Scientific Forecaster Discussion

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Area forecast discussion...update 
National Weather Service Riverton Wyoming
1130 am MDT sun Apr 20 2014

Short term...today through Tuesday night

Sat imagery still showing split upper wave pattern across the Continental U.S..
Wyoming currently under weak nearly zonal flow West/ Ridge north and trough of the
state. Upstream getting into the Pacific northwest...strong jet nosing into
that region...getting ready to fold as large trough rotates around Alaska
Gulf low. At the surface...weak gradient west/ low p to the west and
trough/front still hanging around the southern forecast area...lifting northward
slowly.

Nice warm and low impact Easter Sunday overall. The afternoon will
offer low end chances for isolated to widely scattered showers with
some thunder as a weak upper level disturbance and stalled surface front
race back S through the forecast area. Timing and low end available moisture
will limit the convective area to mainly the S central...southern and southeastern
zones. However...this will be another case of high based showers/few
storms with little rain...and more gusty winds.

Monday through most of the day...upper ridge and stable atmosphere in
place. Monday night...upper ridge axis starts to head out while trough
from the west begins to move in and positive vorticity advection begins to enter the western
portion of the state. Lead vorticity maximum associated with portion jet/trough
mentioned above will then work its way into the western mountains with
some Pacific moisture/precipitation late Monday night/Tuesday morning.

Tuesday then will Herald the beginning of new cyclogenesis from the
Pacific northwest into the northern Great Basin as the stronger dynamics from the
system in Alaska Gulf move in. The addition of modest middle level moisture
into an increasing unstable atmosphere west/ very steep lapse rates
will give chances for isolated to scattered convection by the
afternoon across most of the County Warning Area. An isolated strong storm could be
possible somewhere in eastern Johnson County Tuesday afternoon/evening along
near prefrontal/Lee trough...if moisture return gets back near this
region. Warrants keeping an eye on this area for the next few
days. Otherwise...the best chance for more significant
precipitation will occur across the western mountains as increasing
moisture and orographic lifting combines west/ decent upper jet
dynamics to yield a good region of qg forcing as a strong cyclone
forms to our north into Montana by the end of this period. The current
forecast placement of the incoming trough/jet dynamics suggest a similar
track to that of a few days ago...W/ split cyclone
development...one to the north and the other in the Lee of the central
rockies. Results will likely be similar overall...with most
precipitation...liquid/frozen...in the mountains...and much less
everywhere else.

Long term...Wednesday through Sunday

Pacific cold front will push across area Tuesday night into the High
Plains by 12z Wednesday. Associated upper trough closes off with
stacked low forming near Havre 18z Wednesday. GFS keeps more energy
rotating south of this upper low but trend is toward consolidation
over north central Montana. Thus...this is looking more like a
downslope wind event with and behind the frontal passage east of The
Divide Wednesday morning with tight west-east pressure gradient and cold air
spilling down the east slopes. Most favorable areas for near high
wind criteria will be from the southern ends of the absaroka and
Bighorn ranges to the Interstate 80 corridor. Out west...the
heaviest period of snowfall will occur with the frontal passage
Tuesday night...though numerous snow showers will wrap around the
backside of the low into northwest Wyoming on Wednesday...mainly from
Teton/Togwotee Pass northward. The ec's more stacked upper low
results in a much milder airmass over Wyoming Wednesday afternoon as cold
pool lifts into Montana...700 mb temperatures -2c to -5c east of The Divide.
With downslope dry adiabatic warming...high temperatures were
bumped up a few degrees across central areas and probability of precipitation lowered to
mainly isolated across the lower elevations except closer to The
Divide where some blow-over rain showers are likely. GFS and ec
both slide the main upper low to near ggw by 12z Thursday with
trailing shortwave swinging southeast across Wyoming Wednesday
night...bringing another shot of snow showers to the west and
bighorns and mainly isolated rain and snow showers east of The
Divide in continued downslope flow.

Upper ridge builds in to The Rockies Thursday...getting squeezed
between slow-moving low exiting across the northern plains and next
Pacific system shearing onto the northwest coast. West-northwest breezes will
continue across the area though not nearly as strong as Wednesday
with chance probability of precipitation in the northern mountains

Upper ridge slowly progresses across the plains Friday and through
the weekend. This next series of Pacific systems looks to split as
they head into this ridge. The GFS is much stronger and slower with
the southern stream than European model (ecmwf). However...zonal flow...strong west-east
jet stream along 40n from the central Pacific into the West Coast
will take shape by Saturday with jet gradually nosing into the
western Great Basin Sunday. This favors the faster European model (ecmwf)
guidance...several fast-moving shortwaves with low confidence in
timing on days 6-8. This pattern will bring the best chances of
rain and snow to the west Friday through Sunday. Mild and
occasional breezy/windy conditions will prevail east of The Divide
with slight chances of showers.



&&

Aviation.../18z issuance/

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

VFR conditions will occur today and tonight. Isolated showers are
possible across southern Natrona County from 21z until 02z. The
showers are not forecast to hit the kcpr Airport. A cold front will
continue to push through northern Wyoming through 19z and push south into
central Wyoming around 00z. A wind shift to the north will be the main
impact.

West of The Divide...kbpi/kjac/kpna/krks routes

VFR conditions will occur today and tonight. Isolated showers and
thunderstorms are possible across southern Wyoming from 19z until 02z.
This activity should remain south of the krks Airport during this
timeframe.

&&

Fire weather...

Today...high temperatures will be warm and seasonal...with another
round of isolated to widely scattered chances for mostly light
showers or an isolated thunderstorm possible mainly south of a
Marbleton to Kaycee line. Winds will begin to increase once again
Monday night as will chances for precipitation in the Western
Mountain zones...as the next upper level disturbance approaches from
the west...heralding the beginning of a couple of days of
precipitation out west. Significant mountain snows will be possible
starting Tuesday night through Wednesday morning. Otherwise...warm
and mostly dry across dispatch zones east of The Divide...with
small chances for isolated showers and thunderstorms through Tuesday
evening. Breezy to windy conditions will also commence across all
zones from Tuesday afternoon...lasting through Thursday morning.
As a result...there will be some enhanced fire danger...especially
Tuesday...as the warmest temperatures of the season combine with
the strong winds. Cooler temperatures and better chances for
precipitation are expected both Wednesday and Thursday...moderating
the danger to some extent.

&&

Riw watches/warnings/advisories...none.
&&

$$

Short term...Braun
long term...aem
aviation...lipson
fire weather...Braun