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!

Friday, January 15, 2016

The Sticky Situation with Snow Levels

Another wintry day in Reno with snow levels down to valley floors, which means it was likely a slower and slicker commute for most this morning. Even with my all wheel drive vehicle I lost traction a couple of times. As the traffic signs were suggesting...

Anyways, back to the snow level forecast. Snow levels are one of those forecast parameters that can be very tricky to pin down, and yet can have a significant impact to the overall weather conditions! Snow levels will play a key role in the forecast through the weekend, especially since it looks like northern California and western Nevada will be caught up in a very wet pattern. Check out the details in the latest forecast discussion here

What is the snow level exactly? Think of it as the rain-snow line. We define it as the level at which precipitation is turning into snow, so there may be some rain mixed in there as well, and it isn't where snow starts to accumulate either. So when we are trying to determine where precipitation is transitioning into snow, we can take a look at a few different parameters. Here at NWS Reno we can look at temperatures in a layer (e.g. within 2000 feet of the surface) and temperatures at the Sierra ridge top. We tie these two together by looking at forecast soundings, vertical samplings of temperature throughout the atmosphere. Without getting to deep into the details, the parameters that we use depend on the trajectory of the incoming storm system and also on previous forecaster experience. When looking at the forecast models there is always a bit of a drawback since they can struggle with boundary layer interactions and dynamics. This can lead to some discrepancies and/or variability in the accuracy of snow levels. That being said, we as forecasters can look at a number of forecast models and each one could have a very different solution for what that particular model produces as the actual snow level. We definitely have our jobs cut out for us! 

Let's take a look at this from another angle. Even if we have fairly high confidence in snow levels, let's say within a 500 foot range (which is pretty good!) that still leaves quite a bit of variability on what we have in the forecast. How much is 500 feet anyways? Seems like a lot, but when you compare it to a football field (360 feet) it really isn't that much! Then you talk about nailing down snow level forecasts every 6 hours for 7 days in the future? It's a tough challenge, but we certainly do our best. 

Let's take a look at an early January example. The first graphic shows a most likely scenario for snow amounts with a 70% confidence for a January storm. Notice we have snow amounts for much of western Nevada. Now let's take a look at a warmer scenario, where snow levels would be a bit higher. This would result in no snow for much of the Reno area. You can see where just a difference in a few hundred feet would result in a potentially significant difference in impacts to traffic and travel for this scenario. So what is the best way for you to know what the snow level is going to be? On a day-to-day basis your best bet is to read through the area forecast discussion, where we as forecasters typically discuss our confidence in the snow levels and the impacts that could result if the snow levels are higher or lower than forecast. This will especially come in handy over the next week as we enter a fairly active and wet weather pattern for the west coast. A good rule of thumb is to always apply a +/- 500 foot error bar to any snow level forecast you see, just to be better prepared. Until then, be safe on the slick roads this morning and maybe wait until the snow melts to run errands today. 

Monday, January 11, 2016

[Guest Post x 2] Prescribed Fire and Air Quality in Washoe County

Enjoy this double guest post by Washoe County Health District Air Quality Management Division and the North Lake Tahoe Fire Protection District. They are key partners that work with us throughout the year and we all coordinate with each other for fire, air quality, and weather related concerns. Enjoy!
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Washoe County Health District Air Quality Management Division

The Washoe County Health District Air Quality Management Division  works closely with local Land Managers to conduct Prescribed Fire treatment to our lands. As part of our Smoke Management Program, we have guidelines in place for Land Managers to follow when writing their burn plans, preparing for a prescribed fire, and the day of the burn. Smoke from prescribed fires does not impact our air quality like wildfires do. As discussed in the Wildfire Smoke in Northern Nevada blog post, particulate matter (PM10 and PM2.5) are found in smoke.

As part of the guidelines for prescribed burns, Land Managers are required to follow the daily Burn Code, and cannot burn during a Red Burn Code, which is typically called in the "Unhealthy for Sensitive Groups" range. Smoke from the 2013 and 2014 wildfires reached Air Quality Index levels in the "Unhealthy" range, meaning everyone can be affected by the pollution in the air. The concentrations of particulate matter in these higher AQI ranges due to wildfire smoke is much more harmful than the concentrations of particulate matter from prescribed fires. 

