Area forecast discussion...update
National Weather Service Riverton Wyoming
1118 am MDT sun may 19 2013
Short term...Sunday through Tuesday night
The unsettled weather pattern looks to continue for at least the
next couple of days with some possible drying for Tuesday into
Tuesday night. Radar is currently quite active with some rain
showers across much of central Wyoming. Billings radar is also
showing increasing coverage across much of Montana with echoes
moving to the south. As a result...it looks like a damp day across
much of the area. The next question is how much precipitation will
fall. Models are quite bullish with over a half an inch in many
areas as precipitation wraps around a developing middle level
circulation over the Central Plains. However...we do have some
concerns. One is with the expected widespread convection over much
of the southern and Central Plains states over the next couple of
days. We feel that the thunderstorms will rob a lot of the moisture
that may wrap back into western and central Wyoming so we feel the
models...especially the GFS may be overdoing the amounts of quantitative precipitation forecast. As
a result we did bump up quantitative precipitation forecast a bit although not as high as the models
with the highest amounts in northeastern portions of the County
Warning Area. In the higher elevations...we are not done with the
word that starts with snow and has four letters yet. With northerly
flow and cold advection there will be some snow...mainly above 8000
feet. There will be some accumulation...possibly to advisory level.
However...with recent warm weather and since most of the snow will
melt on roads during the day...we will stick with special weather
statements for now. As we head into Monday...it is a tricky call
across the area since eastern portions of the County Warning Area
will be on the edge of the heaviest precipitation. For now we bumped
up probability of precipitation and quantitative precipitation forecast a bit but did not go as high as the neighbors given
the uncertainty. Things should begin to dry out a bit in areas
further west with the best lift shifting to the east and ridging
beginning to build in from the west. As for Tuesday...a lot depends
on what happens on how quickly the upper level low shifts to the
east and heights can rise with ridging building from the west. Three
days out predicting the position of an upper level low is about as
reliable as a 1985 yugo...so for now we made few changes to
continuity.
Long term...Wednesday through Sunday
A vigorous upper low moves into the Pacific northwest Tuesday/Tuesday
night. Global models keep this strong upper low around the Pacific
northwest area through at least Friday...before medium range
models diverge on its evolution Friday into the weekend. Some
solutions have this system shifting northeast into southern
Alberta/extreme northern rockies...while others have it drifting south
and/or west.
For at least Wednesday and Thursday this large upper low will
result in increasing south to southwesterly flow over the region
and above average temperatures...and possibly some strong
convection at times. On Wednesday...medium range models are
showing a decent difluent flow over the forecast area ahead of a
significant shortwave rotating around the upper low. In
addition...at the surface...models are progging a moist
east/southeast flow developing east of The Divide. Models are
showing the bulk of instability/steep lapse rates on Wednesday
across the west/northwest. Thus could see some strong convection
across the west/northwest Wednesday...and possibly farther east if
more instability occurs than currently forecast. This system could
also surge a cold front eastward into western Wyoming late
Wednesday/Wednesday night...but quickly retreat during the day
back west/northwest Thursday. Models then show a dry slot moving
into southwest Wyoming Thursday...but a continued moist
east/southeast surface flow across the extreme east...with
quickly veering winds just above the surface. Models are showing
the bulk of the instability Thursday across the north and
east...with the west fairly stable...so convection should be
concentrated across the north and east Thursday...with some strong
convection possible especially along and north and east of the
pseudo dry line/front...which should be across Johnson and Natrona
County as well as the Big Horn basin. The basic set up should remain
the same Friday...except perhaps the drier southwest flow pushing
ever so slightly farther east. For next weekend have elected to go
with the upper low remaining out west instead of tracking into
southern Alberta...which results in continued above average temperatures
and mainly dry conditions. However if the solutions pushing this
low into the northern rockies/southern Alberta pans out...it
should push a decent cold front across the area late Friday into
Saturday...and drastically change temperatures and possibly probability of precipitation.
&&
Aviation.../18z issuance/
A weather system will continue to impact the region through 12z
Monday. Widespread areas of rain and High Mountain snow will occur
over the region east of The Divide through 12z 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 12z
Monday. VFR to MVFR conditions will occur in southern and southwest
Wyoming while MVFR to IFR conditions along with mountain obscurations
will prevail over the rest of the region.
&&
Fire weather...
The unsettled pattern continues through Monday...although the
weather looks to be more showery with fewer and less intense
thunderstorms. Locally heavy rain may fall across areas from the
Bighorn range and east from this afternoon into Monday. The west
will begin to see less showers beginning on Monday with drying
slowly spreading from west to east Monday afternoon into Tuesday as
an upper level low moves away to the east.
&&
Riw watches/warnings/advisories...none.
&&
$$
Short term...hattings
long term...wm
aviation...lipson
fire weather...hattings
