Weather

Wetter than Normal in the West, Drier than Normal in the East. But why?


Over the past year, contrasting rainfall over the Cascades has been greater than usual.

The west is usually wet wetter than usual.

Normal dry area east of Cascade drier than usual.

But why?

When you finish this blog, you will know!

Let’s start by showing you the difference from normal rainfall across the state from October 1 to the end of March (see below).

Much wetter than usual on the western slopes of the Cascades and on the northwest side of the Olympics (purple). But east Washington is 0 to 4 inches lower than normal (yellow).


Since eastern Washington is a dry place, if you really want to be dramatic, look at the percentage of rainfall over the same time period. Some places in the west it’s 130% more than normal, but in eastern Washington, along the eastern slopes of the Cascades and downstream of the Okanagan Plateau, Some locations are 50-70% of normal.

The shortfall in inches is small, but if one is talking about an area that is essentially desert, a small drop in inches can be a large percentage.


So why do we get this rainfall, which is clearly tied to the terrain?

The answer is La Nina and the large-scale atmospheric modeling it generates. And remember, La Ninas is associated with colder-than-normal sea surface temperatures in the central and eastern tropical Pacific.

Look at pressure or altitude at 850 hPa (about 5000 ft) – see below.

This plot is showing a difference from the usual. Over the Pacific, the pressure is higher than normal at this level. In western Canada, pressure is lower than usual.

The result of these two barometric features is an area of ​​great pressure variation in the northwest … and large pressure changes resulting in winds. And since winds of this magnitude (about 5000 ft) tend to be parallel to the pressure lines, this situation produces an intensifying northwesterly wind (from the northwest, see arrows)

The northwesterly winds cause the air to be pushed up west of the Cascades … precipitation is increasing there (increased upward movement leads to more precipitation).

But northwesterly winds also produce more downflow east of the Cascades, resulting in more downflow that reduces precipitation.

So what does this have to do with La Nina? Because La Nina situations tend to produce exactly this pressure pattern.

Want proof? Below is a chart of the difference from normal pressure at about 18,000 ft in a La Nina year from a NOAA website. Mom Mia! It looks like the picture above.

So colder-than-normal waters over the tropical central/eastern Pacific have resulted in a strong La Nina this fall and winter, creating highs offshore and lows inland leading to westerly winds. northerly than normal, creating an enhanced countercurrent west of the Cascades and a downhill run to the east, thereby correcting precipitation.

Kind of reminds you of something:





Source link

news7g

News7g: Update the world's latest breaking news online of the day, breaking news, politics, society today, international mainstream news .Updated news 24/7: Entertainment, Sports...at the World everyday world. Hot news, images, video clips that are updated quickly and reliably

Related Articles

Back to top button