Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221651
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1050 AM MDT Sun Apr 22 2018

.SHORT TERM...Sunday through Tuesday night
Issued at 141 AM MDT Sun Apr 22 2018

This morning the short term is what I call a sandwich weather
period. The bread, AKA the boring periods, are on the ends with the
meat, the more active weather, in the middle.

With the active weather coming in, I won`t spend much time on
today. High pressure will control the weather today and bring mild
temperatures and light to moderate winds. There will be some more
high clouds than yesterday, but all in all a nice finish to a nice
weekend. The GFS has a few showers sneaking into the west late
today. But seeing how that model tends to speed things faster
than Burt Reynolds in Smokey and the Bandit and also tends to
exaggerate orographic effects, we kept things dry. A vast
majority of the precipitation from the approaching front still
looks to hold off until after midnight.

Now we get the meat of the forecast. Personally, I prefer roast
beef with swiss and honey mustard on sourdough, yum yum. But in
the forecast, the meat is rain and snow. Heavier precipitation
then spreads into the west late tonight as the front crosses the
area. Initial snowfall amounts do reach advisory levels in some
mountainous areas in the north. Still on the fence about
highlights, since many of the biggest effects are in areas with
little or no population. As a result we will punt to the day shift
to make the final decision. Most of the lower elevations should
remain dry until around sunrise Monday.

The big question is what happens in the lower elevations. Following
the frontal passage temperatures will crash with falling
temperatures possible during the day, especially in the northern
Wyoming if you believe the NAM. This will result in much of the
area seeing snow, especially in the Cody Foothills and in areas over
5000 feet in elevation. The models continue to be consistent in
keeping the best precipitation over northern Wyoming with the best
forcing and jet dynamics. However, this is a tricky situation. This
is where 1 or 2 degrees or a few hundred feet of elevation can make
a big difference in snow accumulation. In addition, with the very
strong late April sun, the snow could have trouble sticking on a
lot of paved surfaces during the day. This could be a situation
where there could be 3 to 6 inches of snow on the grass and the
roads are mainly wet if things set up right. Generally, amounts
will drop off further south as well. The NAM continues to show
some showers in the Wind River Basin Monday evening but the other
models are drier so we split the middle for now. A majority of
precipitation should be over shortly after midnight. Another
problem, could be wind, especially in Johnson County with the cold
advection and northwest flow. Winds at 700 millibars do not
support high winds though, so we kept values below that for now.

At this point, Tuesday looks to see improving conditions. Other than
a few flurries, precipitation should be over. The balance of the day
should feature increasing amounts of sunshine with less wind.
Temperatures will be remain chilly though, averaging 10 degrees
below normal in many locations.

.LONG TERM...Wednesday through Sunday
Issued at 141 AM MDT Sun Apr 22 2018

Wednesday looks dry and warmer with some breezy areas in the usual
places. GFS and ECMWF models now differ on Wed night and Thursday
morning. The ECWMF model keeps a fast moving weather system
diving southeast through WY while the GFS model takes the system
on a track that would miss us to the northeast. Canadian model
clips the northeast zones of my area. Not convinced this system
will miss completely and will keep the low precip chance east of
the divide Wed night into Thurs morning. Plenty of time to adjust
forecast one way or another as time gets closer. West of the
divide will be dry Wed night and Thursday. Dry weather Thursday
afternoon east of the divide. On Friday, most of the region will
be dry. GFS model has isolated showers or storms over and near the
mountains Fri pm and evening while the ECMWF model is dry. Will
keep isolated activity in the forecast. Then Saturday and Sunday,
the GFS and ECMWF models continue to show little consistency in
how to handle an upper low pressure system along the U.S. west
coast. 00Z model runs are hundreds of miles different at 12Z
Sunday. Despite this, they both show isolated shower and
thunderstorm activity over and near the WY mountains both days.
Good enough for now and see how things look tonight. As for temps,
above normal highs will be the rule with 60s to lower 70s Wed and
Fri through Sun. Thursday may be cooler with 50s to lower 60s if
the weather system, discussed above, indeed occurs. Have not
strayed too far from model blend at this time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday)

VFR conditions to prevail for all locations west of the Divide
through the forecast period except for KJAC which will see
MVFR/possible IFR conditions becoming increasingly likely after 09Z
Monday. East of the Divide, VFR conditions will prevail for KCPR,
KLND and KRIW through the forecast period while KCOD and KWRL will
experience deteriorating conditions with MVFR/IFR CIGs and VIS
possible after 11Z Monday.

A strong cold front will push east across Idaho today and tonight,
arriving near/over northwestern Wyoming late this evening.
Isolated/widely scattered rain/snow showers and possibly an isolated
thunderstorm will accompany this activity as the front moves through
Wyoming. The main band of frontal precipitation will move
into/through western and northern Wyoming 8z-18z Monday with
increasing areas of MVFR/occasional IFR CIGs and VIS along and
north/west of a KAFO-KDUB-KWRL by Monday morning. Snow levels will
range from 5.5k-8kft (north to south) with KCOD the most likely to
see snow. The western and northern mountains will also become
obscured during this period.

&&

.FIRE WEATHER...
Issued at 141 AM MDT Sun Apr 22 2018

Expect one more dry day today. Relative humidity will fall into the
teens at times in the lower elevations. However, wind will remain
light to moderate snow critical fire weather is not expected. Mixing
and smoke dispersal will be generally fair to good. A cold front
will move through the state later tonight and Monday and bring much
cooler temperatures along with the chance of rain and snow,
especially across the northern half of the state.&&

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

&&

$$

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


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