Saturday, January 28, 2012

Thunderstorm Outlook for January 29, 2012

A clipper system will quickly move across the Plains through the Lower Great Lakes, putting down some light snows. Tomorrow, on Sunday, the low pressure system should move into the Eastern Great Lakes and produce some showers and thunderstorms along a cold front. This cold front will turn into a weak bowing segment. For that reason. . .have decided to put in the potential for gusty winds and small hail, as bowing segments are typically conductive for some gusty winds.
The threat should progress overnight into parts of the Northeast. However. . .confidence is lower as the model we are using (WRF) does not cover that timeframe.

ECMWF Projecting Ridge to Replace Alaskan Vortex

Click for larger image
0z ECMWF Hour 240 500mb height anomalies
The 0z ECMWF at hour 240 is projecting a very strong ridge to be drifting northeast from the US and possibly into Alaska. From there, things get interesting. If the ridge propels the vortex southeast, where it is currently going, the big low could possibly move into the US. While that is a very long (if not impossible) shot, seeing the direction of the ridge and low system brought up that potential in my mind.
Either way, it looks like there are signs of a +NAO, -EPO in this image.
12z ECMWF Hour 240 500mb height anomalies
The 12z ECMWF has the ridge moving even further towards Alaska, with the trough weaker but still present. The interesting thing is how the trough starts appearing to be trying to swing under the ridge, which would theoretically bring the trough down into the US. Additionally, the 12z ECMWF has a stronger -EPO and a -NAO.

Preliminary Severe Weather Forecast for Spring 2012

"This will be one to remember."
Last Spring, we saw the absolutely horrible April super outbreak of tornadoes, and this year, it looks like a similar situation may be setting up.
Let's start with the Gulf of Mexico.

Caribbean Temperature Anomalies
The Gulf of Mexico is at least 1 degree above normal in many spots. While that may seem like nothing, it actually may have huge implications. You see, the Gulf of Mexico (GOM) is supposed to cool down over the winter from Arctic Air coming south. However, as we have seen, this winter has been very warm. We have already seen tornadoes with fatalities this year. I see that as a very bad precursor indication.
If the GOM, by some chance, is somehow able to stay as warm as it is (which is unlikely given my FEB-MAR outlook posted today at 12:00 PM CST on the day of this post's publishing), the situation could get worse. But if we start seeing some more cold and snow, the GOM may cool down a bit. I think it is too late for the GOM to cool down to normal levels for this time of year given how far we are into 'winter'. With this added GOM warmth, expect to see more moisture and instability associated with storms drawing energy from the Gulf this spring. Additional warmth may also be present, making for a bigger temperature gradient between the cold fronts that produce severe storms in the country this spring, therefore making the storms stronger.

Typical Jet Stream in the Spring
The jet stream in the spring usually factors out to hot weather in the South, wet weather in the Northwest, and cooler weather in the northern tier of the country. The area where the severe storms usually form is in the Plains through the Midwest. As the arrows show the warm, moist air being drawn up, this is the problem I mentioned earlier with more moisture and instability that could make for more severe weather. That added moisture/instability being slammed by the cold, dry air would certainly make for a very robust situation if everything went 'good' for severe storms.
Here's our very preliminary forecast. Keep in mind this is not set in stone at all and has a lot of changes that will need to be made in the long term.

February 2-4 Potential Snow Event (Issued 1-28-12)

This is hour 156 of today's 12z GFS Ensembles. This is the timeframe we are looking at for a potential snowstorm in the East US. As we can see, there are two camps: One that brings all precip north into the Midwest (North Camp) and one that has the most precipitation down south in this page (South Camp). For a simple reference, let's count out how many ensemble are in which camp.
(Out of 12 ensembles)

NORTH CAMP: 8 ensembles (67%)
SOUTH CAMP: 4 ensembles (33%)

Now, of course, many things will change- that was simply for just a glance at it.
Something that makes me think about this storm is checking out the teleconnections.
Here's the PNA forecast.
Most Recent PNA Forecasts
Positive PNA Phase
In a positive phase of the PNA, a ridge sets up over the West US, and the storm track is diverted into the Southwest and then either into the Midwest or Northeast. The forecasts for the PNA in this timeframe (Feb. 2-4) are a strong positive, meaning that the effects of the +PNA will be more pronounced. This is shown in many of the ensembles of the 12z GFS. You can see the +PNA by tracing the red rain/snow line through Canada in the West US then diving back south into the East US.

Something else we can use to check out the storm is the NAO and AO. In the positive phase of the NAO, we see warm temperatures across the East US. The big snowstorms usually fall during a negative NAO. In the ensembles, we have checked and see a -NAO developing on the forecasts. This would bring the biggest snows to the Northeast, theoretically.

The AO is forecast to be negative during this timeframe, which would supply the cold air as well as increase chances for snow for the East US.

