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Area forecast discussion 
National Weather Service Riverton Wyoming
1219 PM MDT Tuesday may 5 2015

Update...
latest radar loop was showing the bulk of the convection in
northeast Colorado. This convection should gradually spread from
the southeast into the eastern portions of the area early this
evening. Otherwise...it appears like it will be mostly dry this
afternoon with scattered showers and thunderstorms in the
mountains.

&&



Aviation.../18z taf issuance/

The main concern with this taf issuance is the timing of when the
cluster of showers and thunderstorms will move into the area.
Latest water vapor loop was showing the upper level low spinning
over northwest New Mexico. This upper level low is drawing in
copious amounts of moisture from the southeast with rainfall
occurring across much of northeast Colorado. This cluster of
convection will gradually spill into the area this evening. In
addition...surface high pressure will begin to build into the area
from southern Montana. This will increase low level frontogenisis
across northern Wyoming overnight and may yield for increase
chances of showers and lower MVFR ceilings across the northern
taf sites including kwrl...kcod...kcpr and klnd towards daybreak.

&&




Previous discussion... /issued 308 am MDT Tuesday may 5 2015/

Short term...today through Thursday night

.Very active weather pattern through Thursday...

Convection was still firing up during the last few hours within and
along the perimeter of the Wind River basin including the Wind River
Canyon where Marble size hail fell late yesterday evening. This
activity has just about died off. Then the next round of stronger
convective activity will begin very early this morning in western
Wyoming...spreading east throughout the course of the day today as the weak
closed 500mb southern stream circulation now over Arizona will track
to slc today and then to southeast Wyoming this evening. The weak 700mb
circulation will take a similar track through tonight.
Already...other lingering weaker convective cells are tracking
toward the northwest as far north as southern Fremont County as opposed to
the northbound cells in the vicinity of the Wind River Canyon farther
north. With widespread convective available potential energy and lifted indices as low as minus 3 in
some areas today along with the influence of the closed circulation
nearby...expect earlier...more widespread and longer lasting
convection with slow moving monsoon like moisture laden thunderstorm
cells to occur today compared to what we have seen over the last
couple of days. Many of the T cells will contain small hail...gusty
winds and locally heavy rainfall today. The convection should last
well into late this evening. After midnight tonight...the surface/bl
flow will shift to northerly as the stacked circulation drifts to
eastern Wyoming. With the low just to our east...showers will become more
numerous along and east of a line from eastern Sweetwater County to
riw...wrl and gey Wednesday with only isolated thunder Wednesday afternoon
west of the dvd as cooler more stable air filters in around the back
side of the low with most of the cool stable air remaining on the
east side of The Divide. An asct surface low will form in the vicinity of
cpr Wednesday as well. With the light wind directional shear from
the stacked weak circulations just to our east Wednesday and the low
local/cloud bases expected today...funnel clouds cannot be ruled out.
These showers will continue into Wednesday night but will taper off
in coverage. A reinforcing surge of cooler air will sweep south
across the area east of The Divide Wednesday nt with 700 mb temperatures dropping to
minus 6 in northern Wyoming where some snow could actually mix in with
any lingering rain showers in Cody early Thursday morning.
Otherwise...the snow level should average around 6500 to 7000 feet
in northern Wyoming and 8500 feet west of The Divide Wednesday nt where light
accumulations of around an inch or so will occur in most mountain
locations.

In the meantime...the strong slow moving northern stream low center
currently spinning along the central British Columbia coast will
drift to the Pacific northwest today and then slow track south along the
Pacific coast to end up near Vegas by Friday morning. Once the low
makes its way into California...a well defined southwesterly
difluent flow will become established over the County Warning Area ahead of this
stronger low resulting in an area of numerous rain showers that will
track from southwest to northeast during the course of the day Thursday.
Some of these showers will persist Thursday night. By Thursday high
temperatures will be around 20 degrees cooler east of The Divide
then today. Another inch or snow could fall in the mountains again
Thursday night above 9000 feet as well as much of ynp.

Long term...Friday through Tuesday

System currently near the southwestern b.C. Coast will be the main weather
maker Friday through Sunday as it lifts NE from the desert SW and
moves just to our south over the weekend. Kicker system is
currently near the south end of the Aleutian chain. This system
will aid in slowly kicking this system out of the desert SW over
the weekend. On Friday...best precipitation chances still look along and
west of The Divide where some daytime heating and low to middle level
convergence is expected to occur as cold front settles in east of
The Divide. Added in isolated storms west of The Divide to align with
this thinking. On Saturday...as the upper low starts to lift NE to
our south...precipitation chances increase east of The Divide as upper low
combines with upslope caused by a surface high over southern Canada. As
the low jumps over The Rockies...Lee cyclogenesis over eastern
Colorado gets going in earnest which will help with our wcb into
at least our eastern zones Saturday night and Sunday along with colder
middle level temperatures. Will have to watch the potential for rain east of
The Divide to change to snow by Sunday morning before ending
Sunday night. Models diverge early next week to what the pattern
evolves into. European model (ecmwf) is the cleanest solution with the low moving
away and being replaced by a weak ridge Monday night and Tuesday
before a weak shortwave moves into the west. The GFS is more
chaotic with a piece of the Canadian energy moving into southern Montana and
getting lost in the flow through Tuesday. Confidence low as to the
details past Sunday so keeping most probability of precipitation in the mountains early
next week. The main weather is still shaping up to occur over the
weekend with significant mountain and foothill snow possible in
central areas along and east of The Divide along with increasing
wind and falling temperatures.

Aviation.../12z issuance/

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

There could be some fog/low clouds this morning...mainly over
Johnson...Natrona and southeast Fremont counties...from recent
moisture or a moist easterly flow. Attention then turns to
convection developing this afternoon. The showers and thunderstorms
could be relatively late starting...but an approaching storm system
will keep chances of convection ongoing through the night. Actually
the best chance of shower activity and MVFR conditions will probably
take place after 06z Wednesday...when dynamics from this system
overspreads the region and a front begins to slide into the area.

West of The Divide...kbpi/kjac/kpna/krks routes

Patchy fog/low clouds could occur in a few spots due to the recent
moisture and small temperature/dewpt spreads this morning. Then attention
turns to convection developing around midday and becoming more
intense/widespread during the afternoon. Will have thunderstorms in the vicinity at all
terminals...but will likely need to have some tempos for thunderstorms and rain in the
20z-03z as time gets closer. Convection should linger well into the
night especially along The Divide and over Sweetwater County.

Fire weather...

A generally wet and stormy week ahead with light winds driven by
terrain and nearby shower or thunderstorm activity. A push of
moisture will arrive this afternoon...which will benefit the
Casper dispatch area the most with wetting rains. Storms are slow
moving so any that do form will likely produce a wetting rain.
There could be a bit of a break from the thunderstorms on
Wednesday...and Thursday for Casper and Cody dispatch areas
although numerous showers are expected. A much larger system moves in with
mountain snow and a very good chance for a wetting rain Saturday
and possibly again on Sunday in the low elevations.

All in all...both the GFS and the Euro have both been in very good
agreement with the track of these weather features for the last
several days. Some of the best consistency seen in a while.

&&

Riw watches/warnings/advisories...
none.
&&

$$

Short term...team Riverton
long term....99
aviation...team Riverton

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