Rosa, jet stream plunge to instigate major weather pattern change to much of western US

By Alex Sosnowski, AccuWeather senior meteorologist

September 30, 2018, 1:21:37 AM EDT

Changes are coming to a large part of the western United States that will break the deadlock of sunshine, dryness and tranquil weather conditions in many areas.

While many parts of the Central and Eastern states have become saturated in recent weeks and month, rainfall has been limited in the West. However, it seems as though Mother Nature will try to spread some moisture into parts of the West in the coming weeks.

Rosa is forecast to produce a swath of soaking rain, flooding and gusty thunderstorms centered on Arizona, southeastern California, southern Nevada, western Colorado, northwestern New Mexico and Utah spanning Monday and Tuesday.

Coinciding with Rosa's race across the Southwest will be a southward dip in the jet stream that will help funnel some Pacific Ocean moisture inland in the form of pockets of rain elsewhere across California, Nevada and Utah.

"This rain can dampen San Francisco, Los Angeles and San Diego which will miss out on rain from Rosa," according to AccuWeather Senior Meteorologist Kristina Pydynowski.

San Francisco can record its first measurable rainfall since May 25 this week.

"The rain is good, but motorists will have to use caution," Pydynowski said. "Roads can turn slippery as the rain mixes with oil residue left behind during the recent dry months."

Where the jet stream dips southward significantly, lower temperatures and unsettled weather often result.

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"High temperatures in 90s F over the San Joaquin Valley of California in recent days will be replaced with highs in the 80s," according to AccuWeather Long-Range Meteorologist Jack Boston.

Elsewhere, highs in the 70s and 80s this weekend will be replaced with highs in the 50s, 60s and 70s.

Recent heat has already been trimmed from northern and central California. The cooling trend will reach more of Nevada and Utah by midweek.

The dip in the jet stream bringing cooler conditions to California this weekend will kick up gusty winds across the interior West.

These winds, ahead of forecast rainfall, may raise the risk of wildfire ignition and cause areas of blowing dust.

The same southward jet stream dip will also allow showers to persist well after Rosa's departure in Arizona, New Mexico and Colorado during the middle to latter part of the week.

So instead of sunshine bouncing right back in hard-hit rain and flooding areas, the weather pattern may remain unsettled for a couple of days over the interior Southwest.

Farther north, the weather will be much less extreme. However, temperatures will trend to 5-10 degrees below average in Washington and northern Idaho to near average in Wyoming and southern Idaho. Highs in most low-elevation locations will range from the lower 60s to the lower 70s.

How dramatic the weather change will be elsewhere will depend on how much the jet stream dips.

At this time AccuWeather meteorologists anticipate only a moderate dip in the jet stream, which will result in showers, spotty thunderstorms and locally breezy conditions centered on California and Nevada.

While the air will cool substantially, any snow will be restricted to the highest peaks and ridges of the Sierra Nevada.

Snow has begun to to fall on parts of the West with 4-8 inches blanketing the Big Horn Mountains of Wyoming and Black Hills of South Dakota to end the last week of September. More snow will fall over the northern Rockies on Sunday.

In many locations, the anticipated pattern will mark the beginning of typical fall conditions. In other areas, conditions may be potentially disruptive for outdoor plans and travel.

Only if a more substantial dip in the jet stream were to occur, would the weather conditions be more dramatic and perhaps locally violent in the form of torrential downpours, thunderstorms with hail, waterspouts and tornadoes and strong wind gusts.

Late in the week and next weekend, temperatures will rebound as the jet stream is forecast to retreat northward. However, there is the potential for more locally strong winds and wildfire danger during the transition in parts of central and Southern California.

"We will continue to monitor the East Pacific since there is the chance of another tropical feature developing and possibly moving toward the Southwest states with rain during the first part of October," Boston said.

The overall weather pattern may allow another cooldown and unsettled conditions to evolve during the second week of October.

Download the free AccuWeather app for the latest forecast and information about Rosa.

