Area forecast discussion
National Weather Service Riverton Wyoming
1125 PM MDT Monday may 20 2013
Short term...tonight through Tuesday (issued at 309 mdt)
Today...large stuck...stacked...closed low over the Dakotas
today...not going anywhere in a hurry. This wave will begin to open
and move Tuesday as another large alaskan system plunges into the
Pacific northwest by Tuesday night. In the meantime...low clouds...scattered
rain and snow showers (above 8500 ft) while surface low develops over
western County Warning Area and cold air continues to pour in aloft. Main threat today
will be strong winds around most the eastern County Warning Area with nice low level
pressure gradient set up this this morning...together with vigorous
nearly unidirectional flow aloft...giving near high wind category for
elevations above 7000 feet in The Owl creek...Bridger...and Big Horn
Mountains. Sustained northerly winds of 40 to 45 miles per hour are common with
occasional gusts to 60 miles per hour across elevated areas. These will persist
through at least late afternoon before quieting down east of The
Divide with the relaxation of the ll pressure gradient. Across the
southern Big Horn basin and Owl Creek/Bridger Mountains...the strong
wind will likely persist a little longer into the evening.
Secondary threat will a more persistent precipitation shield over
the Big Horn Mountains and portions of adjacent eastern counties
including Johnson and Natrona...and secondly along and near the Wind
River and Absaroka Mountains...particularly the mountains that have
good northern exposure and will take advantage of the upslope along
with the region south of the Wind River mountains where ll
convergence and moderate middle and upper lapse rates will be maximized.
Tonight and Tuesday...precipitation chances will begin to wane compared to
the last few days and the stronger winds will subside as the closed
low to out east starts to open and slowly migrate eastward and
surface high pressure also begins to develop over our heads. We
will then get some needed mild ridging building in ahead of the next
big system...currently located in the alaskan/southern Yukon area in WV
imagery. Drying...warming of 5 to 8 degree over today...and more in the
way of sunshine will be the name of the game tomorrow. There will be
some mild thermal instability with increasing lapse rates but
working potential energy through the day will be missing as
combination of overall drying and upper level shear dynamics on the
weak side (substenance the general rule)...only isolated elevated
convection occurring over the highest areas of the Wind
River...absarokas and Big Horn Mountains will be possible.
Long term...Tuesday night through Thursday
Convection will linger over the far West Mountains into Tuesday
evening as a weak shortwave ripples NE across the area. A
potentially more interesting scenario sets up on Wednesday over
mainly the northwest third or so. A surface high develops behind our upper
low and pushes a gentle boundary south and then westward across areas
east of The Divide Tuesday night. A drier S to SW flow over the southern
third will push northward during the day along with a potential weak
shortwave coming up from the south-southwest. Lowering pressures over the northern
Great Basin will also help to draw some of this moisture westward. The northwest
mountains will be the most favored for stronger convection but with the
SW steering flow aloft some storms may form over the mountains and drift
NE across the foothills and potentially parts of the central basins
late in the day Wednesday. Indicated some hail and gusty wind
wording in the storms for this area from about Lander and Riverton
north to Cody and then westward through the northwest mountains on Thursday...the
drier air will likely mix out further north with the
boundary/convergence retreating to the far northern and northestern sections
where a few stronger storms may still fire. Temperatures will be
warming as this warmer...drier air moves northward Wednesday into Thursday.
Thursday night through Monday
A stronger shortwave is expected to rotate around our large pacnw
and northern Great Basin upper low Thursday night which will push a cool
front much of the area with the biggest cool down Friday in the
central and western zones. Still mild but not as warm as Thursday.
Isolated thunderstorms may still fire across the far north where
trough dynamics and daytime heating create enough instability and
lift. Thursday night will be a mild night as this boundary lifts
north and a gusty south to southwest wind blows in many areas
overnight. Models are keeping a fairly dry airmass over the area on
Saturday but with moist ground and daytime heating will keep
isolated thunderstorms going across the north third to half. On
Sunday...the upper low redevelops further south into the Great Basin
again as a strong shortwave drops southeast behind the old low. Strong
dynamics will likely rotate northwest across the area on Sunday with an
increasing chance of thunderstorms. A very moist airmass will be
just to the east of US late in the weekend and if some of this gets
drawn just a little further westward...some strong storms may fire in the
eastern zones. Out west...there will be less moisture but stronger
dynamics from the shortwave rotating northward across the west. Will
indicate scattered thunderstorms over the absarokas with isolated
storms across most of the northwest and northestern zones. Plenty of shortwaves in
the SW flow on Memorial Day with the potential for thunderstorms
continuing across at least the northern mountains and potentially
elsewhere even though the models are showing storms mainly confined
to the northern mountains. Each day will have to be watched closely as
with moist ground and day time heating will likely create more
instability than the models are showing right now.
&&
Aviation.../06z issuance/
Isolated showers will end by 12z in most places. However...isolated
showers are possible until 16z from the Bighorn Mountains south to
Casper Mountain. Otherwise VFR conditions will prevail until 06z
Wednesday. Isolated showers and thunderstorms will occur over and near the
western mountains from 18z until 03z. Local MVFR conditions are
possible in storms along with gusty wind to 25 knots.
&&
Fire weather...
The back end of a cool and wet storm system will continue to impact
the area through this evening before improving conditions return
tonight and through Tuesday. Isolated showers west of The Divide
most areas will produce off and on light rain with high elevation
snow (above 8500 ft) through this evening along with periods of
gusty north to northwest winds running 30 to 40 miles per hour above 7000 feet.
Another storm system will bring a chance of thunderstorms to the
northwest fire zones on Wednesday with gusty south to southeast
winds across the area.
&&
Riw watches/warnings/advisories...none.
&&
$$
Short term...Braun
long term...skrbac
aviation...Arkansas
fire weather...Braun
