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Area forecast discussion 
National Weather Service Riverton Wyoming
1147 PM MDT Tuesday Aug 19 2014

Short term...tonight through Friday (issued at 221 PM mdt)

Tonight we will transition to more of a widespread shower/isolated
thunderstorm with heavy rain scenario as the rich moisture is
stirred up by some positive vorticity advection and decent qg forcing through about 06z.
There will be some flash flood potential...especially early with
heavier storms and over higher terrain overnight where terrain forced
rainfall with convective elements coincide...especially on S to SW
facing slopes overnight and through Wednesday morning. Liked the overnight
shift thinking with avoiding a Flash Flood Watch...since this event
will be partially convective...then stratiform later
tonight...choosing to wait and see how all this evolves.
However...later during this forecast period...a ffw may be necessary
across some of the Western Mountain zones as the ground saturates and
precipitation keeps falling through at least Thursday.

Wednesday and Thursday...abundant moisture remains in the region but
the upper level forcing leaves for Wednesday...expect more of an
airmass/diurnal thunderstorm setup. There have been some model
uncertainties as to what will arrive on Thursday...but there may be
some more broad lift arriving with another short wave quickly
followed by a long wave trough across the Great Basin taking on a
negative tilt and moving our direction...if this should happen then
some more organized thunderstorm activity could visit the forecast
area but have kept a healthy dose of uncertainty in the grids at
this time. In any is likely that much of the forecast
area will continue to see bouts of heavy rain through Thursday and
perhaps even Friday...especially across the western

Long term...Friday night through Tuesday

Medium-range model solutions are in fair agreement today. Broad
trough across the northwest U.S. To begin the forecast period with
high pressure over the southern U.S. Helping to slow the eastward
progression of the trough. Models have slowed some from Monday runs
regarding the upcoming weekend weather system. Overall...cooler
pattern to be in place across The Rockies through the period.

Long wave trough will be positioned over the intermountain west and
Pacific northwest to begin the period. Weak shortwave energy is
likely to be ejected eastward across Wyoming Friday night and favor
precipitation in the northwest and northern mountains. Have nudged
precipitation chances upward in these two locations. This scenario is
likely to play out through Saturday as the trough nears Wyoming and
southwest flow aloft remains over the forecast area. Some additional
sub-tropical moisture will be drawn north into Wyoming Saturday
afternoon and evening. Have increased precipitation chances east of The
Divide to account for this. If models continue to come more in-line
it is likely that precipitation chances would need to be bumped up and
expanded west Saturday afternoon and evening. Middle-level circulation
will move along our northern border Saturday night keeping showers
going all night across the north. This circulation will swing to our
east by midday Sunday subsequently drawing cooler air south across
Wyoming in cyclonic flow. Favorable west-northwest flow should keep
the bighorns and portions of the northwest mountains wet Sunday
afternoon/night. Elsewhere...the southwest looks to be the driest
area Sunday and Sunday night as the upper flow backs to the west and
drier air pushes in from the west. Zonal flow for Monday will lead
to primarily dry conditions. The exception will be the far north
where enough moisture will linger to at least generate
orographically-driven convection. The next piece of energy will dig
southeast along the front side of an eastern Pacific Ridge Monday.
This will reinforce the trough for Tuesday and bring another shot of
cooler air and increase precipitation chances across the west.


Aviation.../06z issuance/

An upper level low pressure system will continue to drift NE across
Wyoming overnight...into eastern Montana Tuesday morning. A line of
weakening showers and isolated thunderstorms moving into far West
Wyoming at 06z will result in higher mountain obscurations and rain
showers moving into vicinity kjac-kafo through around 09z. Otherwise
scattered showers with ceilings mainly fl100-130 and occasional mountain
obscurations will continue mainly vicinity and north of a
kjac-kwrl-kbyg line through 12z...mostly lifting into central and
eastern Montana after 12z.

An upper trough over the northwest U.S. Will eject the next disturbance
across Idaho on Wednesday afternoon and across northern Wyoming
Wednesday night. Expect isolated to scattered thunderstorms to
spread into western Wyoming through the afternoon with the better
coverage vicinity and north of a kjac-kdub line. This activity will
spread into north central Wyoming through the late afternoon and
evening. Shower and thunderstorm activity will be much more
isolated across central Wyoming Wednesday aftn/eve...vicinity klnd-kriw-kcpr.


Fire weather...

Heights and temperatures are showing a lowering and cooling trend
over the next week. Another surge of monsoonal moisture will arrive
today and will bring showers and wet thunderstorms through at least
Thursday. There could be a cold front arriving this weekend that
will drop temperatures well below normal for this time
of year...along with bring another dose of wetting rains to the
region. Relative humidity will remain on the high side and there are
no widespread high wind events on the horizon either.

Forecast confidence is pretty low once we get into next week. There are
some indications that there could be another ridge building in while
other models are pointing to an exactly opposite solution. That
said...while there could be some ignitions with the lightning over
the next several days...there will be plenty of rain to accompany
the storms...especially once we get past this afternoon with more
mud slides...flash flooding...and difficult travel along any back
country or unpaved roads.


Riw watches/warnings/advisories...


Short term...Allen
long term...cnj
fire weather...Allen

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