This Date in Weather History for September 30,2018

Weather History
For Sunday,September 30,2018

1959 - Three tornadoes spawned by the remnants of Hurricane Gracie killed 12 persons at Ivy VA. (The Weather Channel)

1970 - A nineteen month drought in southern California came to a climax. The drought, which made brush and buildings tinder dry, set up the worst fire conditions in California history as hot Santa Anna winds sent the temperature soaring to 105 degrees at Los Angeles, and to 97 degrees at San Diego. During that last week of September whole communities of interior San Diego County were consumed by fire. Half a million acres were burned, and the fires caused fifty million dollars damage. (David Ludlum)

1977 - The temperature at Wichita Falls, TX, soared to 108 degrees to establish a record for September. (The Weather Channel)

1986 - Thunderstorms, which had inundated northern sections of Oklahoma with heavy rain, temporarily shifted southward producing 4 to 8 inches rains from Shawnee to Stilwell. Baseball size hail and 80 mph winds ripped through parts of southeast Oklahoma City, and thunderstorm winds caused more than half a million dollars damage at Shawnee. (Storm Data)

1987 - Afternoon thunderstorms in Michigan produced hail an inch in diameter at Pinckney, and wind gusts to 68 mph at Wyandotte. A thunderstorm in northern Indiana produced wet snow at South Bend. Seven cities in the northwestern U.S. reported record high temperatures for the date, including readings of 98 degrees at Medford OR and 101 degrees at downtown Sacramento CA. (The National Weather Summary) (Storm Data)

1988 - Unseasonably warm weather prevailed over Florida, and in the western U.S. The afternoon high of 94 degrees at Fort Myers FL was their tenth record high for the month. Highs of 98 degrees at Medford OR and 99 degrees at Fresno CA were records for the date, and the temperature at Borrego Springs CA soared to 108 degrees. (The National Weather Summary)

1989 - Thirteen cities reported record high temperatures for the date, as readings soared into the upper 80s and 90s from the Northern and Central High Plains Region to Minnesota. Bismarck ND reported a record high of 95 degrees, and the temperature reached 97 degrees at Broadus MT. Afternoon thunderstorms developing along a cold front produced wind gusts to 60 mph at Wendover UT. (The National Weather Summary) (Storm Data)

This Date in Weather History for September 29,2018

Weather History
For Saturday,September 29,2018

1927 - An outbreak of tornadoes from Oklahoma to Indiana caused 81 deaths and 25 million dollars damage. A tornado (possibly two tornadoes) cut an eight-mile long path across Saint Louis MO, to Granite City IL, killing 79 persons. The damage path at times was a mile and a quarter in width. The storm followed a similar path to tornadoes which struck in 1871, 1896, and 1959. (The Weather Channel)

1959 - A storm produced 28 inches of snow at Colorado Springs, CO. (David Ludlum)

1983 - Heavy rains began in central and eastern Arizona which culminated in the worst flood in the history of the state. Eight to ten inch rains across the area caused severe flooding in southeastern Arizona which resulted in thirteen deaths and 178 million dollars damage. President Reagan declared eight counties of Arizona to be disaster areas. (The Weather Channel)

1986 - A week of violent weather began in Oklahoma which culminated in one of the worst flooding events in the history of the state. On the first day of the week early morning thunderstorms caused more than a million dollars damage in south Oklahoma City. Thunderstorms produced 4 to 7 inches of rain from Hobart to Ponca City, and another round of thunderstorms that evening produced 7 to 10 inches of rain in north central and northeastern sections of Oklahoma. (Storm Data)

1987 - A slow moving cold front produced rain from the Great Lakes Region to the Central Gulf Coast Region. A late afternoon thunderstorm produced wind gusts to 62 mph at Buffalo NY. Warm weather continued in the western U.S. In Oregon, the afternoon high of 96 degrees at Medford was a record for the date. (The National Weather Summary)

1988 - High pressure brought freezing temperatures to parts of Vermont and New York State. Burlington VT dipped to 30 degrees, and Binghamton NY reported a record low of 34 degrees. The high pressure system also brought cold weather to the Central Rocky Mountain Region. Alamosa CO reported a record low of 18 degrees, and Gunnison CO was the cold spot in the nation with a morning low of just five degrees above zero. (National Weather Summary)

