18z 12/24 model disco

[SBUWX23]

Any idea if that is why this run is further west and more like the 12z gfs.. seems plauable but I dont know a whole lot about models and their parameterizations

Not going to speculate at this point.

[needbiggerboat]

swing and a miss for dc/bwi....

[yoda]

850 low right on NC/VA border near ATL ocean it looks like

[mitchnick]

the same result for MA

[stormtracker]

Better, but still a miss for DC.BWI/PHL but NYC and especially BOS should get it

[WidreMann]

T

They all should otherwise it has no conditions along the boundaries since this models domain is smaller than the GFS.

Not to mention the fact that you can't just start a model on nothing but actual observed data.

[AfewUniversesBelowNormal]

8-12" for Cape Cod in 3 hours!

[baroclinic_instability]

MODS, feel free to move. I posted this in 12Z, but it can go here as well...just as relevant for this storm.

This storm is amazingly sensitive, and I will say it again, the GOM will have to play a role. For nobody that believes me, look at the early ECM runs that were bombing this low out. They developed a much stronger GOM depression/surface low once that weak PV Anomaly ejected NE/phased with the northern stream. Look at the 12Z GFS and compare it to the 12Z NAM. Why is this important? Weak convergence in the GOM, and it can be seen on the GFS, is "priming" the surface low far rapid development by releasing latent heat of condensation in the low levels of the troposphere (generally 850 and below) by hour 36. Also note how the surface low is already beginning to take off inside the GOM by 42, then once the system reaches the Gulf Stream it begins to tank out. This is a positive feedback process, and this is why the GFS is so much different as this very small detail results in huge changes with time...it is an example a very non-linear growth of a disturbance. So why is this important? Latent heat release in the lower troposphere acts to decrease the static stability by warming the low levels. It also enhances low level warm air advection. You will see this in the upper level height field ahead of the trough manifested in rising upper level heights, aka a building ridge. Height rises are dynamic, and they don't happen by accident. It can be shown rather simply without pulling out long equations if you develop low level warm air advection in the low levels over an area, what happens? The thickness of that layer of air increases (see hypsometric equation), and therefore the heights above it rise. In QG theory, this is differential warm air advection and it results in height rises. VERY important for that process to occur. As for static stability, it plays a prominent role and decreased static stability helps a cylone rapidly intensify because lower statically stable air will rise more efficiently than very stable air, or high static stability air. It can even result in destabilization and the development of convection near the core. If you look at all prominent vertical motion equations, including the QG omega eqaution, where is static stability?

It is in the denominator of every major forcing term!

Low static stability is KEY.

Height rises? They are important because an increasing ridge in the mid levels results in a larger magnitude of differential cyclonic vorticity advection which increases sy noptic ascent, but it also results in a much more curved and shorter wavelength tropospheric jet stream which also enhances the divergence aloft associated with the mesoscale jet stream. This is a feedback process! Now what happens? More air converges in the low levels, more latent heat is released, warm air advection increases in magnitude as the low continues to develop, heights rise faster, frontogenesis develops strong mesoscale circulations, the jet curves more, etc etc etc. Throw in air-sea interaction instability and you now have a bomb and this system then "hooks" into the coast, in this case into the SNE/NE region. Mid Atlantic is out of the big snow overall, but the coastal and Delmarva region are definitely still in play. This is positive feedback cyclogenesis, and this is why you have such varying solutions. GFS has a GOM influence, and you need it. Look at how flat the

height field is early on:

Then compare as the system develops with an increasing ridge owing to WAA. This is positive feedback at its best.

From a forecasting perspective, folks along the EC, especially NE and into NYC/Long Island will be watching development in the GOM with a close eye in 24-30 hours. If the GFS shows what it is now, the potential is there. If the GOM development is weak, this storm won't verify as the GFS shows it now and it will be a glancing blows to parts of NE. This is becoming a NOWcasting forecast.

[Reale WX]

The sfc low depiction does not seem right at 54.. and with the way the system is beginning to wind up I believe the 18z NAM should have brought some wrap around precip to NJ area etc with the comma head

[VAsnowlvr82]

Shouldnt we be comparing the 18z nam to the 12nam instead of the 12z gfs?

[CT Rain]

18z NAM tucked just inside or over the 40/70 benchmark. Probably needs to be tossed because of GFS errors but interesting.

[Ian]

The NAM was an easterly outlier for a while this shift is mostly insignificant for ppl not north of the MA.

[TUweathermanDD]

Better, but still a miss for DC.BWI/PHL but NYC and especially BOS should get it

Randy, I see. However, 500mb would argue for a further west low once again. Also, the trends are there and if the trough goes negative any earlier, good things may happen.

[Dr No]

Looks like feedback issues in here... a powerful 850mb low over ORF won't be dry on the NW side.. Easterly flow.

