One of the longest November dry spells in Northwest history – and why that also produces thick fog | KNKX

One of the longest November dry spells in Northwest history – and why that also produces thick fog

Nov 8, 2019

Normally early November is a wet and stormy time of year in the Northwest. Not this year. It has been generally dry and sunny over the past few weeks — dry enough to tie a record for lack of rain, assuming no precipitation falls on Friday.

“The record for a period in November is 14 days. That happened in 2002. That’s a very long stretch of no rain,” says KNKX weather expert Cliff Mass, a professor of atmospheric sciences at the University of Washington.

“In fact, I think it’s the longest stretch of this particular year, even including the summer, of having no rain.”

PERSISTENT HIGH PRESSURE

The reason for this long dry spell is persistent high pressure that is locking the air in place over the eastern Pacific, Mass says.

“That’s locked in and held off rain since the 25th of October. We haven’t rained since then,” he says.

At the same time that the high pressure can prevent rain, Mass says it’s also frequently responsible for producing fog, which has been another persistent weather feature people we've been seeing lately.

“Our big fog periods here in the Puget Sound region are associated with periods of high pressure over the region,” Mass says.

There are several reasons for this, Mass says.  First, high pressure is associated with sinking air that tends to drive clouds away. Clouds act like blankets that often keep the earth’s surface warmer than is conducive to fog; the air needs to cool enough to get condensation and the sinking air enables this. High pressure also generally prevents strong winds and mixing at lower elevations, which tend to drive fog away.

“So high pressure allows the cooling and it prevents the mixing — and both are very good for fog.”

NIGHTTIME FOG NOW VISIBLE

Mass says there’s also a big breakthrough in navigating fog, especially at night. Satellite imagery has long been able to see where it is during the daytime. But infrared satellite pictures that sense temperature to calculate dimensions have been limited.

“The trouble is, fog is so shallow, it’s almost the same temperature as the ground, so it’s almost invisible,” Mass says.

“But now we have new satellite capabilities, which look down in various wavelengths and can sense what the clouds are made of. And we can use those to determine the distribution of fog.”

He says the capability has improved dramatically with the launch of the new GOES-7 weather satellite by the National Weather Service.

CLIFF'S FORECAST:

Friday: morning fog will burn off and yield to sunny skies with daytime highs in the mid-50s. “A pretty decent day.” Clouds move in overnight.

Saturday: Cloudy and a 50-70% chance of light showers, ending the dry spell, at 14 days. Temperatures in the mid-50s.

Sunday: “A little bit better.” Cloudy with morning sprinkles possible; temperatures in the mid-50s.

Monday-mid week: Cloudy with a chance of more showers as the ridge of high pressure that has been shielding the region from rain eases. “I think that will weaken,” Mass says. “Cloudy, mid 50s on Monday and a good chance of some showers — probably light showers — as we get into next week.”

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.