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Hi. This is Chris Smallcomb, Warning Coordination Meteorologist at NWS Reno. I’m sort of our
office’s Public Information Officer and work with the media and our emergency management
partners to prepare and respond to hazardous weather events
In this quick briefing, we’ll look at El Nino status and what that means for the upcoming
winter.
I wanted to start by looking back at this past summer.
This chart shows a rainfall analysis using radar estimates + rain gauge data for the
past 90 days. It was an active thunderstorm season over
much of the Sierra and Nevada. NWS Reno issued a well above normal number of severe thunderstorm
and flash flood warnings. Remember those cell phone alerts you were getting this summer?
It’s been a much wilder monsoon season to our south - NWS Las Vegas issued a whopping
303 Flash Flood Warnings!!! The top image here shows you the latest analysis
of sea surface temperatures in the Pacific ocean. The bottom image shows how those temperatures
differ from what is normal this time of year. Blues show temperatures below normal which
is typically associated with La Nina, and reds indicate above normal which is typically
associated with El Nino. Lumped together this phenomenon is called
El Nino Southern Oscillation or ENSO for short. ENSO has been shown to impact weather patterns,
such as the jet stream and storm tracks, over North America including California and Nevada.
Currently the map is showing a mix of above and below normal spots in the tropical Pacific
Ocean. This leads us to conclude that ENSO is in a Neutral state, neither La Nina nor
El Nino. So, what does this say about our winter?
Getting a little bit more into the forecast of ENSO – this chart shows you the forecasts
of the various computer models used to predict ENSO. Computer models are sophisticated programs
that take atmospheric and oceanic data and run them through calculus-like equations to
help predict the weather. The vertical axis is showing you how far the
water temperature in the topical Pacific Ocean is from normal. The horizontal axis shows
you dates are in three-month blocks on this chart. For example DJF is December, January,
and February – the core of the winter months. Each color line represents one of those computer
models. You can see there’s a number of different models used to predict ENSO, each
with it’s own strengths and weaknesses. Anything within ½ degree Celsius of normal
is considered ENSO Neutral. Based on the latest computer model forecasts
– most of the lines stay within ½ degree Celsius of normal during the winter months.
This gives us an increased confidence that ENSO will remain in the Neutral phase through
the winter of 2013-14. Let’s take a look at what ENSO Neutral conditions
have produced in the past for our region. This is based on climate data back to the
1950s. These charts show the percent of past winters
seeing below, near, and above normal temperatures for each of the ENSO types. The left image
is for western Nevada and the right image is for the Sierra Nevada including the Tahoe
region. Focus on the Neutral phase, highlighted by
the red box. Of the past ENSO Neutral winters, 40% have
produced below normal temperatures, 40% above normal temperatures, and 20% near normal.
So – based solely on past winters – temperatures could turn out to be anything.
Now we’re looking at the same charts, but for precipitation. Remember this is total
precipitation, rain + melted snow. Based on past ENSO Neutral winters – 45
to 50% have produced near normal precipitation, 30 to 35% above normal, and 20% below normal.
Taking this information, near to above normal precipitation would be the favored outcome
for an ENSO Neutral winter. However, and this is very important, just
like in the stock market – past performance does not guarantee future results.
This analysis does not explicitly talk about snowfall – just total precipitation (rain
+ melted snow). We have very little guidance on how ENSO impacts snowfall – but it is
important to note that snow levels are snowing a rising trend over the past few decades.
Now let’s look at the most recent Climate Prediction Center forecast for the upcoming
winter. This forecast takes into account historical analog years like we looked at earlier and
uses sophisticated computer models, among other tools.
The latest forecast favors above normal temperatures for the December – February period, however
confidence in that is only low to medium. We can still have cold air outbreaks and warm
days – the forecasts are for the three month period lumped together.
On the right – the precipitation forecast is EC, which means EQUAL CHANCES. This means
there is no favored outcome – precipitation could easily be above, near, or below normal.
Again – this is for a three month period lumped together – we can still have big
storms and dry spells. Bottom line – our confidence in the precipitation
forecast is LOW. ENSO Neutral typically has a larger uncertainty than normal for our long
range forecasts.
That’s a wrap for this winter outlook. Even though our forecast is somewhat low confidence,
hope you found this information useful. It’s probably safe to say – be prepared
for anything this coming winter weather-wise. If you live near a flood prone area or burn
scar – have a plan of action in case of flooding or a flood warning.
Make sure you’ve got a car safety kit for winter driving, that the tires are in good
shape before the snow flies, you’ve got alternate travel plans in case of a major
winter storm, and that your snow blower or snow shovel is in good condition (unlike mine).
Check back at our website and social media for updates and briefings throughout the upcoming
winter. Take care.