State of the NAO

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State of the NAO
Positive phase of the NAO is not conducive for
HSEs in the Delaware Valley. Snowstorms can
happen during a positive phase, but it is rare
for PHL to receive a HSE during a NAO (January
2000 was an exception to the rule). A positive
NAO usually exhibits a fast west to east zonal
flow and/or ridging east of the Mississippi
River.
NAO Site
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State of the NAO
A negative NAO in conducive for East coast
snowstorms because it usually allows for cold air
to spill into the eastern United States. It also
allows for a blocking to take place in Eastern
Canada, a subsequent trough to develop over the
eastern United States, and coastal cyclogenesis
to sometimes occur. A negative NAO is by far the
most important factor in East coast snowstorm
development. Negative NAOs and their associated
blocks usually last 10 to 14 days. Models
sometimes break them down too early.
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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 underwaycontinued

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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 -

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Major Snowstorms and the NAO Index
Most big snowstorms for Philadelphia occur during
periods of negative NAO when it is trending
sharply towards neutral or positive. Sometimes
they occur when the NAO is positive or neutral
trending sharply negative. Click here to see
current NAO values http//www.cpc.ncep.noaa.gov
/products/precip/CWlink/pna/new.nao_index_ensm.htm
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December 9, 2005-2.2 Inches at PHL. Regional 10
Inch Coverage of 2-
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Major Snowstorms and the NAO Index
The NAO index at times can be deceiving. For
example, the NAO can be negative (good for
snowstorms) yet there could be no classic
Greenland block (High Pressure over Greenland).
The block is instead situated elsewhere or not in
the most favorable position for northern
Mid-Atlantic snowstorms.
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Major Snowstorms and the NAO Index (DT)

• There are several of the various manifestations
  of the phenomena   known as  the Negative NAO
• A) IRELAND / - NAO  The height anomaly  seen on
  the Upper level maps may be placed as far east as
  IRELAND and still qualify as a -NAO.

Ireland
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Major Snowstorms and the NAO Index (DT)

• B) GREENLAND BLOCK / - NAO    The height anomaly
  is over central Greenland--- which would qualify
  the -NAO.
• C) DAVIS STRAITS - LABRADOR  / -NAO   The height
  anomaly appears as far west as the Davis Straits
  into Labrador and northeast Quebec.

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Major Snowstorms and the NAO Index
Also, the NAO index can be extreme negative which
can prompt a suppression of storms. In this
case, the lower Mid-Atlantic and/or southeastern
states could receive the heaviest
snow. Therefore, when it comes to the NAO, the
following must be taken into consideration. 1.
Negative or positive? 2. Trend? 3. How
negative or positive? 4. What type of NAO?
Where is the block?
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-NAO Causing COLD or COLD Causing NAO (February
5, 2007)
Attack of cold not because of the -NAO. The
continental cold is what created the NAO. This
occurred as a result of the insurgence of cold
air which caused a storm to bomb out off the east
coast and set up the features necessary for a
NAO. -JB-
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NAO Index (Blizzard of 2006)

• NAO was weak negative prior to the storm, but
  trending sharply positive (this is called
  relaxing).
• This NAO trend of negative to neutral of positive
  is often key in the development and propagation
  of East coast snowstorms.

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NAO (Monthly Indices)
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NAO, Blocking, and U-Shaped Cold

• MY THINKING ON THE PATTERN AHEAD.. If you
  think if the current pattern we are in you see
  the cold air comes down like a U shape. We are
  on the left or center side of the U shape so the
  storm miss off the coast and go out. Once the
  blocking starts and shifts this entire U shaped
  cold you will see the storms roll up the coast.
  You can see the blocking going on at the top of
  the looped maps below. Follow along with your
  mouse and watch the U shaped cold shift west.
  Once you see how that works than you see not one
  but 2 storms roll up the coast. I can see can
  you?
• -Rob Guarino Fox 29- (Jan 2007)

