Winter '11 / '12 - White Christmas Probabilities

Updated below

White Christmas in the US is defined as 1" on-the-ground on 25-DEC.  The criteria are different in other countries.

The probability of an event is determined by dividing the number of times something happened by the number of time it could have happened.  If there was a white Christmas five times over 20 years...the probability is 25% in any given year.

This NCDC map is a little dated b/c it's based on the old 1961 - 1990 period of climate normals.  Good enough for our purposes.

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Folklore predictions...
- A green Christmas; a white Easter.

- If there's thunder during Christmas week...
The Winter will be anything but meek.

- The nearer the New Moon to Christmas Day...the harder the Winter.   [ed.  - new moon is 24-DEC!]

- If Christmas day be bright and clear
There’ll be two winters in the year.

- Thunder in December presages fine weather.
[ed. - somewhat at odds with 'thunder during Christmas week]

- Like in December like all the year long.
[ed. - does this face forward or backward?]

UPDATE:
The NWS has updated their white Christmas probabilities map based on the current 30-year climo period between 1981 and 2010...

Note there is one fewer classes in the new map.  The '< 5%' class has been combined with the '5-10%' class into one '< 10%' class...which is not an improvement for snow crows living along the map's southern boundary. 

Also note the classes are not represented by equal sizes.  Some are 15%-points...some are 10%-points...some are less than 10%-points.

Winter '11 / '12 - Eurasia Snow Cover - November

The latest monthly data from the Rutgers Snow Lab shows the observed areal snow cover over Eurasia was more than one standard deviation above normal during November for the 2nd time in three years.  It was the 5th highest coverage during the period of record (1966 - 2011).

Other Novembers where the snow cover was more than one standard devaition above the long term mean occured in 2009...1993...1973...1972...1970...and 1968.

Note the cluster of above normal Novembers between 1968 and 1973.  What the 3 out of 4 years had in common were negative PDO...negative QBO...and negative NAO.

Areal snowfall data courtesy Rutgers Snow Lab.
Earlier post about Octobers' Eurasia snow cover here.

Winter '11 / '12 - The North Atlantic Oscillation and Sea Surface Temperatures - Part 2

Hi-latitude blocking INVOF Greenland is known to be the main culprit for driving the North Atlantic Oscillation (NAO) index below zero.  The negative state of the NAO is widely held in high regard as a crucial element for the development of severe winter storms in the eastern CONUS...despite strong evidence to the contrary that the Arctic Oscillation (AO) plays a much more important role.

Today's AO reached the 2nd highest value since record keeping began in 1950.  The NAO can be thought of as the eastern extension of the AO.  These two indexes often rise and fall in tandem but not always.

Medium and short range forecasting of the NAO is difficult...at best.  The correlation between NWP model forecasts and observed conditions at ranges of 10 and 14 days are currently 0.596 and 0.447...respectively.  The forecasts are able to explain 36% and 20% of the NAO's variability.  There's barely any skill at 10 days and none at 14 days.

This predictive limitation often leads to the conclusion the NAO cannot be reliably forecast beyond about seven days at best.  The current seven-day correlation coefficient of 0.720 which means NWP explains 52% of the indice's variability.

There may be a predictive a relationship between sea surface temperature anomalies (SSTA) in the northern Atlantic in December and the state of the NAO...as shown below.

- Positive areas on the map indicate a positive correlation between SSTA and the NAO state.  When SSTAs are positive (negative)...then NAO is positive (negative).

- Negative values on the map indicate a negative correlation between SSTA and the NAO state.  When SSTAs are negative (positive)...then NAO is positive (negative).

For example...if above (below) normal SSTAs are observed along the west coast of Greenland in the Baffin Bay they are correlated with a negative (positive) NAO state .

The map shown below depicts SSTAs...as of 01-DEC-11.

Note the above normal SSTAs along the lower SW and SE coasts of Greenland and the negative SSTAs NE of the Azores.  The correlation is negative along the Greenland coast and a positive NE of the Azores.  These anomalies suggest favorable conditions for a negative NAO state.

Also note the above normal SSTAs along the NE CONUS coastline.  These anomalies suggest favorable conditions for a positive NAO state.

There/s no clear signal about the future state of the NAO given the current SSTA observations; however...warmer than normal waters along coastal Greenland could contribute to ridge building should the long wave flow regime shift from its current configuration.

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An earlier post explored the predictive relationship discoved by the UK MET Office between SSTAs observed in May and the state of the NAO during the following winter.

Winter '11 / '12 - Arctic Oscillation - Near Record High

     The daily AO index reached 5.647 today.  It/s the 2nd highest value behind the 5.911 observed on 02-FEB-1990.
     NWP models show the AO in rapid decline over the short term so the record may stand.

