Memorial Day, the unofficial start to summer, will feel more like April from California to Arizona

By Kristina Pydynowski, AccuWeather senior meteorologist

Memorial Day marks the unofficial start to summer, but the weather for this holiday weekend will feel more like April across the southwestern United States.

While temperatures rebounded on Friday across the Southwest after a cool stretch of weather earlier in the week, a new storm dropping down from the Pacific Northwest will send temperatures back down to April-like values for all or a part of the Memorial Day holiday weekend.

The press of cooler air will begin to filter into Northern California on Saturday before encompassing the rest of the Southwest Sunday into Memorial Day.

Sunday is expected to be the coolest day of the holiday weekend across most of California and Nevada. Temperatures may be held to the upper 50s and lower 60s in San Francisco, Sacramento, Fresno and Los Angeles in California. A high in only the middle 50s is expected for Reno, Nevada.

When the reinforcing cool air settles over the interior Southwest, highs on Memorial Day can range from the lower 60s in Salt Lake City to the lower 70s in Las Vegas and near 80 in Phoenix.

Seattle may actually be warmer than many locations in the southwestern U.S. as warmth builds across Washington on Memorial Day.

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More typical highs for the end of May range from the upper 60s along the California coast to the 80s in California's Central Valley and the middle 90s in the deserts of southeastern California and Arizona.

"The cool stretch will add on to what has already been a cool month in the Southwest," according to AccuWeather Senior Meteorologist Brian Thompson. "Temperatures are already running about 5 degrees Fahrenheit below average for May in Las Vegas and Phoenix."

While the cool spell will continue to keep air conditioners quiet and cooling costs down, many residents outside of the deserts may want to keep spring jackets handy for outdoor festivities. That is especially true for events planned in the late evening and morning hours.

"The weather may not be quite as good for hopping in the pool, but it will be far more comfortable for most other outdoor activities," Thompson said.

The storm delivering the fresh cool air will also prevent widespread sunshine from gracing holiday picnics and parades. Instead, showers will be produced daily with the most numerous activity from the Sierra to northern Nevada.

Sunday can be the most active day across California with showers and thunderstorms.

"The storm producing the unsettled weather on Sunday can be potent enough to also produce hail from San Francisco to Sacramento and Fresno," according to AccuWeather Senior Meteorologist John Gresiak. "Funnel clouds may also be seen in a few locations."

"Some communities in California could approach its wettest May on record," AccuWeather Meteorologist Brett Rathbun said.

San Francisco has had its third wettest May on record through May 23. Additional rainfall this weekend could climb them into the top two.

Fresno is currently having its second wettest May on record with 2.17 inches of rain through May 24. About 0.70 of an inch of rain is needed to break the all-time record.

May 2010 was the last time that measurable rain has dampened downtown Los Angeles during the last five days of the month.

There can also be thunderstorms erupting over parts of the region, which can pose a threat to anyone outdoors. You are at risk of being struck by lightning as soon as thunder is heard.

Hikers and campers who decide to still venture out into the mountains will want to ensure they have proper gear to stay dry and warm to avoid hypothermia. Snow can return to the highest elevations, leading to slow and slippery travel along I-80's Donner Summit.

Download the free AccuWeather app to know if rain, a thunderstorm or snow is anticipated for where you are planning to spend the Memorial Day holiday weekend. Keep checking back for updates on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier and Verizon Fios.

Ahead of the storm, gusty winds will get kicked up across the interior Southwest this holiday weekend.

Worse than making it difficult to keep plates and utensils from blowing away at picnics, the wind can once again elevate the fire danger in southeastern Arizona and southern New Mexico.

Patchy blowing dust can impact motorists in the deserts.

"After Memorial Day, it looks like the parade of storms will finally be coming to an end," according to Thompson. "Drier weather will return heading into early June with temperatures climbing back closer to average."