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.

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