Saturday, December 6, 2014

December 31 - January 5 Potential East Coast Winter Storm

I'm watching for the risk of a storm system, possibly associated with wintry weather given the pattern, along the East Coast on December 31 - January 5.

Though we can't see out to January 5th, we can diagnose the weather pattern leading up to this timeframe.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA and NAO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The PNA looks to remain positive throughout the entire forecast period, even rising into well above normal territory towards the end of December. This bodes well for a pattern permitting the entrance of cold into the Central and East United States. The positive NAO, an enemy to these East Cost storms, looks to drop to neutral, and possibly below normal with time as we head to the final days of December. We'll need the NAO to get to its negative state for this storm to even have a chance to hit the region.
Also with time, the West Pacific Oscillation and East Pacific Oscillation (WPO, EPO) look to sink from their positive states to negative states. When the EPO and WPO are negative, the risk for cold weather in the eastern two-thirds of the nation rises substantially. As we saw last winter, a sustained negative EPO can lead to some very cold weather.

All in all, the general weather pattern will gradually become more favorable for an East US winter storm as December draws to a close. Let's keep pushing ahead to see what we can find.

The above image shows the Madden-Julian Oscillation (MJO) forecast from the European model suite (ECMWF), valid from December 6th to December 20th. This chart is complicated for some, so I'll try to break it down. The MJO has eight phases; each phase indicates a different location of enhanced tropical convection/thunderstorms, and those locations for each phase are shown on the perimeter of this diagram. Long story short, when the MJO is in phases 8, 1 and 2 for the winter months, the risk of cold weather rises. Similarly, the MJO in phases 4, 5 and 6 bodes well for warm weather.

The ECMWF suite shows the MJO hitting one phase in particular before moving to the circle in the middle of the image, meaning the MJO is too weak to affect the weather pattern. Can you figure out what phase it is? If you guessed Phase 7, you're correct!

Nicholas Schiraldi
This graphic shows 500mb height anomalies on a time-by-longitude map, averaged out between 40N and 55N latitude. The legend on the left indicates the number of days before (negative numbers) and number of days after (positive numbers) the MJO hits Phase 7. For an example of how to use this map, we might say the region by the 30-W meridian will experience a stormy period about 15 days after the MJO hits Phase 7, due to the deep blue colors in that timeframe and longitude.

Applying this to our situation, if we look up at the MJO phase space diagram above, we see that the ECMWF has us entering MJO Phase 7 in four days, since there are four black dots from the last observation of the MJO to the time when it hits Phase 7. That would put us on December 10th.

Look back for a moment at this composite map immediately above. If you look between the 90W and 60W meridians (about 75W), and move your eyes up to the +25 day level, you'll see a diagonally-moving swath of below normal height anomalies, eventually strengthening quickly at that 75W level. If you're still have trouble understanding, this is saying about 25 days after the MJO strikes Phase 7, a storm system may trek across the United States, and rapidly strengthen when it hits 75W. 75W is located on the East Coast.

Putting all the pieces together, 25 days after December 10th puts us on January 4th, give or take a day. At that time, the pattern may favor a strong storm around the 75 west meridian/East Coast of the US.

So, we now have the MJO supportive of an East Coast storm in early January, with an increasingly-favorable pattern closing out December. But the evidence doesn't stop there...

Tropical Tidbits
The image above shows the forecasted 500mb height values (colored shadings) and mean sea level pressure contours for December 14th in the West Pacific and Bering Sea. Notice a strong storm system crossing into the northern Pacific on this date, with a minimum central pressure of 963 millibars. If you've read this blog in the last month, or follow Joe Renken, you're probably familiar with the Bering Sea Rule. The Bering Sea Rule (BSR) states that weather phenomena occurring in the Bering Sea (typically around Shemya, AK)  correlates to similar weather phenomena here in the US 17-21 days later. Shemya's rough location is in the red circle, and we see this projected storm system just east of that circle.
Work done by Renken suggests Shemya correlates to a location in the Missouri area (I can't remember off the top of my head exactly where), but a storm east of Shemya might suggest the storm appears about 3 weeks later in the Ohio Valley, or even in the East Coast.
As luck (or a little something more?) would have it, extending this forecast graphic's December 14th date out 17-21 days puts us at a potential winter storm in the East US around December 31-January 4th. How convenient!

But that's not the end of the evidence stream just yet...

Those of you on my Facebook and Twitter pages may have taken notice of a new analog system I developed recently, which I then began discussing the other day on the blog. As I had discussed, the system has its good and bad moments, but generally has good success rates from the sample trials I've conducted.

After seeing that there may be the threat for a storm in this timeframe, I decided to see what the analogs were showing for this same timeframe, just for fun.

The image above shows mean sea level pressure values averaged out from all analog years used in this weekly forecasting method. These analogs are centered on January 4th of their respective winters, around the tail end of our December 31 through January 5 forecast timeframe. Notice the swath of blues located in where else but the East Coast, indicating the presence of a storm system.

Precipitation rate anomalies centered on January 4th from the same analogs shows precipitation falling across much of the Eastern Seaboard, with more still falling in the days prior when the storm moved northward from the Southeast region.
I'm still working on these analogs to bump up accuracy rates, and this is by no means correct. Regardless, it is impressive to see the BSR, Analogs, MJO, and teleconnections combine to suggest a potentially active period for the start of January 2015.

To summarize:

- The atmosphere appears to be gradually becoming more favorable for a colder, more active period to close December and start January 2015.
- Analogs and composites suggest this storm threat is definitely a possibility, primarily for the East US.
- Confidence remains very low due to the long range nature of this event.