Monday, January 25, 2016

Wet Weather Returns for the Weekend

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.

Figure 1:  Loop of Total Precipitable Water from Jan 22-Jan 25, 2016.
Courtesy of CIMSS/University of Wisconsin-Madison

The animated gif above shows a loop of "Total Precipitable Water" (TPW) over the last few days. This data is a combination of satellite and model data to help show where moisture is concentrated around the globe. Notice how much of the moisture is concentrated at the tropics with a few streams extending northward into higher latitudes. 

As an example, notice the plume that extends just off the East Coast of the U.S that pushes its way off into the central Atlantic by the end of the loop. What is that? That's the plume of moisture that helped feed the blizzard that buried portions of the mid-Atlantic states last weekend.  See the plume northwest of Hawaii near 170W longitude? That's our moisture for next weekend. 

The Important Matter of Confidence

Confidence in the outcome of a forecast is typically not an all or nothing proposition. Some portions of a forecast can have a high degree of confidence while another aspect of the same forecast can have low confidence. As an example let's take a look at our upcoming storm this weekend.  

Confidence is med-high that we will experience a winter system this weekend but low confidence exists on the timing and especially on the amount of expected precipitation by the end of the event.  How is it that this is the case?  Well as far as the higher confidence in having this winter storm materialize comes for a few reasons. First, this moisture plume already exists, as was evident from the loop of TPW above. Second, models and ensemble runs have been consistently picking up on this feature for days. 

An ensemble model is a lower resolution of the common models we use in forecasting (e.g. GFS, ECMWF, Canadian, etc). which are run many times using slightly different starting points or model physics to come up with a spread of possible scenarios. Say we run the GFS model with slightly different initial conditions 21 separate times, we will come up with 21 different possible solutions. If the majority of those solutions look fairly similar, then we know the model is pretty stable and probably has a firm grasp on the situation, If the outcome shows 21 different solutions, then there is absolutely no confidence in the model and likely the forecast as a result. 

Figure 2:  GFS Ensemble Members for this weekend. 
Courtesy of Plymouth State University

The figure above shows an example of  a case for this weekend where roughly 18 of the 20 members show some form of moisture plume or AR making it across the West Coast this week. This is good agreement but they do differ significantly on timing and intensity. This is where lower confidence comes into play. 

Figure 3:  GFS Total Accumulated Precipitation

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!