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Area forecast discussion 
National Weather Service Riverton Wyoming
1137 PM MDT Monday Jun 1 2015

Updated for 06z aviation discussion.

Short term...(this evening through tuesday)
issued at 331 PM MDT Monday Jun 1 2015

The main concern continues to lie with this evening on the
timing of the convection moving through the northwest corner of
the state.

latest water vapor loop was showing the potential vorticity
anomaly continuing to push towards Utah and southern Idaho. This
feature is prognosticated to lift northeast into northwest Wyoming by
this evening. Based on the latest satellite trends it appears like
the leading edge of the potential vorticity anomaly should reach
the Idaho/Wyoming border around 01z. This appears to be very close
to the timing that the 12z NAM was projecting. As a result...we
could see some strong to severe storms erupt in Idaho during the
next couple of hours. Current 0-6km shear profiles at ksfx were
fairly marginal for severe convection...but as the upper trough
approaches the surface flow may try to back around to south-
southeast which is currently the case in northern Utah. This will
certainly increase the potential for higher shear values and the
potential for supercells. If supercells do get going in eastern
Idaho then some of them could creep into Yellowstone and The
Tetons. As the evening wears on...these storms could form into a
cluster and shift northeast towards before weakening over areas
east of those regions. The rest of the area may see scattered
showers and thunderstorms this evening in response to the jet
streak and upper diffluence. However...the bulk of the convection
should begin to weaken and dissipate by midnight.

Tuesday-Tuesday night:
the potential vorticity anomaly/upper level wave is prognosticated to
lift northeast into Montana early Tuesday morning. However...there
may be some scattered stratiform showers/thunderstorms lurking
behind the wave in response to isentropic lift from rising Theta
surfaces. Otherwise...the southern half of the forecast area may
experience drier and warmer conditions due to downslope flow on

Wednesday-Wednesday night:
quite a bit more uncertainty exists in this timeframe as the
upper level diffluence continues to prevail. This will yield to
more afternoon scattered showers and thunderstorms especially over
the higher terrain. The GFS/European model (ecmwf) are showing a fairly strong
potential vorticity anomaly moving through western Wyoming
Wednesday evening which may help enhance convection during the
late afternoon and early evening. The NAM is not showing much
reflection of this wave...which is not yielding to near as much
precipitation chances. For now...we will need to continue to see
how models evolve with time.

Long term...(tuesday night through monday)
issued at 331 PM MDT Monday Jun 1 2015

The active Spring pattern will continue as persistent SW flow
into the intermountain west develops ahead of a developing late-
week SW Continental U.S. Upper cut-off. Shortwaves embedded within this flow
will be the culprit for daily mainly afternoon/evening convective
chances. The initial negatively tilted shortwave will lift NE
reaching Utah by late Wednesday. Models forecast 500-1000 j/kg of
SBCAPE across much of the County warning forecast area with the highest values found over
Johnson/Natrona counties. Medium range models are consistent in
lingering this convection Wednesday nt as the shortwave trough
axis lifts northeast through the area. 0-6km wind shear will be
sufficient for strong to locally severe potential on Wednesday in
places east of The Divide. Thursdays convection and any
thunderstorm potential will depend on the timing of the initial
shortwave. The latest European model (ecmwf) speeds this ftr it onto
the plains Thursday. Meanwhile...the GFS is slower...lingering
instability west across byg-cpr even into Thursday afternoon.
Transitory ridging will move over the County warning forecast area Thursday nt/Friday morning as
the upstream pattern amplifies ahead of SW Continental U.S. Cut-off low.
Although expect a generally minima in convective chances during
this will be short-lived as the next shortwave lifts NE
across the County warning forecast area Friday afternoon and night. SBCAPE forecasts again
flare to 800-1200 j/kg byg-cpr Friday afternoon/evening. 0-6km
shear values increase to around 45 kts Friday evening which
suggests at least the potential for a few strong storms in parts
of central Wyoming. As split flow strengthens this weekend...the
area will be caught between the northern stream and SW Continental U.S. Cut-off
low. Although some afternoon and early convective chances will
continue Sat/sun...expect the coverage and severe threat. Temperatures
will be warm as highs reach into the upper 60s and 70s across
lower elevations through the period.

Aviation...06z tafs

Isolated showers and storms east of The Divide until 09z.
Isolated showers west of The Divide overnight.

Showers and thunderstorms will develop over northwest Wyoming
after 18z and continue through 03z Wednesday. Most of the activity will
be along and northwest of a line from Afton to Buffalo. Wind gusts
to 40 knots will be possible near storms. Local MVFR conditions will
be possible with storms.

Fire weather... issued at 331 PM MDT Monday Jun 1 2015

Unsettled conditions will prevail during the remainder of the
week. This will yield to scattered chances of afternoon showers
and thunderstorms especially in the higher terrain of northern
Wyoming. This should keep the bulk of the area in green-up
conditions through the week.


Riw watches/warnings/advisories...none.



Short term...Cox
long term....Hahn
fire weather...Cox

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