781
FXUS65 KRIW 201122
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
522 AM MDT Thu Jun 20 2024

.KEY MESSAGES...

- Showers and thunderstorms are possible (15% to 40%) this
  afternoon and into the night, mainly east of the Continental
  Divide. More widespread showers and thunderstorms develop
  Friday.

- Hot Sunday and Monday. Hot and dry weather continues through
  next week.

&&

.DISCUSSION...
Issued at 146 AM MDT Thu Jun 20 2024

Temperatures continue to climb today, with highs sitting right
around, to just above, average for mid-June. A high over the eastern
U.S. combines with a trough sitting over the west coast to funnel
some moisture into Wyoming this afternoon. The NAM projects
precipitable water (PWAT) values in the 150% to 175% of normal range
mid-afternoon. The best moisture looks to remain to the east, with
some isolated to scattered (15% to 40%) showers and storms
developing from Sweetwater County into central Wyoming, including
Fremont, Natrona, and Johnson Counties. Some development over the
Absarokas are also possible (20% to 30%). Isolated chances (about
20%) continue overnight for those mentioned locations, and also the
eastern Bighorn Basin, but an overall downward trend in rain
coverage occurs into the night.

Better moisture begins to make its way into the state late Thursday
night into Friday morning. Aided by some shortwave energy entering
from the west, widespread shower and thunderstorm chances occur
across the area Friday. Some showers may still be going on from
overnight, with some more showers developing in the morning. Showers
really ramp up in coverage during peak heating Friday afternoon. The
least likely (15% to 30% chances) places to get any rain will be far
western Wyoming, including northern Lincoln County, north through
Yellowstone. High-resolution models are projecting a fairly large
swath of scattered to numerous (40% to 70% coverage) showers and
thunderstorms moving across the area between 1pm and 7pm, from
western Wyoming to Johnson/Natrona Counties in that timeframe. This
will be the best shot for rain through the day. Rain quickly comes
to an end Friday evening, leaving a dry night.

Saturday starts what will be a mostly dry and hot next week. High
pressure begins to build in over the Four Corners Region. This
leaves the area hot and dry for the weekend. Highs Saturday begin to
climb. By Sunday, highs will be firmly in the middle 90s east of the
Continental Divide, with upper 80s and a few low 90s west. 700mb
temperatures are around 18 to 19 degrees Celsius, which could bring
100 degree temperatures to places in the eastern Bighorn Basin. For
reference, average highs this time of year are in the upper 70s to
middle 80s. Temperatures look just as hot Monday. To say Tuesday
looks cooler is a bit of an overstatement, but highs are looking a
few degrees cooler. High pressure looks to remain through at least
next Thursday, maybe Friday. This will keep these above average
temperatures in place. Shift of Tails is also on board, highlighting
much of the area in at least the 60th percentile for high
temperatures, with some 80th percentiles Sunday and Monday. A few
shortwaves look to move through during next week, though these seem
to have little impact; perhaps a shower or two is possible depending
if they are strong enough, but overall conditions remain dry.

The GFS and ECMWF are showing a trough in the timeframe of next
Friday (6/28). This could be what breaks the hot and dry pattern,
but being so far out, there is low confidence in this model
projection.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 516 AM MDT Thu Jun 20 2024

High clouds continue to sweep across the state this morning. A
bank of lower clouds associated with increasing surface moisture
has been building toward KCPR, which will bring cigs down to
MVFR for the next few hours, before lifting again by 18Z. Winds
east of the Divide will remain light from the east, generally 10
mph or less. A few higher gusts are possible at KCPR this
afternoon. Areas west of the Divide will see southwest winds 5
to 10kts, with some gusts to 20kts at KRKS. Showers and a few
thunderstorms will develop this afternoon, but will stay mainly
in central and eastern WY, withholding effects to just KCPR
this afternoon. Between 00Z and 06Z tonight, winds will all
shift from the east around 10kts, bringing in more near sfc
moisture. This will build low clouds earlier into KCPR, and
possibly reaching KLND/KRIW around 12Z/21.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Wittmann
AVIATION...Straub