A broad ridge of high pressure set-up over the region, allowing dangerous heat over the course of several days.
Every event with a place to put it; 2 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS MSO wrote it up.
A broad ridge of high pressure set-up over the region, allowing dangerous heat over the course of several days.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Missoula, MT wrote its last area forecast discussion before the first event at Aug 15, 12:45 PM PST, 15 minutes earlier.
Area Forecast Discussion...UPDATED
National Weather Service Missoula MT
245 PM MDT Tue Aug 15 2023
.DISCUSSION...The ridge of high pressure continues its presence
today, leading to increased temperatures and decreased relative
humidities this afternoon through Thursday. Today begins the first
day of potentially record-breaking temperatures for the heatwave,
with the majority of the valleys seeing temperatures in the upper
90s, and a few reaching the triple digits. Notable forecasted
highs for today include Missoula with 101, Libby with 101,
Thompson Falls with 102, and Orofino with 108. An excessive heat
warning remains in effect for the Lower Hells Canyon and Salmon
River Region of Idaho. A heat advisory remains in effect for the
western Montana valleys and locations east of Highway 95 in north-
central Idaho.
Given dangerously hot temperatures, it is important to protect
yourselves and others during the heatwave. Remember to stay
hydrated, drinking plenty of water and electrolytes. Try and limit
outdoor activities to the morning or late evening hours. While
outside, wear lightweight, loose fitting clothing and take
frequent breaks from the sun. It is very important to remember to
never leave people or pets unattended in vehicles, and check on
those most vulnerable to the heat in your community.
Westerly winds are expected to increase Wednesday through Friday,
as the ridge begins to flatten and the pressure gradient
tightens. There is a 60-70% chance of gusts 25 mph or greater in
western Montana on Wednesday, with that chance increasing to
70-80% on Thursday and 80-90% on Friday. For gusts 40 mph or
greater, there is a 30% chance on Thursday and a 40-50% chance on
Friday. The chance of gusts 25 mph or greater decreases for north-
central Idaho, with a 30-50% chance on Thursday and a 50-70%
chance on Friday. The increased winds will lead to choppy lake
conditions Wednesday through Friday. Due to the hot, dry, and
windy conditions, a red flag warning has been issued for the
Flathead Reservation for Wednesday and Thursday afternoon and
evening. The threat of critical fire weather still continues for
the rest of western Montana this week; Ongoing fires may spread
quickly and be difficult to contain, and new fires can start
easily.
As the ridge starts to break down, a trough of low pressure moves
down the northern Pacific coast and combines with a southwest low
pressure system. This will draw moisture northward into the
Pacific Northwest, bringing relief to the area as temperatures
decrease back to seasonal normals. The moisture will also increase
the chance of wetting rains towards the end of the weekend into
early next week. During this period, the 48-hour probability of
experiencing 0.10 inches of rainfall stands at 50-70%. /Dzomba
&&
.AVIATION...High pressure in the Northern Rockies will continue to
cause hot temperatures through Thursday. Excessive heat warnings
and heat advisories are currently in effect from today through
Thursday across a significant portion of the region. The primary
concerns during the period will be heat and high density
altitudes. Isolated visibility reductions are expected,
especially across northwestern Montana due to regional wildfires.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Heat Advisory until midnight MDT Thursday night for
Kootenai/Cabinet Region.
Red Flag Warning from noon to 10 PM MDT Wednesday for Flathead
Reservation.
Red Flag Warning from noon to 10 PM MDT Thursday for Flathead
Reservation.
Fire Weather Watch from Thursday afternoon through Thursday
evening for West Lolo.
Fire Weather Watch from Thursday afternoon through Thursday
evening for Flathead/Glacier Park.
Heat Advisory until midnight MDT Thursday night for
Flathead/Mission Valleys.
Heat Advisory until midnight MDT Thursday night for Lower Clark
Fork Region.
Heat Advisory until midnight MDT Thursday night for
Missoula/Bitterroot Valleys.
Fire Weather Watch from Thursday afternoon through Thursday
evening for East Lolo.
Fire Weather Watch from Thursday afternoon through Thursday
evening for Kootenai.
ID...Heat Advisory until 11 PM PDT Thursday for Northern Clearwater
Mountains.
Fire Weather Watch from Thursday afternoon through Friday
evening for Palouse/Hells Canyon.
Heat Advisory until 11 PM PDT Thursday for Southern Clearwater
Mountains.
Excessive Heat Warning until 11 PM PDT Thursday for Lower Hells
Canyon/Salmon River Region...Orofino/Grangeville Region.
&&The national radar mosaic for the moment the first event began.
2 excessive heat. Local standard time, as filed.
High temperatures reached 105-110 degrees each afternoon for several days. Overnight lows in the low to middle 60s provided minimal relief from the daytime heat.
Daytime high temperatures ranged from 105-115 degrees each afternoon. In addition, overnight lows remained in the mid to upper 60s, and even low 70s in some locations.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2023Every state in 2023This day in weather historyWarnings in Idaho, August 2023
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 1120033, 1120038.