But here's where the buck stops for the Northeast's big snow possibility: The MJO.
MJO Forecast
Basically, when the MJO is closest to the edge of the image, the effects of each phase are stronger. Right now, we are in Phase 6. The Green line is the GFS Ensemble forecasts for the MJO, enhanced by individual ensemble forecasts shown in the yellow lines. The current forecast by the GFS Ensembles and other models are to have the MJO go into a strong phase 7 around the time of the storm. Here are the temperature and precipitation effects of a Phase 7 MJO.
Temperature Anomaly

Precipitation Anomaly
The Northeast is typically dry during Phase 7 and phase 6 (in case the MJO ends up in Phase 6 during the storm). That would be a major blow to the forecast, seeing as the MJO would be at a strong level and therefore could deter the AO/PNA/NAO effects. For temperature, Phase 6 and 7 both involve the East US being warmer than usual.

My Thoughts
In my opinion, I think that while some indices are pointing toward a total Northeast Snowstorm, i'm going to have to go with a Midwest/Northeast shared snow event for now. Based on the strong MJO combined with a strong PNA and subsequent -NAO/-AO, it seems likely that the end result will be a shared snowstorm with these two regions. Another thing that enhances my shared snowstorm theory is how the ensembles are more unified with that idea than a pure northeast snowstorm. In this winter where the models are doing poorly, I find much more confidence in a group consensus rather than just one person's idea of a scenario.

Next System Coming into the Country

Increased water vapor indicates that the next system we are watching is coming into the country. It looks like the jet stream is pulling this system down into the Plains. Because it is a clipper, we will be seeing the system quickly progress east and put down an inch or two across the upper Great Lakes region. The arrows are representing the shield of precipitation, not the direction is is going.
The clipper system will be very progressive, so any accumulations will be 1-3 inches. 

February-March Outlook

Before you read on, bear with me when I say this: This isn't going to be one of those overhype, scream-in-your-face posts practically forcing you to accept that winter is still coming, nor is it going to be an ultimate despair-based moan-and-groan tale of how the world will be a sauna for the rest of eternity.
No. This will be a fair-based, tell-it-like-it-is description of how the rest of winter will turn out to be.

I know I have been proclaiming a cold and snowy February, but this is the first time I am now confident that there will be a cold and snowy pattern coming in.
This is my final prediction. I will not be changing it.

The Pacific North American pattern (PNA) has been in a positive phase for much of the winter- but not correctly matching up with other teleconnections (AO, NAO). As a result, we have not seen the cold and snow. A negative PNA sets up high pressure over the East and a trough in the West- something that has been very common this winter. However, the latest forecasts are calling for a +PNA to develop. A +PNA would result in the opposite of a -PNA.
As we have been seeing, the potential vorticity readings from the North Hemisphere way up by the Arctic Circle have been unstable recently. What I am seeing- and believe is true- is that this potential vorticity will start to be pushed down towards the US as a ridge of sorts sets up. The reason I believe a ridge will set up is because we have seen at least one ridge present in recent weeks up in the Arctic Circle- they just haven't been in proper places to get the storms into the US.
But it's not all in the PNA. It is very much a roll of the dice- certain teleconnections have to combine to make for a certain scenario. With the PNA going positive and the NAO/AO going negative, I think there is at least a chance we see cold and snow.

As for the AO/NAO, take a look at the past several weeks of observation.
We have affixed it with trends.
Just looking at the graphs easily shows how the pattern has changed in the last month. My belief is this: Now that the pattern has changed, I don't believe it will be changing back during February. Now that it has happened (albeit quietly), I just don't find it in the cards for the AO/NAO to spike back into positive territory, and the latest model forecasts even show that it will stay negative in the short term.

People have been trying to use analogues of years to compare to this one in order to figure out what the rest of winter will be like. I think that is just not a good technique this winter. As everyone reading this knows, an incredible amount of defined and undefined records have been set, from temperatures to how many times you shoveled your driveway. That's why I think people have let their guard down.
I have posted about how Tom Skilling (WGN Chicago Meteorologist) has said how winters with comparatively lower snow and warm temps have ended up back-loaded with cold and snow. That is one deal with analogues I am willing to believe. While everyone is comparing analogues with similar things already observed, I think that there is something different that is crucial to these forecasts.
Even though Tom Skilling posted that particular forecast for Chicago, Illinois alone, I think it applies to much of the rest of the East US.

My Thoughts
I believe February will be at least snowier. I think there is not enough evidence to prove otherwise, as we have already seen the pattern change. it has been shown in the AO/NAO and the PNA, among other things. The pattern should get stormier as the +PNA directs storms to a Panhandle Hooker flow and brings strong low pressure systems from the Plains to Great Lakes/Midwest and Ohio Valley.
The reason I focus on the PNA is because my confidence in the AO/NAO being negative (for at least the first week or two of February) is fairly high. A NAO/AO/PNA favorable combo may match up just right to make for a cold and snowy February.
If you have any local questions, you may ask them.

Severe Weather Outlook for Spring will be issued at 5:00 PM CST Today