Why did Jan 12, 2011 turn into a HECS?

[ORH_wxman]

I wanted to do a post mortem this evening while every bit of info was still fresh in my head from the model runs to the nowcasting. There's a reason a HECS is called a HECS...because its historic. HECS do not occur just because a model says it will. Prime example was 12/26/10...that storm turned out to be a fairly routine Nor' Easter in SNE outside of a small jackpot near Boston. But models said it would be much bigger. There's a lot of reasons why these things occur.

Leading up to the event many of us mets said that this dynamical system reminded us of 12/9/05. As you might remember, 12/9/05 was not a HECS, it moved too quickly to drop more than about 15 or 16" of snow. We also said that the best case scenario for this storm might be a 12/9/05 but 6-8 hours longer duration....well that's essentially what we got. So instead of a 6 hour 8-15" snow bomb, we got a 12-15 hour 15-30" snow bomb.

The flow was fairly progressive in this whole event...so why did we get a longer version of 12/9/05 anyway?

Lets look at the 500map and associated sfc map from 06z last night (about 1am)

The 500mb map shows a closed low over the OH/PA border with a nice area of vorticity leading out the southeast side of it off the coast of Delmarva...in response we get a sfc low E of ACY that is rapidly deepening.

Given the progressive flow, you'd expect this low to be up over Cape Cod or even a little further 6 hour later...only one thing can slow it down...its if it gets captured by 500mb low and closes off S of SNE.

Well you see at 12z, that process is occurring:

The height falls are so rapid south of SNE that instead of the low racing NE to E of Cape Cod as it was trying to do...it gets "yanked" back to the N and NNW and tucked near Block Island, RI.

Now another 6 hours later, the process is complete....the low moves only about 60 miles in 6 hours (10mph if you do the math) to a position near Provincetown, MA on Cape Cod...here is 18z:

The track of the 500mb low...or rather the deepening of it south of SNE was the key to all of this. Not only did it slow the low down, but it putt he mid-level centers in perfect position to maximize precipitation over most of the region...we saw the dryslot get swallowed up in the early morning hours in response to the excessive height falls S of RI/SE MA.

Comparing this to December 9, 2005....you can see there's some similarities early on....potent closed 500mb low coming out of the Ohio Valley:

Now, 6 hours later...notice how 500mb is still extreme potent but its actually opening up...

Notice how the sfc low is in similar position to the first map I posted at 06z on Jan 12, 2011...now this is 6 hours later on Dec 9, 2005:

Notice how the low on December 9, 2005 has traveled further in 6 hours than January 12, 2011 traveled in 12 hours. The key was the upper air trough going more negative and the heights falling out so rapidly south of SNE that it closed off a new center there. It temporarily slowed the sfc low down, and did it in the perfect spot for most of interior SNE to receive prolific snow totals.

It didn't happen like that just because the model was painting a lot of qpf over the area...again, we've seen time and time again where models will put out 1.5" of qpf over the region and its not a storm of this magnitude. 12/26/10 is a good example....3/2/09 is another good one. The problem with just forecasting 20" over a large area was that this exact scenario had to happen...and leading right up to the beginning of the event, there was still some uncertainty as to whether it would do that. There has to be a reason to forecast a high amount other than just hoping it happens and the model is painting a lot of pretty qpf colors over your backyard.

Scott has done well talking about the mid-level center tracks....and this is also why we saw NW CT and W MA get a huge hit too. Remember, models did NOT give those areas the pretty pink QPF colors...but we had a feeling there could be a deformation band from hell out there based on the track of the mid-level centers...there often is...even I thought it would progress a little too quickly east for that area, but it didn't...the power of the upper level height falls just worked wonders to really slow everything down all the way to out there.

The end results was widespread 18"+ totals.

[weatherwiz]

Great read!

It was also pretty crazy watching the models, mainly the NAM on this one, show some incredible low-level and mid-level frontogenesis over the region as well...then you had I think it was a +3-5SD LLJ just crashing into the region from the east and southeast which really helped to throw Atlantic moisture into the storm. Good deal of WAA into the system as well as some pretty steep ML lapse rates and the amount of lift over the region was absurd.

This certainly was one for the ages, not only the high totals but seeing a good widespread 2-4''/HR type stuff...pretty crazy.

[donsutherland1]

Outstanding post-storm analysis.

[weathafella]

Great analysis Will...thank you! I explained it to my wife...not sure her eyes weren't glassing over but it's great for me...thanks again!

[nzucker]

Great explanation, Will, very well done.

