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.
After Thursday brings another day of widespread below-normal highs with numerous showers and high-elevation snow from the Sierra to the Rockies, temperatures will rebound on Friday. However, that trend will not last long.
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.
Highs Saturday into Memorial Day are expected to be held to within a degree or two of 70 F in California's Central Valley.
On the coolest day of the holiday weekend, temperatures will only climb into the lower and middle 60s in San Francisco, Los Angeles and San Diego, as well as Reno, Nev.
When the reinforcing cool air settles over the interior Southwest, highs on Memorial Day can range from the middle 60s in Salt Lake City to the middle to upper 70s in Las Vegas and the lower 80s in Phoenix.
Seattle may actually be warmer than many locations in the southwestern United States as warmth builds across Washington on Memorial Day.
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 4 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 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.
If the storm taps into enough moisture, a steady period of rain may sweep from around San Francisco to Los Angeles Sunday night into Memorial Day.
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.
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."