1989 - Seven cities reported record high temperatures for the date, as readings soared into the 80s and low 90s in the Northern Plateau and Northern Plains Region. Record highs included 91 degrees at Boise ID, and 92 degrees at Sheridan WY. The high of 100 degrees at Tucson AZ marked their 51st record high of the year, and their 92nd day of 100 degree weather. (National Weather Summary)

This Date in Weather History for September 28,2018

Weather History
For Friday,September 28,2018

1836 - The first of three early season snows brought four inches of snow to Hamilton, NY, and two inches to Ashby MA. (David Ludlum)

1893 - Albuquerque, NM, was soaked with 2.25 inches of rain, enough to establish a 24 hour record for that city. (The Weather Channel)

1917 - A hurricane hit Pensacola, FL. Winds gusted to 95 mph, and the barometric pressure dipped to 28.50 inches. Winds at Mobile AL gusted to 75 mph. (The Weather Channel)

1987 - Thunderstorms produced up to ten inches of rain in southern Kansas and north central Oklahoma overnight. The Chikaskia River rose 2.5 feet above flood stage at Blackwell OK during the day causing flooding in Kay and Grant counties of north central Oklahoma. Early morning thunderstorms in the Lower Rio Grande Valley of Texas produced 3.07 inches of rain in six hours at McAllen. Thunderstorms produced up to six inches of rain in southeastern Texas later in the day. (National Weather Summary) (Storm Data)

1988 - Thunderstorms developing ahead of a cold front in the central U.S. produced severe weather from northern Texas to the Lower Missouri Valley during the late afternoon and evening hours. Hail three inches in diameter was reported at Nolan TX, and wind gusts to 80 mph were reported at Lawrence KS. Thunderstorms drenched downtown Kansas City MO with up to four inches of rain, leaving some cars stranded in water six feet deep. (Storm Data) (The National Weather Summary)

1989 - Thunderstorms over northeastern Florida drenched Jacksonville with 4.28 inches of rain between midnight and 6 AM EDT. Unseasonably cool weather prevailed in the northeastern U.S. Five cities reported record low temperatures for the date, including Binghamton NY with a reading of 30 degrees. Morning lows were in the 20s in northern New England. Unseasonably mild weather prevailed in the northwestern U.S., with afternoon highs in the upper 70s and 80s. In Oregon, Astoria reported a record high of 83 degrees. (The National Weather Summary) (Storm Data)

This Date in Weather History for September 27,2018

Weather History
For Thursday,September 27,2018

1816 - A black frost over most of New England kills unripened corn in the north resulting in a year of famine. (David Ludlum)

1959 - A tornado 440 yards in width traveled twenty miles from near Hollow, OK, to western Cherokee County KS. Although a strong tornado, it was very slow moving, and gave a tremendous warning roar, and as a result no one was killed. (The Weather Channel)

1970 - Afternoon highs of 103 degrees at Long Beach, CA, and 105 degrees at the Los Angeles Civic Center were the hottest since September records were established in 1963. Fierce Santa Ana winds accompanying the extreme heat resulted in destructive fires. (The Weather Channel)

1985 - A record early season snowstorm struck the Central High Plains Region. The storm left up to nineteen inches of snow along the Colorado Front Range, and as much as a foot of snow in the High Plains Region. (Storm Data)

1987 - While those at the base of Mount Washington, NH, enjoyed sunny skies and temperatures in the 70s, the top of the mountain was blanketed with 4.7 inches of snow, along with wind gusts to 99 mph, and a temperature of 13 degrees. Severe thunderstorms developed along a cold front in the south central U.S. A thunderstorm west of Noodle TX produced golf ball size hail and wind gusts to 70 mph. (The National Weather Summary) (Storm Data)

1988 - Thunderstorms developing ahead of a cold front produced large hail in southeastern Wyoming during the afternoon, with tennis ball size hail reported at Cheyenne. Strong winds ushering the cold air into the north central U.S. gusted to 59 mph at Lander WY. (The National Weather Summary) (Storm Data)

1989 - Freezing temperatures were reported in the Great Lakes Region and the Ohio Valley. Houghton Lake MI reported a record low of 21 degrees. Thunderstorms in the western U.S. produced wind gusts to 50 mph at Salt Lake City UT, and gusts to 58 mph at Cody WY.(The National Weather Summary) (Storm Data)

