Going Negative - Epilogue

The most basic statistical analysis -- the average - for 30 NEWxSFCs further supports the hypothesis that it isn/t nec-es-sarily a good thing to 'go negative' if you want more of what exciting winter storms are all about...which is more snow.

AVG Contest SNFL (n = 30) when...
AO - Negative (Positive) 10.7" (13.2")
NAO - Negative (Positive) 10.2" (11.4")
PNA - Negative (Positive) 11.9" (11.7")

EEBE, max avg station snfl matters not if PNA is plus or minus...as much as it matters whether the NAO or AO are negative (less snfl) or positive (more snfl).

Cross tab analysis
Another way to see if 'going negative' benefits winter wx would be to look at the avg snfl for each teleconnection index's positive and negative phase...as it relates (cross tab) to the two phases of the other two indexes.

MAX avg station snfl (13.2") occurs when AO is positive.
Second ranked MAX avg snfl (12.7") occurs when PNA and AO are positive.

MIN avg station snfl (7.7") occurs when AO is negative and NAO is positive.
Second ranked MIN avg snfl (8.5") occurs when PNA and AO are negative.



Torture the numbers long enough and they/ll confess to anything.

(Ed. - 'light of day' corrections.)

FWIW

Action centers E of GOA and EC.

Going Negative - Part IV

Snowfall amounts for 30 Contest snowstorms since December 2002 were stratified by quartiles to determine whether they were heavy, light, or somewhere in between. The analysis seeks to determine whether negative teleconnection indices are favorable for the best winter wx...defined as producing heavier snowfall events.

Heavy snowfall is defined as belonging to at least the 75 percentile (>= 16") of the 30 Contest storm dataset. Light snowfall is defined as belonging to no more than the 25 percentile (<= 5.6"). The analysis did not control for SN:H2O, areal distributions, events where liquid or freezing precipitation reduced snfl, or the number of stations affected.

NAO
5 of the 8 (62%) max station snfl amts occurred when the NAO was greater than 0.
3 of the 9 (33%) min station snfl amts occurred when the NAO was greater than 0.

Heavier snfl is more likely when NAO greater than 0.

3 of the 8 (38%) max station snfl amts occurred when the NAO was less than 0.
6 of the 9 (67%) min station snfl amts occurred when the NAO was less than 0.

Lighter snfl is more likely when NAO less than 0.

AO
5 of the 8 (62%) max station snfl amts occurred when the AO was greater than 0.
4 of the 9 (44%) min station snfl amts occurred when the AO was greater than 0.

Heavier snfl is more likely AO greater than 0.

3 of the 8 (37%) max station snfl amts occurred when the AO was less than0.
4 of the 9 (44%) min station snfl amts occurred when the AO was less than 0.

Lighter snfl is more likely when AO less than 0.

PNA
5 of the 8 max (62%) station snfl amts occurred when the PNA was greater than 0.
5 of the 9 min (56%) station snfl amts occurred when the PNA was greater than 0.

Heavier snfl is more likely when PNA > 0.

3 of the 8 max (38%) station snowfall amounts occurred when the PNA was less than 0.
3 of the 9 min (33%) station snfl amts occurred when the PNA was less than 0.

Heavier snfl is slightly more likely when PNA less than 0.

What/s It All Mean?
The limited dataset suggests PNA plays a bigger role in the heavier snowfall events. +PNA suggests an amplified 'ridge-W / trof-E' UA flow regime that delivers cold air from the NW NOAM and...as described by the loading pattern and Sutcliff/s 'self-development' theory...is favorable for cyclogenesis along low latitudes of the EC.

Negative values for the AO and NAO suggest an UA flow regime conducive to colder than normal temperatures in the NE but negative values of these teleconnection indices are not necessarily pre-conditions for heavier snfl. In fact...negative values may work against heavier snfl events b/c cold air is dry air and unless the mid-level flow is amplified...then cyclogenesis is unlikely to occur.

The various teleconnection indices are useful tools to diagnose aspects of atmosphere/s configuration but should not be seen a prerequisites for strong to severe winter wx. Good snows have even occurred even when PNA is negative...as have good snows when the AO and NAO are positive.

Standby for Contest #1!

+PNA kick starts winter in the E. As they say: warm AK...kold in the E.

The 6-10 temperature prog is par-tic-u-lar-ly promising for EC snowfalls given this is the coldest time of the year and precipitation is expected to be above normal.





8-10 Day looks great...too.

Going Negative - Part III

More than the other two teleconnection indices...the Pacific-North American index is a reflection of the LW flow regime/s amplitude over North America. When PNA is negative...there/s troughing in the west and ridging in the E. When PNA is positive...the opposite is true -- there/s ridging in the west and troughing in the E. +PNA...especially if it/s increasing with time...often contributes favorable conditions for the process Sutcliff termed 'self-development' which can produce snowstorms in the E.

