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Area forecast discussion 
National Weather Service Riverton Wyoming
250 am MDT sun Aug 30 2015

Short term...today through Tuesday night

A strong trough of low pressure just off the Pacific northwest coast
has ejected a sheared upper level shortwave eastward that appears to
be located over northern Idaho and western Montana. Active convection
continues out ahead of this feature across eastern Idaho near the
Wyoming border and into ynp at the time of this writing. It will be
an early show for convection in western Wyoming today with these storms
about to sweep across far West Wyoming during the remainder of the
night. Then the main trough will approach the northern rockies today
for another round of convection firing up in far western Wyoming by middle
day ahead of the main trough and asct cold front. In the
meantime...east of The Divide...southwest winds will increase over
what they were Saturday as they draw in slightly higher lower level
moisture. Instability will be in place over the entire County Warning Area but only
scant moisture to work with to produce thunderstorms east of The
Divide...at least initially in the early to middle afternoon. There
will be a separate finger of weak high and middle level monsoon
moisture that will stream in across rks and cpr. However the best
instability will be in place across northern Wyoming where the best convective available potential energy
will be. It appears the best chance of any dry lightning will occur
on the periphery of any high based thunderstorms along the
aforementioned weak monsoon moisture plume as well as across
northern Wyoming east of The Divide where relatively high based storms
will correlate with the best convective available potential energy. Dry thunderstorms will morph
into more wet storms as the front draws near. The troughpa will
sweep across the County Warning Area tonight. Then on Monday...another surge of weak
middle/upper level moisture will overlap some lingering instability
over extreme southern Wyoming where some isolated T storms are expected.
Then by Tuesday...another closed low will establish itself along the
southern British Columbia coast. This low will eject a weak vorticity lobe east
northeast across the northern rockies that will result in isolated
convection...mainly over the northwest mountains Tuesday afternoon. Today will be a
couple of degrees cooler than Saturday as clouds and shower activity
decrease the solar insulation. Afterward...temperatures will cool a few
degrees Monday before warming back up Tuesday.

Long term...Wednesday through Sunday

High latitude energy currently over Alaska and the Bering Sea area will
dive southward into our pacnw trough as upstream energy...typhoon
remnants...pumps up the upstream ridge over the eastern Pacific. The
end result is a deepening cold early September trough over the
pacnw and intermountain west the second half of next week that
gradually translates eastward as the flow deamplifys towards the end of
the period and possible split develops over the west early next
week. The midweek period will generally be dry and quite warm with
breezy to locally windy periods and very limited mainly mountain thunderstorms
chances at best. By Friday...the first significant shortwave out
ahead of the trough is already impacting the west and north as a
cold front tries to sag southward into the northern zones. This frontal
boundary will be a significant player through the weekend with
considerably cooler if not colder weather behind it as the main
trough opens up and passes across northern Wyo and Montana over the
Labor Day weekend. A good chance of showers and thunderstorms should occur
in at least the northwestern sections with this first shortwave and then
more of the west and north as the dynamics interact/overrun the
frontal boundary over the weekend. Details are still uncertain but
the GFS and European model (ecmwf) are in pretty good agreement with the big
picture. The source region of this trough is quite cold and model
guidance is dropping 700 mb temperatures down to around zero to even below
with the first shortwave Friday night and then with the main
trough Saturday night so mountains would get a decent shot of some
snow with each of these. It is Labor Day weekend after all.

&&

Aviation.../12z issuance/

A cold front will continue to push east across the intermountain
west today with surface SW flow increasing across west and central
Wyoming ahead of the front...along with areas of low level wind shear. Surface SW winds
at 20-30kts will prevail across the area late this morning into
this afternoon. The main area isolated-scattered -tsra will develop vicinity
and south of a kevw-klnd-kcpr line around 18z today with this
area slowly propagating south and east through the afternoon and
early evening. The cold front will bring drier air and gusty west
wind into northwest Wyoming middle to late this afternoon...pushing across
north and central Wyoming this evening. Clear skies and
diminishing wind will prevail across the area behind the front
early Monday morning.

&&

Fire weather...

Red flag warning through Sunday for portions of
central Wyoming...

On Sunday...overall it will be windier than it was Saturday but with
slightly higher relative humidity. As the cold front associated with
the trough digging to the west advances...it will likely settle into
western...northern and southern Wyoming fairly early in the
day...which would end the fire weather threat in those locations
along with increasing clouds as well as a slight chance for a
wetting rain. Strong southwest flow but still low humidity across
Fremont...Johnson and Natrona counties will occur so these areas
will continue to be under the red flag warning for today. In
addition...a swath of middle level moisture and continued instability
will kick of a few mixed wet and dry thunderstorms...some of which
could contribute to new fire starts with the most activity expected
across northern and western Wyoming...with a secondary swath
expected over Natrona and eastern Fremont counties late Sunday
afternoon...the chance of a wetting rain out of these storms is next
to zero. The best chance of any dry lightning starts will likely be
on the periphery of any thunderstorms that form across northern
Wyoming east of The Divide through middle day where the best
instability will be...then from eastern Sweetwater County to Casper
and the rest of Natrona County later in the afternoon where the
combination of driest boundary level moisture and widest temperature/dew
point spreads will lie.

A repeat of these conditions will be possible middle week.




&&

Riw watches/warnings/advisories...
red flag warning until 9 PM MDT this evening for wyz280-281-283-
285-289-300.

&&

$$

Short term...lipson
long term...plus
aviation...aem
fire weather...lipson

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