Monday, March 30, 2015

Will Skies Stay Clear for Saturday Mornings Total Lunar Eclipse?

As meteorologists we often get asked about anything that happens up in the skies, whether or not it's weather-related. I still remember the "great Google Loon commotion" in May 2014 where our office was inundated with questions about what was floating over Reno for several hours one afternoon. So that brings me to our upcoming total lunar eclipse taking place early Saturday morning. We're not the experts on eclipses, talk to the Fleischmann Planetarium folks about that, but we certainly can talk about the weather expectations for the show.

Picture of the December 2011 lunar eclipse over the slopes of Peavine, taken from NWS Reno
If you're an early riser (like me) or up really late Friday night (not me), here's the particulars on the upcoming eclipse from NASA. Reading other articles, this upcoming eclipse is also the shortest total lunar eclipse of this century, with totality lasting only 5 minutes. Sounds perfect for my attention span!

  • Partial eclipse start - 3:16 AM PDT (roughly mid-way up the southwest sky)
  • Total eclipse start - 4:58 AM PDT
  • Total eclipse ending - 5:03 AM PDT (that's only five minutes long!)
  • Partial eclipse ending - 6:45 AM PDT (moon setting in the west about this time; also note that it will start getting light out between 5:55 and 6:11 AM, and sunrise is at 6:38 AM)

So, we've been in a drought all winter with (too) many clear sky days. Is that going to be the case for Saturday morning? The GFS (American forecast model) simulation is has been indicating that the storm track will remain to our north Friday-Saturday. In the map below, the green and blue color shadings show light precipitation over the Pacific Northwest. The other simulations, including the European forecast model, show something similar with dry weather over our region.

A more likely scenario are high-altitude clouds rolling through, and we don't necessarily need a storm to create those. Forecasting the extent of these clouds especially five days in advance is tricky. This is readily seen in the big differences in cloud forecasts from the two most recent GFS simulations. We run the simulation four times each day.

This map shows the most recent GFS simulation of humidity at about 25,000-30,000 feet for Saturday morning. Based on this one simulation, anywhere in the blue shading has a good chance of seeing thick high altitude clouds, perhaps enough to obscure portions of the eclipse. The green and black areas are drier and have less of a risk of clouds. Note the Reno and Tahoe areas are right on the gradient, and given that the eclipse is taking place in the western sky, there may be some clouds around.

Here's the previous GFS simulation - a bit different, huh? The blues are not as widespread over northern California and Oregon, but there's a few more spots over Nevada. 

Bottom line -- there's potential for seeing some of these high-altitude clouds around Saturday morning, but whether or not they'll impede viewing of the eclipse is too soon to call right now. Hopefully by Wednesday or so our simulations will come into better agreement so we can answer that question with more confidence.

-Chris@NWS Reno

Saturday, March 21, 2015

What is a Downslope Wind? (Caution: Knowledge dropping ahead)

Some of you may have read the forecast discussion the past couple of days and noticed the reference to "downslope winds" or "downslope enhancement". What does it MEAN? Well we are going to explain that to you today in the simplest terms that we here we go!

First of all, wind is typically driven by pressure differences, which are influenced by thermal differences. The greater the difference, the stronger the wind. (Video here) This type of wind is fairly typical, and we would call it a "gradient driven wind". Downslope conditions are characterized by winds encountering a barrier and attempting to get past that barrier. Let's take a look at this by relating it to something we are more familiar with, like water running in a creek!

Let's pretend in a perfect world there is a straight, slow moving river with a smooth riverbed. In this ideal scenario (without friction, physics, etc.), the water flows down the river quietly and calmly with no ripples or eddies within the water flow or on the surface. Depending on your adventure preference via kayak, this would be pretty easy-going and relatively boring. Now let's take a look at what reality would be in a river. Toss in some rocks, riverbed irregularities, and swift-moving water. The water would also move at different speeds at any level within the river, which would change things up. Notice the flow now! Let the kayak adventure begin. 

Mathematically, air and water movement can be calculated with very similar equations, which makes this comparison fairly true to life. So similar to water, when the wind encounters a barrier, propagating waves (eddies/turbulence) result downstream of the barrier. The stability of the atmosphere determines whether the waves will be pushed downward towards the ground (see image on the right) or become trapped midair (trapped lee waves on the left below).

The breaking region of a downslope wind can be thought of as similar to a hydraulic jump region that many are familiar in seeing in rapids, spillways, and even your own kitchen sink at home. This is because the quickly moving fluid, or atmosphere, crashes into a region of relatively lower speed. Just think about being in a big wind event in the Sierra -- winds are almost always stronger at the upper elevations than they are in the valleys below. See the images below for hydraulic jump examples in fluids, with the picture on the right similar to the jump region in a downslope wind event.

Meteorologically, how do we get the setup that we need for downslope conditions to occur? Here are some downslope tidbits:
  • Requires a strong jet perpendicular to the Sierra
  • A ridgetop inversion is needed to develop the wave breaking region, see examples of this in actual upper air soundings from downslope wind events below.
  • Typically occur 3-6 times a year with the core months late October through early May
  • Strongest winds will be immediately in the lee of the Sierra
  • Wind speeds 60 to 90 mph have been recorded in the Reno area during these strong events with gusts of 100 to 150 mph observed in the Sierra. 
 Typical downslope windstorm soundings with the ridgetop inversion level highlighted in the yellow circle:

Downslope winds have been known to cause quite a bit of damage in the lee of the Sierra. Impacts include, but not limited to: power outages, semi trucks blowing over, blowing dust, fence damage, multiple airline cancellations/delays, and roof damage. Even with all of the potential destruction, it is hard to argue that the lenticular clouds often resulting from these winds (plus some moisture) aren't spectacular! 