[Ginx snewx]

Fantastic job Will. Thank you, did the capture also allow NNE to reap further north or was that a function of trajectory.

[Fozz]

Excellent analysis.

[TalcottWx]

It was historic because: Great midlevel and low level support for big snowfalls all over New England. All the levels were vertically stacked in a great position to bury us. Also, the dryslot stayed out of the majority of SNE as a result. The banding was also pretty epic back into ME and NH due to the fact we were right at the perfect place in the NW part of the storm. Snowgrowth was outstanding also. There was also a large level of instability that produced the thundersnow and smoked areas with 3-5" an hour snowfall rates. The large swath of big totals also didn't hurt the fact that it became an HECS. Many of us will remember this one. The r/s line was also limited to the Cape and Island so most of us stayed happy. The winds were impressive due to the rapid deepening and the gradient, lots of fun blowing and drifting snow. Some areas of EMA got some blizzard warnings. I guess its more tough to ask what doesn't make this an HECS rather then what does.

[TheSnowman]

Awesome Awesome Job Will. Love your stuff.

[baroclinic_instability]

Here are a couple images, via CIMSS, regarding the depth of the Dynamic Tropopause and the ensuing tropopause fold discussed the night before the storm. Either way, the event was impressive, and it was quite a sight to see the surface convection along that deep cold front rising rapidly and directly into the active jet region aloft. Divergence levels were incredible. The height field response and the increased activity of the jet circulation as the DT crashed over the coast was amazing and spectacular, and it was almost instantaneous once it began.

[A-L-E-X]

Thanks for the images! My favorite one is number 2 (the middle one) it actually looks like a hurricane turning extra tropical off the coast of Cape Cod as it gets picked up by a front coming in from the west

[free_man]

sick storm and great recap, Will. Fascinating winter underway, hope this continues!

[HoarfrostHubb]

Thanks for the writeup

I will read up on this more later...

[Damage In Tolland]

I got chills reading that..Awesome stuff

[CoastalWx]

Will, nice write up. Good job pointing out to everyone how important the role at 500mb was in order to temporarily capture the low. I saved a bunch of progs too from this storm. Just an incredible event.

[HIPPYVALLEY]

I put this up in multiple threads so people see them.

NWS spotter snowfall reports for the storm:

http://forecast.weather.gov/product.php?site=NWS&format=CI&version=1&glossary=0&highlight=off&issuedby=BOX&product=PNS

Great write up! Thanks Will! Lots to digest there.

[dryslot]

Great explanation Will, Now we can see exactly the reasoning to this phenomenon that occured.....

[Clinch Leatherwood]

Great Will!

I talked a ton about the vorticity and how the NAM was wrong/fast in developing the spinup. It was, but not by as much as the 0z run the night of the storm would have indicated. I used the storm a few days prior as an analog understanding that the trajectory was different at the point of impact on the coast. That busted system was going east, or just north of it. This one was heading NE. So the impact of the later closing of a few hours was not nearly as severe as it was for the storm a few days prior.

instead of a 6 hour delay that resulted in the CCB missing by a few hundred miles east, all this really did is over the course of 4-5 runs gradually push the back edge of the 1" line NE..I believe a few runs had it well into NY/PA/NJ and at that point the model(s) were not doing much way up into NNE because it spun up earlier, did it's little dance earlier, and ended up even with the NAM track a little south once it got off Maine. As the NAM gradually, ever so slightly pulled back it closed the door on the huge totals way to the west in NY/PA, came down from the 3" QPF bombs in CT, and instead opened the door up north.

By not tucking it quite so close early in terms of the mid centers, it was able to slide just a bit further and get Maine/NH before it peaked and dropped out. I always said it mattered not to SNE, it was the difference between the 3" QPF totals and more snow up north.

The SUNY MM5 had some real issues at 12z the day of the storm - timing was terrible. But the 0z nailed the track to within a few miles for the next morning. It also nailed down very well the location of the max lift at the same time. It continues to rock inside of 12-15 hours. HRRR was not as impressive as I expected it to be but maybe I was wrong in looking at the maps. ARW/NMM versions had their moments, but the ARW had precip destroying EPA (think it was the 12z) with too much forcing well west which turned out to be bogus too.

I'm pretty sure the NAM will burn everyone on one of these next events because in this case I'm partially convinced it's normal bias of overdeveloping lows played into it's hand. Even it was low on the strength by a couple of MB's as it passed us which is why IMO it's temp profiles were not good down here, and why a model like the MM5 did better in hinting at precip type problems down here/to my NW with a stronger low.

