Descending stratospheric warming

The major stratospheric warming that began in early January continues to descend toward the tropopause. Notice the two nodes in the temperature anomalies ... the first at the initial warming, near 2mb, the second around 5 days later around 20mb. Now we have a third and final node in development around 50 to 70mb.

What does this mean for the Northern Hemisphere? There could be a renewed period of easterly anomalies imposed on the background state of the polar vortex. At this time, the AO has recovered back toward neutral after dipping below -2SD ... well below 7-day model forecasts. Models have the AO continuing to rise toward +1SD by February 1st, however this latest node in the warming may to some extent suppress/delay the resurrection of  the polar vortex over the Arctic.

Increased storminess for last week of January

Stuck in northwest flow for the next week, any current weather discussion here at OU has not been able to escape the use of the word "boring". However, a change is gonna come.

One of the distinct characteristics that I'm seeing of the pattern over the next week is that within this wave system, the sub-polar vortex over eastern Canada and the Northeast overwhelms the ridging over western North America. I compared it to RKW theory and the balance of vorticity in the system ... In this case the stronger trough leads to the eastern edge of the ridge slowly getting broken down ... can watch packets of anticyclonic vorticity erode away and get sucked underneath the trough

This is also why I have been very cautious about getting excited about any storm prospects for the East. Essentially we have a smooth long wave trough with periodic impulses of stronger flow consequence of s/w ridging swinging south of the trough and increasing mass gradients ... NOT conducive to amplifying disturbances for the East.

Anyway, back to the point -- this is NOT a stable pattern. Eventually, all the anticyclonic vorticity with the western ridging will get eroded southeastward ... probably by the end of next week as indicated by the long range models.

The immediate result will be a warm up for most of the Plains for a day or two.

The second result will be opening up the CONUS to Pacific disturbances ... with the blocking ridge now out of the picture.

IMPORTANT -- All the while, we still have the sub-polar vortex sitting off the northeast, over Newfoundland and the North Atlantic ... the effect being persistent background upper level confluence over the Great Lakes, Northeast, and eastern Canada.

I could see the first in the series of disturbances off the Pacific being a significant storm for the Ohio Valley to Northeast next weekend as some ridging still stands in the West.

However, as the pattern shifts to more of an RNA configuration by the following week, the next s/w could have a chance to dig earlier into the central Plains ... then remember the confluence to the northeast to keep the short wave from breaking north and maintain upper level dynamics ... could be a significant winter storm for someone to the north and perhaps a thunderstorm/rain threat for someone to the south.

Anchorage Wind Event Poster

January 15th Pacific Bombogenesis

From Ryan Maue:

I used the full-resolution (T574) surface pressure grids from NCEP GFS for the data points:  

1000 mb ... (13/12z)
994 mb  ... (13/18z)
989 mb  ... (14/00z)
978 mb  ... (14/06z)
960 mb  ... (14/12z)
946 mb  ... (14/18z) 
940 mb  ... (15/00z)

Check out the DT pressure map I attached below. You'll never see a better signature of a tropopause fold. And obviously all sorts of upper level support with massive divergence aloft.

Pressure Gradient Breakthrough

I decided to investigate the Cordova-Anchorage pressure gradient a little more given the relatively weak correlation compared to the intuitively strong association, as well as the lag that exists between strong wind events and the time of the maximum gradient.

I realized that the change in the gradient over time might have a better physical connection as it would account for the isallobaric wind. Without pulling any new data, I just used the gradient at the time of the report and the maximum gradient to determine the change over time. I plan on trying to analyze the pressure gradient change centered around or just prior to the wind report next.

The result was a 0.56 correlation to the wind magnitude. This was a notable increase from the maximum gradient correlation. More importantly, when replaced in the empirical formula, it makes a significant difference.

In addition, I have rerun the pressure gradient program to be able to pull data from one event that was missing from the initial database. This is the weakest case, and thus an especially important case.

First of all, it actually reverses the dominant group ... the pressure correlation is 0.61 for the wet class and 0.54 for the dry class. The new correlation for the bestfit values is 0.84 overall, which is excellent ... 0.85 for the wet class, and 0.83 for the dry class. This is a huge improvement. Case-by-case, this also removes the biggest outlier! There are two cases with error greater than 10kt. The average error is 4.4kt or 6.0%

Splitting cases into wet and dry events

Following the information I found from the mid level (3.5km) dew point, I split the cases into wet and dry events, using -30C as a benchmark.

I reanalyzed all of the variables in the empirical formula given the class of the event (wet/dry), for a new average, standard deviation, and correlation to the actual data.

In addition, I added the dew point variable itself into the formula given that especially the dry cases show a strong correlation to the wind magnitude (drier mid levels = stronger winds).

The latest formula incorporates the following variables:

• Cordova-Anchorage SLP gradient
• Depth of the mid-level temperature inversion
• 2.5-5km shear magnitude
• Change in low-level cross-barrier flow over time
• 3km temperature
• 3.5km dew point

The results are very good. Of most significance, the new formula reduces the error of the outliers.

For the wet cases: Correlation = 0.73, Error = 5.0kt, 6.7%

For the dry cases: Correlation = 0.83, Error = 3.9kt, 5.2%

Overall: Correlation = 0.75, Error = 4.6kt, 6.1%

The 0.83 correlation for the dry cases is excellent.

Dew point

Examining the vertical profile of dew point in the Anchorage soundings, there is significantly greater variance than with temperature. The most interesting feature in the composites is between 3000m and 4000m where there is a dry layer in the strong cases and a relatively more moist layer in the weak cases.

I plotted 3500m dew point against the magnitude of the wind event. The huge variance is clear but what stands out the most is that there are two very distinct groups of dew point values. There is a moist group and a dry group. I went ahead and split the data accordingly (color coded on the graph), and analyzed them separately. For both there is a negative correlation between dew point and the wind events ... drier mid levels are conducive to stronger wind events. The correlation is much greater for the dry events (-0.40) then the moist events (-0.22).

Together, there is actually very little correlation between the dew point and wind magnitude, as both the moist and dry classes span a wide spectrum of wind speeds. It's when they're split that we can better identify possible relationships.

This poses the question: Should all events be initially identified as dry or moist before any further analysis be done? Next plan is to redo some of the previous analysis split into these two categories.

Below I have included sounding composites for the dry (red) class and moist (blue) class. These classes also maintain themselves through the 12-24 hour period leading up to the event. So for instance a dry event in this classification maintains a dry profile during the time of the event itself.