Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Riverton Wyoming
321 am MDT Friday Aug 28 2015

Short through Sunday night

The shortwave trough now exiting southeast on the front side of the
ridge over the Idaho/Wyoming border produced its share of
convection that was peppered across much of the County Warning Area earlier this
evening and elicited some good wetting rains. Over one half inch
of rain fell just southeast of Casper as well as above Bull Lake
on the Little Bob Lake fire on the east sloe of the Wind River
mountains. These showers have stratiformed out and still linger
south and east of the Lander Riverton area at the time of this
writing. Downdrafts from these earlier convective cells have
compressed the smoke moving in from the Pacific northwest and were
pushed across northern and central Wyoming along the asct outflow
boundaries. This smoke will likely disperse somewhat today once we
mix out. Today will be influenced by the ridge with dry weather
and a warming trend as the ridge axis passes by. All we should see
today is very isolated convection over the higher peaks of our mountain
ranges with light winds aloft and lack of sufficient moisture
along with some instability. Then on Saturday a massive trough
will carve out just off the Pacific northwest coast...thus
increasing our difluent southwest flow which will help initiate
some convection in the west Saturday afternoon along with negative
lifted indices and weak cape. In the meantime...southern Wyoming and
areas east of The Divide will become increasingly windy from the
southwest along with lowering relative humidity through the low to middle teens.
This has prompted a Fire Weather Watch for these areas Saturday
1100 through 2100 Sunday. Saturday will likely be the warmest day
of the forecast period. This Pacific trough will weaken as it
comes onshore and encounters the ridge. As it gets sheared by the
ridge...there will still be enough dynamics with the trough and
increasing instability for a better chance late day
thunderstorms...especially in the northern and western mountains late
Sunday. Even though the relative humidity will increase Sunday...a wind shift
associated with the related cold front and the possibility of dry
lightning will be enough to keep the Fire Weather Watch
going...but just from the Green Mountains to cpr.

Long term...Monday through Friday

Warm and mainly dry conditions are expected Monday and Tuesday
ahead of an upper low/trough digging southeast along the British
Columbia coast. A weak surface pressure gradient should keep winds
below critical fire weather thresholds...and afternoon relative humidity appears
to be marginal as well. There could be isolated convection each
day...mainly in rks-cpr Monday where some residual middle level
moisture from the last system could set up...and then just in the
mountains Tuesday.

On models are indicating a good possibility of
extreme fire weather conditions developing. A strong late Summer
trough digging into the Pacific northwest Wednesday...will induce
Lee cyclogenesis and the corresponding breezy to windy southwest
winds. 00z models show very little or no chance of convection
ahead of the trough Wednesday with warm to very warm and
critically low relative humidity expected.

The mainly dry cold front associated with this system is currently
expected to push across the forecast area Wednesday night as the
main system heads into southern Saskatchewan. A secondary cold
front is expected Thursday/Thursday night with a trailing system
pushing across the northern rockies. The track of this second
system could be far enough south to give the area some
precipitation...mainly over the northwest and north. Although
there will be some concern for extreme fire weather conditions in wake
of the front Wednesday night...current thinking is the cooler
conditions will keep relative humidity levels below critical thresholds on

Another trough is prognosticated to dig across the West Coast Friday with
a warm southwesterly flow developing...and the possibility of
elevated/extreme fire weather conditions again.


Aviation.../12z issuance/

Areas of MVFR fu ushered in by a northerly flow Thursday evening
vicinity and east of The Divide will likely linger through 16z-20z
Friday before dissipating. Only a few -shra/-tsra are expected over
and vicinity mountains Friday afternoon with minimal chances of affecting any
terminals. Light surface winds less than 12kts will prevail across the
area Friday afternoon. A backing S-SW flow should keep most of the
area smoke free with prevailing VFR conditions Friday night.


Fire weather...

A generally quiet beginning to the fire weather period. Weak
moisture may introduce a few mountain showers and thunderstorms
this afternoon... with high pressure otherwise bringing light
wind and dry conditions across the lower elevation. The weekend
will bring a different story. A system moving over the Pacific
northwest will tighten the pressure gradient over the
area...resulting in strengthening winds across much of the area.
Humidity expected to remain low ahead of the trough...and the
combination of low humidity...strong wind...and dry fuels has led
to the issuance of a Fire Weather Watch for much of central and
southern Wyoming. Stay tuned for updates relating to this
developing situation.


Riw watches/warnings/advisories...
Fire Weather Watch from Saturday morning through Saturday
evening for wyz275>277-279-281>283-285-287.

Fire Weather Watch from Saturday morning through Sunday evening
for wyz280-289-300.



Short term...lipson
long term...wm
fire weather...lipson

National Weather Service Glossary of Abbreviations