[retusnmete8]

For SE VA this model is very similar to 12z, slightly higher QPF. Very close to 5-8", but solidly 2-4" for ORF/NTU

Maybe 4-6" ECG

[weatherweather]

if this run verifies, boston is measuring in feet...

[usedtobe]

The NAM was an easterly outlier for a while this shift is mostly insignificant for ppl not north of the MA.

We get screwed by the initial impulse that moves thru and brings a weak 850 low to our north. that prevents us from getting warm advection and makes it late for us when the eastern flow finally gets north of the second mores southern low that becomes the main one. That's still our problem. It will be interesting to see what the gfs does.

[Reale WX]

MODS, feel free to move. I posted this in 12Z, but it can go here as well...just as relevant for this storm.

This storm is amazingly sensitive, and I will say it again, the GOM will have to play a role. For nobody that believes me, look at the early ECM runs that were bombing this low out. They developed a much stronger GOM depression/surface low once that weak PV Anomaly ejected NE/phased with the northern stream. Look at the 12Z GFS and compare it to the NAM. Why is this important? Weak convergence in the GOM, and it can be seen on the GFS, is "priming" the surface low far rapid development by releasing latent heat of condensation in the low levels of the troposphere (generally 850 and below) by hour 36. Also note how the surface low is already beginning to take off inside the GOM by 42, then once the system reaches the Gulf Stream it begins to tank out. This is a positive feedback process, and why the GFS is so much different because of this very small detail results in huge changes with time...it is very non-linear process. So why is this important? Latent heat release in the lower troposphere acts to decrease the static stability by warming the low levels. It also enhances low level warm air advection. You will see this in the upper level height field ahead of the trough manifested in rising upper level heights, aka a building ridge. Height rises are dynamic, and they don't happen by accident. It can be shown rather simply without pulling out long equations if you develop low level warm air advection in the low levels over an area, what happens? The thickness of that layer of air increases (see hypsometric equation), and therefore the heights above it rise. In QG theory, this is differential warm air advection and it results in height rises. VERY important for that process to occur. As for static stability, it plays a prominent role and decreased static stability helps a cylone rapidly intensify because lower statically stable air will rise more efficiently than very stable air, or high static stability air. It can even result in destabilization and the development of convection near the core. If you look at all promient vertical motion equation, including the QG omega eqaution, where is static stability?

It is in the denominator of every major forcing term!

Low static stability is KEY.

Height rises? They are important because an increasing ridge in the mid levels results in a larger magnitude of differential cyclonic vorticity advection which increases sy noptic ascent, but it also results in a much more curved and shorter wavelength tropospheric jet stream which also enhances the divergence aloft associated with the mesoscale jet stream. This is a feedback process! Now what happens? More air converges in the low levels, more latent heat is released, warm air advection increases in magnitude as the low continues to develop, heights rise faster, frontogenesis develops strong mesoscale circulations, the jet curves more, etc etc etc. Throw in air-sea interaction instability and you now have a bomb and this system then "hooks" into the coast, in this case into the SNE/NE region. Mid Atlantic is out of the big snow overall, but the coastal and Delmarva region are definitely still in play. This is positive feedback cyclogenesis, and this is why you have such varying solutions. GFS has a GOM influence, and you need it. Look at how flat the height field is early on:

Then compare as the system develops with an increasing ridge owing to WAA. This is positive feedback at its best.

From a forecasting perspective, folks along the EC, especially NE and into NYC/Long Island will be watching development in the GOM with a close eye in 24-30 hours. If the GFS shows what it is now, the potential is there. This is becoming a NOWcasting forecast.

Thank you very much for this very insightful. If I recall the additional latent heat release from convection and the positive feedback associated with that played a big role in the cyclogenesis and more inland track of the Suprise storm in Jan 2000 which the models did not do well with until hours before the storm lol

[A-L-E-X]

MODS, feel free to move. I posted this in 12Z, but it can go here as well...just as relevant for this storm.

This storm is amazingly sensitive, and I will say it again, the GOM will have to play a role. For nobody that believes me, look at the early ECM runs that were bombing this low out. They developed a much stronger GOM depression/surface low once that weak PV Anomaly ejected NE/phased with the northern stream. Look at the 12Z GFS and compare it to the NAM. Why is this important? Weak convergence in the GOM, and it can be seen on the GFS, is "priming" the surface low far rapid development by releasing latent heat of condensation in the low levels of the troposphere (generally 850 and below) by hour 36. Also note how the surface low is already beginning to take off inside the GOM by 42, then once the system reaches the Gulf Stream it begins to tank out. This is a positive feedback process, and why the GFS is so much different because of this very small detail results in huge changes with time...it is very non-linear process. So why is this important? Latent heat release in the lower troposphere acts to decrease the static stability by warming the low levels. It also enhances low level warm air advection. You will see this in the upper level height field ahead of the trough manifested in rising upper level heights, aka a building ridge. Height rises are dynamic, and they don't happen by accident. It can be shown rather simply without pulling out long equations if you develop low level warm air advection in the low levels over an area, what happens? The thickness of that layer of air increases (see hypsometric equation), and therefore the heights above it rise. In QG theory, this is differential warm air advection and it results in height rises. VERY important for that process to occur. As for static stability, it plays a prominent role and decreased static stability helps a cylone rapidly intensify because lower statically stable air will rise more efficiently than very stable air, or high static stability air. It can even result in destabilization and the development of convection near the core. If you look at all promient vertical motion equation, including the QG omega eqaution, where is static stability?

