So Long...+ENSO

This winter/s +ENSO...oft a convenient scapegoat for the abnormally warm temperatures across much of the CONUS...appears to be fading...and fast.

NOAA/s ENSO definition...based solely on SSTAs in region 3.4...holds that if the anomaly is at least +0.5°C...it/s el Nino. If the anomaly is less than or equal to -0.5°C...then it/s la Nina. The Australians consider the Southern Oscillation Index (SOI)...Outgoing Long wave Radiation (OLR)...trade winds...sub-sea surface temperature anomalies...as well as SSTAs.

There are significant signals...using the Australian criteria...that +ENSO is on-goin..but in its last throes.

ENSO signals...week ending January 21:



1) SOI stands @ -5.5...doubling down from December/s end-of-month value of -2.4. +ENSO is indicated when the SOI is negative.



2) ENSO region 3.4 SSTA 0.57°C...barely above the 0.5°C threshold.

3) Trade winds weaken and reverse direction during +ENSO. Trade winds are currently normal to above normal in the ern portion of region 3.4 and anomalous near the International Dateline. Some have suggested the west wind comes courtesy the MJO...but MJO is still largely voo-doo...so who can say for sure what it means.



4) Above normal sub-sea surface temperarture...such as those shown during November...are supportive of +ENSO conditions. Below normal sub-sea surface temperatures are now reaching well into the ern PAC.



5) OLR has been negative since early January...indicating widespread active convection in the region...and a sign that el Nino is on-going.

Recap
SOI 30-day moving average still weakly negative. SSTAs still above +ENSO threshold but falling fast. OLR negative. Trade winds in flux. Sub-sea surface anomalies trending negative.

+ENSO doesn't usually end during the srn HEMI/s summer...so it probably has some life left...but it/s getting about time to say...'so long.'

Is A Big One A'comin'?

This winter/s evil-doers -- mT air masses from the ern PAC -- were overthrown last week when the PV made its long-awaited return home to nrn CN. Since then...LR progs have been honking about more regime change late next week...evidenced by NAO and AO forecasts going negative -- NAO strongly so -- and the PNA flirting with +1.

Do these expected changes in three important winter teleconnection indices portend big snows for the NE and M-A toward the end of the upcoming week?

For the answer to that musical question...we look to Ms. Heather Archambault (University at Albany...State University of New York) who co-authored a study in 2004 titled 'Cool-Season Regime Transition and its Impact on Northeast Precipitation' (PPT).

The study/s premise was to examine...
1) the meteorological wisdom that holds there/s an increased threat of a major storm during large–scale regime transition...
2) past research pointing to a connection between synoptic-scale cyclogenesis and reconfiguration of the planetary-scale flow (e.g. Colucci 1985)...and
3) Dave Groenert (CSTAR...2002) documentation of an apparent tendency for increased precipitation in the Northeast during phase transitions of the North Atlantic Oscillation (NAO).

The study took a statistical look at cool-season (Nov – Apr) weather regime transitions and Northeast precipitation anomalies and created a composite analysis for one kind of regime transition. The regimes were defined as +/- NAO and +/- PNA. Study cases were selected if the index anomaly was at least one standard deviation (STD) from the mean.

Regime change was defined as...
1) a significant change in teleconnection index magnitude (2 stdev) over a 7 – day period...
2) teleconnection index phase change...and
3) index at start of transition must be strongly positive or negative (Index>1 stdev).

Bottom line result of the study -- significant changes in LR forecasts for NAO and PNA would give forecasters a one-week advance 'heads up' of an increased potential for major winter storms.

Are Forecaster's Getting Their Heads Up?
No doubt these/s a large scale regime transition going on in across the NHEMI. Geo-potential heights are expected to dramatically rise / fall over Greenland / Azores this week. +PNA is on the way...too.

Will It Be Enough?
At the start of the seven-day run-up period...today/s NAO is -0.37 (slightly below the mean) and forecast to fall to ~ -1.5 STD below the mean...which is not even close to the study/s -2.0 STD threshold. It/s worth noting the study did not indicate whether a sensitivity analysis was done to determine whether a range of slightly higher or lower NAO values altered the final results. It/s possible the STD constraint is too strict and a smaller deviation from the mean...such as -1.5 STD... would sustain the study/s main conclusions.


Like the NAO...the PNA starts out near the mean (0.3) as well. It/s forecast to increase to a smidge more than 1 STD above the mean and like the NAO...the increase in PNA does not meet Archambault/s 2 STD criterion.