The 'self-development' process occurs when the Rossby (AKA 'long wave,' planetary wave') wavelength shortens from warm air advection (WAA) pulling the downstream ridge back to the W and cold air advection (CAA) pushing the upstream trof to the E. This causes the LW to stretch in the N/S direction which increases its amplitude, divergence aloft, and curvature. The increase in curvature produces greater vorticity and the advection of the vorticity by the geostrophic wind (PVA) generates upward vertical motions above the surface cyclone...which in turn generates more WAA / CAA and the cycle repeats itself. But I digress.

In the first two installments of 'Going Negative,' the NAO and AO teleconnection indices for 30 NEWxSFC snowstorms were analyzed to see if the popular idea that 'winter just ain/t happenin' and it ain/t gonna happen until they go negative' was a reasonable conclusion. It turns out...it just isn/t so. If +PNA if conducive to the 'self-development' process of cyclogenesis...what kind of snowfalls can be expected when the PNA goes negative?

Pacific-North American
The PNA was positive for 18 (60%) of the 30 Contest storms and less than or equal to zero for 12 (40%).

Seven storms (39%) had a max station snowfall amount of at least 12" when the PNA was greater than zero. Of those storms...four (57%) had a max station snowfall greater than 20".

Four storms (33%) had a max station snowfall amount of at least 12" when the PNA was less than or equal to zero. Of those storms...two (50%) had a max station snowfall amount of at least 20".

Contest storms were 50% more likely when the PNA was greater than zero; however...when a Contest storm occurred...the storms during a -PNA regime were almost as likely as the +PNA regime storms to produce a max station snowfall about of at least 12" and 20".

In tomorrow/s final installment...we/ll try to answer the musical question: what/s it all mean?

Going Negative - Part II

As winter approaches its midpoint next week...seasonable wx in the E has...by and large...been anything but seasonable. Save for a brief cold snap and a smattering of light snowfalls early on...winter/s wx has little to show for itself. El Nino/s been blamed for the current run of well-above normal mean monthly temperatures...as has the North Atlantic Oscillation. Some have even pointed an accusing finger or two at global warming.

Whatever the cause of this winter/s fitful start...hope for a full recovery during its second half is...in some quarters...heavily pinned on the getting the NAO and Arctic Oscillation (AO) teleconnections indexes below zero.

As yesterday/s analysis showed...good snowstorms have occurred in the NE and M-A when the NAO has been above zero. Most of the better storms happened when the NAO was not negative. The NAO and AO often oscillate in tandem...but not always.

When these indices are negative...cold wx does tend to dominant the eastern US but it also can work against snow b/c cold air is dry air.

The 30 snowstorms used in the analysis are not necessarily representative of all snowstorms in the NE and M-A regions. Forecast contests have not been held for all snowstorms...mainly b/c the progs were not conclusive early enough to warrant a decision to hold a contest or the storm was not expected to produce more than nuisance amount or affect more than a few stations. Some storms in the analysis sample did not produce heavy snowfall.

The Arctic Oscillation
The AO was negative for 17 storms (57%). Six of those storms (35%) had a max station snowfall of at least 12". Two of the storms had a max station snowfall of at least 20".

Of the 13 storms where the AO was greater than or equal to zero...five (38%) had max station snowfall amounts of at least one foot and four (31%) had at least 20". Only one storm where the max station snowfall was at least 12" had an NAO less than zero. Max station snowfall where the AO was greater than zero was 27.3"

There/s a 57% chance that a Contest snowstorm occurs when the AO is negative. When the AO is negative...there/s a 20% chance of one station measuring at least one foot of snow and a 7% chance of a measurement of at least 20".

There/s a 47% chance that a Contest snowstorm will occur when the AO is positive. When the AO is positive...there/s a 38% chance of at least one foot and an 8% chance of at least 20".

There/s a slightly better chance for a snowfall forecasting contest when the AO is negative than when the NAO is negative. Chances are about equal for a max station snowfall of at least 20" when the AO is either positive or negative; however...when the AO is positive...there/s a better chance of getting a max station snowfall of at least 12".

When the NAO and / or the AO are negative...winter air temperatures are characteristically cold; however...sometimes it/s too much of a good thing b/c cold air is dry air. Regardless of any particular indice/s sign...what/s needed for snow is cold air...moisture...and forcing to provide synoptic-scale upward vertical motion.

In tomorrow/s installment...we/ll look at another important winter teleconnection index -- the Pacific-North American -- to see what happens when it goes negative.

