We could see cool troughs bring unsettled weather to the northern Sierra the weekend of the 30th-1st and again the weekend of the 7th-8th. The pattern is starting to become slightly more active as we head into a new water year and the 2023-24 winter season.
Short Term Forecast
We are enjoying beautiful weather this week. Things will start to change on Friday with southwest winds starting to pick up a bit as a cool trough digs in from the northwest.
That trough will be over the region for the weekend with cooler and unsettled weather.
A low-pressure system spinning up in the trough will sit over the northern CA/NV region. We could see some showers over the mountains by later Friday night through Sunday. The best chance for steadier showers looks to be Saturday with scattered showers possible into Sunday before the system lifts out of the region.
The latest model runs show light precipitation amounts with up to half an inch of total precipitation possible by Sunday night.
Temperatures in the 60s this week may struggle to get out of the 40s at lake level over the weekend. It will feel like fall with cooler and unsettled weather around the Tahoe basin and northern Sierra.
Snow levels will start high but could dip near 8000 ft. by early Saturday morning, and then possibly dip as low as 7000 ft. in spots by early Sunday morning. The heaviest precipitation could fall through Saturday with showers becoming more scattered by Saturday night into Sunday, so any snowfall will likely be limited to the highest peaks and only a coating of snow.
Our snowfall models are forecasting up to an inch of snow by Sunday above 8000' along the crest, as seen below on our forecast for around 8500' near the Granite Chief peak.
Not much if any snowfall for most locations, but maybe a coating for some of the peaks and a sign that winter is around the corner.
As the system departs by Monday we will see high pressure building back in for next week with the nice weather returning. We should see mostly sunny skies with highs back into the 60s by Tuesday.
Another trough is forecast to develop over the West Coast by the following weekend of the 7th-8th of October.
That could bring another cool and unsettled weekend to the region. Another low-pressure system may spin up over northern CA with showers possible, similar to the previous weekend. The long-range models show
Show another few tenths of an inch of precipitation possible on top of the few tenths from the weekend of the 30th-1st.
The uber long-range models are suggesting that troughing could remain near the West Coast into the week of the 9th. Some suggest a wetter storm is possible that week, but we'll see... That is pretty far out for models to forecast, especially during the fall as the seasons change.
The good news is that the start of the water year on October 1st looks to start with some light precipitation that could return the 2nd week of October. So not a dry forecast to start out the new water year.
The Upcoming Winter Season:
If you have already been following along this month, you know that I've been writing about some signals for the upcoming winter season pattern and what it could mean for northern CA/NV.
We have fairly strong El Nino conditions in the Pacific in the ENSO regions near the equator off of South America. If that were to hold through winter it could enhance and extend the southern branch of the jet stream into southern CA at times through the winter season.
Looking at the sea surface temperatures this week, I noticed a dramatic cooling of the waters in the Pacific NW, likely due to upwelling from the storms that started hitting the Pacific NW and Canada.
I like a colder northeast Pacific during the winter with the warmer water staying out near Hawaii, as I think that tends to keep any prolonged ridging farther west away from the West Coast.
Unlike the 2015-16 El Nino season with warm water along the coast and the ridge that sat near the West Coast and bumped the jet stream farther north into the Pacific NW, giving us the opposite of a typical El Nino season with a drier southern CA and wetter Pacific NW.
So could be a good sign developing. Also during the 2015-16 season and some other recent El Nino seasons. The warmest waters along the equator shifted farther west away from South America, which didn't enhance the jet stream as far east into Southern CA and the southwest. There was some cooling off of South America as well this week.
The forecast models are not in agreement on how strong the El Nino stays with the water staying well above average, and they don't agree on whether the warmest water shifts toward the central equatorial Pacific in a "Modoki" El Nino-type pattern. I'd prefer the warmest water to the east for what should increase our chances of a wetter season.
Kind of the opposite for the northern vs equatorial Pacific for what I'd prefer. Warmest water farther west in the northern Pacific and farther east in the equatorial Pacific I think would lean more towards a wetter season in CA historically.
Reading the discussions at WxBell it seems they are keeping an eye on similar signals. Their latest winter forecast maps show below-average temperatures for southern and central CA. That is due to an active storm track with precipitation and cloud cover helping to keep temperatures cooler than average.
They show above-average snowfall across the southern tier of the country as well.
They have the northern Sierra slightly above average with the average line just to our north in the southern Cascades, and below average farther north into the Pacific NW.
A few posts ago I went through some of the stats for moderate-strong El Nino seasons. Around the Tahoe basin it was a 50/50 split for above/below average precipitation during the winter season Oct-May, and around a 60/40 split above/below average for snowfall.
If the water cools off the coast and stays warm in the eastern equatorial Pacific, it's this forecaster's opinion that the stats lean toward slightly above-average snowfall during seasons with similar conditions. I would be prepared for another decent snowfall season, not ridiculous like last season, but possibly above average. If we are near to below average at least you were ready to deal with it.
Mentally, if you prepare for a big year you could always be disappointed. I'll continue to look at the winter season signals as we go through the fall, and I'll do more posts comparing actual historical stats as well.