[Ian]

nice post.. i hate you all tho.

[cdmwx]

Here is a Wxscope radar loop of the storm overlayed with wind arrows to visualize the track of the surface low over the Cape.

Link to loop...

(If your browser supports it you can click to play/pause and use keyboard arrows to advance/go back.

[Clinch Leatherwood]

With NAM graphics now.

IMO, if the dividing line between what ended up being mainly rain v snow ran from PVD to Boston versus the Plymouth to New Bedford area the accuracy of the NAM would be perceived differently. It did nail the track, the idea, and was no doubt the best model. But it still had it's issues. It never really mattered for most of you which is what I was saying all along, it mattered to the north in NH/ME, down here with temp profiles, and to some extent in EPA/E NY etc.

The 0z Wed NAM at 500 continued a multi-run trend of diminishing the vorticity centered on 12z Wed. It had gradually backed off over the course of a few days. The result was a slow shift towards a cooler solution, and a slow trend of increasing precip up north. It reversed itself totally beginning at 6z that night. For days it had gone one way, hard to believe it was anything but the low over-performing which in the end as others have said everything went right...unless you were in the Blizzard Warning on the Cape and were relying on the NAM profiles which is normally a good bet. It had gradually tapered some of the dynamics in the extreme west...which in reality turned out to be a erroneous trend.

When it made the jump back towards a stronger feature as you can see on the 500mb maps, it also adjusted the low/mid level lows a bit north or stronger. The result was a bust in temp profiles down this way. Instead of being just cold enough, it was too warm. Nominal error, but interesting trend reversal that again - if that dividing line was over the cities may have yielded a different perception. You can also see this increase directly attributed to a much better presentation at 700 which arguably helped it to attain the advertisted NAM solution.

It actually came together better than the 0z NAM advertised, and better than several runs had trended.

It got this one right. It's advertised a solution that would come together at least one other time in the last few weeks and busted, and in several cases in the last year/18 months. Many of those the same exact arguements were used to say why it would be as severe as the NAM, only to have it fail. So 100% agree this time it did the best job but I'm pretty confident it will get schooled by the Euro most of the time still.

The 0Z NAM still went towards the other models, only to reverse itself over the subsequent two runs. Perfect/overperforming low, great model...we'll see in the coming months/years.

[wokeupthisam]

Great post - thanks for boiling down alot of complexity and explaining it in terms a non-met can understand. That's not easy to do.

[Typhoon Tip]

A Will -

I think you are spot on with your reasoning as to why the two comparable events differentiated as they did. This last system was larger spatially, in addition to have large mass field involved in its mechanics; it would quite simply seem more than reasonable that it would take longre to translate all that through.

Harv and I used to comment how progressive bowling balls are technically not entirely closed off, because by stricter definition, a closed low has been disconnected from the westerlies.

But... the points you made are hugely valid and I concur. Great post partem -

[Organizing Low]

nice read Will

and i here was thinking the low was booking lol

but it does verify my thoughts that the precip shield was pretty compact for a strong coastal low, and thats why it appeared on radar it was moving quicker than it actually was at the surface.

[ski MRG]

Just for the record, the highest total recorded for this storm was in Savoy, MA with a tidy 40.5".

[ORH_wxman]

nice read Will

and i here was thinking the low was booking lol

but it does verify my thoughts that the precip shield was pretty compact for a strong coastal low, and thats why it appeared on radar it was moving quicker than it actually was at the surface.

Even with the "stall"...really just a slowing down....the storm was still quite fast moving for a HECS. Most HECS are over 24 hour events. This was really a 15-18 hour event. The feature that really made it a HECS was that it had a precip shield that was extremely intense...basically like 12/9/05. In order to get a HECS or HECS type totals in under 18 hours, you need a precip shield like that which is uncommon in itself...and then have it last 15+ hours on top of that....something that 12/9/05 failed to do. Everything had to come together perfectly to get widespread 18"+ amounts...and that's what occurred.

As I mentioned earlier in the original post, these storms don't happen because you have 1.5" of qpf painted over you....we've seen many storms in the past 3 seasons that show that and didn't work out to give monster totals here.

[phishn]

Excellent Post Mortem Will! Thanks.

[cheetah440]

Very nice write up, easy to follow for a novice. Thank you.

[SacrydDreamz]

I like this... I think I will be rejoining this community :-)

[Damage In Tolland]

Bump..

What if any differences are there with this storm compared to the Jan 12th superbomb?