Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181756
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1155 AM MDT WED MAY 18 2016

.SHORT TERM...Today through Friday night

Imagery shows virtually the same pattern as yesterday with a general
longwave across most of the CONUS with an extended trough axis
stretching from Maine back through southern California. A closed low
within the trough...in a near/weak Rex block pattern...is now
centered over the desert southwest at the interface between
California and Arizona. Surface has general high pressure across
Wyoming into the high/central plains with lower pressure off over
the desert southwest. No precipitation present across WY.

As the upper closed low regains contact with a westerly component in
the large scale flow, it will finally open and kick eastward into
the central/southern Rockies. This in turn will allow modest
moisture to begin to encroach on western Wyoming as the mid/upper
flow turns westerly once again. With low end warming beginning
across the CWA today, there will be slightly better instability
across the state and particularly the west. However,(mainly) only
mountain areas will be able to overcome capping and allow some
terrain forced convection to begin west of the divide and across the
south. These showers/storms will move from the northwest to
southeast slowly at 10 to 20mph and will likely produce some gusty
winds, moderate to brief heavy rain and some very small hail into
the evening period. Overall impact looks fairly low - with all
storms generally handled well by an SPS. However, a couple of these
showers/storms may become almost terrain tied, having a hard time
moving off the mountain...causing some ponding.

Tonight, the blocking pattern continues to break down and the trof
moves out over the high/central plains of Colorado and Kansas while
another large trough moves into the PAC NW. Flow begins to
strengthen some aloft along with ascent ahead of the trough, while
lee side troughing occurs over WY. Then, through the day Thursday,
relatively weak low pressure tries to spin up across central Wyoming
ahead of a quickly approaching front from the west. For a period of
time Thursday afternoon and evening, in the warm sector ahead of the
front, a few isolated strong to marginally severe storms will be
possible mainly across western/southwestern WY. Strong wind gusts
will be the mostly likely form of severe with DCAPE values likely to
approach/exceed 1000 J/KG while storms will already be moving at 30
to 35 kts from the southwest. A little large (.75 to 1 inch) hail
cannot be completely ruled out either - with VILs around 40 looking
to produce near severe based on storm tops of around 35k ft.

Thursday evening and overnight, the front will push into/through
western Wyoming, with good low to mid level forcing combining with
the terrain and increasing potential vorticity aloft squeezing out
decent precip from the moisture feeding into the area from the
eastern Pacific/PAC NW. Snow levels will remain fairly high however,
at or above 10000 feet to start, only falling to between 9000 to 9500
feet by early Friday morning...with mixed precip perhaps down as low
as 8500 feet. So, while QPF will be decent, snowfall should be
limited...well below advisory levels for all elevations. By Friday
morning, the front should wash out as it traverses Wyoming from west
to east and moves away from the upper level support of the main
trough while the trough itself slows down to a crawl as another
blocking type pattern develops. With the only real forcing now coming
from the mid/upper levels around the trough and the orientation of
flow now from the drier southwest, precipitation chances Friday
afternoon/evening will generally be confined to the mountains of the
west. Isolated to widely scattered very fast moving showers and
storms will ensue...cruising to the north at 40+ mph... with quite a
bit of wind, but not much in the way of rainfall. There will probably
be some severe gusts from these storms just based on storm
movement/speed itself...but shower/storm coverage should be fairly
isolated...dying off through the evening period.

.LONG TERM...Saturday through Wednesday

The stalled strong upper low/trough over the Pacific NW/West Coast
will gradually open up and push across the Intermountain West and
Rockies late Saturday through Sunday evening. The associated cold
front is expected into western Wyoming Saturday and then across
the rest of the area Sunday. This will result in best chance of
convection across the west Saturday...with probably little east of
the Divide...with convection spreading east of the Divide Sunday
with the front. Instability Saturday and Sunday is progged to be
low. This combined with the cold front not being as strong as it
will be initially on Thursday...should keep any strong
thunderstorms to a minimum across the forecast area. Otherwise the
big concern will be continued active mountain snowmelt this
weekend east of the Divide with above average temperatures.

In wake of this system this weekend...a long wave trough will
remain over the western third of U.S. early next week. This will
keep the weather unsettled at times...with generally below
average temperatures. Looks like some weak energy both Monday and
Tuesday followed by perhaps a more organized system on Wednesday.

&&

.AVIATION.../18Z ISSUANCE/

West of the Continental Divide...KBPI/KJAC/KPNA/KRKS Airports

Mid level moisture this afternoon is already leading to moderate
cumulus development over the mountains and surrounding valleys and
basins.  Scattered rain showers with isolated thunderstorms are
expected over the mountains with isolated showers and thunderstorms
impacting KJAC, KBPI, and KPNA from about 19Z to 02Z.  Possible rain
showers will move from NW to SE and impact KRKS from about 00Z to
06Z, as well as north of KJAC through 06Z. Higher terrain certainly
obscured this afternoon and evening. May see some gusty wind of 20
mph from the showers.  Southwest flow to increase after 15Z Thu as a
line of convection from eastern ID moves info far western WY Thu
afternoon.

East of the Continental Divide...KCOD/KCPR/LND/KRIW/KWRL Airports

VFR conditions expected for the next 24 hours east of the divide.
Isolated showers possible late this afternoon and early evening at
KCOD and KLND as showers move off the mountains.  Otherwise mostly
clear skies with generally light wind, except for gusts near 20 mph
through 21Z at KCPR.  Southwest flow will pick up after 15Z Thu
across central WY with gusts increasing to 30 mph Thu afternoon.

&&

.FIRE WEATHER...

Fire danger low today as green-up and elevated humidity levels
continues across Wyoming. A few showers and thunderstorms will be
possible today, mainly across the higher mountains west of the
divide. Otherwise, warming and drying begins...persisting east of
the divide until Saturday. West of the divide will see the arrival
of the next spring storm system Thursday afternoon and evening with
good chances for more wetting rains.

&&

.RIW Watches/Warnings/Advisories...None.

&&

$$

SHORT TERM...Braun
LONG TERM...WM
AVIATION...McDonald
FIRE WEATHER...Braun



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