Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 171716
Area Forecast Discussion
National Weather Service Riverton WY
1116 AM MDT SUN JUL 17 2016
.SHORT TERM...Today through Tuesday night
Imagery shows broad flat ridging across most of the CONUS with weak
bookend trofs across each coastal zone and embedded weak
disturbances moving through this flow. Surface has front through
most of WY, draped across the southern portion of the state and
linked up with weak low pressure set-up over southeastern Wyoming.
A few light showers currently falling near and south of this
Today/Tonight, ridging aloft and post frontal with low pressure
trying to develop into southwest WY while some low level return flow
moves out of the central plains into eastern/central WY. The western
portion of the state will see winds back out of the southwest. Dry
again across most lower elevations, however winds across area where
fuels are critical...in particular Natrona county...will see
generally lighter winds in vicinity of a building weak surface trof
thereby keeping fire danger elevated but not extreme. Most
shower/thunder action this day will occur across portions of
southern to central WY as well as the mountains as another ridge
running disturbance moves across the FA and aids in triggering
convection. The best chance for a few stronger storms with brief
heavy rain, small hail and near severe wind gusts will occur
across the eastern zones...particularly Natrona county. However,
most severe activity will occur east of the FA over eastern
Wyoming where better deep moisture and dynamics will allow for
near supercell type structures.
Monday and Tuesday, upper ridge moves a bit further east into the
plains while the ridge center sets up across the southern plains. It
appears that a monsoonal type flow is setting up across the southern
and central Rockies although mainly only into eastern/southern
portions of WY. A lee side trof/surface will also build into
central/eastern WY. With the upper ridge axis now tilted slightly
from southeast to northwest upper level flow over most of the CWA
will be from the southwest and most of the FA will be well capped.
Most of Monday and Tuesday`s convection will then lay across the
southeastern quad of WY with only isolated showers/storms over
mainly the higher terrain of the Absaroka, Uinta and Medicine Bow
mountains. Fire danger looks to remain elevated but not extreme both
.LONG TERM...Wednesday through Sunday
GFS and ECMWF models are closer tonight with the weather pattern.
Wednesday and Thursday will feature a southwest flow aloft with
more of a west southwest flow aloft on Friday. The main forecast
issue the past few days has been the mid and upper level moisture
return and the potential for thunderstorms. The trend is now for
less thunderstorm activity over the north and a little better
coverage over the south half of our area. Will adjust Wed through
Fri forecast in this direction. Model differences show up
Saturday and Sunday with respect to a weather system that moves
east out of the Pacific NW. The GFS is stronger and slower with
the progression and has it moving into Montana next Sunday.
Meanwhile, the ECMWF model is weaker and faster and has this
system moving through Montana on Saturday. No clear cut favorite
this far out. Still expect isolated storms over the south
Saturday. Isolated activity over the far north near the Montana
border Saturday and Sunday due the proximity of the weather
system those days. Otherwise dry next Sat/Sun. Very warm to hot
high temps during the period with little day to day variance.
Maybe a little cooler over the north Saturday or Sunday depending
on when the weather system moves through Montana. Fire danger will
continue to increase in this pattern.
Showers and thunderstorms will be found across much of central and
western Wyoming this afternoon. Coverage is expected to increase
through 00z. Gusty wind and small hail will be possible, with
associated periods of MVFR conditions and mountain obscurations.
Showers will weaken across the area by 06z, with mainly mountain
activity lingering through 12z. Widespread VFR conditions then
expected after 12z. Wind will start to increase after 16z, mainly
across southern and western Wyoming.
Behind the front today, RH values will be slightly higher than on
Saturday. And while it will still be on the dry side, winds will not
generally be as strong as those Saturday except for brief periods
under/near thunderstorm activity this afternoon. Therefore fire
danger will remain elevated but not extreme for all zones including
those where fuels are critically dry. Similar conditions will
persist through Tuesday while most thunderstorm activity shifts
east and south of the forecast area.