CHINOOK OF THE WEST...

terryswails1
2 minutes ago
4 min read

The winds at the jet stream level are setting up in a favorable pattern to drive mild Pacific air across the Midwest for much of the next two weeks. In doing so, it enables the development of Chinook winds, named after the Chinook Tribe who first noticed them and the significant impacts they had on temperatures.

WHAT IS A CHINOOK WIND?

As warm, moist air moves inland along the west coast, it encounters a 10,000 foot wall, known as the Rocky Mountains. Forced to ascend over the mountains, the air cools as it rises. During this ascent, the air mass drops much of its moisture as precipitation. Once the now dry air mass reaches the Continental Divide, it begins to descend the eastern slopes. As the air lowers (being a gas) it warms rapidly due to compression. The dried air then picks up speed and warms as it races down the leeward slopes, leading to the gusty snow eating wind known as the Chinook. They can cause rapid changes in temperature on the lee side of the Rockies, and that’s precisely what happened on the morning of January 22, 1943 in Spearfish, South Dakota (and other towns on the east slopes of the Black Hills).

At 7:32 a.m. with a temperature of -4 in Spearfish, Chinook winds began blowing in earnest, causing the temperature to jump to an astounding 45° in just two minutes. This 49-degree temperature change was the most extreme ever recorded by the National Weather Service.

Since the Midwest does not have the Rockies to create the downsloping effect, the powerful winds and extreme short term changes in temperatures are something we don't have to deal with. However, the warmth that's created hundreds of miles away can streak east in a modified way, creating significant thaws with temperatures 20–30 degrees above normal. Most of our January thaws are often the offshoot of a Chinook. And because the air is extremely dry and mild, it can melt or even sublimate a bunch of snow in a single day.

You probably figured I was leading up to something, and if it was a two-week Chinook loaded pattern of thaws and mild weather, you are right on the money. In the animation below, you will see the projected 500mb pattern for the period December 20th through January 1st. Aside from a couple of days, you can see the strong persistent westerly flow off the Pacific that is downsloping east of the Rockies.

Get a load of the exceptional Chinook driven temperatures the GFS shows in Denver, Colorado the rest of the year. That's the foundation of the warmth much of the nation east of the Rockies will be seeing. Rest assured it will be modified coming east, but nonetheless will bring much above normal temperatures to the Midwest leading up to the holidays.

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WHAT A CHANGE...

Getting to the nuts and bolts, these are the daily temperature departures for the 10-day period December 22nd to January 1st. Only the NE has anything resembling normal.

Christmas through New Year's Day (7 days), temperatures are shown averaging 13 to 20 degrees above normal per day! Some parts of western Iowa are as much as 22 degrees above normal over the same 7-day period. It's a big deal to have a 2-3 day stretch of readings 20 or above, 7 or more is getting into uncharted territory.

This is what the GFS has for highs through January 1st in the Quad Cities. About 50 Christmas, and that could be low. Have fun with that new sled Santa gave you, little Johnny.

Actually, the EURO is even warmer and shows 59 Christmas, and that's down from readings over 60 in earlier runs.

With all the downsloping, restricted moisture, and a storm track that's well north, storms and any worthwhile precipitation are not likely to be a factor. This is the 15-day temperature departure on the EURO through the end of December. A very dry look from the Rockies east.

In fact, much of what precipitation does occur is shown falling as light rain late Wednesday night and Thursday morning as a stinger digs southeast across the upper Midwest. Ahead of it Wednesday, temperatures in most areas will be above freezing in the mid 30s north to near 40 in the far south. Clouds will arrive Wednesday night along with brisk south winds ahead of the disturbance. This will cause slowly rising temperatures, with readings ranging from 38 to 44 at daybreak Thursday. By then, showers will have entered the picture, and those will be around until a cold front slips west to east across the region during the morning hours. The cuts off the showers and brings strong NW winds and a quick dip in temperatures Thursday night. Lows of 5 north to 15 south are expected, with wind chills of zero south to 10 below north. As I mentioned, this is a stinger, meaning the cold hits and rapidly departs, exiting by Saturday. Here's what the GFS and EURO show for rain totals.

The GFS

The EURO

After that, a warm-up ensues Saturday that is cut short by another fast moving but dry front Sunday. Early next week the ridge builds, allowing the flow to flatten out further to the north, and we head into the meat of the warmer weather that should largely dominate until early in the New Year. I'm not thrilled about it, but it looks inevitable, and so I have no choice but to embrace it and enjoy it. Roll weather...TS