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At a Glance Excessive heat has gripped the Southwest and California this week.

The hot temperatures are the result of a large heat dome in the region.

This type of hot weather pattern is typical before monsoonal moisture arrives in the summer. Dangerously hot temperatures have gripped the Southwest this week, which led to the all-time record-high temperature being tied in Las Vegas on Tuesday.

A large dome of high pressure in the upper atmosphere developed over the Southwest. Beneath the dome, sinking air is causing temperatures to soar well over 110 degrees in many areas.

(MORE: This is Where 120-Degree Temperatures Have Officially Been Recorded the Most )

This is a classic pre-monsoon heat event for the Southwest region. Some of the highest temperatures of the year can be recorded before the onset of the summer monsoon. Humidity levels are low, so the sun's energy can be used to heat the air rather than being absorbed by water vapor or used for evaporation.

(MORE: How Often Your City Reaches 100 Degrees )

The hot conditions this week will be particularly dangerous for vulnerable groups, such as the sick and the elderly. The NWS offered useful heat safety tips that can be incorporated into a daily routine when extreme heat sets in.

Job sites: Stay hydrated and take breaks inside as often as possible. Remember that in temperatures above 110, you will not know that you are sweating.

Indoors: Check up on the elderly, sick and those without air conditioning.

In vehicles: Never leave children or pets unattended – look before you lock.

Outdoors: Limit strenuous activities and find shade. Drink plenty of water and avoid alcohol.

Also, remember that flights planes may be delayed or canceled in temperatures of 115 or more due to the lost of lift that planes need to fly.

Record-High Temperature Recap

Daily record highs were set Wednesday in Needles, California (123 degrees), Las Vegas (114 degrees), Phoenix (117 degrees), Tucson, Arizona (115 degrees), Redding, California (110 degrees - tie), and Palm Springs, California (118 degrees - tie).

The 115 degree high on Wednesday allowed Tucson to set a new record for longest streak of 115 degree temperatures, with three days.

Needles, California, tied its all-time record high Tuesday when it reached 125 degrees. That temperature had been reached three times previously (2016, 2005 and 1925).

Las Vegas also tied its all-time record high by reaching 117 degrees Tuesday afternoon.

Tucson, Arizona, missed its all-time record by just one degree Tuesday, with an actual high of 116 degrees.

Daily record highs were set Tuesday in Phoenix (119 degrees), Tucson, Arizona (116 degrees), Yuma, Arizona (120 degrees), Palm Springs, California (122 degrees - tie), Corpus Christi, Texas (100 degrees), Salt Lake City (101 degrees - tie) and Denver (99 degrees).

Several daily record highs were set Monday, including Death Valley, California (125 degrees), Phoenix (118 degrees - tied), Tucson, Arizona (115 degrees), Palm Springs, California (119 degrees), Fresno, California (108 degrees) and Reno, Nevada (103 degrees).

The 115-degree high in Tucson, Arizona, on Monday was the third hottest temperature ever recorded in that city and only the sixth time it has been 115 degrees or hotter since the late 1800s.

A number of daily record highs were set on Sunday, including Death Valley, California (124 degrees), Fresno, California (107 degrees), San Jose, California (103 degrees), and San Francisco (97 degrees - airport).

Summer's Peak Heat Arrives Early in Southwest

The North American monsoon typically begins to take shape in the Southwest as we head through July. This seasonal shift in wind direction brings increased moisture, fueling more frequent showers and thunderstorms.

(MORE: 5 Things to Expect During the Summer Monsoon )

As a result, parts of the region see their hottest readings, on average, from the latter half of June into early July when the air mass is still very dry. For example, average highs in Phoenix and Tucson reach a maximum of 107 and 102 degrees, respectively, during this time.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/summer-heat.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/summer-heat.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/summer-heat.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Average warmest day of the year across the Lower 48 states. (NOAA/NCEI)

The hottest temperatures on record in Phoenix (122 degrees, June 26, 1990) and Tucson (117 degrees, June 26, 1990) also occurred in early summer.

MORE ON WEATHER.COM: The Most Extreme Temperatures in All 50 States