Monday, February 18, 2013

February 21-23 High-Impact Significant Winter Storm (Updated 2/18)

500 millibar analysis reveals the storm system in question remains a while away from the United States, with enhanced wind readings being found in the Gulf of Alaska. This is the storm system in question, and is the storm system that is at the center of this big mess of confusion amongst the models.


There continues to be a large spread amongst the forecasting models as far as which solution is the most likely to verify. Shown above is the North American Ensemble Forecasting System, or NAEFS forecast for Hour 96, which converts to 4 days away from today. We see our system is in Kentucky at this time. If you've been reading my recent posts, you will understand that there has been a north trend with this storm that some models have had. This trend then resulted in the storm going to Wisconsin and other areas in the Northern Plains/Upper Midwest. It has been within the last 48 hours that we have seen a sudden revolt, so to speak, among the ensemble systems. This includes the NAEFS, the Canadian ensemble system and the American ensemble system. All three are considered big ensemble forecasting systems and have all been in fascinating agreement with the system going south rather than north.

So why is there so much confusion? Well, shown above is the NAEFS 500 millibar height anomaly forecast for 3 days away. Colder colors correspond to low pressure systems, while warm colors are supportive of high pressure systems. The model disagreement appears to stem from how strong high pressure in the Central and East US will be. The high pressure originates from persistent low pressure in the West US. As a result of below normal heights in the West, there must be above normal heights in the East US, as Newton's Third Law of Motion would indicate. These lower heights in the West and higher heights in the East are called the negative Pacific North American index, or negative PNA. In the negative PNA, storm systems that form in the Southwest as a result of persistent low pressure in that area are forced north into the Northern Plains, due to high pressure formation in the Southeast US. The models are catching onto this idea, but why are the ensembles going south? One reason is the presence of high pressure to the east of Greenland, called an east-based negative North Atlantic Oscillation. You can probably guess why the East based negative NAO is called east-based, but the negative NAO implies high pressure is over Greenland. This high pressure then acts to strengthen the subtropical jet stream, which is essentially another jet stream that sits along the Gulf Coast and Southern US rather than the North US (which is the big time jet stream). The subtropical jet stream, which is also called the STJ, is strengthened in a negative NAO. This negative NAO also acts to suppress the Southeast Ridge of high pressure, which would then act to verify the ensembles prediction of the storm sticking to the south.

Now that we've identified the indices at play here, let's look over that 500mb anomaly map and identify why the models are not handling this well. In that image, we can see high pressure centered over Canada and stretching into the US. This stretch into the US is helped by the negative PNA. By Hour 72 (when this image is valid), the high pressure system has started to extend north, leaving a gap of high pressure in the Southeast that is weaker than the high pressure in the Upper Midwest. Some models, like the European model, want to take this system north and have it ride the stronger high pressure wave into the Upper Midwest. The ensembles seem to believe it will go towards the weaker area of high pressure and this take a more southern route. The logical solution would be to take this system to the south, towards the weaker area of high pressure, as the ensembles are showing. I can understand why the models want to take the system more into the Upper Midwest, and it's due to where the system ejects from the Rockies. The farther north it comes out of the mountains, the farther north the track is going to be. Likewise, the further south the system ejects from the Rockies, the further south the storm track will be, like the ensembles are forecasting. In my personal opinion, if the system will be moving out of the Rockies from Southeast Colorado and the jet stream will be flowing towards the northeast (with winds over 100 knots), I could see the Northern track work out.


I combined both possible storm tracks in the image, with the yellow 'L's illustrating the north track and purple 'L's showing the southern storm track. Amounts were put at a wide spread of 2-6 inches for those in the light blue. Most should reach the middle of that spread, while some closer to the dark blue will flirt with the half foot mark. The dark blue contains an even wider spread of 6 to 12 inches. The wide-ness of the forecast spread is due to how low confidence still remains. The system is not yet onshore; we still have another day or two before the system makes landfall on the US. The pink shows amounts surpassing one foot. Southeast NE and north KS will bear the brunt of the system if either track succeeds.

Andrew

16 comments:

Ray T. said...

If the further south track is realized, that 6-12" will be expanded eastward, and a little bit southeast. That will depend in exactly how fast the system moves. The faster it moves, the further south track it will take. However, at this time, the most favored track looks to be the northern-most as this thing really looks to weaken as it heads northeast, it gets occluded. Thanks for the updates, Andrew, this is good news in terms of drought-relief for the Plains.

Ray T. said...

It obviously will not break the drought, but it will definately help to some degree. Something is better than nothing, with some of these areas likely to see around an inch of water-equivalent snow. Nice parade of storm systems could bring more moisture to the drought-striken areas as well over the next 2-weeks.

Anonymous said...

would the freezing line be north or south or on the low pressures you put in your map? Just wondering if it will be rain or snow for me :)

Greg Carbin Jr. said...

