Humidities surge Saturday night and Sunday with weekend high temps jumping from 80s Saturday to 90s Sunday—a wind-shifting “back-door” cold front sweeps through Sunday afternoon/evening bringing “NNE” winds and a temp pullback.

A HUGE DOME OF HOT AIR IS BUILDING ACROSS CENTRAL UNITED STATES

• A huge dome of hot air is building across central U.S., a development which is to boost Chicago weekend temps until a “back door” cold front reaches the city and shifts winds “north/northeast” by mid-Sunday afternoon and evening. Temps will plunge with that windshift, especially near Lake Michigan. Northeast winds will also subdue heating though Monday, but temps are to surge and current forecast indications put the hottest air and potential heat wave conditions into Chicago Tuesday, Wednesday and Thursday. Meantime southerly winds on the hot dome’s back side is behind Hurricane Hilary’s move on the southwest United States.
• We have a lovely weekend coming—perfect for the Chicago Air and Water Show. Saturday is to open with low humidity, but humidities will begin to increase later in the day and the air over Chicago is to become humid Saturday night into Sunday. High temps surge to 86 Saturday afternoon, drop only to 72 Saturday night, then surge to 94 Sunday with a peak heat index approaching 103 degrees.

BUT CHICAGO’S WEATHER IS TO UNDERGO BIG CHANGES SUNDAY AFTERNOON AND EVENING WITH THE ARRIVAL OF A “BACK DOOR COLD FRONT”

• This type of front races southward the length of the lake and shift winds NNE, tugging lake-cooled air into the city. Temps may drop from 94 degrees early Sunday afternoon to the mid 70s along Lake Michigan by nightfall.
• THE COOLING MAY LINGER INTO MONDAY with northeast winds expected to ease. That should allow heat to build back into the Chicago area, heat which may require advisories and send readings to the highest levels of the year by Wednesday and Thursday. If current forecast trends hold, mid to upper 90s aren’t out of the question, even the potential for a near 100-degree reading in the Wednesday/Thursday time frame.
• Much can happen between now and then, so this is a situation which will be monitored closely and for which there will be updates.

A FIRST-EVER “TROPICAL STORM WATCH” ISSUED FOR FAR SOUTHERN CALIFORNIA FOR THIS WEEKEND INTO EARLY NEXT WEEK

• Powerhouse category 4 Hurricane Hilary—with 145-mph sustained winds and 1,193 miles south/southeast of Los Angeles and northwest bound—will now begin weakening over cooler Pacific waters west of Baja California, Mexico. But, Hilary is to arrive in the southwest U.S. later this weekend into Monday as a drenching, potentially flood-generating rain-producing system with rains which could impact an area extending across a swath of California, parts of western Arizona and Nevada even as a pounding Pacific surf sweeps into the California coast.

WE’RE TALKING ABOUT A RARE SET OF WEATHER CONDITIONS DESCENDING ON CALIFORNIA LATER THIS WEEKEND IF CURRENT PROJECTIONS COME TRUE

• Not since 1939 and the landfall of the so-called “Long Beach Tropical Storm” has a TROPICAL STORM MADE IT INTO CALIFORNIA. THAT HAPPENED 84 YEARS AGO.
• The National Hurricane Center (NHC) Friday morning issued its FIRST EVER SOUTHERN CALIFORNIA TROPICAL STORM WATCH. This is something to be added to the list of STRANGE SUMMER, 2023 meteorological occurrences which have included record Canadian wildfires and their smoke, record heat in Illinois and more tornadoes to date than any other state—135 twisters reported to the Storm Prediction Center in 2023 to date. The next closest state is Alabama with 93!
• The TROPICAL STORM WATCH was issued for extreme southern California Friday morning as powerhouse Hurricane Hillary—still over warm waters more than 1,000 miles south/southeast of San Diego and Los Angeles—begins a trek north/northwestward over cooler, lower-heat content waters off Baja California. NOTHING saps a hurricane’s wind energy faster than denying it the supply of nourishing latent heat energy than tropical ocean waters provide.

