Dec 11-13 MW/Lakes/OV Snow Event? Part III

[Hoosier]

reading various afd's and notced this from Springfield Missouri

THE FOCUS FOR THE WEEKEND IS ON A SYSTEM THAT WILL TAKE SHAPE OVER MONTANA FRIDAY MORNING AND MOVE INTO THE PLAINS. THERE REMAINS SOME MINOR INCONSISTENCIES IN THE MODELS...GFS/ECMWF/NAM-WRF IN REGARDS TO THE TRACK OF THE SYSTEM THOUGH MOST ARE NOW IN AGREEMENT WITH A LOW DEVELOPING EAST OF THE UPPER LEVEL TROUGH OVER IOWA AND MOVING EAST OVER THE GREAT LAKES REGION FOR THE WEEKEND. THERE ARE INDICATIONS HOWEVER THAT THE MODELS MAY NOT BE HANDLING THE STRENGTH OF THE UPPER LEVEL JET VERY WELL. CURRENT MODELS ARE SHOWING A 160 TO 180KT JET DIVING INTO THE PLAINS AROUND 00Z SUNDAY. THIS JET DOES NOT BEGIN TO ROUND THE BASE OF THE TROUGH UNTIL BETWEEN 06 AND 12Z SUNDAY DEPENDING ON THE MODEL. WITH SUCH A STRONG JET AND IT SLOWER TO ROUND THE BASE OF THE TROUGH...WOULD EXPECT THE TROUGH TO DIG MORE THAN CURRENT MODELS ARE DOING. THE GFS/ECMWF AND NAM-WRF ARE IN THE CAMP OF LESS DIGGING WHERE THE UKMET AND CANADIAN DO INDICATE MORE SOUTHERN PROGRESSION TO THE TROUGH AND THUS A MORE SOUTHEASTERLY TRACK TO THE DEVELOPING SURFACE LOW. IF THIS WERE TO OCCUR...THERE IS A CHANCE FOR A BIT MORE PRECIPITATION FOR THE REGION.

[Stevo6899]

NOT SURPRISING FOR A DEVELOPING 3

TO 4 PLUS SIGMA EVENT WHICH BY DEFINITION IS ROUGHLY A 1 IN 100...UP

TO OVER 1 IN 1000 PLUS PERCENTAGE PHENOMENA WHICH MODELS AREN/T

EXPLICITLY DESIGNED FOR.

Covered standard deviation in stats last year but dont recall it much and was wondering how it relates to weather. What do they mean 3-4 plus sigma and 1 in 100. They saying this will be a historic storm or that the models cant handle standard deviations too far from normal.

[Harry]

Yes I do. As for our individual storm thoughts, we have laid them out and discussed them. No bother continuing. I feel I am right, you feel you are. I guess we can wait and see. Seems many NWS offices seem to agree with me though, and I am rather confident in my thoughts.

We will see come verification time. AGAIN my debating point was with HOW the NAM gets it there. NOT saying the track cant happen it is HOW it happens/gets there. Hack it could go further nw with a good set up. I know better then to argue that.

I just know the euro and this model has blown it alot recently with trying to take these systems to far north only to settle back south a bit. I strongly suspect that is again what is happening here. We should have our answer though once it all gets sampled.

[roardog]

We will see come verification time. AGAIN my debating point was with HOW the NAM gets it there. NOT saying the track cant happen it is HOW it happens/gets there. Hack it could go further nw with a good set up. I know better then to argue that.

I just know the euro and this model has blown it alot recently with trying to take these systems to far north only to settle back south a bit. I strongly suspect that is again what is happening here. We should have our answer though once it all gets sampled.

Where do you feel the center of this storm will track?

[csnavywx]

NOT SURPRISING FOR A DEVELOPING 3

TO 4 PLUS SIGMA EVENT WHICH BY DEFINITION IS ROUGHLY A 1 IN 100...UP

TO OVER 1 IN 1000 PLUS PERCENTAGE PHENOMENA WHICH MODELS AREN/T

EXPLICITLY DESIGNED FOR.

