Download NAO

State of the NAO
NAO Site
One of the biggest keys to any winter weather
pattern across the US is the amount of high
latitude blocking that is present. The two most
frequently used indices to look at northern
hemispheric blocking is the AO and NAO.
High latitude blocking (negative NAO) will result
in an amplified pattern with the potential
for deep trough and ridges and also
strong storm systems. A negative phase of
the NAO corresponds with increased
pressures/heights in and around
Greenland/Iceland and lower
pressures/heights near the Azores region
while a negative phase of the AO corresponds
with blocking near the north pole and a
displaced polar vortex and the associated
cold air with that vortex. Often times these
indices are in phase together.
State of the NAO
NAO Site
Positive NAO
• The Positive NAO index phase
shows a stronger than usual
subtropical high pressure center
and a deeper than normal
Icelandic low.
• The increased pressure difference
results in more and stronger winter
storms crossing the Atlantic Ocean
on a more northerly track.
• This results in warm and wet
winters in Europe and in cold and
dry winters in northern Canada and
Greenland
• The eastern US experiences mild
and wet winter conditions
Negative NAO
•
The negative NAO index phase shows
a weak subtropical high and a weak
Icelandic low.
•
The reduced pressure gradient results
in fewer and weaker winter storms
crossing on a more west-east pathway.
•
They bring moist air into the
Mediterranean and cold air to northern
Europe
•
The US east coast experiences more
cold air outbreaks and hence snowy
weather conditions.
•
Greenland, however, will have milder
winter temperatures
Correlation of NAO and SST
• Notice the anomaly pattern that accompanies winters
with a –NAO.
• There is an area of above normal water temperatures
around and north of Newfoundland to the southern tip
of Greenland and then up the Davis Strait, with an
equally important below normal area south of
Newfoundland to just off the NE coast of the US.
• Now to think that this SST anomaly couplet is alone
the cause of the NAO is naïve. It is possible it is the
result of the NAO phase, more than likely it is a
feedback process between the two.
Correlation of NAO and SST
State of the NAO (Aug 2006-Nov 2006)
State of the NAO (Prediction for 2006-07)
This data (cyclic NAO) and prediction (+NAO through December) from a professional
met indicates a high likelihood of a warmer than normal December and 1st half of
January across the eastern U.S. 2nd half of January and 1st half of February more likely
to see seasonal or below normal temperatures due to lower NAO values.
State of the NAO (General Trends in
2006)
2006 Data:
Negative NAO in October 2006, Positive NAO in
November 2006
2006-2007 Winter Outlook:
“Positive NAO 1st half of winter, negative NAO 2nd half of winter.” Rob G.
“Overall positive NAO.” Misc. Pro Met
“Overall positive NAO.” Misc. Pro Met
Note: Most agree that the winter NAO will be + overall but the 2nd half of
winter will be less positive than the 1st half , possibly even neutral or
negative.
State of the NAO (General Trends During
2006-07 El Nino Winter)
 -NAO in October favors a + NAO winter
 + NAO in October favors a –NAO winter
 NAO state in November dictates NAO state for winter during 9 of 12 El
Nino winters, including the last 6.
 Of these 9, only 2 have flipped back again during the winter.
 Thus, if it flips in 2006-2007 from October to November there is a 77%
chance that the NAO state for the winter will be positive.
 If the NAO flips from October to November it usually does not flip back and
the NAO holds the November state for the winter.
Note: October 2006 NAO was negative, November 2006
NAO was positive. So there was a flip. Therefore, 2006
data suggest a +NAO for winter which favors less cold
and snow.
NAO Viewpoint from DT (Fall 2006)
WxRisk View: While many see the SSTA set up for the Atlantic
Ocean as NOT ideal for sustained intervals of
the negative phase of the NAO I see some promise. Many
forecasters assume that there can only be one particular
manifestation of the negative phase of the NAO... which is
called the “Greenland Block.”
NAO Viewpoint from DT
However sometimes you can get a -NAO
manifestation that shows up as a blocking
pattern over the Davis Straits...or
northeastern Canada or northern
Québec. Indeed we saw that several times
during the months of September and
October. And the strong pools of very warm
SSTA in and around the Davis Straits and of
the coast of southeastern Canada strongly
supports the idea of a westward positioned
blocking pattern developing over northern
Québec or the Davis Straits from time to time
this winter. I do not see an" ideal“ SSTA
configuration at this time although it would
not take much to change the Atlantic SSTA’s
into a pattern that would favor sustained NAO.
Importance of High Latitude Blocking
(-NAO)
“Of course we are going to have to watch this very carefully as there is a
window of opportunity for some sort of wave to develop warm front
over the southeast JAN 16-18. But notice what most of the models ate
doing here. Without any High latitude blocking the trough becomes very
deep and the strong High pressure area drops or falls into the trough...
tracking from western Canada along the east side of the Rockies all the
way into Texas and Oklahoma. This is a very bad sign for significant
snowstorm activity for the East Coast and especially the Northeast.
This is WHY the blocking pattern over Eastern Canada is SO important
because it forces the arctic High pressure systems to stay to the North
and track along the US Canada border.... instead of falling into the
central and Lower Plains. The High to the north keeps temperatures
cold over the East coast but also provides access to ocean moisture as
well as being BIG player in the set up all of ageostrophic flow when
East coast Low pressure development is underway…continued…
Importance of High Latitude Blocking
(-NAO)
On the other hand when you do NOT have any high latitude blocking then you have
got real problems in terms of getting a good winter pattern cooking. There is a
tendency among weather hobbyists to really downplay the significance of high
latitude blocking ....such as a sustained negative phase of the NAO. I know that
there are some out there who say while you can still get big snow oin the Northeast
without a Greenland block as long as you have lots of cold air. Often times the
February 1995 noreaster is cited as a classic case of a good snow for the I-95 cities
that and as rain and there was no high latitude blocking.
Well that may be true but also proves the rule. You see the issue is not whether you
can get a good snow in the Northeast without any high latitude blocking or negative
phase of the NA O. You most certainly can BUT it won't stay all snow and that in
my opinion is always the big issue. The February 95 system was such a rapid
moving system that by the time the warm air got into the big cities of the Northeast
most of precipitation had already fallen so we ended up with 85- 90% of the precip
fell as snow and then the rest as drizzle or freezing drizzle. If the 2/1995 had
noreaster move at a more typical speed OR if the Noreaster had closed off in the
upper levels the High latitiude blocking would of been far more important. Tthe
warm air would have had more time to drive inland into the I-95 cities.... meaning
less snow and more rain. – DT on January 11, 2007 -