Daily CPC data here.

Winter '11 / '12 - Arctic Oscillation - Day One

The Arctic Oscillation (AO) index continues to stick its thumb in the eye of east coast winter weather enthusiasts by climbing to more than three standard deviations above 'normal' in its 7-day moving average.

Today...on the 1st day of meteorological winter...the AO/s daily value stands at 4.830!

There are 19 other days during the AO/s period-of-record beginning in 1950 where the index has been at least as high.

Hyper-AO appears to favor weakly positive ENSO and weakly negative PDO.  QBO swings both ways although there is some preference to its negative state.  No apparent association with this year's conditions b/c ENSO is moderately negative.

Just out of idle curiosity...various length running means of the AO index for the first day of meteorological winter (D-J-F) is shown below.
 

Not sure what...if anything...this reveals about the AO other than the first day of winter...during the '90s...the northern annular mode was generally above zero and during all other periods...it was generally below zero.  The '90s were a time of numerous el Niños and a PDO in its positive phase. (corrected chart 12/10).

There is no indication in the latest 10-day forecasts for the polar vortex at 10 mb to weaken (west winds observed currently +170 kts INVOF AK) or the development of hi-latitude blocking.  In fact...today/s ECMWF 10 mb forecast has the PV wind speed increasing to 220 kts by D+10.

Winter '11 / '12 - Season-total Snowfall Forecast Contest - Forecast Summary

Updated below.

Ten Nine forecasters.
Welcome back veteran forecasters and welcome aboard to Rookie forecaster Chris Tingus.

Blue (red) shading indicates the 25th (75th) percentile in the above table.
PORN is the Period of Record Normal.
Individual forecasts and summary statistics at the Contest/s web site.

Consensus...as a percent of climatology...favors above (below) normal snowfall across northern (southern) forecast stations.  Closer to normal amounts expected for southern New England and the Mid-Atlantic.

Consensus and the min-max range of forecasts.

UPDATE:
NEWxSFC received a late entry which was submitted in good faith and accepted.  The station-by-station forecasts and summary page at the Contest/s web site has been updated.

Winter '11 / '12 - Arctic Oscillation and Eurasia Snow Cover

New research from Judah Cohen of sCast fame shows a strong correlation between the rate of change in Eurasia snow cover during October and the state of the Arctic Oscillation (AO) during the coming winter.

The paper detailing the results of the study were published in the 05-NOV-11 issue of Geophysical Research Letters Vol 38.

The "...develop(ment) a snow advance index (SAI) derived from antecedent observed snow cover...explains a large fraction of the variance of the winter AO."  The new index follows work relating monthly areal snow cover to NOAM temperatures and the implicit implication for the state of northern annular oscillation modes (AO / NAO).  The snow advance index (SAI) is the regression coefficient of the least square fit of the daily Eurasian SCE equatorward of 60°N calculated for the month of October.

Note how the observed AO and surface temperature values for meteorological winter (D-J-F) are correlated in c) above.  The positive correlation in the eastern CONUS (red) means when the AO is positive...temperatures are warmer (positive anomalies) and vice versa.  When the AO is negative...temperatures are colder (negative anomalies).

A strikingly similar depiction appears in d).  Cohen's new snow cover index derived in October is highly correlated (r = 0.86) with temperature anomalies in the eastern CONUS and all but mirrors the observed AO / T correlations during D-J-F.

Cohen and Jones...

"The implications of this discovery are potentially significant. Currently the AO is considered a product of the stochastic behavior of internal atmospheric dynamics and therefore chaotic.

"The fact that we discovered a single predictive index that explains close to 75% of the variance of the winter AO (though the period is short and the degraded SAI over a longer time period explains less of the AO variance) is inconsistent with this thinking and demonstrates that the AO, while thought to be unpredictable, may in fact be one of the most easily predicted  phenomenon known in the climate system.  [emphasis added]

"Even the most sophisticated GCMs achieve only marginal skill on the seasonal time scale in the extratropics. Implementation of the SAI in winter seasonal forecasts could potentially be a sea change in operational seasonal forecasts." 

Full paper here (.pdf 920 kb).
Harvey Leonard's (WCVB Boston) interviews with Dr. Cohen here and here.

Winter '11 / '12 - Great Appalachian Storm of 1950

Kevin Myatt at the Roanoke Times reminds us of a blockbuster snow storm from a Thanksgiving weekend long ago. 

