La Nina stuff

Scott Cronk n7fsp@linux.n7fsp.ampr.org
Wed, 30 Sep 1998 20:39:42 -0700 (PDT)



 For those of you that saw the 'doom and gloom' messages (not previously
 sent to 'seatcp') of imprending winds and major amounts of snow for the
 Puget Sound region... here is the straight scoop (according to) from
 the National Weather Service in Seattle: 


> Subject: La Nina stuff
> Author:  Ted Buehner at w-wr-sew
> Date:    9/30/98 3:46 AM
>
>    To all concerned -
>
>    In the past two weeks, we have become aware of two '98-99 winter
>    weather messages from unnamed and unknown sources somewhere in
>    Snohomish county.  Both messages offered significant mis-information
>    about the outlook for this coming winter season.  The following is
>    intended to help provide more clear information.
>
>    The first message generated some reaction.  Our response to this
>    message is given at the bottom of this message.
>
>    The second message released around Sept 24 created a much more
>    significant reaction.  In talking with emergency managers in our area,
>    all of us have taken our share of phone calls and e-mail messages
>    regarding this message.  At a state emergency management workshop
>    today, I had one emergency manager show me a copy of the message that
>    was routed through Australia, pointing out one of the negatives of
>    e-mail and the Internet.
>
>    We appreciate all of you who have forwarded your concerns about these
>    messages and your desire for the "straight scoop".  Both of the
>    messages expressed the "preparedness message", which we fully endorse.
>    Unfortunately, these messages also provided a forecast of extreme
>    conditions that do NOT in any way, shape or form, mirror those that
>    the NWS has issued.
>
>    Last winter was an El Nino winter, one of mild winter temperatures and
>    seasonal precipitation.  Basically, we had the year off from inclement
>    weather.  El Nino is now over, and whether we have La Nina or a more
>    common "neutral" season, more usual inclement winter weather is likely
>    and we need to ensure citizens are prepared and are not complacent
>    since last winter was so tranquil.
>
>    Just below is our response to the second message drafted by Brad
>    Colman, our Science and Operations Officer.  It thoroughly explains
>    the process behind the Climate Prediction Center's long-range
>    forecasts, and the current outlook for this winter.  Thank you, Brad!
>
>    This message was addressed to all those listed on the messages I have
>    received thus far since Sept 24.  Please forward this message to
>    anyone you feel needs to receive it.
>
>    We also plan to issue "Public Information Statements" (AFOS header -
>    SEAPNSSEA or WMO header ABUS34 KSEA) following the next few Climate
>    Prediction Center long range forecasts issued near the middle of each
>    month to help provide a "local" prespective.  The CPC web site address
>    is given at the bottom.  If you have further questions, please do not
>    hesitate to ask.  We hope this message helps "clear the clouds" on
>    this matter.
>
>    Ted Buehner
>    NWS Seattle Warning Coordination Meteorologist
>
>    BACKGROUND....We've all heard by now that NOAA's Climate Prediction
>    Center (CPC) has forecast that we will have a La Nina this coming
>    winter.  You might also have heard that as a result the Pacific
>    Northwest is going to experience one of the worst winters in years.
>    Much of this has been exagerated and inconsistent with our current
>    understanding and predictive skill.  Below is a summary of the latest
>    conditions and the CPC forecast.
>
>    La Nina is the cool phase of an oscillation in the equatorial Pacific
>    sea surface temperatures between cool and warm anomalies.  El Nino is
>    the warm phase.  Many years, the temperatures are normal and neither
>    phase is present.  Last year we experienced one of the more extreme El
>    Nino's this century with a dramatic impact on west coast weather.
>    Winter was rather stable and somewhat benign in the Pacific Northwest,
>    but was very stormy and wet in California.  CPC was able to forecast
>    this nearly nine months in advance.  So with La Nina what can be said
>    about this coming winter?
>
>    La Nina does favor a weather pattern that produces colder, wetter, and
>    stormier weather in the Pacific Northwest.  However, the pattern is
>    somewhat unstable and frequent variations in the storm track and
>    weather pattern is typical.  As a result, the correlations between La
>    Nina and our weather is somewhat less that with El Nino.  The logical
>    extension of this is that the confidence in a forecast based on the
>    occurrence of La Nina should also be less than one based on the
>    forecast of an El Nino.
>
>
>    FORECAST...The CPC issues long range forecasts on or about the 15th of
>    each month.  The latest forecast was issued on 17 September 1998.  The
>    forecasts are presented as probabilities of occurrence of above normal,
>    below normal, or normal weather.
>
>    First, what is meant by these terms. The CPC divides a long record of
>    temperature and precipitation into thirds.  The coldest (driest) third
>    is "below normal," the middle third is "normal," and the warmest
>    (wettest) third is "above normal."  In other words, without any other
>    information there is equal probability (33.3%) for each of these
>    categories.  All CPC forecasts are based on this distribution.  When
>    the CPC doesn't feel there is enough information to increase the
