Forecast Credibility

Wx web logs...forums...and message boards were all aflame yesterday with full-throated forecasts for a 'significant' snowfall in advance of the winter storm coming out of the GOM this evening.

Amateurs and pros alike hoisted 48-60 hour forecasts for wide swaths of 4-8" (meets and exceeds NWS warning criteria for heavy snow)...pockets of 6-12"...and 8+".

In light of more recent information...these collectively absurd forecasts have little chance of verifying.

Back in the day...forecasters knew better than to post snowfall forecasts with such long lead times. And they knew better not to segue the MR with the SR. Details portrayed in the the MR are pure fantasy. Timing...intensity...centers of circulation depicted in the MR are purely guidance of impending events. Same is often true about the SR progs...but they are much closer to the truth.

The lead time for a snowfall forecast with the best chance of verifying should be issued no more than 24 hours before the event. The current state of NWP is such that timing...intensity...and centers of circulation is too crude to attempt a forecast at such an extended time range.

A forecaster/s credibility needs to be preserved if they are to be taken seriously. Issuing a snowfall forecast more than two days before the first flakes fly is frought with danger and should be always be avoided.

FWIW - Verification of NCEP Week2 5H Z Anoms

Remember this?


It/s NCEP/s Ensemble Mean forecast for 500 mb height anomalies VT 1/26/2007...issued January 12. So...how good were the action center predictions?

500 mb height anomalies

Negative height anomalies...
...over NE CONUS verified farther N and E over the CN Maritimes
...over SW CONUS had minor displacement error N and W
...along and W of the IDL verified along IDL but farther E over the Aluetians
...over Greenland missed the NAO ridge.

Positive height anomalies...
...NW CN verified spot on
...ATL along ~40°N missed broad trof from CN Maritimes to NW Africa.

Good skill with PNA-type ridge-W...but completely missed the NAO dipole.

500 mb heights

Coastal Teaser #4

Click to animate. Loop speed = 1 frame / 3 sec

Wave forms on tail end of strong baroclinic zone courtesy arctic frontal boundary across the GOM. Dynamics from hot STJ provides the UVM.

Forecast problem is whether the storm will chug harmlessly out to sea or be held closer to the coast from the fx of a shortening wavelength.

Trying to forecast the timing and intensity of the numerous short-waves rotating around the PV is like playing roulette...even at shorter time ranges.

Sooner or later...something good has to come from the combination of the ern 2/3 of the CONUS flooded and continualy refreshed with arctic air and a hi-amp...short wavelength pattern.

Japan Also Waits for -AO

Japan is on the verge of breaking a long-standing record for latest snowfall.

The current record was set in 1876...when winter/s first snowfall was observed on February 10.

Like much of the CONUS...Japan has experienced a relatively mild winter. The anomalous warmth has been attributed to the persistently positive phase of the AO and ENSO.

Cold air is contained @ hi-latitude during +AO. +ENSO keeps HIGH pressure anchored off Japan/s SE coast in a fashion similar to a La Nina ridge over the SE CONUS.

Resurgent SOI

(Updated below)

Latest 30-day moving average of the SOI is -9.7.

The last time the SOI was that low was the 30-day period ending November 15.

Update:
The -9.7 SOI value is in the top 40% of all negative values since the index went negative last May. It/s also in the top third since the +ENSO began, base on NOAA/s definition (>= 0.5°C), in late September.

FWIW - Week2 5H Z Anoms

Week2 500 mb height anomalies from the PSD, ECMEF, and NCEP (VT 2/9/06).


Good agreement wrt Ridge-W / Trof E over CONUS...but does it foreshadow any snow?

Some support for -AO tripole with above normal heights over the Pole and below normal heights INVOF Aluetians. Height anomalies in the ern ATL don't fit the pattern...so it/s two out of three.

CPC index forecast from OP GooFuS (blue bars) and the Ensembles (yellow band) suggest at AO will be least one standard deviation below the mean; however, the coefficient of determination (R²) for Op (26%) and Ensemble (21%) are meager. Less than 30% of the AO/s variability is forecast at D+14 by the model. The AO loading pattern can been seen here.



One of the
NAO/s dipoles finds above normal heights over Greenland @ D+14 ...although not a strong action center. Not much of a weakness across the ATL Ocean...either. CPC/s NAO forecast has the index near zero @ VT. The Op (Ensemble) D+14 forecast has an R² of 21% (7%). Numbers like those don/t lend themselves to awe inspiring confidence.

Note the current state of the NAO; strongly negative which goes along way in explaining why there has been so few EC snowstorms to date. The UA flow regime responsible for the intrusions of arctic air is too strong to support nearshore EC cyclogenesis. The winter storms are out there, it/s just that they/re too far out there...going through their explosive development well offshore.



The PNA dipole of above normal heights over SW CN and below normal heights INVOF the Aluetian Is. is well supported by the three models. PNA index is not particulariliy high -- less than one standard deviation above the mean -- but ridge-W is a 'typical' mid-winter feature. The D+14 forecast for slightly above normal PNA suggests a continuation of arctic airmasses invading the lower 48.



The OP GooFuS captures only 19% of the PNA/s variability; however...the Ensembles are much better (~60%) at getting the magnitude and direction correct.

The current D+14 forecasts has the AO negative to strongly negative..the NAO neutral...and the PNA weakly positive. A recent four-part study of past NEWxSFC storms suggested the best snowfalls...on average...occurred when the AO was positive or when the AO and PNA were both positive. The lowest snowfalls occurred when the AO was negative and the NAO positive.

The present forecast combination of -AO and +PNA @ D+14 has not produced the best Contest snowfalls. Instead of big snows in the E by the end of Week2...what may be more likely is a continuation of arctic cold and arctic dry.

Coastal Teaser #3

Most promising MR prog in weeks!

Today/s 12z ECMWF coughs up a long overdue Miller A storm for the EC that has a lot going for it. Even though the chart is not 'perfect,' that/s a good thing at this time range. The last thing you want is perfection in the MR b/c the VT analysis won/t look anything like the 216 HR chart.

Nonetheless...there/s little doubt about the antecedent environment across the ern CONUS that will feature plenty-o-arctic air and a highly amped UA flow regime with short wavelength.

Locked & Loaded

A four Rossby-wave configuration is stationary...meaning once established...it/s not going anywhere...anytime soon. Arctic cold in the E has arrived and the ECMWF/s D+10 prog suggests it/s inclined to take its shoes off and sit a'spell.

In the run-up to D+10...storms are advertised to continue their explosions over the open waters of the wrn ATL due in large part to the rapid succession of upstream kickers. The interval btwn these nrn stream short waves is too short to allow any one of them to dig in its heels over land; however...


...the ECMWF/s D+7 prog shows an important break in the wave train interval that would allow a different scenario to evolve over the EC during the first week of February...just in time for the full Snow Moon.

As the short wave climbs the AK ridge...the ridge amplifies. A second short wave has already rounded the crest and is poised to descend into the LW trof on a direct-deposit trajectory toward the wrn GOM. Unlike previous short waves that are being abruptly kicked downstream and subsequently well offshore into the wrn ATL...this one has ample time and opportunity to dig deep into the base of the full-latitude trof b/c the upstream short wave is still ascending.

The increasing amplitude of the AK ridge has the downstream effect of shortening the wavelength over CONUS...which would allow cyclogenesis to occur much closer to the EC than earlier cyclone deepenings.

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?

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.