Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 060709

Area Forecast Discussion
National Weather Service Riverton WY
109 AM MDT Thu May 6 2021

.SHORT TERM...Tonight through Saturday

We still have some showers out there this afternoon in association
with some unstable northwest flow and some leftover moisture from
the showers of the past few days. These are largely confined to the
eastern half of the area. With cool temperatures and not a ton of
CAPE, we don`t anticipate any severe weather this afternoon.
However, with the low wet bulb zero levels any storm or even shower
could have some small hail with it. There could also be some gusty
winds with any shower or storm, but that is pretty much every storm
in Wyoming. Any precipitation should end around sunset with dry
weather later tonight.

Transitory ridging should then build across the area and bring
around a 42 hour period of dry weather. Thursday looks to be the a
really nice spring day for most areas with plenty of sunshine, mild
temperatures and light to moderate wind. We will leave it at that.
Get out and enjoy it if you can.

Friday will be the transition day as a Pacific cold front and trough
approach from the west. The models have trended slightly slower
though. As a result, we kept the morning dry and confined most of
the POPS to the west during the afternoon hours. The other concern
is elevated fire weather. With a combination of temperatures
expected to rising to around 80 in some areas as well as a gusty
southwest wind developing, some fire weather statements may be
needed. As the front slides into northern and central Wyoming, areas
favored by the cold advection / northwest flow pattern will see some
gusty winds, but mid level winds are not strong enough for high

Saturday looks to be a contrast across the area with cooler and damp
weather across the north with much cooler temperatures than Friday.
The models have also trended cooler in the south as well, so fire
weather concerns may be eased somewhat. All models are in agreement
with more showers in the north. Uncertainty is greater across the
south with coverage of showers through. For now, we made few changes
to continuity until hopefully the models come into better


Upper low expected to be to our north with northwest flow aloft
across Wyoming Sunday. Proximity of the upper low, favorable
instability, and pesky northwest flow should favor showers over the
north half of the forecast area, especially Sunday afternoon.
Temperatures below 0C indicate below normal highs in the 50s for
Sunday. Jet streak begins to dig down the west side of the trough
Sunday night bringing with it a reinforcing shortwave. This will set
the stage for a better chance of precipitation across the entire
forecast area late Sunday night and particularly Monday. Cluster
analyses showing a more progressive pattern in northwest flow aloft
during this period with the deeper, slower GFS ensembles the
outlier. The preferred ECMWF deterministic solution shows a 700mb
low spinning up over eastern Wyoming Monday providing a bit better
chance of upslope precipitation in central Wyoming. However, 700mb
wind speeds remain unimpressive and QPF has been tempered to one-
quarter to one-half inch for many areas. Temperatures Monday remain
below normal with lower elevation highs 45-55F. This will result in
snow levels likely hovering around 7000 feet MSL, so accumulating
snow will occur. Favored areas should be the east slopes of the Wind
River and Bighorn ranges along with the southern Absaroka Range.
Shortwave energy and the associated jet will traverse the region
Monday night and Tuesday morning with precipitation lingering. A
flat ridge to our west should begin to dry conditions from west to
east late Tuesday and Tuesday night. The departing shortwave with
the jet crossing the region may lead to gusty west-northwest wind
Tuesday. Nothing worthy of highlights, but something to be watched.
This trend continues into Wednesday with drier northwest flow aloft
over the forecast area. Temperatures begin to rebound Tuesday and


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 108 AM MDT Thu May 6 2021

VFR conditions will prevail through the TAF period. Winds will
remain mainly light and variable at all terminals. The exception
will be a bit of a breeze at KBPI and KRKS during the afternoon
hours. Any gusty winds will diminish around sunset and return to
light drainage flows overnight.



Isolated to scattered showers and thunderstorms should end this
evening. Any shower or storm could have small hail or gusty winds.
High pressure will then bring dry weather from late tonight through
Friday morning. Relative humidity will fall into the teens at times
Thursday afternoon. However, wind will remain light to moderate.
Mixing and smoke dispersal Thursday afternoon will range from fair
to poor in the northern basins to good to excellent in the west and
south. Elevated fire weather is possible Friday afternoon across





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