East Wind Events: Windy Up "There", Not-So-Windy Down "Here"

Gusty Ridges, Calm Valleys. 

We have gotten a lot of questions lately about why it's so windy up in the mountains and why there are no winds down in the valleys, so we figured we would write up a quick blog post about it. Let's talk observations...these are just a spattering of observations from an event we had this week (Dec 6-8, 2020). Notice the big difference between the ridges and the valley locations. Let's not get stuck on the actual numbers at the ridges, since some of those sensors can get a little 'off'once wind speeds really pick up, the more important thing to note is the large differences in the wind gusts from ridges to foothills to valleys. (note obviously Peavine was having a bit of trouble too) We want to dig into how this occurs and why it happens! So if you're brave and ready for some science, then continue onward.



General Weather Setup 


It is a well known set up and similar to previous events, but the magnitude of the event this week is

not clearly not as strong as others the region has experienced.



  1. Low with northeast to southwest oriented jet passing south

  2. Surface high building into northern Nevada - strong offshore pressure gradient

  3. Post frontal strong cold air advection

Confused yet? Let's dig into these one by one. I will be using images from the February 2020 event,
which was remarkably strong. That way we can really see these signals that we look for when we're
forecasting for these events. 

We are going to start at the 'top' of the atmosphere and work our way down. We will be talking about
pressure heights in mb, so here is a little chart that will give you an idea of where in the atmosphere
these signals are located (generally). 


Low with Northeast to Southwest Oriented Jet Passing South


The images below show the forecasted position of the jet from the NAM model simulation. The black
 lines are the isobars at 300 mb and they show us the position of the upper level low, while the colors
show us the strength of the speeds with the 300 mb flow. The deeper reds indicate higher wind speeds
 with the jet. Confused about what 300mb means? Go here. When we see a pattern like this in the
upper levels of the atmosphere in our model analysis then we start to dig in a bit more.  Note the
northeast to southwest direction of the winds. 



Migrating down in the atmosphere to 500 mb, we can see the winds are quite strong (deep purples)

and also draped across NE California and portions of the Tahoe Basin. This is showing us that the

strong northeast to southwest winds are not only confined to the higher levels of the atmosphere. 



Going down a touch more in the atmosphere we arrive at 750 mb winds. Here at the NWS Reno office

we consider ridge winds to be right around 650-750 mb height level. Some things to consider when

looking at model data. We don't necessarily grab onto the exact numbers that any model or simulation

products, but we use our forecaster knowledge and experience to determine what we think the wind

speeds will be. I am only highlighting one model here for instructional purposes only, in reality we

actually look at a suite (or ensemble) of model solutions to produce the forecast. 





Surface High Building into Northern Nevada - Strong Surface Offshore Pressure Gradient


This part will be a little confusing if you don't understand how wind forms. We suggest watching this short explanation of how wind forms! We took a look at the high levels of the atmosphere, now we need to consider what is happening at the surface. When the high levels and the surface "work" together, the outcome can be a lot stronger. Picture water at different levels in a reservoir or even the ocean. It's easy to visualize that when all levels of the water are flowing the same direction that the outcome can be stronger than if they were moving in opposite directions (directional shear). We aren't going to go there though. From this quick analysis, we can see that the surface pressure gradient (indicated by black lines) are close together across the Sierra from roughly Tahoe southward. This is increasing our forecaster confidence that winds will be strong across the crest.





Post Frontal Strong Cold Air Advection

Ok, now this sounds way more complicated than it intends to be... let's break it down. Post-frontal: after the cold front Cold Air Advection: cold air moving in to stay! NWS Los Angeles produced a nice little graphic summary. Now, why is LA talking about Santa Ana winds when we are talking about east  wind events?! Well, these dry cold fronts that drop down through our region in Nevada and the eastern Sierra result in major impacts for central and southern California as well. Weather knows no boundaries! You can see on the chart that as the surface high strengthens (cold air builds up in the Great Basin), and a surface low exists across the West that we (Reno-Tahoe-Mammoth) end up with east winds, while Sacramento, Hanford,  Los Angeles and San Diego contend with winds too. These are downslope winds on the west side of the  Sierra (is your mind blown yet?) Go ahead and check out this post on Downslope winds to brush up on your meteorology even more!


So now to the short question that everyone was asking.. which is why don't we get these super strong winds "down here". The easy short answer is that the bulk of the strong winds are trapped in the upper atmosphere. Down here in Reno-Carson City-Gardnerville (you name a lower valley) when that cold air pushes into the region, we end up shielded from the stronger winds under an inversion layer (yep, we have a video for that). Now the depth or strength of these inversions will determine which valleys or foothills get to experience the east winds and which do not. Anecdotally, the NWS Reno office sits in the foothills above Reno Airport and the other night when winds were beginning to pick up there would be periods of increased winds (as the inversion fluctuated up/down) and periods of no winds. Pretty neat huh?

Well, that is enough from us, be sure to message if you have any questions. We know this is a lot to digest, but hopefully you learned some new terms and processes.

-NWS Reno 

weather.gov/reno 

Location: Reno, NV, USA

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