This Date in Weather History for September 26,2018

Weather History
For Wednesday,September 26,2018

1936 - Denver, CO, was buried under 21.3 inches of snow, 19.4 inches of which fell in 24 hours. The heavy wet snow snapped trees and wires causing seven million dollars damage. (26th-27th) (David Ludlum) (The Weather Channel)

1950 - Residents of the northeastern U.S. observed a blue sun and a blue moon, caused by forest fires in British Columbia. (David Ludlum)

1963 - San Diego, CA, reached an all-time record high of 111 degrees. Los Angeles hit 1S09 degrees. (David Ludlum)

1970 - Santa Ana winds brought fires to Los Angeles County, and to points south and east. Half a million acres were consumed by the fires, as were 1000 structures. Twenty firemen were injured. (25th-29th) (The Weather Channel)

1979 - In the midst of a hot September for Death Valley, California, the afternoon high was 104 degrees for the second of three days, the coolest afternoon highs for the month. (The Weather Channel)

1987 - Freezing temperatures were reported in the Northern and Central Appalachians, and the Upper Ohio Valley. The morning low of 27 degrees at Concord NH tied their record for the date. Temperatures soared into the 90s in South Dakota. Pierre SD reported an afternoon high of 98 degrees. (The National Weather Summary)

1988 - Unseasonably warm weather prevailed across Florida. Afternoon highs of 92 degrees at Apalachicola and 95 degrees at Fort Myers were records for the date. (The National Weather Summary)

1989 - Rain spread from the southeastern states across New England overnight. Cape Hatteras NC reported measurable rainfall for the fourteenth straight day, with 15.51 inches of rain recorded during that two week period. Phoenix AZ reported a record high of 108 degrees, and a record 134 days of 100 degree weather for the year. Afternoon temperatures were only in the 40s over parts of northwest Wisconsin and Upper Michigan. (The National Weather Summary)

2004 - After making its infamous loop east of the Bahamas, Hurricane Jeanne made landfall the night of September 26th, 2004. Jeanne came ashore as a major category 3 hurricane just a few miles away from where Hurricane Frances made landfall a few weeks before. Jeanne produced extensive damage along the east central Florida coast from Volusia County south to Martin County. The highest wind gusts occurred over extreme Southern Brevard County as well as Indian River County with 110 - 120 mph estimates at the peak of the storm. (NWS, Melbourne, FL)

This Date in Weather History for September 25,2018

Weather History
For Tuesday,September 25,2018

1939 - A west coast hurricane moved onshore south of Los Angeles bringing unprecedented rains along the southern coast of California. Nearly five and a half inches of rain drenched Los Angeles during a 24 hour period. The hurricane caused two million dollars damage, mostly to structures along the coast and to crops, and claimed 45 lives at sea. ""El Cordonazo"" produced 5.66 inches of rain at Los Angeles and 11.6 inches of rain at Mount Wilson, both records for the month of September. (David Ludlum) (The Weather Channel)

1987 - Hurricane Emily crossed the island of Bermuda during the early morning. Emily, moving northeast at 45 mph, produced wind gusts to 115 mph at Kindley Field. The thirty-five million dollars damage inflicted by Emily made it the worst hurricane to strike Bermuda since 1948. Parts of Michigan and Wisconsin experienced their first freeze of the autumn. Snow and sleet were reported in the Sheffield and Sutton areas of northeastern Vermont at midday. (The National Weather Summary) (Storm Data)

1988 - Low pressure off the Northern Pacific Coast brought rain and gale force winds to the coast of Washington State. Fair weather prevailed across most of the rest of the nation. (The National Weather Summary)

1989 - Twenty-three cities in the south central U.S. reported record low temperatures for the date, including Topeka KS with a reading of 33 degrees, and Binghamton NY with a low of 25 degrees. Showers and thunderstorms in the southeastern U.S. drenched Atlanta GA with 4.87 inches of rain, their sixth highest total of record for any given day. (The National Weather Summary) (Storm Data)