Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
319 am MDT Tuesday Sep 23 2014
Short term...Tuesday through Thursday night
For once it looks to be a fairly quiet period through most of the
short term. The upper level low that brought the scattering of
showers and thunderstorms on Monday is now located near the
Wyoming-South Dakota border and continues to move away. In its
wake...ridging will build northward across Wyoming. The main
problems today will be for some fog in areas west of The Divide that
saw more rain yesterday. Also...with a weak shortwave moving through
the flow we have continued the isolated probability of precipitation across the higher
elevations this afternoon for spotty thunderstorms.
Otherwise...expect a mild to warm and dry day today.
The ridge then amplifies into the through Thursday night. The result
will be plenty of sunshine along with high temperatures well above
normal. Some of the warmer locations in the Big Horn basin could see
high temperatures climb into the 90s possibly as early as Wednesday
but with a better chance for Thursday.
Long term...Friday through Tuesday
Synopsis...an upper level low pressure system will dig into the
Great Basin over the weekend and then lift across Wyoming into the
northern High Plains Sunday night and Monday. Dry and unseasonably
mild conditions...near record high temperatures...will
precede this system on Friday. Showers and thunderstorms will push
into mainly western Wyoming on Saturday and into central Wyoming
on Sunday. Chances of precipitation will be highest over the north
and west on Monday.
Medium range models in better agreement on 23/00z runs than 12 hours
ago. European model (ecmwf) has reverted back to a closed SW low scenario over the
weekend with GFS/Gem/ECMWF showing a closed 500 mb low making landfall
and ending up somewhere between Medford and Reno by 12z Saturday.
This system then drifts east-southeast into the Great Basin...into strong
downstream ridge east of The Rockies...over the weekend. Medium
range models also cluster the "kicker" system in the vicinity of
55n/145w by 12z Sunday. Filtering out all the details of vortices
rotating within SW low complex...the European model (ecmwf) is slower overall with
the progression of the system and the onset of precipitation into
western Wyoming than the GFS. Slower guidance usually wins out
with these SW lows and is frequently not slow enough this far out.
Forecast has trended the onset of precipitation a tad slower over
the weekend with the highest chances of precipitation in the west
on Saturday while above normal temperatures and mainly isolated
precipitation chances prevail east of The Divide. Currently...Sunday
looks to be the main washout day as the SW low begins to lift
across the area with Wyoming in the NE quadrant of the system
where the best moisture and instability will reside.
European model (ecmwf) and GFS show completely different scenarios on subsequent
interactions between the SW low lifting out and the upstream
kicker. Without getting into all the nitty gritty details that will
likely continue to change over the next few days...confidence is very
low on how this will all pan out next week. Forecast has kept the
higher chances of precipitation over the west and north Monday and
Tuesday with temperatures slightly below normal across the area.
Areas of low clouds and fog will occur in northwest Wyoming as well
as in the western valleys until 17z. MVFR to IFR conditions are
expected with some mountain obscuration. After 17z VFR conditions
will prevail in these areas through the forecast period.
Elsewhere...VFR conditions will occur today and tonight. Isolated
showers or storms will occur in the mountains after 18z until 02z.
This activity is not expected to impact any terminal forecast sites.
A ridge will build across Wyoming into the middle of the week and
bring warmer and drier conditions. A weak upper level disturbance
moving through today could bring isolated afternoon thunderstorms
today in the mountains but most areas will remain dry. Relative
humidity will fall into the teens by Thursday. However...with wind
expected to remain light to moderate critical fire conditions are
not expected at this time.