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The following is an updated outlook from the National Weather Service Western Region Operations
Center for the upcoming 2013-2014 winter and was prepared especially for Cal-OES on November
27, 2013. The overall message for the Winter Outlook
is much the same in that uncertainty is high as to whether or not above or below normal
conditions will occur. While uncertainty is always high for long-term forecasts, the uncertainty
is even higher this year due to the combination of a lack of El Nino or La Nina conditions
and additional variability in the other forecast model guidance and indices that are used for
long term forecasts.
As a result, there are no clear, reliable, long-term signals that exist for temperature
or precipitation forecasts for this coming winter. This does not mean that the prediction
is for normal conditions, but it does mean that there are no strong indicators leading
us to forecast either above normal, below normal or normal conditions for much of California
this winter. However, we can look historically at years
where there was no El Nino or La Nina present and get an idea for what was observed during
those years.
For temperatures in years without El Nino or La Nina, there has been little to no bias
in CA with regard to temperature. Basically the data shows that average temperature conditions
during those years were within one-half to one degree of long term normal conditions.
With precipitation on the other hand, the data shows that years where there was no El
Nino or La Nina have resulted in slightly below average precipitation (by 1-2 inches)
throughout most of CA. This is especially true in northern and central CA.
Now that we know what a typical year without El Nino or La Nina looks like, letís look
at the official forecasts from the Climate Prediction Center. First letís take a look
at just December.
The forecast for temperature is for Equal Chances of below normal, normal, or above
normal conditions throughout nearly the entire state. Only right along the Colorado River
is there a slight favor for above normal temperatures.
The forecast for precipitation is for Equal Chances of below normal, normal, or above
normal conditions state-wide. Even with the data from the previous slide, recall that
other forecast model data and indices are included in the forecast, so that is why the
forecast is not calling for drier than normal conditions across much of CA.
Similarly, during the heart of the winter, December through February appear to mostly
have Equal Chances for below normal, normal, or above normal conditions. The one exception
is the area where above normal temperatures are slightly favored along the Colorado River.
Next, let us take a brief look at the latest water supply issues. For Californians, 2013
thus far has had some of the driest conditions in 118 years, despite the recent rains. This
is especially true across portions of northern and central CA where you see the record driest
conditions in the areas of red for the January through October 2013 timeframe.
This translates directly into the water supply state-wide with nearly all reservoirs still
well below capacity, and most are below the historical averages for where they should
be for this time of year. So, in summary, with this CA winter without
El Nino or La Nino: 1) a weak correlation exists for below average precipitation; 2)
this correlation strengthens the further north and coastal you go in the state; and 3) no
real correlation exists regarding seasonal temperatures thus equal chances either way
this year nearly state-wide. One significant caveat is that we need to
remember that CA and the Sierra are still prone to individual storms (like last December)
that can produce significant amounts of the seasonal precipitation. One storm can make
a disproportional contribution to the water supply and can still play a role in developing
impacts related to flooding. Also, remember that 2013 has been in some places in CA the
driest in 118 years, and this has translated to the latest regional and statewide water
supply. Reservoirs have significant space available to accommodate any large early season
storm runoff, which means the first few storms may only create flooding concerns on uncontrolled
rivers, in urban areas, and near recent burn scars.
Thanks for your attention. Here is our contact information at the National Weather Service
Western Region Headquarters. Remember your local National Weather Service office is also
available for more localized information.