Sunday, September 13, 2015

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

Chris Dolce
Published: September 13,2015

Big weather changes are on tap to start the week as the jet stream pattern rearranges itself once again across North America.
If you've been enjoying the taste of fall this weekend in the Midwest and East, get ready to warm back up. The opposite can be said for the West Coast, which will see a welcomed cooldown after dealing with record heat and dangerous wildfire conditions.
(MORE: Two Damaging 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 have been enjoying 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. Another cool, pleasant morning is expected on Monday with below-average temperatures in parts of the South, Ohio Valley and the Mid-Atlantic.
But a sign of the changes in the week ahead can be seen in the Plains, where above-average warmth has already returned. This warmth will now spread through the Upper Midwest into the Great Lakes and Northeast throughout this week. 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.

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 return back into the low to middle 80s this week. 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 Tuesday-Friday. Though warmer-than-average, the middle 80s we expect later 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 three consecutive days with measurable rain through Saturday. 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, and it's positive news overall.
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 5-15 degrees below average 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 to start this 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 seven days with highs 90 degrees or warmer through Saturday.
Another bonus to this weather pattern change is the increased chances of rainfall in 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 Monday through Wednesday there.
Although we don't anticipate widespread, heavy precipitation across the West, the increased moisture levels and rainfall chances are a welcome sight given the dangerous wildfire conditions we've seen recently.

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

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