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Area forecast discussion 
National Weather Service Riverton Wyoming
247 am MDT Tuesday may 26 2015

Short term...today through Thursday night

WV imagery showing...showing general lw trough across western 2/3s of the
Continental U.S. With embedded SW trough over the northern...cntrl...and into the Southern
Plains. Upstream...another SW trough with closed low resides down through
the western coastal states with center of circulation over Washington. In between
these two...weak SW ridging over Wyoming. Surface has a weak broad area of
high p across the intermountain west...with a weak trough/developing
weak front from central Montana...SW to southeastern or.

Showers mostly ending this morning except for small chances remaining
primarily over the mountains and northwest Wyoming as a small upper wave brushes
the region. Otherwise...influences from the upstream trough/closed low
system pushes the weak ridge east...while divergent upper southwesterly flow
begins to overtake the forecast area from the west while a modest embedded SW
moves through this flow across the western forecast area in the afternoon...catching
up with this mornings upper wave...and helping to initiate showers and
thunderstorms...especially west of The Divide...especially across SW
Wyoming. Increasingly deep and unidirectional flow out of the southwest
will likely create a mesoscale-low and convergence zone from the South
Pass area...eastern Fremont County...and into at least western Natrona
County. Generally...more isolated storms expected east of The Divide.
However...any storms near/over this convergence zone may be somewhat
stronger than most by evening...offering up half inch to three
quarter inch hail if/when they fire. Convection will decrease in
coverage overnight...with a few being most persistent right under
the upper wave as it moves quickly off to the NE overnight.

Wednesday...upper trough continues to move toward the forecast area...with weak up SW
ridge moving over Wyoming on the tails of the overnight SW trough...and surface
Lee troffing over eastern Wyoming. Some dry entrainment still expected across
much of the SW to central forecast area...with adequate cape...upper wind speed and
deep shear breaking out isolated to scattered thunderstorms and rain elsewhere. Deep
shear will begin to favor some storm organization...with ml cape
rising above 500 j/jg for most areas...and sb cape eclipsing 1200
j/kg in some areas east of The Divide. Storms will be able to rotate
sustain themselves better with a some producing small to near severe
hail...especially from eastern Fremont County...eastward and into Natrona and
Johnson counties along/near/east of another developing convergence
zone. Storms will also move at a faster clip...20 to 30 miles per hour and
hopefully with somewhat less potential of
flooding.

Thursday...main...but weakening...upper trough moves into/across Wyoming...with
upper winds decreasing in speed...losing some of the better middle/upper
forcing and shear profiles from the day before along with more dry
air entrainment. Still...modest moisture and warmth will help to
produce relatively good cape...with decent middle/upper vorticity
rotating within the base of the upper trough over the swrn/srn...to central
and then onto the eastern County Warning Area. The set up will produce scattered
rain showers/thunderstorms and rain west and east...with the stronger more robust storms of
the day over Natrona and Johnson counties...especially in the area of
maximum ll convergence from northern to eastern Natrona County.
Overnight...precipitation will lose a good portion of the convective
component and be driven by the dynamics left with the upper
trough...with areal coverage decreasing rather quickly as surface to middle
level high pressure builds in across the forecast area.

Long term...Friday through Tuesday

Upper low currently over far eastern Washington/northern Idaho will eventually get
kicked southeast as current upper low near the southern Alaska coast moves into
western b.C. At the start of this period. Both the GFS and European model (ecmwf) have
a weakly closed off low over southwestern Wyoming at 12z Friday with surface
high and associated cold front pushing south...east of The Divide
..as a shortwave skims the far northern High Plains. The combination
of the upslope east of The Divide and middle level low should provide
a rather wet period through the day Friday...especially along and
east of The Divide. Gradual improvement later Friday night into
Saturday morning as circulation weakens and most likely moves off
to the east or Southeast. Ridge is then expected to build over the area
through the weekend with considerably warmer temperatures. The GFS
has more moisture and weak ripples underneath the building high
and thus much more afternoon and evening convection. Most of the
convection should be over the mountains and forecast indicates
this for now. Early next week...an upper low currently near
40n/150w is kicked eastward by a piece of energy that comes off of eastern
Asia and moves across the Pacific. This upper low is forecast to
move into the northern California/Oregon area early Monday and then slow down
as it moves into the far northern rockies before stalling out into
midweek. Potential for increasing thunderstorm chances exist in
our northwestern corner with potential cold front lying in the area
combined with afternoon heating and jet streak early next week.
Some strong storms could certainly develop in this area in this
favored pattern so with time we'll just have to see how the details
work out. For now...another unsettled period to start the medium
range period...followed by a warmer weekend with most
showers/thunderstorms in the mountains followed by a potential increase in
thunderstorms in the west/northwest sections Monday into Tuesday with approaching
cold front and upper trough. Continued warm east of The Divide
with some cooling in the far west/NW.

&&

Aviation.../12z issuance/

Patchy fog will partially obscure terrain through 15z...especially
where significant rains have recently fallen across central
Wyoming.

A weak middle-level disturbance is expected to push scattered rain showers/isolated thunderstorms and rain
from SW into central Wyoming Tuesday afternoon/evening with most thunderstorm
activity across the southern half of Wyoming. Showers will linger
across central Wyoming past 06z Tuesday while the west will
partially clear with patchy fog redeveloping early Tuesday morning.

&&

Fire weather...

Fire danger low and will remain low through most of
the work week. Seasonally cool to warm temperatures will combine
with relatively moist atmospheric conditions to produce decent
chances for showers and thunderstorms each afternoon and evening.
Lightning and storm specific gusty winds will be the main threat.
Generally too...smoke dispersal each day...except for short periods
in the very early morning hours...will be good to excellent.

&&

Riw watches/warnings/advisories...none.

&&

$$

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

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