Big pattern change brewing in the Pacific

MONDAY NOVEMBER 30 5:14 AM….Tahoe’s weather for most of this week will be calm and mild as a warming trend sets in beneath a ridge of high pressure sitting over the west coast. The winds that raked the region over the weekend have mostly subsided and should die down almost completely by the end of today. The absence of the wind will also mean that an inversion layer over the region will be difficult to displace, and we will likely see a narrowing of the typical temperature gap between the mountain passes and nearby valleys as the valleys remain cooler than normal. The only other action in the short term will be a low pressure system passing to our north which could bring some cloud cover and cooler temperatures on Wednesday and Thursday. But that system is not expected at this time to bring much if any precipitation to Tahoe.

The big news today is the growing body of evidence pointing to a major pattern shift in the Pacific that could unleash an extended period of wet weather for Tahoe, starting late next weekend.

It looks like a ridge of high pressure will build this week from the Eastern Pacific into the Gulf of Alaska, and then the southern end of that ridge will get pinched off, allowing a series of storms packing subtropical moisture to ride underneath the ridge on a track headed straight for California. This is the first sign of the effects of the moderate to strong El Nino condition that has developed in the Pacific this fall but has yet to affect our weather. The issue for Tahoe will be how much of that moisture we get, and how much cold air gets combined with the moisture when it arrives. For the first slug of moisture, currently forecast for the Sunday-Monday time frame, two of the computer forecast models are showing a burst of cold air from the north well timed with the arrival of the Pacific moisture plume. If that comes to pass, we would see a major snowstorm next Sunday or Monday. Another forecast model, however, is showing that cold air drifting down the interior west over Idaho and then Nevada but not necessarily combining with the warm, wet surge coming in from the ocean. That scenario would be bad news, because it could mean a big wet rainstorm for Tahoe just as we are trying to get our season underway.

It is still too early, seven or eight days before the event, to know which of these scenarios will play out. But one thing does seem certain: the second week of December will bring some unsettled weather to the West Coast and the Sierra.

Stay tuned for details as the change develops.

Forecast Highs

Tahoe City

Monday 47
Tuesday 49
Wednesday 49

7000 feet

Monday 50
Tuesday 49
Wednesday 49

SUNDAY NOVEMBER 29 5:24 AM….A ridge of high pressure is beginning to build over California in the wake of Friday’s storm system, and it will bring a slow warming trend to the Tahoe Basin over the next few days. Sunday, however, will remain on the cool side and windy, with a Lake Wind Advisory still posted for Lake Tahoe and large waves expected on the lake through this afternoon. Monday should be calmer and warmer, and Tuesday will also be pleasant. The only complication will be an inversion layer that could keep the Sierra valleys as cool as the mountain passes.

The next chance for unsettled weather will be midweek, but the computer forecast models are having an unusual amount of trouble pinpointing the track of a storm system forming now in the Gulf of Alaska. One model has the low pressure system settling off the coast of Oregon on Wednesday, which would bring rain or snow to Tahoe. But another model has the system drifting all the way down the coast to Baja California, which would leave us dry most of the week. At the moment it looks as if the second, drier scenario is more likely, but either one could come to pass.

The bigger weather issue is when the moisture track in the Central Pacific will begin feeding our storms. With the El Nino condition in place, we should start seeing some wetter storms any week now, and the models are beginning to show the movement of that moisture plume into position to send additional precipitation our way. That could happen as soon as next weekend, but if not, it looks as if it will happen at some point in the first two weeks of December.

Stay tuned for reports on the possible midweek storm and for more details on the bigger pattern change as well.

Forecast Highs

Tahoe City

Sunday 40
Monday 47
Tuesday 47

7000 feet

Sunday 43
Monday 47
Tuesday 45

Saturday: cold, windy

SATURDAY NOVEMBER 28 5:12 AM…..A low pressure system that brought about a half a foot of fresh snow to the Tahoe region Friday has moved south and east of the area, leaving high winds and cool temperatures in its wake. A few scattered snow showers remain on the backside of the system, especially south of Tahoe. But Saturday should see mostly clear skies with highs around the Lake in the low 30s. The big issue will be the winds, as a cold northeasterly flow rakes the Sierra Crest and nearby valleys. Gusts have ranged from 30 to 50 mph overnight and could strengthen this morning, then east off before returning Saturday night. A wind advisory issued by the National Weather Service is in effect until Sunday morning.

The forecast for the week ahead is a tricky one. Conditions should be dry from Sunday through Tuesday, but a low pressure system will be drifting off the coast of California with a destination that at this point remains uncertain. It could swing inland over Central California, bringing rain and snow to the region. Or it could continue to drift southward and leave us dry. We will just have to keep watching its track and the forecast models to get a better fix on it as the system approaches.

Another system will be forming off the coast by Friday, but the forecast models are split on its ultimate trajectory.

The bigger picture news is that a plume of moisture is forming in the Central Pacific that could bring us wetter weather at some point in early December if the overall pattern in the eastern Pacific cooperates. That’s the pattern change we would be looking for to get the snow season started in earnest.

Stay tuned for updates as these storms develop.

Forecast Highs

Tahoe City

Saturday 31
Sunday 40
Monday 45
7000 feet

Friday 32
Saturday 41
Sunday 45

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