Smoke from prescribed fires, while visible, very rarely impact Washoe County. The worst wildfires we've had in the Truckee Meadows were 8 to 10 times higher in particulate matter concentrations and 6 to 8 times higher than the health standard than the prescribed fire concentrations shown in the chart below. 

Prescribed fire treatment on our lands is a very important factor in reducing the number and intensities of wildfires, therefore preventing major air quality impacts to our community. For a little more detail about prescribed fires see the portion below by North Lake Tahoe Fire Protection District. 

North Lake Tahoe Fire Protection District 

Annually. fire and land managers implement prescribed fire projects throughout their managed land. Each project includes specific goals or objectives, to reduce hazardous fuels and improve natural resource benefits predetermined by land managers. Two common prescribed fire methods are pile and understory (broadcast) burning. Pile burning involves burning slash piles that were created by hand and mechanical slash piling. 

Understory (broadcast) burning involves implementing a light to moderate intensity fire through the surface or ground area of an identified area, to reduce fuel loading, overstock reproduction, and to accomplish natural limbing of lower branches of more mature and desirable trees. Both methods consist of preparing a prescribed burn fire "Burn Plan". 

Why Prescribed Fire? 
After more than 100 years of fire exclusion practices, an ecosystem that requires periodic fire has become unhealthy and extremely hazardous. Trees are stressed by overcrowding, fire-dependent species are disappearing, invasive non-native species are thriving, flammable fuels have built up and become hazardous, and the general public has dramatically increased development and usage within these unhealthy lands. 

What is the result if land managers do nothing? Catastrophic wildfires!!
  • Fires have become more difficult to control and are more damaging, due to higher intensities and faster rates of spread
    • Increasing threat to public and firefighter life and safety
    • High costs associated with damage/loss of infrastructure
    • High costs associated with suppression and rehabilitation efforts
  • Damaging effects on plants and wildlife
  • Damaging effects to watersheds
  • Destructive post fire effects (floods, loss of timber, erosion, vegetation change, etc.)
  • Increased carbon emissions and poor air quality 
What are the benefits of prescribed fire?
Fire in the wildland plays an essential role in the ecosystem. A "non-catastrophic" fire, whether natural or prescribed has many resource benefits. Prescribed fire is the pre-planned controlled application of fire to the land to accomplish specific land management goals and objectives, to achieve numerous benefits. Some of these benefits include but are not limited to: 
  • Provide for firefighter and public safety and minimize loss/damage to infrastructure during catastrophic wildfire events
  • Reduction of hazardous fuels (natural and/or logging debris)
  • Wildlife habitat, grazing, and forage improvement
  • Insect and disease control
  • Aesthetic appearance enhancement
  • Native vegetation and seeding/planting improvement
  • Perpetuate fire-dependent species
  • Watershed protection and improvement
  • Reduces carbon emissions and greenhouse gases
  • Provide training and qualification opportunities to firefighter personnel
What is a Burn Plan, and who implements the burning? 
The Burn Plan is a document created by land managers to help ensure that the objectives of the project are met in a safe and efficient manner during implementation and post burn efforts. Elements within the burn plan include environmental factors, number of resources/equipment needed on site, contingency planning, fire behavior parameters, smoke management, safety/medical plans, permitting/notification requirements, and pre/post burn requirements. All elements within the plan must be met or followed prior to burn implementation. 

Prescribed fire operations are conducted by trained and qualified fire management personnel who have studied and have experience in fire behavior and fire management/leadership techniques. 

What about the smoke? 
Controlling the amount, duration, and where the smoke goes is an essential part of every prescribed fire operation. Prior to any burning land managers look very closely at possible smoke impacts to schools, residents, roadways, and other smoke sensitive sites proximal to the project area. The burn plan is drafted to minimize negative impacts of smoke to such areas. 

However, smoke is a natural byproduct of fire and some amounts are unavoidable during prescribed fire operations. Periodic prescribed burns prevent heavy fuel accumulations that typically would produce larger amounts and longer duration smoke impacts in the event of an uncontrolled wildfire.