Cold Outbreak/California Fire Review, and a Hurricane Zeta Update

The next two weeks look unseasonably calm. We’ll see a moderate system swing through on Friday, but other than that, our weather looks warmer and drier-than-average for the next 10 days as a strong, broad ridge of high pressure builds across the Western US. Given the boring weather ahead, I thought I’d do a quick post-mortem on the cold snap we saw this weekend and the extreme Santa Ana/Diablo Winds California experienced Sunday/Monday, as well as a brief update on Hurricane Zeta in the Gulf of Mexico.

Folks in Western Washington and Oregon saw chilly weather this weekend and Monday morning as the first real “cold trough” of autumn/winter plunged south from Canada. But temperatures east of the Cascades were downright frigid, and those over the Rockies were even colder. There were unverified reports over Northern Montana saying that the cold weather had caused property taxes to freeze, but unfortunately, I expect any taxes to thaw with the warmer weather headed our way.

Spokane saw 6.9 inches of snow Friday and another 0.6 inches early Saturday. With 7.5 inches, they’ve already set the record for the snowiest October on record, where records go all the way back to 1881!

One of the side roads in West Spokane. #wawx @NWSSpokane pic.twitter.com/Tu830Jv6RQ

— Benjamin Jurkovich (@BenjaminJurkovi) October 24, 2020

The fresh snow could be seen on Sundays’s satellite image. Although I confess that I’m not a huge fan of the cold weather associated with the snow (yes… I’m a wimp!), it’s so so wonderful so see a blanket of white covering the Cascades and Inland NW. These early-season snow dumps don’t have any bearing on how our 2020-2021 ski season will turn out, but it sure gets skiers like me excited!

The temperatures measured over the Intermountain West were simply extraordinary. The Great Falls, MT NWS office listed all the record cold temps broken in just their CWA (county warning area) yesterday. Many of these records began in the 19th century, and they were absolutely obliterated by this cold spell.

And if you think the snow in Spokane was heavy, it was nothing compared to the blizzard that came through Montana on the 23rd and 24th ahead of the dry cold front that gave record low temperatures on the 25th and 26th. Helena, MT saw 20″ of snow in 12 hours! Quite a storm for late October.

California Fire Danger

But as is so often the case, this early-season arctic outbreak over the Rockies and Inland NW brought extraordinarily dangerous Diablo and Santa Ana winds to North/Central and Southern California, respectively. This is a very typical late autumn/early winter pattern; the deep trough responsible for this cold, dry Canadian air plunges into the Great Basin, bringing the cold and dry airmass with it. Because cold air is denser – and heavier – than warm air, cold air at the surface is often associated with high pressure (just like our heat waves have a “thermal trough” at the surface even though there is ridging aloft).

The below graphic shows the difference between “cold core” and “warm core” low and high pressure systems. The cold, upper-level troughs that drop south from the arctic are “cold core highs” (picture B below), with high pressure at the surface but low pressure aloft due to the cold/dense airmass at the center of the cold-core high.

The strongest winds occurred from Sunday night into Monday morning. Note the upper-level pattern 5pm Sunday, with a strong ridge offshore and a deep trough plunging south over the Great Basin. This allowed cold, dense air to filter into the Great Basin while the California lowlands saw much milder weather, creating a huge temperature and pressure gradient across the Sierra Nevada.

❄️Shiver me timbers!❄️ 45 daily low records were broken today across the western US, 11 of which were by double digits! Missoula, MT took the 🍰 beating it’s record by 20 deg. Casper, WY was close behind, beating it’s record by 19 deg. Did your area break a record today? #Brrrr pic.twitter.com/tEJH3TGEWV

— NWS Weather Prediction Center (@NWSWPC) October 26, 2020

These extreme gradients caused winds to gust to 140 mph at Kirkwood ski resort near Lake Tahoe. I’d like to get my hands on one of those weather stations… my anemometer (from a Davis Vantage Pro II) lost a cup after the August 29, 2015 windstorm!

This event shared many similarities to the record-breaking October 26-28, 2019 event that resulted in over 3 million Californians, primarily PG&E customers, seeing “public safety power shutoffs.” Over the past year, PG&E and other California utilities have worked to make these shutoffs affect fewer customers and last a shorter time, but PG&E still had to shut off power to 361,000 customers (over one million Californians) with this event. I had a bit more trouble finding shutoff data for Southern California Edison and San Diego Gas & Electric, California’s other two big investor-owned utilities, but my guess is that SCE shut off power to 20,000-30,000 and SDG&E shut off power to 5,000 – 10,000 customers.

Here are some gusts around the San Francisco Bay area with this event compared to the 2019 event. It’s no coincidence that both occurred in late October – October to mid November is prime Santa Ana/Diablo season, though they can frequently as late as January and even February on occasion.

How did last night’s wind event compare with the big wind event from Oct 27-28 of last year? In terms of maximum wind gust, it looks pretty close. Both are historical events. #cawx pic.twitter.com/KHRbwgnM1D

— NWS Bay Area (@NWSBayArea) October 27, 2020

Both events had minimum humidities in the 1-5% range and warranted a “PDS (Particularly Dangerous Situation) Red Flag Warning from the Los Angeles NWS for extreme Santa Ana winds. There are rumors that the Silverado Fire in Southern California was caused by downed, energized power lines, but the situation would have been far, far worse if utilities (especially PG&E) did not have widespread public safety power shutoffs.

Hurricane Zeta

Hurricane Zeta made landfall last night over the Yucatan Peninsula as a category 1 hurricane, causing flooding due to extremely heavy rain and light structural damage from the hurricane-force winds. It re-emerged this morning over the Gulf of Mexico as a tropical storm and had very little thunderstorm activity around its center of circulation, but convection has since intensified as the storm reorganizes itself over the warm waters of the Gulf of Mexico. An eye is now apparent on satellite, and although the 8pm PDT update still had Zeta as a high-end tropical storm with 70 mph winds, I’d guess that it has reached Category 1 status (74 mph winds or higher) as of writing (8:43 pm).

Even though cloud bands extend all the way to the Gulf Coast, Zeta is a relatively compact storm. This type of compact structure is favorable for further strengthening, and I believe Zeta has a good chance of intensifying overnight and reaching category 2 status by daybreak.

Zeta should continue to strengthen gradually tomorrow morning, and I believe it will make landfall as a category 2 storm near the Mississippi Delta around 5-6pm CDT (3-4 pm PDT) tomorrow. Zeta will encounter increasing wind shear and cooler water as it approaches the coast, but due to its fast forward speed, these factors won’t have much time to disrupt the storm. As a result, Zeta will likely stop intensifying several hours before landfall, but the shear and cooler ocean temperatures won’t have enough time to actually weaken the storm before it makes landfall.

The main threats with Zeta are strong winds, moderate surge, and a risk of weak tornadoes over the deep SE with the rain bands ahead of the storm. Due to the fast forward speed, Zeta will not pose a huge rainfall threat, but it may be able to bring hurricane force gusts into parts of inland Alabama before it weakens dramatically Wednesday night. By Thursday afternoon, the remnants of Zeta should pass near New York City.

Thanks for reading and have a great evening! I’ll have Zeta updates tomorrow on my Facebook and Twitter pages.

Charlie