Winter '18 / '19 - North Atlantic Oscillation (NAO) Analog Verification

The analog forecasting technique seeks similarities to the NAO state in the run-up to the coming winter with NAO run-up states of winters past.  Presented here is the verification of NAO analogs for the '18 /'19 winter.

The NEWxSFC method ranks analog years by their the sum of square errors (SSE) statistic.
Lower SSE errors ==> stronger analog

Constraining the number of analog winters for analysis to five is arbitrary.

North Atlantic Oscillation (NAO) Index Analog Forecast Verification

In the run-up to Winter '18 / '19 ... '84 / '85 was the leading analog followed by '72 / '73 ... '59 / '60 ... '89 / '90 ... and '91 / '92.  Winter '18 / '19 NAO started weakly positive ... approached zero at meteorological winter's end ... then climbed above 1 in MAR.

A qualitative assessment of the analog forecast's accuracy would rate the analogs #2 or #5 as 'best' with #1 ... 3 ... and 4 'poor.'

A quantitative assessment of the other '72 / '73 and '91 / '92 teleconnections ... not so much.

Winter '72 / '73 and '91 / '92
ENSO: -0.7 / 1.7
SOI:  -28.6 / -51.4
AO:  1.085 / 1.095
PDO:  -0.4 / -0.01 (sign flip in JAN)
QBO:  -7.03 / -13.66

Winter '18 / '19
ENSO:  0.8
SOI:  1.5
AO:  0.182
PDO:  -0.26
QBO:  0.88

Winter '18 / '19 VERIFICATION RESULTS:
Only two of the five analogs provided useful qualitative guidance for Winter '18 / '19 and few of the other teleconnection indexes were in agreement.
 
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An alternative NAO forecast technique looks at its 'sign' -- i.e., positive or negative -- for each calendar-year's month preceding the pending winter as a potential leading indicator of the NAO/s sign during upcoming D-J-F period.

Results from a chi-square 'test for independence' test infers a statistically significant relationship (95% CI; p-value <= 0.05) between NAO/s sign in NOV and NAO/s sign during the upcoming D-J-F period.

If NOV/s NAO is positive ... chances are likely (73%) the NAO state will average 'positive' during D-J-F.

If NOV/s NAO is negative ... chances are even NAO/s state will average 'negative' during D-J-F.

Period-of-Record NAO for NOV and D-J-F.  Marker indicates both NAOs > 0

Winter '18 / '19 VERIFICATION RESULTS
Predictor:  NAO/s NOV '18 sign was -0.11 (negative).
Response:  Average NAO for Winter '18 / '19 was 0.497 (positive)
Outcome:  The 2x2 contingency table suggests equal odds when NOV NAO > 0 (positive)