Good evening, Andrew. You ought to take a look at what's being shown for next week, Tuesday into Wednesday, Feb. 26-27. This storm is stunning. It's so early, but should it continue to be shown like this, could see another very stout blizzard on it's north side, and a massive severe weather outbreak on it's south side. These storms are very concerning for what may be about to come for the Spring season. This arctic air butting up with the warm, Gulf air seems to be increasing, and March could be quite interesting to say the least in terms of possible severe weather and enhanced tornado risk across the central part of the US! Just incredible. In 23 years, I honestly do not recall a pattern this active.

Alexander R. said...

@Andrew

Do you think Central NJ will get snow from this?

Anonymous said...

Hey Greg/Ray, I am a 14 year old kid who is interested in weather. I'm sure you get this a lot, so sorry, but how does one land a job in the NWS? I have a particular interest in severe storms, so the SPC is particularly appealing to me, but any job in meteorology will be enough to make me happy. I know it is highly math based, but math really does come to me so I am not worried about that. Overall I'm a good student too, and I got a perfect score on the practice ACT. Academic-wise, I think I have the brain to do it. What courses/career choices do you recommend I take in order to get a job in the SPC or NWS in general? Please answer me!

Andrew said...

Ray T: Yes, If the south track does work out, totals will need to move east. I based this off of the north track mainly to follow the operational models, but it was a hesitant call due to ensemble resistance. Fun times ahead, keep the great comments coming!

Andrew said...

Anonymous at 5:17: The Freezing line will be on a separate basis. This system is very fickle, and I will only be able to figure out the freezing line's position when the ensembles and models are

Andrew said...

Continued reply to Anonymous at 5:17: when they are in good agreement.

Andrew said...

Greg Carbin Jr: Yes, this pattern is definitely an exciting one! The storm you are referring to would certainly bring heavy snows to regions that may miss out on snows later this week. Thanks much for the great insight, don't hesitate to keep the comments coming!

Andrew said...

Alexander: The main system will be dying off as it heads east, but could rise again off the coast. Nothing extreme.

Greg Carbin Jr. said...

Anonymous at 8:51-

There are many volunteer jobs you can do, not necessarily for pay, but just as your own excitement or even for someone, like you, who wants to become a meteorologist. CoCoRaHs is one of them, which stands for Community Collaborative Rain, Hail and Snow network. It allows you to report rainfall, snowfall or hail which is posted to the NWS's LSR (Local Storm Reports) as long as you are a valid user and have not been flagged in the past for false information, which does happen sometimes. However; to become employed with the NWS as a meteorologist, a 4-year college degree is required in order to be directly employed with the NWS/NOAA/SPC. Back when I was your age, only a 2-year coarse was required, but now that things are so much more complex, especially technology wise, a longer education is required to assure one's full comprehesion of the suject. You're only 14 so you've got a long ways to go. Don't let that get you down or deter you from doing what you love. You can do whatever you set your mind to. If you are very interested in the field, odds are you'll know a lot about it once it's time to jump into college. One thing you can do for now is go to one of your local severe weather semiars and/or classes to become a certified storm spotter, as long as you are in at least 9th grade, which is required to attend the class and become certified. This will allow you to be associated with the National Weather Service! Actual employment, however; doesn't come about until you've graduated college for meteorology.

If you let me know your City and State, I can send a link to you for exactly when and where the spotter training classes are.

Vince said...

Andrew, I just happen to come across your blog by chance and wow, I'm amazed. It's refreshing to get a more personal take on model forecasting without having to trudge through NWS forecast discussion for Chicago and suburbs. I'm actually in the Rockford Il area and we have been right on the cusp of a couple storms that just passed to y North and West which it appears this one will do too. Hope the warm air advection gives us at least 3 or 4 inches though. I volunteered with Tom Skiillng at WGN nearly 20 years ago and I sure learned a lot. Weather has always been my passion sine childhood. I do have one question. Being on the southeast track of this upcoming storm unless an outlier verifies, how can I stay at or below freezing through all layers of the atmosphere to produce snow? Does this have more to do with the storm being negatively tilted or that fact that its occluded or neither. Thx for the info and keep up the great work. There s definitely an audience for this blog...Vince

Anonymous said...

NWS STARTING TO ISSUE WATCHES & WARNINGS FOR NORTHER KANSAS
SOME PLACES CAN SEE 12 TO 16 INCHES WITH THE POSSIBLITY OF SOME PLACES GETTING UP TO 20" OF COURSE THESE ARE EARLY ESTIMATES AND COULD CHANGE
THE BAD SIDE TO THIS GOOD SNOW WILL BE THE WIND - BLIZZARD CONDITIONS ARE GOING TO HAPPEN
I AM NOT COMPLAINING I AM VERY THANKFULL!!! HOPE IT STAYS ON TRACK

Anonymous said...

I am hoping that the storm takes a southern track and strengthens over Colorado, as we seriously need the moisture here, we have yet to see a single decent snowfall anywhere east of the continental divide here in Colorado, we are bound to get pounded sooner or later, I mean, every single solitary storm can't possibly track unfavorably, at least one will produce significant moisture, so I am hopeful.

Anonymous said...

Hopefully you will do a update soon as i do not trust anyone or site but yours!
I know the weather is always changing & want to know if this pattern is changing?
Thank you!
bree