HILARY WINDS WILL COME DOWN OVER COOLER OCEAN WATERS IN COMING DAYS

• The southwest United States—including California, sections of western Arizona and a swath of Nevada—are in for potential major impacts from Hilary later this weekend, heading into early next week. Among them the risk of serious flooding and battering coastal waves. There’s even the potential tornadoes may spin up in parts of that area.

A FIRST-EVER “TROPICAL STORM WATCH” ISSUED FOR FAR SOUTHERN CALIFORNIA FOR THIS WEEKEND INTO EARLY NEXT WEEK

And, smoke produces a hazy look to Chicago area skies Friday.

THE LAHAINA BLAZE ON MAUI IS ALREADY THE DEADLIEST U.S. WILDFIRE SINCE BLAZES SWEPT MINNESOTA IN 1918 KILLING HUNDREDS

• Beyond death, there have been serious burn injuries and children are feared among the dead. The number of wildfires in Hawaii has quadrupled in recent decades, and the increased presence of invasive grasses on the Hawaiian Island—which more easily burn in drought conditions—has only added to the risk of fire.

• Sixteen percent of Maui was in a state of severe drought at the time of last week’s fire, and drought conditions were particularly acute on the west side of the island. The region which was home to Lahaina burned to the ground amid roaring 60 to 70 mph wind gusts, much of the fire occurring in the darkness of night. What transpired was horrifying. Imagine trying to escape smoke and fire in the darkness of night with winds roaring around you. It’s hard to imagine a more terrifying environment.

• The west sides of the Hawaiian Islands are areas in which the prevailing easterly TRADE WINDS, for which Hawaii is renowned, are dry. Historic Lahaina was such a community, located on the dry west side of Maui.
• Those easterly winds, coming into Hawaii after a trek across thousands of miles of ocean, deliver frequent rains to the eastern sides of the Hawaiian Islands. There, lush tropical forests and vegetation flourish. The easterly winds pile up once on land and are forced to rise over the west-facing slopes of Hawaii’s volcanic mountains. When air rises, it cools to saturation producing rain and clouds.
• But, while rains are frequent because of those easterly winds on the EAST SIDES of the Hawaiian Islands, those same winds descend from the volcanic peaks and the higher elevations as they move to the west sides of the Islands, warming and drying in the process. Meteorologists refer to the process in which temps surge and humidities drop in the descending out-of-mountainous regions as KATABATIC WARMING—warming which occurs as air descends from higher elevations, subjecting it to compression as it sinks into the lower reaches of the atmosphere where air pressure increases. It’s a fact that gases—including the air we breath—warms when compressed, and warming dries air. All this made the west side of Maui especially vulnerable to the deadly fires, made even more devastating because of the winds which fanned them.
• The meteorological disaster assessment team at AON Insurance puts together WEEKLY SUMMARIES of the world’s most deadly and destructive disasters. Prominent in this week’s report is the disaster which has unfolded as a result of the deadly, wind-driven wildfires which swept Maui in Hawaii.
• The scale of destruction caused by multiple wildfires across Hawaii, most especially in Lahaina, continues to grow by the day. The wildfire there—now the worst U.S. wildfire in terms of human toll in over a century—has claimed 111 lives as of August 16. This number is still expected to grow as recovery efforts are underway.
• Aggregated economic and insured losses from all wildfires across Hawaii are likely to reach into the billions USD.

THE MOST DEADLY U.S. WILDFIRES FROM THE AOL DISASTER ANALYSIS TEAM

• The Lahaina wildfire in Maui County, Hawaii that began on August 8 is now considered the state’s worst natural disaster since it joined the United States in 1959. As of August 17, 111 people are confirmed dead while hundreds more remain missing. Given the current death toll, the Lahaina wildfire is now the deadliest U.S. wildfire in over 100 years and the 5th deadliest in all of recorded U.S. history. The death toll is expected to climb further as only 40% of burned area has been searched.