512
FXUS65 KCYS 092334
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
534 PM MDT Tue Jul 9 2024

.KEY MESSAGES...

- A potentially significant heat wave is likely to impact the
  region from Friday through Monday.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 216 PM MDT Tue Jul 9 2024

It`s another fairly quiet afternoon across the area today underneath
northwest flow on the fringe of the powerful ridge building in over
the western third of the CONUS. A weak vort-max passed through the
area earlier today and brought some scattered mid-level clouds and a
few showers to areas west of the Laramie range. Temperatures have
warmed back up to near average for this time of year for most
locations, and this has helped to mix out the inversion that
prevented any convection yesterday. Without this cap and with a
little synoptic lift from the vort-max, we`ll have a few high-based
showers going up over the higher terrain this afternoon. A few
rumbles of thunder will be possible too, but rainfall probably won`t
be much more than a trace to a few hundredths of an inch at most.
These may bring some locally gusty winds.

The ridge builds further east on Wednesday and Thursday. By 00z
Thursday, we`ll have a strong gradient of 700-mb temperatures across
the area ranging from +11C near the eastern boundary to near +18C
near the western boundary. 24-hours later, these values increase to
+15C and +20C respectively. This will support highs climbing to
perhaps about 5F above average Wednesday, then closer to 10F above
average Thursday. Thursday will be the first really hot day, with
almost the entire area outside of the mountains expected to breach
the 90s, and some 100s expected in the North Platte River Valley and
near Chadron. Each afternoon will also feature a few isolated
showers and thunderstorms. The low-level warming combined with a
slight increase in moisture aloft from a weak shortwave rotating
over the top of the ridge will help lead to a little more
instability compared to today. Wednesday`s activity should be
confined to near and over the higher terrain, but with a little
better coverage than today. With models showing a more defined
dryline in the NE panhandle on Thursday, we can`t totally rule out
some thunderstorms going up over this boundary, but confidence is
too low to add PoPs into the forecast yet. &&

.LONG TERM /FRIDAY THROUGH MONDAY/...
Issued at 216 PM MDT Tue Jul 9 2024

A potentially significant heat wave is likely to impact the area
through the extended forecast period, and possibly beyond with a
highly anomalous 597+ dam 500-mb ridge centered over the central
Rockies through the period. The deterministic GFS/ECMWF/GEM, and
their ensembles remain in excellent agreement w/ H7 temperatures
in the range of 18-22 deg C w/ at least 80% of ensemble clusters
showing 18 deg C or warmer for the entire period. It appears the
highest probabilities of H7 temperatures of at least 21 C should
be Sunday with a broad swath of ~50 percent probabilities across
Carbon County at 6 PM Sunday per the grand ensemble. Such values
are near the climatological maximum (above the 99th %ile per the
NAEFS) during the climatological warmest time of year which does
suggest potential for some of the warmest temperatures this area
is capable of. Our overall confidence in more extreme values for
Friday and Saturday has diminished, as it does seem there should
be sufficient mid and high level moisture to generate clouds and
perhaps a few very light (or virga) showers as several pieces of
mid-level energy pivot around the edge of the massive ridge each
afternoon. While we should easily achieve 95-105F for most areas
along/east of I-25, this could be met shortly after 18z with the
potential to level off thereafter. It does seem probable that we
will still need Heat Advisories in later updates as these values
are near the upper percentile of our climatological distribution
for any time of year. Greater concern for the potential need for
Excessive Heat Watches/Warnings has shifted toward Sunday as the
center of the ridge moves overhead, and the resulting subsidence
limits potential for cloud and precipitation development. Latest
NBM output shows a mean MaxT of 95/104/106 with 90th %ile values
of 99/107/109 for CYS/BFF/CDR for Sunday. More concerning is the
overnight lows Sunday night w/ increasing clouds likely limiting
the potential for radiational cooling. The latest NBM shows a 10
to 20 percent chance (minimum) of overnight lows remaining above
70 F for all areas along/north of a Wheatland to Kimball line w/
a 20% chance of remaining above 65 F even for Cheyenne. With yet
another hot day expected for Monday w/ highs 95-105 F, this will
definitely warrant the discussion of Excessive Heat headlines at
some point down the road.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 531 PM MDT Tue Jul 9 2024

North northwest flow aloft will continue. Clear skies will
prevail for the Nebraska terminals, with scattered to broken
clouds near 10000 feet for the Wyoming terminals early this
evening, then skies will become clear. Occasional showers will
occur at Laramie until 01Z, producing wind gusts to 40 knots.
Winds will gust to 25 knots at Rawlins and the Nebraska
terminals this evening.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...CLH
AVIATION...RUBIN