Area forecast discussion
National Weather Service Riverton Wyoming
1122 PM MDT Sat may 18 2013
Short term...tonight through Sunday (issued at 358 PM MDT )
Today...morning showers with some elevated thunder embedded in Hot
Springs and Washakie counties this morning...other showers continue
over the western border and northern County Warning Area in and around the rest of Big Horn
basin. This morning...shortwave disturbance seen in WV imagery
moving north-northeast through northern and northwestern County Warning Area supporting shower activity over
Big Basin and mountains west. This wave should exit the Big Horn
basin and be into Montana by 11am...while the trailing portion of
the disturbance takes longer to exit the northwestern County Warning Area...probable late
afternoon. Middle level moisture streaming into eastern Wyoming this morning
will help in supporting increasing chances for strong thunderstorms
this afternoon over Johnson and northern Natrona counties by early to middle
afternoon. 850 mb to 500 mb shear vector magnitudes of 35 to 40 kts will
help support more prolonged rotating storms in the area. Tricky part
of the process involves the main trough axis out west becoming more
negatively tilted...we should lose some upper level support by middle
to late afternoon due to this. Instability will be on the modest
side and will somewhat be dictated to some extent by the amount of
sunshine we get with the moisture and what effect the cloud push
from the south headed toward the eastern County Warning Area has. Even then...forecast surface cape
values coming off the big horns will hover around 1000 j/kg by
midday with with little or no cap in place...so storms will just go
and this will limit storm strength somewhat. Over southern half of Natrona
County...storms will be more elevated than further north into
Johnson County. Marginal severe hail is a possiblity and some minor
street and field flooding could also happen.
Tonight and Sunday...transitioning western trough opens and broadens a bit
right over our heads...the end result - cool cloudy conditions with
widespread light rain showers nearly everywhere. Some snow
tonight...mainly above 8000 feet mean sea level.
Long term...Sunday night through Friday by Sunday night the 700 mb
and 500 mb closed low will be reestablishing itself and strengthening
over Nebraska resulting in a northerly upslope flow that will be
deepening through Sunday night and Monday. This deepening upslope
flow will result in convective activity Sunday evening morphing
into stratiform precipitation late Sunday night and into Monday morning
east of The Divide. Rain and mountain snow will be fairly
widespread and have indicated likely probability of precipitation in upslope favored
areas. The best likelihood of snowfall will occur along The East
Slope of the Wind River mountains Sunday night and Monday as the
northerly upslope flow will be most favorable for the orientation
of that mountain range where up to 4 inches of snowfall is
expected there. In fact...once the low bombs out in the Central
Plains so to speak...northerly winds will become quite brisk by
Monday morning...especially from the Great Basin to Johnson
County. 2 to 3 inches of snowfall is expected in the Big Horn Mountains
and the Absaroka Mountains where the 700 mb circulation will track
east across northern Wyoming. Overall the best likelihood of precipitation
will be over the north half of the state. In addition to the cold
air air advection around the backside of the closed low...further
cooling from adiabatic cooling due to upslope flow as well as lack
of solar insulation due to precipitation and cloudiness has prompted the
cooling of high temperatures east of The Divide Monday by around 5
degrees in the forecast with most areas remaining below 60
degrees. Snow levels should be around 6500 to 7000 feet in the
northern mountains and 8000 feet in the Wind River mountains. This
will make 6 weeks in a row minus one week last week where the
mountains will pick up accumulating snow early in the week.
Surface high pressure will begin to build into the area later
Monday when the precipitation will begin to diminish during Monday
afternoon. Clearing will take place Monday night as a short lived
high pressure ridge builds in from the west. Then the attention
shifts to the massive closed low that Will Park itself over the
Pacific northwest by 06z Wednesday. A difluent southwest flow
ahead of this low along with instability as well as low level
moisture feeding in from the southeast will increase the chances
of late day convection...especially over the north half of the
County Warning Area...Wednesday and Thursday with Wednesday seeing the best
chances for storms given the weak shortwave being ejected from
this closed low that day. Another shortwave will eject from the
parent low Friday but it appears the surface boundary or dry line
will push northeast and shift the low level southeasterly moisture
advection farther north and east of most of our County Warning Area. Expect a
warming trend beginning Tuesday with the southwest flow with highs
in the 80s at the lower elevations east of The Divide by the
latter half of the week.
&&
Aviation.../06z issuance/
A weather system will continue to impact the region through 06z
Monday. Areas of rain and High Mountain snow will occur over the
region east of The Divide through 06z Monday. West of The Divide
rain and mountain snow will occur in northwest Wyoming. Scattered rain showers
will be common in southwest and southern Wyoming through 06z Monday. VFR
to MVFR conditions will occur in southern and southwest Wyoming while
MVFR to IFR conditions will prevail over the rest of the region.
&&
Fire weather...
Another round of thunderstorms are firing this afternoon...starting
in the rind river basin and along and east of the Big Horn Mountains
in Johnson County. Storms in these areas may produce gusty wind and
small hail...with a few storms becoming strong and producing 3/4 to
1 inch hail in Johnson County. Isolated street and field flooding
may also ba possibility where storms are training over the same area
for a length of time. Isolated thunderstorms expected
elsewhere...with gusty wind possible. This unsettled pattern
continues through the weekend...but will be more showery and
somewhat less the thunder type storms. Northwest flow will begin to
move over western Wyoming Monday morning...though some thunderstorms
may again be found across most of the County Warning Area save Johnson County in
this case.
&&
Riw watches/warnings/advisories...none.
&&
$$
Short term...Braun
long term...lipson
aviation...Arkansas
fire weather...Braun
