Friday, December 26, 2014

January 3-5 Potential Winter Storm

It's looking like the next threat of a winter storm will arrive in time for the first few days of the New Year.

On December 31st, a strong trough is expected to drop into the West US and close off, as the circular 500mb geopotential height contours show above. This cyclone closing off in the West slowly drifts eastward, and that's the first thing we must watch for. A common model bias involves guidance closing off these troughs too quickly. For those located in the Plains and Midwest hoping for a snowstorm, you want this trough to close off quickly. A later close-off of the trough would result in a further south track, which might be in the cards of this is an instance of that model bias. For now, however, the trend has been to close off the energy earlier and earlier.

Tropical Tidbits
This image shows the projected 500mb vorticity values for January 3rd, the same type of chart as above. Off of the latest GFS model projection, we can see the energy now located in Indiana, taking on a negative tilt, as the contours seem to be "pushing" in a southeast-ward direction. When the system initially exits the Southwest, it will have a positive tilt. In order for those north of Kentucky to have a shot at a good snowstorm, this storm needs to attain a negative tilt rather quickly, as the GFS has done above.

Tropical Tidbits
The 500mb geopotential height anomaly chart off the ECMWF, valid for January 3rd, shows a similar situation as the GFS is predicting. We see our energy in southern Minnesota and northern Arkansas attaining a negative tilt. Although I don't have access to the smaller-interval charts from the ECMWF, it's not a stretch to assume this storm takes a similar path as the GFS model, as well as the Canadian GGEM model (not shown).

There are a few, rather significant items standing in the way of this being a 'likely' winter storm for the Midwest.

Tropical Tidbits
The image above shows the projection for 500mb geopotential height values (colored regions) and MSLP values (contour lines) for December 29th over the West Pacific. We see a low pressure system has formed on the southeastern coast of Japan, and is moving east-northeastward along the coast. As time progresses, this low will eventually shift out to sea and away from Japan.

Our Christmas Eve storm was expressed well by this Typhoon Rule application, and this storm looks rather similar to the track the storm took in Japan preceding the Christmas Eve event. The only difference here is this time around, the low pressure system drifts out to sea instead of cutting up the eastern coast of Japan. This is a huge red flag, and tells us that this storm may very well go further south and east than current model guidance is suggesting. Given the success this predictor has had in the past few years I've used it, there is reason to believe this system could go further south and east.

Tropical Tidbits
Also of concern is that massive Arctic high pressure system to the north in Canada, pressing southward into the United States. It is well known that model guidance has a bias to be too slow with the progression of Arctic high pressure systems southward. Applying this to our storm here, this means that model guidance might be forecasting this low pressure system to be too far north. Consequentially, this would result in the storm going further south and east.

When you shed all the more 'uncertain' parts to this forecast, you end up with a model consensus favoring a northward track, and the Typhoon Rule in favor of a southward track. Usually, I would go in favor of the Typhoon Rule and predict a further south track, but we have a strong streak of mid-level winds rounding the base of the trough on the latest GFS, which is a big signal for a storm to go negative-tilt, and in this case, go north. Add up all the model biases and predicted phasing of the subtropical & polar jet streams for this event, as well as a suspicious timeframe for the system in Japan, and there's too much uncertainty to decide on one particular track at this time.

To summarize:

- There is the potential for a winter storm to impact the Central and Eastern United States between January 3-5 (good confidence).
- There is too much uncertainty to tell whether this storm will hit the Midwest and Great Lakes, or stay south in the southern Ohio Valley.
- With good Arctic air already in place prior to this storm, accumulating snowfall is a good bet at this time.


January to Commence With Wintry Central US, Balmy East

The pattern to kick off January 2015 is looking cold for the central section of the country, while the East looks to bask in warmth.

Tropical Tidbits
The above image shows 500mb geopotential height anomalies, valid for December 28th, when we expect this pattern to start establishing itself. The atmosphere is quickly turning very La Nina-like, as we see atmospheric angular momentum values begin to circulate in anomalously low regions. This La Nina-like set-up will kick off with a strong ridge forming in the Gulf of Alaska, the primary indicator of a negative East Pacific Oscillation (EPO) pattern, which favors cold weather in the United States. Energy coming down the pipe from the Pacific will ride the jet stream and crash into the West US, setting up a nightmare scenario for the East US' cold weather fans.

Shown above is the typical jet stream alignment for a La Nina pattern. Notice the aforementioned ridging in the Gulf of Alaska, as well as the polar jet stream arcing over that ridge and then pushing southward into the West. Due to all the energy crashing into the West, we will see a ridge form over the East US, shown by the push north in the polar jet stream over the eastern CONUS in the above image. What ends up happening is a dream scenario for snowstorm fans in the Central US, and a nightmare scenario for all winter weather fans in the East US.

Energy that does crash into the West US will have the potential to become a substantial winter storm in the Plains and Midwest. This is due to what looks like phasing of the jet streams over the Central US. Notice how the polar jet stream shifts south from Canada and then mixes in with that other jet stream coming in from Mexico. This phenomenon supports potential strengthening of any storm systems that form and maintain their structure as they eject into the Plains, eventually progressing east-northeast into the Ohio Valley.

To summarize:

- A pattern change is currently in the works, and is expected to take hold in just a couple of days.
- This new pattern resembles a La Nina pattern.
- This pattern will favor cold and possibly snow in the Plains and Great Lakes, as well as very wet conditions in the West.
- Warmth and relative quiet will prevail to end 2014 and start the month of January in the East US.