The crocuses are in bloom. In February. In Oregon and Washington. Thank the polar vortex and unusually warm water off the coast.
Decoded Science has named the unusual spell of warm, dry weather in the Pacific northwest Fair Weather Event Crocus — a fitting name.
Sometimes The Jet Stream Giveth
The undulations in the jet stream broadly correspond to high and low pressure centers at the surface of the earth. Low pressure is located under dips (troughs); high pressure under ridges.
In the most general description of the atmosphere, high pressure wants to spread out its surplus, while low pressure wants to suck in air to compensate for its deficit.
The inflow into a low forces air near the center to rise — rising air makes precipitation.
The air under high pressure sinks as the air mass spreads out at the surface. Sinking air warms, and since warm air can hold more moisture than cold, the water vapor does not condense.
An Unusually Stagnant Jet Stream
As Decoded Science has repeatedly noted, weather doesn’t move the way it used to: Highs and lows persist for a long time in the same place. If you’re stuck under a low, as the residents of the northeastern US have been stuck under the polar vortex this winter, you get the snowy end of the stick. If you are under high pressure, you bask in the golden glow (ok, purple glow) of a Flower-Named Fair Weather Event. Crocus encompasses the entire west from the Rockies to the Pacific coast, but particularly Washington and Oregon, which are unused to warm, dry weather in winter.
Crank Up The Amplitude
The degree of warm and cold, wet and dry, depends on how high and low the pressure gets at the jet stream level. Just as ocean waves vary from tiny ripples to gigantic breakers, so the waves in the atmosphere have different heights, generally referred to as the amplitude of the waves.
This jet stream currently features a large swing from north to south, with a bulge in the west and a dip in the east (the polar vortex) of exaggerated proportions. The result is record warmth out west and record cold in the east.
How Warm Has It Been?
There were 3500 high temperature records set in the United States in January, most of them in the west. There were only 775 cold temperature records.
During the past week, the warmth in the west has been especially noticeable at elevations above 4,000 feet. Some record temperatures for the last few days:
- Buffalo, Wyoming, 4,600 feet, Thursday: 50
- Sandberg, California, 4,200 feet, Thursday: 72
- Klamath Falls, Oregon, 4,200 feet, Wednesday: 72
- Ely, Nevada, 6,400 feet, Wednesday: 66
- Prescott, Arizona, 5,400 feet, Wednesday: 71
- Bellingham, Washington, 10,800 feet, Tuesday, 59
The Warmth Has Been Accompanied By Dry Weather — Mostly
The ridge in the jet stream that has encompassed the west coast for much of the last two years has occasionally broken down; storms blasted the Pacific northwest in early February with so much rain that Settle has already surpassed its normal rainfall for the month.
Farther south, the rain has been more sporadic. San Francisco went bone dry — not a single drop of rain — in January, before the early February deluge. And farther south, the severe drought conditions continue.
Water Temperatures May Be The Key To The Weather Pattern
The warm water (all things are relative; it’s in the 40s) in the Gulf of Alaska appears to have been the driving mechanism for the jet stream pattern for the last two winters. The warm water anomaly produces a temperature anomaly at the surface. The expanding column of air (warm air is lighter and therefore more spread out) begins the process of jet stream ridging over the west coast and adjacent waters. This in turn produces the wrinkle that becomes a trough over the eastern US.
El Niño Eggplant Competes With The Warm Alaskan Water
Since summer, El Niño conditions have prevailed over the central tropical Pacific Ocean. Though these conditions are not sufficient to meet the official (NOAA) definition of El Niño, Decoded Science has considered warm temperature anomaly significant enough to name it El Niño Eggplant.
An El Niño tends to produce a low latitude branch of the jet stream which can bring rainfall to California. In November, powerful storms reached as far south as San Diego. The February storms only got as far as central California.
These events are evidence that Eggplant is making some headway against the persistent ridge. With warm water anomalies in place over both the equatorial Pacific and the Gulf of Alaska, periods of stormy and dry weather will probably alternate on the west coast for the rest of the winter.
The Weather For The Next Few Weeks
Enjoy Crocus because it may not last much longer. After some showers today in Seattle and Portland, the Pacific northwest will experience an unusual midwinter interlude of four warm and cloudless days. After that, most long-range forecasts show the ridge flattening out. That would result in more typical, showery, drizzly weather interspersed with single days of sunshine.
As the battle goes on between competing water temperature anomalies, Crocus is a nice break from normal for Washington and Oregon. But if you want to see Crocus actually blooming, head that way soon.