Is A Big One A'comin'?
One huge assumption about MR forecasting of significant NE snowstorm lurks beneath the conclusions of 'Cool-Season Regime Transition and its Impact on Northeast Precipitation.' Can MR / LR NWP models accurately predict the state of the NAO and PNA such that the study/s findings can be put to use in an operational environment or do they merely make for interesting post-mortem analysis?

Two measures of skill are the 'correlation coefficient' (R) and its square...the 'coefficient of determination' (R²). R establishes whether there/a a relationship between the forecast and the observed. Does the observed value increase when the forecast indicates an increase? The closer to one the ratio is...the stronger the relationship. The NAO/s (PNA) seven-day forecast correlation co-efficient is ~0.83 (~0.88)...which is a good indicator that when the forecast indicates a change in direction of the index...it will be correct. The model/s ability to capture the variability (as in variance) of the index is measured by R². R² is the square of the correlation co-efficient. The NAO/s (PNA) R² is 69% (77%) ...meaning 69% (77%) of the variability in the observed NAO (PNA) is captured by the model. Respectable values...but not overly so.

If Archambault et. al./s results are applied to next week/s expected evolution of events along the EC...then nothing 'big' is apt to occur and we/re left waiting and still wanting for the start of some serious winter wx.

Fresh Air

T/weren/t all that long ago when one mT anticyclone after another...filled to the brim with warm PAC air...was flooding the Lower 48. Monthly T departures of 6°F were not uncommon. The UA flow was just plain flat and that was that.

Well into MET winter...winter was a no-show. We/d all been stood up...again.

The PV...anchored over ern Asia...was directing wave after wave of LOW pressure into wrn CN and srn AK. Whistler Mt. in British Columbia and Alyeska...AK benefited from the atmosphere/s early winter largess by piling up history-making snowfalls.



For whatever reason... weakening el Nino...Comet McNaught...Democrats taking control of Congress...the PV began a huge pivot and lunged more than 90° E to positon INVOF Hudson Bay. What followed was the beginning of winter wx across much of the CONUS.



In recent days...the ECMWF has been advertising another migration of a cold anomaly headed for the Hudson Bay...once again originating from the ern shores of Asia. The following progs pick up this feature near the IDL. Once it reaches it/s final destination...it would ensure a continued supply of fresh...well-chilled arctic air masses over the EC.

D+0 @ 100 mb. Warm stratosphere ==> cold troposphere.

D+7 @ 100 mb

Note the new Hudson Bay vortex in the 5H Z prog below...and the well-inland wrn CONUS ridge axis.



So with all this cold air lolly-gagging around...where/s all the snow?

FOTMI...the current and forecast planetary flow regime...where the NAO is expected to hover below zero...is not conducive for nor'easters. Sure...there/s a ridge-W / trof-E configuration...but the W-ridge is positioned too far E. Cyclogenesis is occurring...but it/s occurring well offshore where the fishes sleep.

A little retrogression wouldn/t hurt the cause. Nor would it hurt if the the new Hudson Bay vortex sets up a tad west of the old one...but alas...the D+7 prog suggests otherwise.

Most years past there have been ~10 contest storms. It/s long past midnight this year for Contest #1, eh?

High Latitude Easterlies

Until recently...this winter/s planetary flow regime has looked just like the left side of this conceptual model with its depiction of the positive...warm phase of the Arctic Oscillation. HIGH pressure in the E PAC...a sprawling Azores/ anticyclone in the E ATL...and a hi-index (zonal) flow regime across NOAM and Europe.

The source region for CONUS air masses...the +ENSO enhanced warm waters of the PAC. Every few days...anticyclogenesis would occur in the W...then migrate E in the flat...fast zonal flow. The mT air masses were modified as they descended the Rocky MTS thru compressional warming before arriving in the E with surface air temperatures well above normal.

Signs of change became evident in mid-to-late December as mid-latitude storms tracking across the PAC began veering toward AK and away from the PAC NW. The omega block over ern Europe was breaking down and progs from the LR models showed the PV moving from an abnormal position over wrn Asia to its climatologically favored position over nrn CN.


A month later...the change...albeit gradual...is well under way as the first of several arctic anticyclones build into the ern two-thirds of the CONUS. The animation of latitude-height sections shown at the left from the ECMWF indicate a continue presence of easterlies around the pole...a configuration supportive of HIGH pressure @ hi-latitude akin to the right-hand side of the negative...cold phase of the AO / NAO.
Today/s LR progs depict a hi-amp ridge over Greenland anchored by the PV to its W and a multi-contour closed LOW to its E.

Click to animate.

Year to Date: Mid-winter

UGH!




What little snow this winter has coughed up has been observed along the extreme northern tier of stations with BTV getting the relative best of it.



YTD '06 compared to YTD '05 is butt ugly...too.

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?