Big Weather Changes This Week Include Warmer, Drier East; Cooler, Wetter West

Chris Dolce
Published: September 15,2015
Big weather changes are underway so far this week as the jet stream pattern rearranges itself once again across North America.
After a taste of fall this past weekend in the Midwest and East, temperatures are now warming back up above average. The opposite can be said for the West Coast, which is seeing a welcomed cooldown after dealing with record heat and dangerous wildfire conditions.
(MORE: Destructive Wildfires in California)
Let's lay out the two big weather changes taking shape to start the new week.

Change #1: Warmer Plains, Midwest and Northeast

Portions of the Midwest, Plains and East enjoyed a pleasant taste of fall the last few days, giving a break from the hot temperatures that were in place to start the month of September.
(MAP: Current Temperatures)
A small part of the Upper Midwest even saw a frost or freeze during the weekend. Morning lows Saturday dipped into the 20s in a few spots in northeast Minnesota and western Upper Michigan.
Low temperatures in the 50s were reported all the way into the Deep South Sunday morning, with some 40s in the southern Appalachians. On Monday morning, lows in the 40s and 50s were reported from the Great Lakes into the Ohio Valley and parts of the South.
But big changes are now taking shape, with above-average warmth spreading from the Plains to the Midwest and Northeast. While parts of the Midwest will see temperatures return to near or below-average levels late week, the warmth will stay in place across the Northeast into next weekend.

Forecast Highs Compared to Average

Here are a few examples of the temperature changes we will see in the East this week:
Chicago: After recording its first low in the 40s (O'Hare airport) since early June on Sunday morning, highs will be in the low to middle 80s through Thursday. Average highs in the Windy City during mid-September are typically in the middle 70s.

Forecast Highs

St. Louis: High temperatures will trend back towards 90 degrees later this week after struggling to reach 70 degrees on Saturday. The average high in mid-September is around 80 degrees.
New York City: Highs will warm back up 5-10 degrees above average Wednesday-Saturday. Though warmer-than-average, the middle 80s we expect this week will be much more tolerable than the record heat from last week.
This weather pattern change will also give the Northeast a break from the cloudy, wet conditions we've seen at times in the last several days.
Philadelphia had seen five consecutive days with at least a trace of rain through Sunday. This week, plenty of sunshine is in the forecast for not only Philadelphia, but also the rest of the Northeast as high pressure takes control.
(MORE: Forecast Weather Maps)

Change #2: West Cools Down; Increased Moisture

Forecast Highs

 The West Coast is also seeing a major change due to this weather pattern flip.
Last week featured yet another round of record heat and extreme wildfire danger. This resulted in two major wildfires in California that have burned tens of thousands of acres, forced evacuations and damaged numerous structures.
In some cases, temperatures to start this week will be more than 30 degrees cooler than we've seen in recent days. The West as a whole will see highs 5-20 degrees below average through Thursday from Washington state, Idaho and Montana into portions of California, Nevada and northern Arizona.
Here are a few examples of the temperature drops we'll see compared to last week:
Sacramento: Record high of 107 degrees this past Thursday will be replaced with highs in the middle to upper 70s continuing Wednesday, then warming into the 80s late week.
Portland, Oregon: Highs will only be in the 60s this week, which comes on the heels of consecutive days in the 90s on Friday and Saturday.
Los Angeles: Temperatures will top out mainly in the 70s or low 80s this week. This follows a streak of eight days with highs 90 degrees or warmer through Sunday.
This weather pattern change will also bring increased chances of rainfall to the Pacific Northwest, Intermountain West and even Southern California as disturbances aloft move through those regions.
The National Weather Service in Los Angeles says that moisture from former Hurricane/Tropical Storm Linda in combination with an area of low pressure in the upper atmosphere will help enhance rainfall through Wednesday there. As of Tuesday evening, Los Angeles had picked up more than two inches of rain. Only one other September day since 1877 has had more rain in Los Angeles.
Although we don't anticipate widespread, heavy precipitation across the West, localized bouts of heavy rainfall could result in flash flooding. We saw an example of this on Monday evening, with deadly flash flooding in the southern Utah.

Meteorology 101: Why Is This Change Happening?

This latest pattern change is all tied into the jet stream.
The jet stream is a ribbon of strong-winds aloft that helps play a role in the weather conditions we see near the surface of the earth.

Jet Stream Pattern Midweek

In the last couple of weeks, the jet stream has been amplified across North America, meaning it's lifting north in one area while diving south in another geographic region.
Late last week and into the weekend, the jet stream surged north across the West, resulting in the widespread warmth mentioned earlier. Meanwhile, the jet stream dove south across the East, providing a taste of fall.
This week, the jet stream pattern is flipping. A southward dip in the jet stream will bring cooler, wetter conditions to West, while parts of the Midwest and East warm back up in response to the jet stream lifting north over those regions.

MORE: Valley and Butte Fires in California