At a Glance
An upper-level pattern change will usher in much colder temperatures to parts of the West.
Snow is also expected to fall in some of the higher elevations of the northern Rockies.
Areas from the Plains to the East Coast will experience mild conditions into mid-September.
A noticeable pattern change is taking shape, especially across the West where the first snow of the season may fall in some locations.
In the West, a dramatic change in the weather is expected late this week as a strong upper-level low-pressure system will push southward into the northern Rockies, spreading much colder temperatures, along with rain and some higher-elevation snow.
This change is courtesy of a shift in the jet stream. Over the West, the jet stream will take a southward dip in the region, allowing colder conditions to infiltrate farther south. Conversely, the jet stream will be pushed northward over the East, with mild temperatures streaming northward into the Midwest and Northeast.
In addition, a significant low-pressure system will produce widespread showers and thunderstorms from Idaho and Montana into northern Arizona and New Mexico through Thursday. As the colder air flows in behind this system, the rain will change to snow in the mountains of southwestern Montana, western Wyoming and portions of Idaho Thursday night into Saturday.
Here's what to expect with this pattern change.
Chilly, Snowy West Forecast
Summer (June-August) was one of the top-five warmest summers on record in much of the West, according to the
Southeast Regional Climate Center, and the unusual warmth has persisted into September, so the cold blast will be even more noticeable.
Warmer-than-average temperatures continued through Tuesday but, now, cooler temperatures will begin to push south, making it feel more like fall. Much of the northern Rockies will see high temperatures 30 to 40 degrees colder Friday compared to the start of the week, as temperatures will only climb into the 40s and 50s.
Low temperatures will be even chillier, with temperatures dropping into the 20s and 30s for much of western Montana, eastern Idaho and Wyoming Friday through Sunday mornings. Widespread frost is possible and, in some spots, there may even be a hard freeze Sunday morning in low-lying areas.
These below-average temperatures will allow the rain to change to snow in many spots. Significant snow is possible across portions of the higher elevations of western Montana and northern and western Wyoming.
As cold air filters into the region, snow may even spread down to the valley and basin floor in parts of western Wyoming by Friday evening.
Accumulation of snow is expected generally above 6,000 feet, with the higher mountain peaks picking up 1 to 2 feet of snow through Saturday.
Winter storm watches and winter weather advisories have been issued for portions of western Montana, eastern Idaho and northern and western Wyoming – the firsts of the season here.
It will likely still be too warm for much snow to accumulate on major roadways, unless the snow becomes heavy.
This pattern change is also anticipated to assist with slowing the ongoing wildfires in the area.
Snow in mid-September does happen in the northern Rockies, especially in the higher elevations, so this is not all that unusual.
Pikes Peak and Mount Massive in the Colorado Rockies already saw the
first snow of the season in early August. Many areas of the West, including Denver, Cheyenne and Billings, have seen snow as early as early September.
However, given the recent warm conditions, campers and hikers need to be prepared for
freezing temperatures and snow.
Mild East in Mid-September
Meanwhile, parts of the East saw cooler-than-average conditions early this week, due in part to the clouds and rain from the remnants of
Hurricane Irma.
Several cities set daily record-cold high temperatures on Monday (new record in parentheses), including: Asheville, North Carolina (60 degrees), Knoxville, Tennessee (62 degrees), Greenville, South Carolina (62 degrees), Huntsville, Alabama (65 degrees), Birmingham, Alabama (66 degrees), Montgomery, Alabama (68 degrees), and Jacksonville, Florida (76 degrees).
High temperatures remained below average in portions of the South and Ohio Valley into Wednesday, where highs were generally in the 60s.
Late this week, temperatures will return to near average across the South and into the Ohio Valley and mid-Atlantic, with warmer-than-average temperatures prevailing from the Plains into the Midwest and Northeast.
Highs will generally reach into the 80s, with 70s in parts of the Northeast, while lows in the 50s and 60s can be expected. The southern Plains will see highs top out in the 90s.
It will also feel more humid for much of the South and East late this week.
The mild conditions are expected to last at least into the weekend across the East.
Many areas will also see drier conditions once the remnants of Irma depart the East by late week. Early next week, a cold front is expected to push eastward bringing a chance for showers and thunderstorms.
