Winter '11 / '12 - Sudden Stratospheric Warming Watch - Update 4

Tervetuloa Sääbriefing!

Hi-latitude 10 mb temperature well above average.  These warm temperatures extend to near 30 mb.

Source:  CPC/s Stratosphere:  Global Temperature Time Series

NWP models continue the forecast trend for a significant weakening of the polar vortex (PV) and a bold reversal @D+10.   The blue area above 10 mb between 50°N to 90°N latitude (upper right corner) depicts a mean zonal wind flow from the east (-u:  into the board).  The orange area in the troposphere centered above 30°N latitude (lower left corner) is the strong sub-tropical jet blowing from the west (+u:  out of the board).

In the study of dynamic meteorology...the 'u' wind is the zonal component (negative for east / positive for west) of the 'total' wind.  The 'v' wind is the meridional component (positive for north / negative for south) of the 'total' wind.

If the forecast reversal of the PV is observed...the effects of this change would be felt at the surface about three weeks...which portends a very cold February.

Previous update here.

Winter '11 / '12 - Sudden Stratospheric Warming Watch - Update 3

The 'minor' warming event -- indicated by areas shaded in red -- having started on 25-DEC-11...has gained significant latitude as it propagated east from ~140°E longitude over Asia to a position near 80°W latitude over NOAM.

The temperature @10 mb over the North Pole has risen from -80°C in mid-DEC to almost -50°C...as of 02-JAN-12.

The lower stratosphere/s extension of the polar vortex (PV) @100 mb has split in two.  The split is forecast to upwell into the 50 mb level next week as shown in the graphic below.

The upwelling is forecast to accompany a broad...deep column of frigid air imported from Siberia into Canada's Hudson Bay region around D+10.  Of particular interest is the short-wave ridge downstream of the cold pool suggesting the feature may b/come a closed circulation.  The ECMWF 500 mb heights within this feature approach 492 dm....GooFuS ~120 meters deeper.

Finally...the PV continues to lose strength...decreasing to less than 40 kts (20 m/s) by the end of the period.  Also note 1) the mean zonal wind from the east throughout the depth of the polar troposphere...quite suggestive for a developing anti-cyclone and a long awaited return of the Arctic Oscillation to its negative state and 2) the robust 80kt (40 m/s) sub-tropical jet (STJ) INVOF 30°N latitude.

All the ingredients for severe winter weather in the eastern CONUS are forecast to come together during the start of winter's second half.

Previous update here.

Winter '11 / '12 - Arctic Oscillation and Chi Square

Geo. Washington...Valley Forge...PA (1928)

Wes Junker...of the WaPo/s Capital Weather Gang blog...crafted an interesting post last month about January/s temperature and precipitation prospects.  He included a rough analysis on the likelihood of December/s persistently positive Arctic Oscillation (AO) carrying over through January.  The article concluded the chances were high for such an outcome.

One measure of whether two events -- such as the AO for two months -- are independent of each other or whether a strong association exists between them is the chi-square test.  The distribution of observed events (positive one month, negative the next or vice-versa) is compared to the distribution of the 'expected' events.  Small differences between the 'observed' and 'expected' are interpreted as evidence of an association.  Large differences mean they are independent.

No statistical testing was presented in Junker/s post; however...strong evidence for an association does exist...as shown in the table below at the intersection of DEC and JAN.  The probability is 0...to three decimal places...the association between  December/s and January/s AO state occurs by chance.

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What about the other 'cool season' months?  Can more predictive information be mined from these data?  Is there a statistically significant association between the AO state lurking elsewhere?

Monthly AO data between 1950 and 2010 from CPC were analyzed using the 'chi-square' statistic to determine if there were statistically significant associations between months.

There is a fairly strong association between SEP/s AO state and that of FEB (a 5.5% probability the association occurs by chance) and a good association between JAN and FEB (8% probability)

What does AO state for SEP-11 'predict' for FEB-12?  The SEP-11 AO was 0.665...so FEB-12 'should' follow suit and be positive.

What does AO state for JAN-12 'predict' for FEB-12?  The JAN-12 AO is unknown but since JAN follows DEC....JAN-12 AO 'should' also follow suit...which implies FEB-12 will be positive.

Winter's where the AO ran hot between DEC and FEB have occurred nine times since 1950 (1972/3...1974/5...1975/6...1988/9...1991/2...1992/3...1998/9...1999/00...2007/08).

Not lookin'good for the home-team...altho all may not be lost this winter.

The so called 'Storm of the Century' roared out of the GOM and tore up the east coast in MAR-93 after +AO in D-J-F (1.627; 3.495; 0.184) and MAR was 0.764!  There was a 'notable' NESIS storm in MAR-99 and the unforgettably infamous JAN-00 storm.

Winter '11 / '12 - Meteorological Winter - Month One

One down.
Two to go...altho the Contest does run until the storms stop -- assuming they do get started.

As bad as it/s been for areal snow cover over the forecast area so far this winter...DEC-03 was worse with DEC-06 a close second.

December/s end-of-month snow cover between 2002 (upper left panel) and 2011 (lower right panel).  Images courtesy NOHRSC.

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Disappointed snow crows blame the villainous duo of la Nina...which favors a storm track west of the Appalachian mountains...and the persistently positive state of the Arctic Oscillation from a paucity of hi-latitude blocking.

The above table -- sorted by descending 'percent of climo' -- shows December snowfall totals for the 'season-total' forecast contest stations.  Trace amounts are reported as 0.05.  Green (red) cells are the top (bottom) 25th percentile of values in each column.

