Weekend Severe Possibilities

The big consensus among model guidance at this point is that a piece of energy from the trough over the Northwest Pacific will jump to the Gulf of Alaska by Wednesday morning, then drop southeastward into California Thursday night into Friday.

Similar to the last event, there is a weak disturbance in the southern stream that gets ejected out ahead of the trough dropping into the Southwest ... and able to lift northeastward as the polar vortex over southeast Canada retreats poleward. So the result is some moisture return ahead of the disturbance Wednesday and Thursday ... maybe some rain too, but let's not get carried away.

The real differences between the GFS and CMC and ECMWF is with the handling of the individual impulses within the western trough. CMC and ECMWF like swinging the lead impulse into eastern Colorado Saturday afternoon. Amplitude, tilt, and timing of this particular piece of energy will be crucial. At this point, the projected evolution brings a warm front through Oklahoma on Saturday with round 1 of thunderstorm potential, and a surface low developing in eastern CO.

The ECMWF and CMC deepen this surface low into western Kansas Saturday night ... really pretty awesome placement for OK. Euro increases CAPE substantially Saturday night. The cold front catches the dryline sometime Saturday night into Sunday morning for round 2 ... so timing is an issue, but verbatim this would no doubt be a significant overnight event.

The trough continues eastward with that primary upper level impulse sharpening over the southern Plains, and the surface low intensifying across the central Plains to Midwest, introducing severe threats for Sunday and Monday for the Dixie Alley area.

Below I've included images from the GFS leading up to the weekend. The first image is a hysplit trajectory model output indicating the moist source of the boundary layer airmass, and the warm/dry source of the airmass aloft, providing a cap ... which will be important as the stronger upper level dynamics move into the southern Plains Saturday night into Sunday.

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.