sCAST - Winter '08 / '09

AER/s sCAST has been released in past years in late NOV or early DEC. No sign of it yet...but while we wait...here/s a thumbnail sketch about how it works.

sCAST considers these elements when making its winter forecast:

Eurasia/s OCT snow cover

Sea level perssures

Air temperatures

Atmosphereic energy flux

ENSO state

Global warming trend

NAO / AO state

sCAST developer Dr. Judah Cohen explains...

"...the link between October snow cover in Siberia and the Northern Hemisphere's winter temperatures, and snowfall.

"October is the month when snow begins to pile up across Siberia. October is also the month that the Siberian high, one of three dominant weather centers across the Northern Hemisphere, forms.

"In years when Siberian snow cover is above normal, a strengthened Siberian high and colder surface temperatures across Northern Eurasia develop in the fall.


""The result is a warming in Earth's stratosphere that occurs in January," said Cohen. "This eventually descends from the stratosphere to Earth's surface over a week or two in January, making for a warmer winter in Northern Hemisphere high latitudes. However, in mid-latitudes it turns colder, so winters in the northeastern U.S. and eastern Europe are likely to be colder and snowier than normal."

Eurasia/s OCT 2008 snow cover of 8.88 km² was 13% below PORN (10.19 km²) this year... suggesting a generally mild winter b/c NAO/AO will be positive in JAN. Positive NAO/AO are a consequence of a strong polar vortex which keeps the coldest air at high latitudes.

sCAST Launched

News about this LR winter forecast method has dribbled out once and a while over the past few years. Appears the model is ready for prime-time. The sCAST developers have a paper about their model in the current Journal of Climate.

Improved Skill of Northern Hemisphere Winter Surface Temperature Predictions
based on Land-Atmosphere Fall Anomalies.

Abstract
A statistical forecast model, referred to as the sCAST model, has been developed using observed October mean snow cover and sea level pressure anomalies to predict upcoming winter land surface temperatures for the extra-tropical Northern Hemisphere. In operational forecasts since 1999, snow cover has been used for seven winters and sea level pressure anomalies for three winters. Presented are skill scores for these seven real-time forecasts and also for thirty-three winter hindcasts (1972/73-2004/05). The model demonstrates positive skill over much of the eastern United States and Northern Eurasia; regions that have eluded skillful predictions among the existing major seasonal forecast centers. Comparison with three leading dynamical forecast systems shows that the statistical model produces superior skill for the same regions. Despite the increasing complexity of the dynamical models, they continue to derive their forecast skill predominantly from tropical atmosphere-ocean coupling, in particular from ENSO. Therefore, in the Northern Hemisphere extratropics, away from the influence of ENSO, the sCAST model is expected to outperform the dynamical models into the foreseeable future.



From TerraDaily...
"(Dr. Judah) Cohen and colleagues outline the link between October snow cover in Siberia and the Northern Hemisphere's winter temperatures, and snowfall.

"October is the month when snow begins to pile up across Siberia. October is also the month that the Siberian high, one of three dominant weather centers across the Northern Hemisphere, forms.

"In years when Siberian snow cover is above normal, a strengthened Siberian high and colder surface temperatures across Northern Eurasia develop in the fall.

""The result is a warming in Earth's stratosphere that occurs in January," said Cohen. "This eventually descends from the stratosphere to Earth's surface over a week or two in January, making for a warmer winter in Northern Hemisphere high latitudes.

"However, in mid-latitudes it turns colder, so winters in the northeastern U.S. and eastern Europe are likely to be colder and snowier than normal."

Eurasia Snow Cover - OCT '08 - Part II

October/s numbers for Eurasia areal snow cover are in and it ain/t pretty if you/re on the east coast and in the market for cold this winter.

Eurasia snow cover...Siberia specifically...is correlated negatively to east CONUS winter temperature in AER/s sCAST model...so as Eurasia snow cover goes...so goes sCAST.

Observed snow cover (L) Departures (R)

The Global Snow Lab @ Rutgers U. reports Eurasia areal snow cover for OCT @ 8.88 km² (25th percentile)...which is 13% below the period-of-record average (1967-2008) of 10.2 km². This compares to an -8% departure in NHEMI snow cover.


Eurasia/s OCT snow cover has been above average six (6) times during the past 10 years...most recently in 2006. Last year/s departure was -16%...which is not substantially different than this year. The '07 / '08 winter produced bonus snows across northern NE and disappointing totals elsewhere.