Going Negative

To hear some people tell it...winter just ain/t winter unless there/s a lot of negativity going around. Negativity regarding certain teleconnection indexes...that is.

Take for instance the North Atlantic Oscillation. Conventional wisdom has it the NAO/s gotta be negative. The NAO/s at or above zero? Well then....winter just ain/t happenin' and ain/t gonna happen until it falls below zero.

Same 'thinking' goes for the Arctic Oscillation. AO/s gotta be negative. Can/t get good winter wx without a negative AO.

It/s not that a little negativity is necessarily a bad thing. Positive things can and do come from being negative...but does it have to be that way?

To test the hypothesis that good winter wx requires negative indices of the NAO and AO...30 NEWxSFC snowstorms since December 2002 were reviewed. The results shows there has been plenty of good winter wx when those indices were positive.

The 30 snowstorms used in the analysis are not necessarily representative of all snowstorms in the NE and M-A regions. Forecast contests have not been held for all snowstorms...mainly b/c the progs were not conclusive early enough to warrant a decision to hold a contest or the storm was not expected to produce more than nuisance amount or affect more than a few stations. Some storms in the analysis sample did not produce heavy snowfall.

The North Atlantic Oscillation
The NAO was negative for 16 storms (53%). Seven of those -NAO storms (44%) had a max station snowfall of at least 12". There was one storm where the max station snowfall was at least 20".

Of the 14 storms where the NAO was greater than or equal to zero...five (29%) had max station snowfall amounts of at least one foot. In fact...all five storms had least 20" and three storms had at least 25". Only one storm where max station snowfall was more than one foot had an AO below zero.


There/s a 53% chance that a Contest snowstorm will occur when the NAO is negative. When the NAO is negative...there/s a 23% chance of one station measuring at least one foot of snow and less than 1% chance of a measurement of at least 20".

There/s a 47% chance that a Contest snowstorm will occur when the NAO is positive. When the NAO is positive...there/s a 20% chance of at least one foot and an equal chance of at least 20".

Seems hard not to conclude that -NAO 1) isn/t necessary for Contest snowstorms and 2) isn/t the best for heavy snowfalls in the NE and M-A!

Tomorrow we/ll look at the Arctic Oscillation to evaluate how important it is for this teleconnection index to be negative if you/re looking for a good snowstorm.

Wait No Mo' for the -AO

At long last...the AO is poised to come crashing down...if you believe in the EC/s 240-HR prog. Note the ridge with the big blue H over the pole...anchored to the E by the PV over CN and to the W by another cold gyre in the Bering Sea.



Another perspective...the 192-HR latitude-height x-sect/s depiction of deep easterlies fm sfc into the lower reaches of the ignorosphere.



Not a perfect match with the loading pattern...but there/s definitely more mass over the pole and that/s what counts.

QBO - December '06

Nine months into its positive phase...the latest QBO comes in a solid 6.21.

Still a relatively strong value...a smidge more than +1 SD above the long-term monthly mean for December of 6.12...as well as being in the 77th percentile. December/s QBO ain/t no slacker.

When QBO is W...the PV is deeper and stronger. Considering the PV has been over the north-central PAC for weeks during a moderate +ENSO...this strongly suggests the westerly stratospheric winds along the equator played a major role in maintaining the anomalously strong PAC jet and the near continuous flux of mild PAC air masses over the CONUS.

Now that the PV is shifting quickly E to a position over north-central CN...the temperature gradient driving the jet should shift into western CN where a hi-amp...+ENSO enhanced PNA-ridge is expected to develop. The thermal contrast won/t be as strong as it was when the PV was over the PAC...but strong enough to excite the polar jet and drive arctic air masses into the lower 48.

We/re coming to the end of the QBO/s west phase...so the stratosphere/s zonally-averaged wind speed should continue its decline during the remainder of '07. As QBO decreases during the second half of winter...the PV will weaken...which would allow more opportunities for more polar cold to encroach upon lower latitudes.

From Russia with Love

850 mb analysis from 12z 12/31 centered over North Pole. Note position of -35°C cold pool over Asia and -20°C cold pool INVOF the Hudson Bay.



12z analysis @ 100 mb from 1/1 ECMWF shows warm temperature anomaly over Asia INVOF the -35°C pool @ 850 mb. This anomaly has been nearly stationary for weeks. It may have played a role in sustaining the persistent jet anomaly across the PAC by enhancing the thermal contrast with the warming contributed by the on-going +ENSO.



D+8 ECMWF forecast from 1/1 shows warm stratosphere anomaly has propagated E to a position INVOF Hudson Bay. This is where the cold air...currently positioned over Asia will set up next week and take the place of the 'warmer' -20°C pool.



The arctic airmass forecast to enter the lower 48 next week had its origins over the snow fields of Siberia.