bluewave Posted February 7, 2014 Share Posted February 7, 2014 I just ran the numbers on the last twenty 6"+ snows at NYC from 12-05-02 through 2-3-14. The two most important factors for getting a 6"+ snowfall at Central Park have been a -AO/+PNA combination. The NAO and EPO have been less important indices to look at for warning level snows in NYC. The 500 mb composite for the last 20 starting dates of 6"+ storms at Central Park shows the importance of the Davis Strait block for us. Percentages for 6"+ snows in NYC out of 20 storms since 12-05-02 -AO......80% +PNA...75% -NAO....55% +EPO...50% 20 storm 500 mb composite Link to comment Share on other sites More sharing options...
REDMK6GLI Posted February 7, 2014 Share Posted February 7, 2014 Great read and an even better reference for forecasting future storms chris Link to comment Share on other sites More sharing options...
Dsnowx53 Posted February 7, 2014 Share Posted February 7, 2014 The thing about the NAO, though, is that it's often rising as a storm hits and is negative before a big storm. But yes, this is a good post. Link to comment Share on other sites More sharing options...
Ralph Wiggum Posted February 7, 2014 Share Posted February 7, 2014 I'm surprised the nao correlation is so low...good post. Link to comment Share on other sites More sharing options...
Juliancolton Posted February 7, 2014 Share Posted February 7, 2014 I'm surprised the nao correlation is so low...good post. I may be completely off the mark, but I think people like Henry M. have played a role in ingraining the "-NAO = big snowstorm" myth. It helps of course (per KU and others) but an idle observer of weather blogs may well be misled into thinking it's the most important factor. Link to comment Share on other sites More sharing options...
donsutherland1 Posted February 7, 2014 Share Posted February 7, 2014 I just ran the numbers on the last twenty 6"+ snows at NYC from 12-05-02 through 2-3-14. The two most important factors for getting a 6"+ snowfall at Central Park have been a -AO/+PNA combination. The NAO and EPO have been less important indices to look at for warning level snows in NYC. The 500 mb composite for the last 20 starting dates of 6"+ storms at Central Park shows the importance of the Davis Strait block for us. Percentages for 6"+ snows in NYC out of 20 storms since 12-05-02 -AO......80% +PNA...75% -NAO....55% +EPO...50% 20 storm 500 mb composite COMP.gif Great information. It should be noted that as the wavelengths begin to shorten, the AO becomes less of a factor e.g., there's almost a 50-50 split between AO-/AO+ when it comes to 6" snowstorms that commenced 2/16 or later. The same is true with respect to the PNA. Hence, at least for me, it becomes more difficult to identify a large-scale snowy pattern let's say for March 1-15 than it was with respect to the current 1/25-2/15 period. Little details matter far more late in the season. Link to comment Share on other sites More sharing options...
donsutherland1 Posted February 7, 2014 Share Posted February 7, 2014 I may be completely off the mark, but I think people like Henry M. have played a role in ingraining the "-NAO = big snowstorm" myth. It helps of course (per KU and others) but an idle observer of weather blogs may well be misled into thinking it's the most important factor. It isn't really a myth. There's a difference between the much larger pool of 6" or greater snowstorms and the smaller set of blockbuster storms identified in the KU book (the majority of which have a NAO-). Link to comment Share on other sites More sharing options...
PB GFI Posted February 7, 2014 Share Posted February 7, 2014 It isn't really a myth. There's a difference between the much larger pool of 6" or greater snowstorms and the smaller set of blockbuster storms identified in the KU book (the majority of which have a NAO-). Yeh if you want the real bombs , you need that confluence to cut it off and slow the flow down . 6 inch storms can always get thru the flow . Link to comment Share on other sites More sharing options...
bluewave Posted February 7, 2014 Author Share Posted February 7, 2014 You can see the -NAO come up a bit for the twelve 10"+ events at Central Park over the same period along with the +EPO. The best ridging on the 500 mb composite can also bee seen extending further east from the Davis Strait over to Iceland. The other important point about the NAO is that the 5 positive cases out of the 12 storms were only in the +0.1 to +0.8 range which was weakly positive. Twelve 10" events at Central Park since 12-05-02 -AO.....83% +PNA..83% -NAO..58% +EPO..67% Link to comment Share on other sites More sharing options...
CIK62 Posted February 7, 2014 Share Posted February 7, 2014 So what are the current values of these parameters and will they help or hinder any precip. activity showing for 2/13-20? Most action occurs when NAO and PNA are crossing the 'neutral zone' together I think. I don't know if the direction in which each is approaching neutral matters. Seems like we have been contemplating snowstorms only because of the polar vortex (product of the stratospheric warming of Nov-Jan) being farther south than normal and now a more active southerly jet. Indexes did not seem favorable. Thanks to whomever answers. Link to comment Share on other sites More sharing options...
bluewave Posted February 7, 2014 Author Share Posted February 7, 2014 Here are the percentages for the 11 greatest snowstorms with 15"+ at NYC since 1960: -AO......82% +PNA...73% -NAO....64% +EPO...64% Link to comment Share on other sites More sharing options...
uncle W Posted February 7, 2014 Share Posted February 7, 2014 I can't find the post I made about NYC snowstorms and what the ao nao and pna was when they happened...the +pna had the highest pct... Link to comment Share on other sites More sharing options...
forkyfork Posted February 7, 2014 Share Posted February 7, 2014 here's the composite for all nyc 15"+ storms since 1948, two days before each storm Link to comment Share on other sites More sharing options...
bluewave Posted February 7, 2014 Author Share Posted February 7, 2014 I can't find the post I made about NYC snowstorms and what the ao nao and pna was when they happened...the +pna had the highest pct... It came out very close in importance to the -AO. But the the big point is that NYC didn't have any trouble reaching 40"of snow on a mostly +NAO since the AO became more negative after an early positive start. Link to comment Share on other sites More sharing options...
forkyfork Posted February 7, 2014 Share Posted February 7, 2014 here's the composite for all nyc 15"+ storms since 1948, two days before each stormhere's the same composite rolled forward to the storm dates Link to comment Share on other sites More sharing options...
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