Covered standard deviation in stats last year but dont recall it much and was wondering how it relates to weather. What do they mean 3-4 plus sigma and 1 in 100. They saying this will be a historic storm or that the models cant handle standard deviations too far from normal.

#SD = % of data contained within that number of standard deviations (assuming a normal distribution -- bell curve shaped)

1 SD = ~68% of the data

2 SD = ~95% of the data

3 SD = ~99.7% of the data

4 SD = ~99.9% of the data

as you may recall. Therefore a 3-4 SD storm would be 1 in 300 to 1 in 1000.(It's actually somewhat different, because I assure you we aren't dealing with a perfectly normal distribution.)

[Indystorm]

It has been absolutely riveting to read these discussions. For someone like myself with a bachelor's in earth science years ago with only a few met courses it is amazing to see how the models have evolved and been tweaked over time. And with this particular scenario I am amazed that there is so much discrepancy with so little time left. The sampling will tell the tale. Data and initialization are certainly the key as has been mentioned.

[Harry]

Where do you feel the center of this storm will track?

Somewhere close to the euro ensemble mean and canadian. I'll feel better ofcourse once this thing has been sampled. A jump south/se would not surprise me once it is. See the discussion Hoosier posted.

[Tony]

With all the jumping around with all the models the past few days, I am guessing it is still not over. Each day it has been something different and I suspect based on afd's from all around the Midwest that these Mets feel the same way. Not etched in stone...YET.

[Stevo6899]

#SD = % of data contained within that number of standard deviations (assuming a normal distribution -- bell curve shaped)

1 SD = ~68% of the data

2 SD = ~95% of the data

3 SD = ~99.7% of the data

4 SD = ~99.9% of the data

as you may recall. Therefore a 3-4 SD storm would be 1 in 300 to 1 in 1000.(It's actually somewhat different, because I assure you we aren't dealing with a perfectly normal distribution.)

Ah yes clear to me now. Thanks.

[baroclinic_instability]

What is getting lost in this discussion regarding jet streams "digging" is the models ARE digging the jet stream and have a very curved upper tropospheric jet.

However, the development of incipient cyclogenesis happens for a reason, and jet streams happen for a reason. Incipient cyclogenesis with the lead wave is forecasted to take off rapidly with the leading shortwave with the jet stream "digging" farther S with time.

The lead shortwave which forces rising motion on the synoptic scale owing to differential cyclonic vorticity advection.

This sets in motion positive feedback cyclogenesis in which low level cyclogenesis along the baroclinic zone alters the upper level height field which increases synoptic forcings, frontal forcing, jet stream circulations, etc. etc.

In reality what is happening is cyclogenesis farther N with the jet stream digging S in response to the push of CAA on the backside of the low. Jet streaks are found residing over regions of enhanced baroclinic zones.

Note that in the NAM, the lead shortwave (circled red) initiates cyclogenesis with the overall trough (blue line) diving southward in response to the cold front and CAA behind it. The NAM and latest GFS and SREF are modeling a realistic solution. Positive feedback cyclogenesis dominates the initial forcing, and the surface low rapidly deepens as that baroclinic zones tightens. As you can see, phasing is important, and cyclogenesis can be a sensitive process.

I can get deep into synoptic meteorology and dynamic meteorology, but that could take a long time. These solutions are not "model garbage" because they are off-hour runs, and that can be easily proved.

[Stebo]

What is getting lost in this discussion regarding jet streams "digging" is the models ARE digging the jet stream and have a very curved upper tropospheric jet.

However, the development of incipient cyclogenesis happens for a reason, and jet streams happen for a reason. Incipient cyclogenesis with the lead wave is forecasted to take off rapidly with the leading shortwave with the jet stream "digging" farther S with time.

The lead shortwave which forces rising motion on the synoptic scale owing to differential cyclonic vorticity advection.

This sets in motion positive feedback cyclogenesis in which low level cyclogenesis along the baroclinic zone alters the upper level height field which increases synoptic forcings, frontal forcing, jet stream circulations, etc. etc.

In reality what is happening is cyclogenesis farther N with the jet stream digging S in response to the push of CAA on the backside of the low. Jet streaks are found residing over regions of enhanced baroclinic zones.