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NAO, Blocking, and U-Shaped Cold

• LONG WAVE TROUGH SWINGS WESTWARD.BLOCKING
  STARTS JAN 28TH AND BEYOND Oh you can see the
  long wave trough swing westward. The digging of
  the cold will shift to the west a bit and that is
  the SIGNAL OF THE NAO KICKING IN. This
  correlates with the NAO going negative and the
  pattern changing from southern sliders to
  noreasters. It is interesting to watch on the
  loop below
• -Rob Guarino Fox 29- (Jan 2007)

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NAO (U-Shaped Cold Swing to the West)

• THURSDAY SYSTEM AND THE NAO...IS IT TOO SOON !
  The NAO is on the negative side today but is
  the "U" shaped cold not fully in swing yet ? My
  previous blog I mentioned that "U" shaped cold
  has to swing back to the west to get a phasing
  going on or get a system up the coast. CLICK
  HERE THE NAO INDEX Maybe it is too soon and the
  GFS jumped the gun ? I also showed the comparison
  to the Jan 2000 NAO and the result on the UPSWING
  of the NAO as it came from negative to neutral
  after the dip. CLICK HERE NAO PATTERN WE ARE
  GOING INTO
• -Rob Guarino Fox 29 (January 2007)-

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NAO (U-Shaped Cold Swing to the West)

• In January of 2000 the NAO dipped on the 13th
  and we had snow by the 22nd, which is 9 days
  after the dip. We are now negative today and 9
  days from now it the 30th. See my point here is
  the 30th may be the real deal for snow and this
  is the transition snow. JANUARY 2000 DATALOOK
  HOW WARM 1ST HALF, COLD SECOND HALF Notice how
  we got .1 of snow the other and probably .5 to
  .8 in Philly for this one. Each system gets a
  little closer as the WHOLE PATTERN starts to
  swing NAO. This is not a GUARANTEE be any means
  that it will follow this pattern but I do see so
  many similarities in the 2 months. Notice the
  last GFS run and all the snow is after the 30th.
  Yeah, this is beyond or near 240 marker on the
  GFS but it is the pattern you must watch. This
  has been very frustrating for the snow lovers on
  here, including me. This as I noted is the tiny
  steps or STEP DOWN to winter that we have to deal
  with this season.
• -Rob Guarino Fox 29 (January 2007)-

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February 6, 2007 (Cold Has Nothing To Do With NAO)

• NAO neutral AND IS NOT, AND HAS NOT HAD
  ANYTHING TO DO WITH THIS COLD. It is one week of
  negative did not change the pattern as the seeds
  of this pattern change have been outlined and
  tracked since December, and formed the basis for
  the winter forecast of a harsher mid and late
  winter. However, if it pops negative again, and I
  think it will there are some very interesting
  repercussions, and later today I will explain
  that. I just don't buy the pat answers as to what
  is causing this or that change from "El Nino" to
  "negative NAO" since it is a combination of
  things that not only I, but others have talked
  about

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February 6, 2007 (Cold Has Nothing To Do With NAO)

• As far as this goes, if the snow can come in
  the areas still lacking, then this will go down
  with the great El Nino flip winters that we had
  looked at. Keep in mind that we were colder than
  normal when it was rising into the lower end of
  the scale and now that we are falling back we are
  there too. The big message here is that I don't
  believe simple answers from models, or labeling
  one feature as the reason is anything I would
  consider correct. I have taken great pains, right
  or wrong to make sure our subscribers knew how
  this can evolve, and again to be fair, I do think
  there are others that have seen it too. The
  weather occurs in the real world, not the virtual
  world and the only way to judge right and wrong
  is to look at the facts. One week of a negative
  NAO should not and cannot be the reason for this.
  In any case, later today I will explain how if it
  forced negative again it can keep this going into
  mid-March, which would be the normal course of
  the great arctic outbreaks we have seen before as
  they reluctantly pulse back out -JB-

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