Photo taken from NWS Reno office, March 15, 2015

Time lapse of a long duration lenticular cloud on November 7, 2009. This was a trapped wave event with strong winds at ridge top level, but only moderate winds at the surface.

Need more information? Here are some resources for our weather geeks: 

Monday, March 9, 2015

Tools of the Trade (well one of them...)

This entry is part of our series called "Tools of the Trade".  The idea behind the series is to share with you some of the cutting edge tools that go into our decision making process.

Today we're going to look at relatively new tool, West Coast Atmospheric River Landfall Tool, that is becoming a go-to way to help us examine the potential for atmospheric rivers reaching the west coast. We may abbreviate atmospheric rivers as AR, because sometimes we don't want to type out all the letters of a word. The tool has been developed out of several years of research from the CalWater project at NOAA's Earth System Research Laboratory with assistance from Plymoth State University and the University of California-San Diego Scripps Center for Western Weather and Water Extremes. A significant portion of our winter precipitation comes from just a handful of these atmospheric rivers making landfall in northern and central California. During drought winters we may only see 1 or even zero, in normal winters it can be 3-5.

The image below is the graph of the AR tool from a model run on 00Z Sunday March 8th. In local time, that is the late afternoon of Saturday March 7th. The horizontal scale is days from the current Model Run, while the vertical scale is latitude along the west coast. The color bar indicates the probability of an atmospheric river reaching the west coast. Red and purple are a high probability, while colors in blue and green are a very low probability. In this example, this highest probability is from the afternoon of Wednesday March 11, (+3 days from the model run time) to the afternoon of Thursday March 12th (+4 days) between 48.0 and 42.0 north latitude. A second small AR may reach the coast on March 14th and 15th.  From a forecast standpoint for the Sierra and western Nevada, and a personal one too with this (insert your own adjective) drought, we want a lot of purple moving into the west coast between 42.0 and 36.0 north latitude. Unfortunately this tool doesn't show the magnitude of the atmospheric river, but we have other tools for that.

The probabilities in the graph are calculated from several model runs, we call ensembles.  Ensembles are atmospheric models run with slightly different initial conditions. The big take away here is if several ensemble runs are showing the same outcome, our forecast confidence increases.  Now if the ensembles show the same solution over several days (we call this model consistency in our discussions), well that's more gold in our bucket of forecast confidence.

The next loop below is from the atmospheric river event that occurred in early February, which resulted in much needed rainfall across central and northern California. AR events are usually accompanied by very high snow levels and this event was no exception. There was a moderate increase in the snowpack but only for elevations above 8000 feet. Notice the large area of purple reaching the west coast, which gave NWS offices along the west coast a huge amount of confidence in this event. 
The atmospheric river tool loop from the February 5-8, 2015 storm.

Now that we are somewhat familiar with what this tool displays, let's check out the loop for next week.  Notice how there isn't a lot of red and purple, booo!!!!  For now it appears that we are going to see a pattern change, which means the ridge will breakdown by Tuesday, with two systems possible for Wednesday and sometime over the weekend. Unfortunately it doesn't appear that we will be getting very much precipitation. For now our interpretation is we're going to a get a little precipitation moving into the west coast, but the confidence in a big event for the Sierra and western Nevada is pretty low.
AR tool for the potential "storms" the week of March 8th, 2015.
As always thanks for your interest and always check out the latest Forecast Discussion for more details on the official forecast.

Thursday, March 5, 2015

NWS Reno Blog

Welcome readers!

NWS Reno is venturing out into the world of blogging. Our goal is to provide a resource for the science behind the forecast, random explanations of weather phenomena/factoids, and anything else that we deem interesting with respect to the atmosphere that affects the Sierra and western Nevada. We hope to post a blog entry once a week, perhaps a bit more frequently as interesting topics arise.

For those of you who aren't as familiar with us, the National Weather Service in Reno is responsible for a portion of  eastern California that spans from the Surprise Valley and Lassen County to the Lake Tahoe area and southward to Mammoth Lakes. Across western Nevada, our responsibilities cover Carson City, Churchill, Douglas, Lyon, Mineral, Pershing, Storey and Washoe Counties.  The map below shows our forecast area outlined in black.

We issue new forecasts twice a day, typically around 3 am and 3 pm. Yep we're up all night, 365 days a year to bring you the latest weather updates. If high impact weather is changing rapidly, we will update the forecast much more frequently. You can find our official products, forecasts and warnings at You can also interact with us on our Facebook page and Twitter feed.

This blog should not be utilized as your sole resource for decision making or even what to wear for the day. It is designed to be a creative and fun outlet for weather across our amazing and very unique forecast area. We encourage you to provide feedback, questions, or comments by emailing us at

In weather news, it is going to be warm the rest of this week with afternoon highs climbing into the 60s for the weekend. We could be in store for a pattern change by the middle of next week.

In the next blog we will highlight some of the tools that we have available to examine the potential for pattern changes, the probabilities of it occurring and what weather may result.