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Hello everyone. My name is Tom Dang, a meteorologist with the National Weather Service in Sacramento.
We would like to take the opportunity to discuss the weather conditions over the winter, as
well as an outlook through at least the early stages of summer.
This winter certainly has been one of extremes. We started the winter season off with a ***,
with one of the wettest November and December time periods on record. Then around Christmas
time, the storms simply shut off, replaced by a very persistent ridge of high pressure.
All told, the 8 Station index, which tracks precipitation totals across the Northern Sierra,
is running right around normal for early April. Additionally, northern California reservoirs
are running around historical averages. Let’s start taking a look at some maps and
charts. Here is graphic depicting the percentage of normal precipitation for the current water
year, dating back to October 1. We’ve highlighted two areas to draw your attention to. Considering
the dry stretch California has been through the last several months, the northern half
of the state has fared remarkably well. Much of the northern half of the state, and especially
along the mountains, are running at 75% to 100% of normal. Southern California has not
held up near as well this winter, unfortunately. The area highlighted in red has received only
5 to 25% of precipitation for the entire water year.
This is a timeline graphic that plots the 8-Station Northern Sierra index. Anytime the
dark blue line is above the shaded light blue area, the northern Sierra has received above
normal rainfall. As you can see, especially during the month of December, the northern
Sierra received bountiful rain and snow fall. However, the storm door closed at the start
of the new year. The last several weeks, the northern Sierra has received just enough rain
and snow to keep up with normal. Here is a graphic showing the water levels
at the major reservoirs across the state. As you can see, most of the reservoirs are
very near their historical averages, marked in red, for this time of year.
This is a graphic depicting Water Supply Volume, or the amount of water out of melting snow,
and into the reservoirs. This is directly correlated to the lack of snowfall we received
between January and March. As you can see, the farther south in the Sierra you go, the
less snow there is available to melt. For this year, the peak snowpack occurred around
March 1st, where the normal peak snowpack occurs in early April. Additionally, most
of the rivers across northern California have already had their peak runoff this spring,
which doesn’t normally occur until late April or early May.
Here is the latest drought picture, compiled by the National Drought Mitigation Center,
the Department of Agriculture, and NOAA. As you can see, most of northern California is
shaded at the D0 level in yellow, just below drought levels. This is largely due to the
vast amounts of rain that we received in December. Here is the latest statement from the Climate
Prediction Center regarding the El Nino / Southern Oscillation. El Nino neutral conditions have
persisted through much of this past winter, and is forecast to continue through at least
this summer. This graphic depicts Sea Surface Temperature
anomalies, and is the most direct observation of El Nino or La Nina conditions. Typically,
with stronger El Ninos, you would expect to see very strong warm anomalies off the coast
of South America, and cool anomalies along the western Pacific Ocean. That is not the
case here, with most of the Pacific basin not recording much if any anomaly. Of note
is the slightly warm color along the Eastern Pacific, which is residual of the weak El
Nino conditions recorded early in the winter season.
This graphic is a plume of various long-range weather models and gives an idea of where
El Nino or La Nina may go over the next several months. There are two things that we can pick
out of this graphic. One is that there is a very large spread among the models. That
doesn’t give us very much confidence in the El Nino forecast moving forward. However,
the general trend and cluster of long-range models does tend to be centered right around
0.0. Thus, the forecast moving forward is that El Nino neutral conditions will persist.
Now let’s take a look at the outlook for later this spring, and getting into early
summer. On the left is a graphic that depicts the likelihood of above or below normal temperatures
in April, May, and June. The graphic on the right also shows probabilities, but for precipitation
instead of temperatures. As you can see, the outlook for the next 3 months is that there
are “Equal Chances” between above, below, or near normal temperatures. However, the
graphic on the right states that there is a greater than 40 percent chance of below
normal precipitation. This doesn’t mean too much however, as April, and especially
May and June are fairly dry months across much of northern California.
Looking deeper into our crystal ball, let’s take a look at the 6 month outlook, which
covers July, August, and September. As you can see, much of the country has a high probability
of above normal temperatures for the latter half of the summer. This is pretty far in
the forecast period, and usually confidence is quite low in these temperature outlooks.
However, the Climate Prediction Center has indicated a fair bit of confidence in this
forecast. Let’s recap what we’ve talked about in
this presentation. We started off with one of the fastest starts on record from a water
standpoint. Then that abruptly shifted into one of the driest January through March stretches
in California’s history. Moving forward, El Nino neutral conditions are expected to
persist through at least the summer of 2013. For the next few months…forecast temperatures
are trending normal. However, longer term charts are suggesting that temperatures could
heat up for the second half of summer. Water reservoirs in California remain in decent
shape as of early April, owing largely to the winter storms back in December. However,
our extremely dry January through March stretch has left the Sierra in a large snow deficit.
Rivers and streams are already showing poor inflows into reservoirs this month. In the
end, reservoirs are likely to trend below normal levels in the upcoming months. The
lack of snow has the potential to prolong this summer’s fire season, and cause extremely
dry fuels toward the end of summer. Fire starts will be highly dependent on wind and lightning
events, which are an unknown at this time. Drought stress may also become a growing problem
toward the end of summer. For additional information, check out the
Climate Prediction Center’s homepage, or our own homepage at weather.gov/sacramento.
You can also check out our growing social media presence on Facebook, Twitter, and Youtube.
Thank you for viewing this presentation! If you have any additional comments or questions,
the contact information for Dan Keeton has been listed. Thanks, and have a good day!