Snowfall departure analysis courtesy NCDC.

Other NEWxSFC posts about sCAST here.

sCAST Forecast Verification - Winter '07 / '08

More...

Last October/s post about sCAST/s forecast for Winter '07 / '08.

Other NEWxSFC posts about sCAST here.

Eurasia Snow Cover - OCT '08

The sCAST forecast model from AER looks to October/s snow cover over Eurasia...Siberia in particular...as a leading indicator for winter temperature departures in the CONUS.

OCT/s global snow cover climo from Rutgers U. 'Global Snow Lab' is shown on the left. What does OCT-08/s snowcover suggest about the coming winter?

Good question.

Even though mid-NOV is coming 'round the mountain...it/s apparently too early to tell b/c GSL has yet to post OCT/s numbers / analysis.

We can; however...look @ the bellwether month/s starting point...which includes Week 1.

September/s climo (L) and observed (R).
Click images to enlarge A lot of ground to make up. Notice how little snow had accumulated over the Himalyan/s and more importantly...Siberia.

Other NEWxSFC posts about sCAST here.

sCAST - Winter '07 / '08

Met Winter '06 / '07 ------------- Met Winter '07 / '08

AER/s briefing slides (.pdf) about the sCAST model...this year/s forecast...and verification of last winter/s forecast.

Seems odd AER issued a forecast this early since an important model input is October snow cover across Siberia.

From the National Science Foundation (NSF)..."The model uses October Siberian snow cover, sea level pressure anomalies and recent temperature trends in its winter forecast."

AER/s rationale for its Winter '06 / '07 Forecast...
"October 2006 snow cover was observed to be above normal that coupled with a predicted negative winter Arctic Oscillation (a pattern that favors high heights in the Arctic and more cold air intrusions further south) produces the colder than normal temperatures in the East. The above normal snow cover also favors a stratospheric warming event this winter.

"A weak to moderate El Nino is currently predicted for the winter, which we anticipate will not have a strong impact on winter temperatures in the eastern US. However, if the El Nino turns out to be stronger than predicted, in the moderate to strong range, then the predicted temperatures in the East may be too cold. (Also the above normal snow cover in Siberia and observed atmospheric conditions in the North Atlantic during October once again favor a higher probability of a major East Coast snow storm this winter season).

"October is the month where snow cover undergoes its greatest expansion in the Eurasian region. During October, Eurasian snow cover can increase by as much as 10-15 million square kilometers, which is greater than the total land area of the United States, including Alaska. NSF-funded research has shown that variability in the extent of Eurasian snow cover can be used to predict cold or warm winters across the entire mid-latitudes of the Northern Hemisphere."

Trends in NHEMI Snow Cover

Tony Wood of the Philadelphia Inquirer writes in his Weather or Not column today about trends in NHEMI snow cover. Areal coverage has been decreasing and the spring melt has come earlier. An earlier post on NEWxSFC suggested similar trends.

Tony reports some interesting observations made by Rutgers University/s geography professor David Robinson and AER atmospheric scientist Judah Cohen on the subject and what it all might mean.

"First, and curiously, the snow cover in other seasons (summer, fall and winter) hasn't shown big changes, Robinson says.

"Second, after falling off in the late 1980s, reaching a record low in 1990, the snow cover rebounded - until three years ago. So the decreases in complete calendar years 2004, 2005 and 2006 could constitute an anomaly.

"Third, Robinson, who contributed to the blue-ribbon Intergovernmental Panel on Climate Change (IPCC) reports, says that one could make the interesting argument that the world is warmer because of less snow.

"It's a chicken-egg question," he says.

Judah Cohen adds...

"The physical mechanism isn't fully understood, Cohen says, but fall snow coverage evidently affects air pressure patterns that influence other changes: An above-normal snowpack in Siberia in October correlates with below-normal temperatures in the United States in winter.

"October is the key, Cohen believes, because when the sun is strongest (summer), the snow is scarce; when the snow cover peaks (winter), the sun is absent. October has both."

More...
Rutgers University Snow Lab
sCAST from AER...Inc

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.

AER - Winter Outlook '08 / '09

sCAST modeler Judah Cohen of AER expects 'warm' temperatures in the east this winter. Cohen mentions the below normal snow cover in Siberia as a contributing factor in this year/s forecast.

More...

sCAST Money Shot

Note the negative correlation (increased snow cover associated with decreased temperature) between Siberia and the cold signal across the OH / TN River Valleys and ern GL regions of the upper M-W.