515
FXUS65 KRIW 100436
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1036 PM MDT Tue Jul 9 2024

.KEY MESSAGES...

- Likely the last day of near seasonable temperatures as hot and
  dry conditions are expected to prevail through the remainder
  of the week.

- The real heat begins to build into the region by Wednesday as
  a potent high pressure continues to gradually slide east.

- Heat Advisories have been issued for parts of the Bighorn
  Basin, Fremont, Natrona, and Johnson Counties starting
  Thursday afternoon until Sunday evening.

- Near record breaking heat is looking more likely with highs
  10-15 degrees above normal from Thursday through Sunday.
  Temperatures may reach the upper 90s across much of the CWA
  with some places nearing or exceeding 100F.

&&

.DISCUSSION...
Issued at 250 PM MDT Tue Jul 9 2024

The potent high pressure that has been plaguing the West Coast with
record breaking heat is knocking on the door of the Cowboy State.
Today will likely be the last day of near seasonable temperatures as
the heat intensifies through the remainder of the week. Highs today
range from the mid to upper 80s across most of the CWA with warmer
temperatures in the low 90s over parts of the Bighorn Basin. Similar
to Monday isolated showers and thunderstorms are possible this
afternoon and evening with most remaining over the mountains in the
northern and central parts of the CWA.

Wednesday looks to be the start of the heat as the center of a
potent high from the West Coast makes its way over the Great Basin.
The eastern edge of the high will start to move into western WY
during this time. Models are showing some nasty heat associated with
this potent high. Above normal 700 MB temperatures start to move
into the state, with values near 20C making their way into western
WY. These values would likely translate to surface temperatures near
or exceeding 90 degrees in places such as Jackson Hole, Star Valley
and the Upper Green River Basin. 700 MB temperatures in the upper
teens are forecast for the remainder of the CWA, meaning hot surface
temperatures in the mid to upper 90s.

Models for what seems like the first time this year are being pretty
consistent, continuing to show near record breaking heat for the
second half of the week. Temperatures will continue to gradually
increase through the remainder of the week as the center of the high
slides east. The end of the week will see the high just to the south
of the CWA, with some of the hottest temperatures expected for
Thursday through Sunday. Models are showing 700 MB temperatures on
these days ranging from 20-23C over most of the CWA, especially
central locations. If this does actually occur, surface temperatures
would likely near or exceed 100F in areas east of the Divide. Those
to the west of the Divide will still see hot, above normal
temperatures, with surface values in the low to mid 90s. Western
valleys such as Jackson Hole could likely see highs in the mid 90s
nearing record values.

The most recent models continue to persist the high likelihood of
this near record breaking heat for the end of the week. Ensemble
data shows much of the CWA seeing temperatures within the 90th or
greater percentile. Looking at climate data for all time high
temperatures, some areas current forecasted highs fall within the
top 20 with some records going back to 1891. The NBM is showing a
(40-70%) chance of seeing 100 degrees or higher temperatures from
Thursday to Sunday in parts of the central and eastern CWA. Western
valleys such as Jackson Hole and Star Valley have a (30-60%) chance
of seeing highs meet or exceed 95 degrees. The Green River Basin and
the remainder of the southern CWA have a (40-80%) chance of seeing
highs greater than 90 degrees. Multiple climate sites have the
potential to see record highs met or broken with many areas seeing
forecasted highs within 3 degrees of records. The heat lingers for
multiple days with some areas seeing near record high values for 3
or more days. Overall, this high pressure definitely means business
with many records already being broken in parts of the Western
CONUS. Thankfully, we are not expecting anywhere near as extreme of
heat here.

Heat Advisories have been issued across parts of the Bighorn Basin,
Fremont, Natrona, and Johnson Counties starting Thursday afternoon
until Sunday evening. The main concern regarding this upcoming heat
is the prolonged period of high temperatures nearing or exceeding
100 degrees over the course of 3 or more days. Climate data is
showing that some places highlighted within the advisory may see a
period of heat that nears all time record highs. While overnight
temperatures are forecast to cool and provide some relief to the
heat, this only looks to be for a short period of time during the
early morning hours. Areas west of the Divide will need to be
monitored as they may see near record heat over the course of
multiple days as well. However, at this time there is not enough
confidence to issue any highlights.

The last thing to mention is the potential for some showers and
thunderstorms by the end of the week. Models show a few weak
shortwaves try to slide by to the north brining the possibility for
afternoon convection. The key thing will be to see if anything will
be able to develop and remain in such a dry and hot atmosphere. The
most likely outcome will be very isolated convection with
precipitation struggling to reach the surface, strong gusty outflow
winds would also be possible.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1036 PM MDT Tue Jul 9 2024

VFR conditions will prevail through the TAF period, however
scattered convective showers/isolated thunderstorms will develop
Wednesday afternoon. Cloud bases will be quite high (above 10k ft)
and it will be very dry, so most of the showers will be virga. The
dry nature of the convection will result in strong
downdrafts/outflows, so gusty/erratic wind will occur near
showers/storms. The prevailing wind will generally stay below 10
knots at terminals over the next 24 hours, but the
downdrafts/outflows will result in stronger wind at times Wednesday
afternoon.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon Thursday to 10 PM MDT Sunday for
WYZ004>006-010-011-017-018-020.

&&

$$

DISCUSSION...Dziewaltowski
AVIATION...Rowe