Not a single station is even close to normal snow...altho the distribution is typical for a la Nina winter.  December's total 'normal' snowfall is 214".  The month's observed snowfall is 44", a deficit of 169" and a mere 21% of normal.

Winter '11 / '12 - Arctic Oscillation - Day 30

500 mb height anomalies
associated with
three states of the
Arctic Oscillation

Thirty days into the season...the Arctic Oscillation (AO) remains well above zero despite a weakening polar vortex (PV) and hi-latitude easterly winds.

The 500 mb height anomalies (below right) have looked more like the top chart which has kept arctic cold away from the mid-latitudes.  The ECMWF continues to depict a weak anti-cyclone building over the Pole; however the on-set is always at least seven days away. 

Today/s 500 mb height anomalies

The minor stratospheric warming event @10 mb has contributed to a weakened PV but is occurring well away from the pole INVOF northeast Asia and has not propagated below 30 mb.

Minor warmings do not cause the PV to break down or cause a wind reversal from west to east so it's debatable how much influence this event will have on establishing hi-latitude blocking.  Best guess:  very little.

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Below is a time-series of the AO on 30-DEC throughout the 60+ year period of record (1950 - 2011).  The blue line is a trace of the daily AO index on 30-DEC.  The dark red line is a a trace of the same data after applying a nine-point binomial filter.  The filter removes low frequency noise and highlights decadal trends.

Not sure what this says about the AO at the end of December other than there is a decernable cycle every seven years or so.

Winter '11 / '12 - Coastal Teaser #1

Bit of a disagreement between us and them about next Wednesday/s weather along the eastern CONUS...

GFS:  27-DEC  12z
VT:  04-JAN-12 12z

ECMWF:  27-DEC  12z
VT:  04-JAN-12 12z

Both models were in fair agreement in the short range but then diverged as the ECMWF deepened a GOM Low which popped a high amplitude PNA-type ridge.  OTOH...GFS did not close the door to the flood of PAC air into the western CONUS and maintained a hi-index flow regime across the lower 48.

Suspect GooFuS will catch up eventually.

POSTSCRIPT:  The 12z Canadian and ECMWF are in excellent agreement through 144 hours in their depiction of a full latitude trof.

Winter '11 / '12 - Sudden Stratospheric Warming Watch - Update 2

It's official!

A 'minor' stratospheric warming...no longer a figment of NWP/s imagination...was observed at 10 mb by the Japanese Meteorological Agency (JMA) for the seven-day period ending 25-DEC-11.

A minor warming event is defined as an observed 25°C temperature increase over "... a week or less at any stratospheric level in any area of winter time hemisphere. The polar vortex is not broken down and the wind reversal from westerly...to easterly...is less extensive." ¹

The heavy red shadings denote the 25°C threshold.
The image is approximately oriented north-south along 140°E longitude.
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Previous update here.

Winter '11 / '12 - Late December Snow

Another storm manages to thread the needle and slip by the forecast stations.  This winter is only the second season where NEWxSFC did not have a contest-worthy storm in December.  Every winter... with the notable exception of '06 / '07...had contest storms during January...so the odds are favorable for this season.

Small glimmers of hope have been showing up recently in the extended range...especially the ECMWF...as the progs depict a highly amplified ridge-West / trof-East flow regime developing across the CONUS.

Whatever evolves eventually from the weak warming in the stratosphere would likely take several weeks to affect the lower troposphere.

Winter '11 / '12 - VCP32 - The Day After Christmas

Winter '11 / '12 - Sudden Stratospheric Warming Watch

A sudden stratospheric warming (SSW) event appears to be getting underway this week.

NWP output from the ECMWF continues to show an expectation for significant weakening of the stratosphere's polar vortex  (PV) by year's end.  The PV's west winds...having reached a maximum speed ~210 kts (110 m/s) a few days ago...are forecast to fall off a fookin'cliff and decrease to ~50 kts (25 m/s) by the end of the period.  Diminishing PV wind speeds can lead to a reversal in flow where east winds become dominant.  These are highly favorable conditions for a negative state of the Arctic Oscillation (AO) to develop if they persist and propagate into the troposphere.

Also note the increasing intensity of the sub-tropical jet (STJ) INVOF 30°N @200 mb.

The decay in the PV's intensity comes courtesy of warming in the atmosphere's highest level millibars between the pole and the equator.  The warming all but eliminates the strong temperature gradient which in turn causes the wind to diminish.

The American Meteorological Society's 'Glossary of Meteorology' defines stratospheric warming...also called sudden warming...as...

"(a) rise in temperature of the stratosphere in the polar region in late winter resulting from enhanced propagation of energy from the troposphere by planetary-scale waves."

From Wiki...

A major warming is defined as "...westerly winds at 60°N and 10 mb...become easterly [...].  A complete disruption of the polar vortex is observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole."  

The D+10 ECMWF panel shown above depicts east winds ~20 kts in the layer just above 10 mb.

From Wiki...

A minor warming is defined as "...the westerly...winds are slowed; however... (they) do not reverse. [...] a breakdown of the vortex is never observed."

An alternate definition of a minor warming considers an observed temperature change of "...25°C...in a...week or less at any stratospheric level in any area of winter time hemisphere. The polar vortex is not broken down and the wind reversal from westerly...to easterly...is less extensive."

This is excellent news for anyone waiting for winter to get underway in the eastern CONUS.