>    likelihood of one of these classes by at least 10% then the forecast is
>    for normal conditions.
>
>    So, what is the forecast for this coming winter for western
>    Washington.  Let's first look at the temperature forecast....  For all
>    three month periods (nov/dec/jan, dec/jan/feb, and jan/feb/mar) the
>    official forecast is "normal" temperatures.  The reason the CPC is not
>    forecasting a strong shift in probabilities toward below normal
>    temperatures is that for the Pacific Northwest there has been a long
>    term trend toward milder readings.  As a result, there is some
>    indication that the La Nina favored cooling will be somewhat offset by
>    the long term warming trend.
>
>    To give you a sense on the magnitude of how much cooler is below normal,
>    the 50% occurrence threshold for Seattle for the months Jan/Feb/Mar is
>    43.1 F.  In other words, half of the winters have an average temperature
>    for these months that falls below this number and half fall above.  The
>    threshold for below normal temperatures is 1.0 F colder....42.1 F.  For
>    above normal temperatures it is simply 44.1 F.  This doesn't seem like
>    much of a shift and that is because we don't have a lot of extremes in
>    our weather and most winters fall quite close to average.
>
>    For Precipitation, the forecast for most of the winter is in fact for an
>    increased likelihood of above normal or wetter conditions.  This shift is
>    strongest in the Oct/Nov/Dec period when the probabilities are 53.3%
>    chance of above normal, 33.3% chance of normal, and only an 18.3% chance
>    of below normal precipitation.  The 50% threshold of occurrence for
>    precipitation for this three month period is 14.7 inches.  The below
>    normal threshold is 13.2 inches and the above normal threshold is 16.3
>    inches.  So the CPC forecast is a 53.3% chance of SeaTac airport receiving
>    more than 16.3 inches of precipitation during Oct/Nov/Dec.  The other
>    three month forecasts are also for wetter but at less confidence than
>    these.
>
>    SUMMARY....
>
>    The CPC is NOT forecasting the most severe winter in years.  They are
>    forecasting near normal temperatures with more than normal
>    precipitation.  On the other hand, this does not rule out severe winter
>    weather and given the favored weather pattern during a La Nina the
>    scales are tipped in the direction of colder, wetter, and stormier
>    weather.  It is just at this time the CPC doesn't feel the indicators
>    are strong enough to shift the probabilities of occurrence more than
>    10% in that direction.
>
>    -------------------------------
>
>    To all concerned in Snohomish county -
>
>    Our office received a message from Everett Fire today regarding the
>    winter weather outlook based on La Nina.  The e-mail message copy was
>    based on a NOAA (Natl Weather Service parent organization) "alert", as
>    the message put it.  The message referred to "70 MPH winds and 5 to 8
>    feet of snow for extended periods of time".  I am NOT aware of any
>    alert that NOAA or the NWS has issued with such alarming information.
>
>    I'd like to ensure accurate information is being circulated about La
>    Nina so everyone can make sound decisions in preparation for this
>    winter season.  In concurrance with local emergency management, we
>    encourage preparedness measures, especially after a quiet El Nino
>    winter season last year.
>
>    The current outlook for this winter based on La Nina is for a tendency
>    for above normal precipitation, and near normal temperatures. However,
>    there are too few recorded La Nina events to strongly indicate how
>    much above or below normal with any confidence.
>
>    As a result, the odds of having one or more significant winter snow or
>    ice storms, river flooding events, or damaging wind storms are
>    enhanced, but not in any way certain.
>
>    For example, the arctic outbreak in Feb 1989 was during a La Nina
>    event.  The Dec 12, 1995 wind storm was during a La Nina event as well
>    as the Nov 1995 and Feb 1996 flooding events.  However, the winter
>    storm events during Dec 1996 and Jan 1997 were NOT during either a La
>    Nina or El Nino event.  This was true for the Dec 1990 winter storm
>    events as well.
>
>    The message here is that we have greater odds of having one or more
>    significant weather events this winter.  Following last winter's mild
>    weather, we need to remind ourselves that we need to prepare for the
>    better likelihood of significant winter weather this season.  However,
>    winds of 70 MPH and 5 to 8 feet of snow for extended periods of time
>    are very unlikely.
>
>    We have a press release from our Sept 11 La Nina forum, attended by
>    over 100 emergency managers, city officials, media, school district
>    officials, and private businesses, that summarizes the presentation
>    given by Ants Leetmaa from the NWS's Climate Prediction Center.  If
>    you desire a copy, we can fax it to you.  Another source is a La Nina
>    web site found at:  www.noaa.gov/
>
>    You are also welcome to attend our next media/emergency management
>    workshop on the morning of Oct 8 here at our NOAA facility in Seattle.
>    A part of the workshop will provide an update on La Nina and our
>    area's winter weather outlook.  We can fax workshop information and
>    registration to you as well.
>
>    I hope this information helps you.
>
>    Ted Buehner
>    Warning Coordination Meteorologist
>    NWS - Seattle
>    206-526-6095 x223
>           -6094 fax