Note that in the NAM, the lead shortwave (circled red) initiates cyclogenesis with the overall trough (blue line) diving southward in response to the cold front and CAA behind it. The NAM and latest GFS and SREF are modeling a realistic solution. Positive feedback cyclogenesis dominates the initial forcing, and the surface low rapidly deepens as that baroclinic zones tightens. As you can see, phasing is important, and cyclogenesis can be a sensitive process.

I can get deep into synoptic meteorology and dynamic meteorology, but that could take a long time. These solutions are not "model garbage" because they are off-hour runs, and that can be easily proved.

I think the question then becomes doe we believe the lead shortwave will end up being that strong though. If it doesn't then it really will end up changing things.

[baroclinic_instability]

In other words what I am saying is this. Look at the 12Z UK. Note how it develops a significant low well south. Why? It isn't phasing the shortwave with the polar vortex. In other words, weaker and non-feedback cyclogenesis occurs north with all that jet energy everyone keeps referencing of "digs" south. What the difference is between NAM/GFS/EURO/SREF is they phase the vortex with the wave and that results in a much beefier baroclinic zone as much colder air develops along the boundary. This results in the leading wave initiating incipient and rapid cyclogenesis along this intense baroclinic zone and that jet "digs" southward along the cold front.

[IWXwx]

A little off topic, but no matter how this storm works out, it has been very educational to me to read all of the posts here by mets a and hobbyists alike. Who needs met classes when I have American Wx?

[baroclinic_instability]

I think the question then becomes doe we believe the lead shortwave will end up being that strong though. If it doesn't then it really will end up changing things.

An interesting point, but with this type of cyclogenesis, a really strong shortwave is not needed. In other words, all guidance is relatively similar with the lead wave. Better question, why should we not believe those solutions? In that vein, should we always believe every thing the models show dynamically such as that leading wave will be wrong?

[Stevo6899]

What is getting lost in this discussion regarding jet streams "digging" is the models ARE digging the jet stream and have a very curved upper tropospheric jet.

However, the development of incipient cyclogenesis happens for a reason, and jet streams happen for a reason. Incipient cyclogenesis with the lead wave is forecasted to take off rapidly with the leading shortwave with the jet stream "digging" farther S with time.

The lead shortwave which forces rising motion on the synoptic scale owing to differential cyclonic vorticity advection.

This sets in motion positive feedback cyclogenesis in which low level cyclogenesis along the baroclinic zone alters the upper level height field which increases synoptic forcings, frontal forcing, jet stream circulations, etc. etc.

In reality what is happening is cyclogenesis farther N with the jet stream digging S in response to the push of CAA on the backside of the low. Jet streaks are found residing over regions of enhanced baroclinic zones.

Note that in the NAM, the lead shortwave (circled red) initiates cyclogenesis with the overall trough (blue line) diving southward in response to the cold front and CAA behind it. The NAM and latest GFS and SREF are modeling a realistic solution. Positive feedback cyclogenesis dominates the initial forcing, and the surface low rapidly deepens as that baroclinic zones tightens. As you can see, phasing is important, and cyclogenesis can be a sensitive process.

I can get deep into synoptic meteorology and dynamic meteorology, but that could take a long time. These solutions are not "model garbage" because they are off-hour runs, and that can be easily proved.

Shouldn't the shortwave initiate cyclogenesis further south where the trough is? Thus having a low a little further south? Is this action phasing along with cyclogenesis? I guess it all matters where they both meet. It seems like it could change rather easily, thus an unpredictable track. Just wanted to clarify if this is phasing or if the phasing is with the two separate low pressures or energies. Last questions from me as you are probably tired of questions.

[Stebo]

An interesting point, but with this type of cyclogenesis, a really strong shortwave is not needed. In other words, all guidance is relatively similar with the lead wave. Better question, why should we not believe those solutions? In that vein, should we always believe every thing the models show dynamically such as that leading wave will be wrong?

With the shortwave barely being sampled I think the strength of it is still in question, unfortunately by the time it is fully sampled we are looking at probably 12z runs tomorrow.