Known at the time as 'The Storm of the Century' ... Miller 'B' cyclogenesis occurred along shore of the Carolinas on the 24th ... rapidly deepened (26mb / 12hrs) over the Chesapeake Bay  / Delmarva on the 25th...then became trapped beneath a strong 1042 mb blocking HIGH to its north.

Kevin notes...

"The Great Appalachian Storm of 1950 left behind 9 inches of snow in Roanoke and 14inches in Wytheville, with 4 to 12 inches common across our region (ED:  SW VA).

"But it was far worse elsewhere. Parts of West Virginia, eastern Kentucky and western Pennsylvania saw 2 to 4 feet of snow. Winds topped 100mph in New York. The storm is blamed for more than 300 deaths and about 1million people cut off from electricity."

Some other interesting minutia about this historic early season snow storm...

Snowfall totals...
Youngstown...OH - 29"
Pittsburg...PA - 30"
Elkins...WV - 34"

Notable wind gusts were observed at...
Newark (EWR)...NJ - 108 mph
Bear Mountain (N of NYC) - 140 mph
Concord (CON)...NH - 110 mph
Mount Washington (MWN)...NH - 160 mph

Other storm effects...
- Coastal flooding breached dikes and flooded runways at LaGuardia Airport in New York.
- Crop damage and record minimum temperatures in the deep south (Birmingham:  5°F; Atlanta:  3°F;  Nashville:  -1°F)
- Some coastal areas in New England reported greater damage than the famed '38 hurricane.
- "...(T)he Ohio State-Michigan football game went on as scheduled (in Columbus), despite blizzard conditions. Nearly 50,000 fans showed up to watch the “Blizzard Bowl,” with a Rose Bowl berth the payoff. Michigan won the game 9–3 without making a single first down and only gaining 27 yards on offense." (Weatherwise Mar-Apr 2011)

Charts and graphs after the jump.

Read more »

Winter '11 / '12 - The North Atlantic Oscillation and Sea Surface Temperatures

Conventional wisdom states it's next to impossible to skillfully forecast the state of the North Atlantic Oscillation (NAO) more than a week to ten days in advance.

Even that's a stretch given the generally low correlation between forecast and observed conditions.  The current ten-day and 14 day correlations are 0.72 and 0.55...respectively which means the forecast explains 52% and 30% of the NAO's variability.  There's some skill at ten-days...not so much at 14.

The UKMET Office produced an NAO winter-season forecast up until the fall of 2009.  It was based on sea surface temperature anomalies (SSTA) in the mid-Atlantic Ocean.  They claimed a 67% skill level.  It's no longer published...possibly b/c of recent controversies and adverse publicity surrounding their 'failure' to forecast the severity of the past two winters.

Image courtesy UKMET Office

"By taking the observed SST anomaly for May and calculating how it projects onto the predictor pattern we (ed:  UKMET Office) can make a prediction for the winter NAO.

"If the projection is positive (i.e., the anomaly pattern looks similar to the predictor pattern shown above) then the prediction is for a positive winter NAO.

"Conversely, if the observed May SST anomaly projects negatively onto the predictor pattern (i.e., it looks like the reverse of the predictor pattern) then we (ed:  UKMET Office) would predict a negative NAO."

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So...where were SSTAs lurking in the Atlantic this May?
What...if anything...might they predict about the state of Winter '11 / '12 NAO?

If you're rooting for Team NAO to come through and deliver a halfway decent winter...then you can't be happy about the positive-negative-positive SSTA pattern along the northwest rim of the Atlantic Ocean last May.

The observed pattern predicts low 500mb heights will prevail over Greenland this winter and therefore...a predominantly positive NAO.
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More about SSTAs and the NAO during winter in a few days.

Winter '11 / '12 - Sunspots and The North Atlantic Oscillation

From 'Bits of Science'...

"Recent Nature Geoscience publication shows correlation between sunspot cycle and the AO and NAO index. This in turn influences northern hemisphere temperature distribution. Current NASA observations meanwhile show the 2008-2010 sunspot minimum has come to an end – shown NASA forecast predicts new peak around the year 2013."

"Judging by solar activity a pattern of consecutive cold winters [2008-2009, 2009-2010 & 2010-2011] has come to an end. High pressure blockades (over the North Atlantic, Scotland or Scandinavia) are therefore unlikely to prevail and for Western Europe there could even be another episode of some 5 or 6 years of mild and rainy winters ahead.

"The theory is quite straightforward – and has been known to climatologists for many years: during the winter months solar activity influences air pressure patterns over the Arctic and lower northern hemisphere latitudes.

"In this the number of sunspots is key. When there are few, northern hemisphere westerlies are weaker – and cold air is capable of escaping from the Arctic towards for instance the USA and Europe."

More here.