Tahoe Weather Geek
Forecast summaries for Tahoe residents and Sierra-bound travelers

For weather updates, Tahoe cams and our adventure sports blog, go to www.tahoeloco.com

December 3, 2009

Tahoe still has at least two more days of comfortable fall weather before a major change in the pattern is expected to take hold. Things will be getting colder this weekend, then wetter by next week. But the details beyond Sunday remain elusive. Here is what is in the mix:

A high pressure system over the Eastern Pacific is retrograding toward the north, opening the door to a series of storms that should impact the Sierra starting this weekend.

But first we will have an "inside slider," a storm that has ridden up and over the ridge, into British Columbia and then south through the interior west. These storms tend to be cold and also dry, because they don't sweep over the Pacific scooping up moisture before they arrive in the Sierra. And that's how this one looks. It will be cold, with temperatures dropping at least 10 degrees from Friday's highs. It will also be windy. And it will be dry, with well under an inch of precipitable water. But the conditions will be ideal for snow creation if there is any moisture at all to work with. Depending on how far west the system drifts as it heads south, this could mean a dusting for Tahoe late Saturday and early Sunday or several inches of snow.

What happens then is the real question. A cold low from the north is expected to take hold over the Great Basin for several days. At the same time, a warm, wet moisture plume is setting up in the Pacific on a track headed for California. How these two systems interact as they collide will determine how much rain and snow the Sierra gets next week. If the cold low prevails, it will push the moisture track to the south, leaving the Sierra with less precipitation but also colder temperatures. If the cold low stays east of the Sierra, however, that subtropical moisture plume would likely bring a series of warm, wet storms to the area.

The first one, due to arrive Sunday and Monday, will probably be all snow, mixing with the cold air already present. The Tahoe high country could see a foot or more of beautiful dry powder from this event. The second wave, now timed for a Wednesday or Thursday arrival, looks like snow at the passes and rain below. But each successive wave should be warmer as the cold air gets pushed out and replaced by the tropical influences from the west. And if that happens, it's possible by the end of the week we could be looking at rain at 7000 feet.

It's a quantity versus quality thing. The more moisture we get, the higher the snow levels are going to be, and the greater chance that we will get rain at 7000 or even 8000 feet. We tend to root for the colder, drier storms here, but with the snow pack still so meager, it wouldn't hurt to get a nice big, wet base down to start the season. As long as it doesn't rain.

Sorry we can't be more definitive. But it's early yet. With the computer forecast models giving mixed signals, it's impossible to say this far out when it will snow, or how much.

Check www.tahoeloco.com for updates at least daily as these storms develop.

New Local Doppler at TahoeLoco.com

If you want to know exactly where that big yellow blob on the radar is going next, check out our new up-to-the-minute micro local doppler at TahoeLoco.com. Created specially for TahoeLoco by Intelliweather.net, the radar image zooms in on the greater Tahoe region, from Emigrant Gap on the west to Carson city on the east. It will have great detail during this winter's storms, and animation to show the movement of the storms. See it now at the Weather Geek page on TahoeLoco.com.

El Nino Watch

As we ended November, the El Nino condition in the Pacific Ocean officially went into the books as an El Nino event, with five consecutive overlapping three-month periods in which sea surface temperatures have been at least .5 degrees Celsius above normal. And while the full data for November are not yet available, we ended the month with temperatures at 1.7 degrees Celsius above normal in the central subtropics, which, if it held for three months, would qualify as a "strong" El Nino event. So the ocean is a little warmer than it normally is. What's that supposed to mean for our weather?

In a strong El Nino, the warmer ocean temperatures will generate more than the usual amount of moisture over the Central Pacific, and that moisture will get pushed by the prevailing winds into the weather pattern that brings storms into California. Because the moisture is coming at us from the south and west, however, Southern California is usually assured of the heaviest rain, while Tahoe can get some of that precip but not all of it. The Pacific Northwest, meanwhile, is usually on the dry side. If the big storms heading south are big enough, we can get a massive dump of snow, or a big rain storm with high-elevation snow levels. If the storm veers too far south, though, we can be left with little to show for it, looking more like our friends in Washington and Oregon. Generally, though, in strong El Nino years, Tahoe is more likely to get heavier precipitation than normal.

For more detail on how past El Ninos have been correlated with Northern California precipitation, see our latest El Nino watch at www.tahoeloco.com.

MONTHLY WEATHER ALMANAC FOR NOVEMBER

November was drier than normal for Tahoe, with Truckee getting 32 percent of its normal precipitation, and South Lake Tahoe just 13 percent. It looks like December will be better. Let's hope so. The detail:

TRUCKEE

High: 72 on Nov. 3

Low: 10 on Nov. 21

Average temp: 35.4

Departure from norm: -0.3

Precipitation: 1.24

Normal precip: 3.83
Percent of normal: 32%
Total snowfall: 9.1 inches.

SOUTH LAKE TAHOE
High: 68 on Nov. 2
Low: 13 on Nov. 30

Average temp: 38.7

Departure from norm: +4.3

Precipitation: .59

Normal precip: 4.70 inches
Percent of normal 13%

Total snowfall 4.5 inches.

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