Wet Weather this weekend? Where is it coming from?
We start off this week relatively quiet as high pressure builds across the region. However, the main forecast item of interest turns to our next potential winter storm this weekend. If you have been reading our recent forecast discussions, you've read about the potential for a deep layer of moisture in the atmosphere to make its way across to the West Coast this weekend.
These plumes of moisture are known by several different names such as "Atmospheric Rivers" or a "Pineapple Express" due to their ability to transport large amounts of moisture from tropics near Hawaii to the West Coast. We often use the term Atmospheric River or (AR) for short in some of our forecast discussions.
The figure above is a model representation of total accumulated precipitation through Monday morning. The white bulls-eye across the northern Sierra represent the highest totals in the region that model came up with. How much is it? Well, it depends when you decide to look at the model on a particular day. This model has shown wild variability in terms of precipitation amounts in just the past 24 hours. As little as 2 inches of liquid equivalent accumulation in the northern Sierra to as much as a whopping 16 inches. Not to mention that does not agree with other models which have their own amounts. As another example take a look at what some of these ensembles yield for rainfall for Reno. You don't need to fully understand the plot but the takeaway is that as little as a few tenths of an inch to above 3" is possible. Not very helpful, and a low confidence proposition due to large spread in solutions.
Figure 4: Ensemble Spread for precipitation this weekend in Reno.
In the internet age with quick access to data and model output, its becoming increasingly common to find one big model run being posted on the internet without regard to its stability with recent runs. So what to believe? This is where the forecaster's experience and pattern recognition comes into play. So far this winter and over the past several years, very few storms have lived up to the models long range expectations and without a favorable pattern with a slowly evolving system, don't think this one will either.
At this point in time, the most likely solutions begins with what we've seen so far this year, that is, amounts more in the 2-3" liquid equivalent along the northern Sierra versus a monster run of 16"+. Once individual model runs begin showing more agreement in both their individual and ensembles runs, confidence will improve and the precipitation forecast will be adjusted accordingly.
Takeaways for this Weekend
Confidence is increasing that we'll see a winter storm impact the Sierra this weekend with a potential for periods of heavy precipitation. Confidence is lower in regards to the impacts and intensity. These will become much more refined in the upcoming days in which we can then start speaking specifics.
Snow levels are likely to begin high which may lead to heavy rains for Tahoe and increased river flows, but then fall as the event wears on. Sierra pass travel above 6500 ft is likely to be a Sierra cement mess, with periods of strong shadowing and high winds likely Friday/Saturday in western Nevada and areas along Hwy 395.
Bottom line -- for anyone with weather sensitive events, travel, or projects Friday-Sunday please keep an eye on the forecast and updates!