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.

Public Enemy #1

Another week of obsessive / compulsive anticyclogenesis from the ern PAC.

Preliminary SOI - December '06

(Updated below)

The SOI is a 30-day moving average and with one day to go before the end of the month the index comes in near -4.9. After flirting with zero during the last few days of November and into the middle of December...by month/s end the SOI had been in steady decline.




+ENSO Lives...but for how long?

More evidence that +ENSO is holding its own is found in outgoing long wave radiation (OLR) anomalies. Decreased OLR caused by increased cloudiness produced by deep...moist convection is typical during +ENSO events. EEBE, increasing OLR (decreasing cloudiness) would suggest the current +ENSO is fading...but the data for December do not suggest that is happening.

Red areas depict cloudy areas where OLR is below climo.


December - Week 1


December - Week #2


December - Week #3


December - Week #4



Working against the current state are 1) the trade winds that have returned to near normal 2) cooling sub-sfc water...and 3) the continued steady decline in region 3.4 SSTAs. Weekly US and AU SSTA data cover different 7-day periods. The US lags AU by ~4 days.


Weekly anomalies through 12/24
US 1.4 1.3 1.2
Au 1.3 1.2 1.1

A steady decline...indeed.



Waters are still warm and producing plenty-o-convection...for now. The trade winds appear to be working against an eastward propagation of the warmer SST near 180°. It/s only a matter of time before the fuel in region 3.4 is exhausted and it/s over.

Update: End-of-month SOI was -2.4.

Time for a Change

There/s plenty not to like in the LR progs...not the least of which is the rabid PAC jet flooding the lower 48 with unseasonably mild temperatures and a dearth of EC snowfall. Run after run its little mo' than mo' of the same.

Hints of change to the current pattern are hard to come by. Progs alone aren/t enough. Time to strap on the synopticoscope.

In recent days...several events have occurred that suggest changes to the status quo. Whether it/s a change for the better remains to be seen.

The most significant event is the relatively large anticyclone (1040+ mb) coming off the Asian continent. This has shifted a persistent PAC storm track along 40 / 50 into NW PAC to one where coastal storms rise rapidly in latitude into the Bering Sea and wrn AK. Previous HIGHs coming off the continent were relatively small and weak (1024 mb) with new 'fish' storms forming every three days off the coast of Japan. The storms crossed the PAC...crashing ashore INVOF SEA and with time...BC. Whistler Mt...BC has measured 20' of record snowfall YTD with four months left to go.



Over the past few days...the PAC storm track has farther shifted N into ANC and toward the Aleutian Is.

The second notable change is the erosion of the mid-lvl omega block with its closed center over England...France...and Germany. The area is in transition to a fast...zonal flow as the closed anticyclone sets up to the S over the NW coast of Africa.





Third...the Icelandic LOW...one of AO/s three poles... is W of Greenland today. This feature has generally been E of Greenland since November...a characteristic of +AO phase. A position W of Greenland is favorable for the negative AO phase.



Last...LR models retrograde the sub-tropical heat ridge into the Caribbean Sea and GOM by end of the forecast period.



The change in the PAC storm track is the best sign of improvement for downstream conditions b/c it strongly influences one pole of the AO teleconnection. Add in the fx of a strengthening pole in the N Atl. The third pole over the Azores is weak b/c of its proximity to the strong ridge.

Retro

Does Thursday/s 5H NHEMI T chart -- with its depiction of three distinct waves --suggest a looming pattern change via retrogression.

Back up a few of the main ftrs @ 5H and see if you don/t see winter in all the right places.

12/26 Storm - Contest Worthy?

Most likely not.

All guidance strongly suggests this is a synoptic-scale rain storm with the potential for mesoscale snowfall along the comma head/s SW to NE axis.

Barring significant changes in tomorrow/s NWP solns...this storm is not in play.