[A-L-E-K]

Shouldn't the shortwave initiate cyclogenesis further south where the trough is? Thus having a low a little further south? Is this action phasing along with cyclogenesis? I guess it all matters where they both meet. It seems like it could change rather easily, thus an unpredictable track. Just wanted to clarify if this is phasing or if the phasing is with the two separate low pressures or energies. Last questions from me as you are probably tired of questions.

If mets are going to take the time to explain things, take the time to read it through.

Anyways, time to:

[baroclinic_instability]

Shouldn't the shortwave initiate cyclogenesis further south where the trough is? Thus having a low a little further south? Is this action phasing along with cyclogenesis? I guess it all matters where they both meet. It seems like it could change rather easily, thus an unpredictable track. Just wanted to clarify if this is phasing or if the phasing is with the two separate low pressures or energies. Last questions from me as you are probably tired of questions.

No prob.

To answer your question, shortwave troughs result in synoptic ascent if there is differential cyclonic vorticity advection present. This can be derived using the vorticity equation. For a simplistic thought...think conservation of angular momentum. Anyways, if a sufficient low level baroclinic zone exists, this synoptic ascent will "initiate" low level cyclogenesis (if the level of non-divergence is in between). So, because the atmosphere follows the laws of mass continuity (mass can't be destroyed or created), then a region of rising air above the level of non-divergence must be compensated by convergence in the low levels. Low level convergence is the incipient cyclogenesis, and if a sufficiently strong baroclinic zone exists (helped even more if it is moist in the lower atmosphere which results in large amount of diabatic heat release), this low level convergence results in surface pressure falls, frontogenesis, warm air advection which alters the upper level height field, curved jets which increase divergence, rising air condenses and results in latent heat release in the low levels, and this feedback continues. This, in short, is why negative tilt troughs are observed to be the most intense.

[JoMo]

Excellent explanation baroclinic_instability.

[baroclinic_instability]

With the shortwave barely being sampled I think the strength of it is still in question, unfortunately by the time it is fully sampled we are looking at probably 12z runs tomorrow.

This can be discussed a lot, but satellites are quite advanced and can sufficiently sample upper tropospheric waves. I don't disagree with your point as I too have seen systems over the ocean change, but a pretty consistent height field trend is coming in line with both the control runs and the SREF runs. If you were a forecaster, you simply can not keep "waiting" to alert the public to potential storms. That is what makes forecasting hard, and I always advocate that all mets should consider that when they look at weather.

[tornadotony]

The one thing that has me intrigued with this is the Euro's insistence on a powerful, long-duration LES event here Sunday afternoon-Monday afternoon. It has a 24h+ period of persistent northerly flow down the fetch of LM...

[goknights]

Wow now I know why I chose biology and not meteorology

[Hoosier]

18z UKMET FWIW has the low in southern Indiana at 60 hours.

[baroclinic_instability]

18z UKMET FWIW has the low in southern Indiana at 60 hours.

I didn't even know there was an 18z UK, link?

[buckeye]

18z UKMET FWIW has the low in southern Indiana at 60 hours.

yup stubborn little guy. possibly a tad north of 12z but basically same idea. Absolutely amazing considering it's 60 hrs out

[buckeye]

I didn't even know there was an 18z UK, link?

http://moe.met.fsu.edu/tcgengifs/ukm/2010120918/

[baroclinic_instability]

http://moe.met.fsu.e...ukm/2010120918/

Thanks for the link.

[JoMo]

yup stubborn little guy. possibly a tad north of 12z but basically same idea. Absolutely amazing considering it's 60 hrs out

Clipperish

[buckeye]

Thanks for the link.

6z and 18z only go to 60 hrs. I also can't seem to get a 500 map with those

[baroclinic_instability]

yup stubborn little guy. possibly a tad north of 12z but basically same idea. Absolutely amazing considering it's 60 hrs out

It did track it a decent amount N, and that is meaningful. Numerical models use the previous forecast as the first guess analysis field. 0Z will prolly be even farther N.

You can see it better when you open the same forecast times up and switch back and forth.

http://moe.met.fsu.e...20918/slp10.png

http://moe.met.fsu.e...20912/slp11.png