunrise services. In addition, with ridging over the area there
should be less wind so any bonnets will not be lost. The one small
downside is with the dry air in place and a clear sky, it will be
chilly this morning so people may be hippity hopping to stay warm.
But all in all the Easter weather should be egg-celent for the most
part.

We made a few changes to the weather for tonight. The models
continue to trend slower and weaker with Pacific cold front and
trough. As a result, we removed all POPS for the evening hours and
limit POPS late at night to isolated. Future shifts may want to
remove the POPS completely if the slowing trend continues. At this
point, the heaviest precipitation looks to be during the morning in
the west. However, the models are now keeping the most favorable
dynamics across Montana so we lowered precipitation amounts in the
west. The system also is fairly warm, with 700 millibar temperatures
not dropping below minus 3 or minus 4 so valleys will be mainly
rain. In addition, it is looking even more unfavorable for showers
and thunderstorms east of the Divide. We kept some low end POPS
across the northern border but confidence is low for that. What we
are confident about is the wind. It will turn breezy to windy
across much of the area. At this point, it looks to stay below
high wind thresholds for the most part. However, I have some
concern about the Cody Foothills where the right front quadrant of
the jet could enhance some downward momentum. This system should
move away by sunset with most Monday night largely quiet.

Another more potent system will then move into the west later Monday
night and into Tuesday morning. This also looks like a fairly warm
system on the onset. The difference with this system is some of the
precipitation will likely spread East of the Divide. The question is
how far. And right now, with the models diverging we do not want to
put all of our eggs in one basket. The NAM keeps the best dynamics
further south and confines things across southern Wyoming. The GFS
is more bullish and pulls it further north, even developing some
upslope later Tuesday night as low pressure develops over Colorado.
For now, we made few changes given the uncertainty.

.LONG TERM...Wednesday through Sunday

Synopsis...An upper level disturbance will lift across
southern/eastern Wyoming into the northern High Plains Wednesday
morning with heavier precipitation out ahead of this system shifting
into the Plains.  Cooler north to northwest flow and scattered
mountain snow and valley rain showers will be left in its wake
across most of the area through Wednesday.  A brief ridge of high
pressure will bring mainly dry weather Wednesday night and Thursday
morning.  The next Pacific storm system will dig into the Four
Corners region Thursday night and move across the southern Rockies
on Friday.  This system will bring chances of showers mainly across
the south half of Wyoming Thursday night and Friday while high
pressure over Montana keeps most of northern Wyoming dry.  Warmer
and dry weather is expected to prevail over the area Saturday.  An
upper level disturbance will bring chances of showers to northern
Wyoming on Sunday.

Discussion...Medium range operational runs show some spread with
shortwave ejecting northeast out of the Great Basin across southern
and eastern Wyoming Wednesday morning, with Canadian on the fast end
of guidance, GFS slow, ECMWF the in between solution.  GFS appears
to continue to have some feedback noise with this system, producing
copious QPF in excess of 1" over NE Wyoming Wednesday morning.
Little confidence in placing precipitation that far north and west
with heavier precipitation more likely to be associated with
deepening surface low over NW Kansas and its eastward extending warm
front.  Models do agree on shortwave ridge building over the area
Wednesday night between this shortwave moving into the Upper
Midwest and next Pacific system digging across the western Great
Basin. Overall ECMWF is further east/north with this storm`s track
than GFS, with GFS offering the more consistent solution here
with a trend toward a further southward track across the Four
Corners region and Southern Rockies Thursday and Thursday night.
This track maintains the higher PoPS across the south half of the
area Thursday night and Friday. GFS also shows more consistency
than ECMWF with upper ridge building in behind this system on
Saturday. Both models bring a shortwave across the Northern
Rockies on Sunday with the GFS showing a further northward track
the ECMWF. ECMWF seems to be the outlier here with its flatter
flow across the eastern Pacific and western U.S. than GFS or
Canadian. Forecast has some low PoPs across the north to account
for this system with a warming trend next weekend.


&&

.AVIATION...18Z Issuance

VFR conditions will occur at all the terminal sites through 12Z
Monday. High level clouds will be increasing over the area today.
Mid level clouds will be increasing over the west after 03Z Monday.
Isolated mountain snow showers will develop after 09Z Monday in the
far west mountains. LCl MVFR conditions/higher mtn obscn are expected
in the far west including the vcnty of KJAC by 12z Monday due to
lowering cigs and vsby due to scattered rain/snow showers.

&&

.FIRE WEATHER...

High pressure will bring dry weather to the area today. Expect a mix
of sun and high clouds along with less wind then the previous couple
of days. Relative humidity will fall into the teens again in some
locations but conditions should remain below elevated with the
lighter wind. Mixing and smoke dispersal will range from fair to
poor across northern and western areas to good to very good in the
south and east. A cold front will bring some light rain and snow to
the west on Monday.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Ross/Lipson
FIRE WEATHER...Hattings



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