|500 mb height anomalies|
three states of the
CONTEST STATUS - Updated: SUN...20-JUL-14 @ 7:35 PM EDT
Friday, December 30, 2011
Tuesday, December 27, 2011
|GFS: 27-DEC 12z|
VT: 04-JAN-12 12z
|ECMWF: 27-DEC 12z|
VT: 04-JAN-12 12z
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.
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.
Previous update here.
Monday, December 26, 2011
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.
Catalog: Regular Season
Friday, December 23, 2011
Thursday, December 22, 2011
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.
"(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."
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."
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."
Wednesday, December 21, 2011
|E. 165th St - Bronx...NY|
Slow starting seasons aren't at all common but they have happend.
The '06-'07 season didn't start until 12-FEB. Once the storms started rolling tho...they came somewhat fast and maybe a little furious with another one in late February... two during the 1st half of March...and one last pathetic gasp in mid-April.
Back then...a weak el Niño was fading...AO averaged 1.612...PDO averaged 0.06...and +QBO was well on its way to flipping to the east.
The only common factor between the two seasons was the persistently positive AO.
There are signs the AO may flip but that would be several weeks away.
Catalog: Regular Season
Tuesday, December 20, 2011
This three day Height-Latitude Cross Section of Zonally Averaged Zonal Wind loop shows deep-layer easterly winds blowing over the pole...a weakening polar vortex (PV)...and a strengthening sub-tropical jet (STJ)...all of which are favorable ingredients for the Arctic Oscillation to finally go negative.
As a result...the AO has ducked below zero for the first time in what seems like ages.
Whether this trend continues remains to be seen.
Recent daily AO data here.
Contours show zonal wind speed with an interval of 2.5 m/s.
Easterly winds are shaded in warm colors.
Graphics courtesy the Japanese Meteorological Agency (JMA).
Saturday, December 17, 2011
Sunday, December 11, 2011
Last Sunday...the Arctic Oscillation (AO) reached a near-record high of 5.647...or did it?
CPC has revised their data...without comment...for several days between 02-DEC and 08-DEC.
Despite the change...the AO is still damn too high...altho the ECMWF has been signaling the past few runs this may be about to change...albeit not until early January...if the PV breaks in two and a sudden stratospheric warming event develops as suggested by yesterday/s 240hr 100mb prog and the split continues its upward propagation.
Thursday, December 8, 2011
Yesterday/s snowfall over portions of the forecast area came courtesy of a soggy nor'easter with frozen precipitation falling from the mid-level deformartion zone along the NW edge of the comma cloud shield.
These type of systems are rarely contest-worthy and this event was no exception. HPC/s 'Day 1' snow accumulation prog suggested a
high decent probability for plowable snowfall along a narrow corridor threaded neatly in between the forecast stations.
HPC Winter Wx Forecasts
HPC Surface Analysis
NOHRSC Snow Analysis
GOES Image Search Engine
Wednesday, December 7, 2011
White Christmas in the US is defined as 1" on-the-ground on 25-DEC. The criteria are different in other countries.
The probability of an event is determined by dividing the number of times something happened by the number of time it could have happened. If there was a white Christmas five times over 20 years...the probability is 25% in any given year.
This NCDC map is a little dated b/c it's based on the old 1961 - 1990 period of climate normals. Good enough for our purposes.
- A green Christmas; a white Easter.
- If there's thunder during Christmas week...
The Winter will be anything but meek.
- The nearer the New Moon to Christmas Day...the harder the Winter. [ed. - new moon is 24-DEC!]
- If Christmas day be bright and clear
There’ll be two winters in the year.
- Thunder in December presages fine weather.
[ed. - somewhat at odds with 'thunder during Christmas week]
- Like in December like all the year long.
[ed. - does this face forward or backward?]
The NWS has updated their white Christmas probabilities map based on the current 30-year climo period between 1981 and 2010...
Tuesday, December 6, 2011
The latest monthly data from the Rutgers Snow Lab shows the observed areal snow cover over Eurasia was more than one standard deviation above normal during November for the 2nd time in three years. It was the 5th highest coverage during the period of record (1966 - 2011).
Other Novembers where the snow cover was more than one standard devaition above the long term mean occured in 2009...1993...1973...1972...1970...and 1968.
Note the cluster of above normal Novembers between 1968 and 1973. What the 3 out of 4 years had in common were negative PDO...negative QBO...and negative NAO.
Areal snowfall data courtesy Rutgers Snow Lab.
Earlier post about Octobers' Eurasia snow cover here.
Sunday, December 4, 2011
Today's AO reached the 2nd highest value since record keeping began in 1950. The NAO can be thought of as the eastern extension of the AO. These two indexes often rise and fall in tandem but not always.
Medium and short range forecasting of the NAO is difficult...at best. The correlation between NWP model forecasts and observed conditions at ranges of 10 and 14 days are currently 0.596 and 0.447...respectively. The forecasts are able to explain 36% and 20% of the NAO's variability. There's barely any skill at 10 days and none at 14 days.
This predictive limitation often leads to the conclusion the NAO cannot be reliably forecast beyond about seven days at best. The current seven-day correlation coefficient of 0.720 which means NWP explains 52% of the indice's variability.
There may be a predictive a relationship between sea surface temperature anomalies (SSTA) in the northern Atlantic in December and the state of the NAO...as shown below.
- Negative values on the map indicate a negative correlation between SSTA and the NAO state. When SSTAs are negative (positive)...then NAO is positive (negative).
For example...if above (below) normal SSTAs are observed along the west coast of Greenland in the Baffin Bay they are correlated with a negative (positive) NAO state .
The map shown below depicts SSTAs...as of 01-DEC-11.
Also note the above normal SSTAs along the NE CONUS coastline. These anomalies suggest favorable conditions for a positive NAO state.
There/s no clear signal about the future state of the NAO given the current SSTA observations; however...warmer than normal waters along coastal Greenland could contribute to ridge building should the long wave flow regime shift from its current configuration.
An earlier post explored the predictive relationship discoved by the UK MET Office between SSTAs observed in May and the state of the NAO during the following winter.
The daily AO index reached 5.647 today. It/s the 2nd highest value behind the 5.911 observed on 02-FEB-1990.
NWP models show the AO in rapid decline over the short term so the record may stand.
Daily CPC data here.
Saturday, December 3, 2011
The Arctic Oscillation (AO) index continues to stick its thumb in the eye of east coast winter weather enthusiasts by climbing to more than three standard deviations above 'normal' in its 7-day moving average.
Today...on the 1st day of meteorological winter...the AO/s daily value stands at 4.830!
There are 19 other days during the AO/s period-of-record beginning in 1950 where the index has been at least as high.
Hyper-AO appears to favor weakly positive ENSO and weakly negative PDO. QBO swings both ways although there is some preference to its negative state. No apparent association with this year's conditions b/c ENSO is moderately negative.
Just out of idle curiosity...various length running means of the AO index for the first day of meteorological winter (D-J-F) is shown below.
Thursday, December 1, 2011
Welcome back veteran forecasters and welcome aboard to Rookie forecaster Chris Tingus.
Blue (red) shading indicates the 25th (75th) percentile in the above table.
PORN is the Period of Record Normal.
Individual forecasts and summary statistics at the Contest/s web site.
NEWxSFC received a late entry which was submitted in good faith and accepted. The station-by-station forecasts and summary page at the Contest/s web site has been updated.