It is in the denominator of every major forcing term!

Low static stability is KEY.

Height rises? They are important because an increasing ridge in the mid levels results in a larger magnitude of differential cyclonic vorticity advection which increases sy noptic ascent, but it also results in a much more curved and shorter wavelength tropospheric jet stream which also enhances the divergence aloft associated with the mesoscale jet stream. This is a feedback process! Now what happens? More air converges in the low levels, more latent heat is released, warm air advection increases in magnitude as the low continues to develop, heights rise faster, frontogenesis develops strong mesoscale circulations, the jet curves more, etc etc etc. Throw in air-sea interaction instability and you now have a bomb and this system then "hooks" into the coast, in this case into the SNE/NE region. Mid Atlantic is out of the big snow overall, but the coastal and Delmarva region are definitely still in play. This is positive feedback cyclogenesis, and this is why you have such varying solutions. GFS has a GOM influence, and you need it. Look at how flat the height field is early on Then compare as the system develops with an increasing ridge owing to WAA. This is positive feedback at its best.

From a forecasting perspective, folks along the EC, especially NE and into NYC/Long Island will be watching development in the GOM with a close eye in 24-30 hours. If the GFS shows what it is now, the potential is there. If the GOM development is weak, this storm won't verify as the GFS shows it now and it will be a glancing blows to parts of NE. This is becoming a NOWcasting forecast.

Jason, youve been right on this thing in terms of needing more GOM influence. I dont know if the 18z NAM is "tainted" but it certainly did trend west. I wont dismiss any solution until 0z at the earliest. The only certainty is continued uncertainty!

[Bluescat1]

Looks like feedback issues in here... a powerful 850mb low over ORF won't be dry on the NW side.. Easterly flow.

I think the models are getting a better fix with all of the players on the field, but timing is everything with this particular set up.

[weatherweather]

geez from 60-66 it basically stalls out east of boston

[baroclinic_instability]

Jason, youve been right on this thing in terms of needing more GOM influence. I dont know if the 18z NAM is "tainted" but it certainly did trend west. I wont dismiss any solution until 0z at the earliest. The only certainty is continued uncertainty!

It will definitely have to be watched real closely as RealeWX posted above. The potential is there if the models have the GOM influence right, and honestly, that will have to be nowcasted to a degree as the event unfolds.

[Eskimo Joe]

If you look at the 500mb features, the longwave trough seems to be sharper with the past two runs. I'm curious to see what 18z GFS and the subsequent 00z runs have.

[baroclinic_instability]

Thank you very much for this very insightful. If I recall the additional latent heat release from convection and the positive feedback associated with that played a big role in the cyclogenesis and more inland track of the Suprise storm in Jan 2000 which the models did not do well with until hours before the storm lol

Good example there. This storm can surprise folks if they aren't keeping close eye. The 18Z NAM is not a joke, nor is the 12Z GFS.

[Dalfy]

18z NAM tucked just inside or over the 40/70 benchmark. Probably needs to be tossed because of GFS errors but interesting.

I thought you said that it was the assimilation of data @ initialization on the 12z gfs run that caused problems so shouoldnt the nam be unaffected since it uses different parameters? Also I thought the nam only used GFS data on lateral boundary conditions not on other things?

[WhiteoutWX]

Anyone else see the crazy thing the NAM does with the 850 low at hr 36-48. Seems to jump it around quite alot. At 30 hrs, trough is digging south looks like it should close a low off the NC/SC coast. Then by hr 36 it closes a center over NJ. Then by hr 42 its got a closed center in SC, then finally by hr 48 its off the NC coast

Anyone think thats strange?

[Reale WX]

Good example there. This storm can surprise folks if they aren't keeping close eye. The 18Z NAM is not a joke, nor is the 12Z GFS.

You are completely right and I havent been able to quite rule this storm out because of that and hints from past runs. Unortunately it will most likely come down to nowcasting, which also means most likely work for me on Sunday :-/

[chris87]

the 18z NAM would have been initialized with any new observations from the past 3 hours (i don't know the actual assimilation cycle off hand, it may be all new observations since 12z). the first guess field comes from the 6-hour forecast of the 12z NAM.

it does use GFS as lateral boundary conditions, but HPC noted the discrepancies [i'm not comfortable enough calling them initialization errors...just because an assimilation system, in this case the one used to initialize the NAM and GFS, resolved features different than the Euro (although likely a better assimilation system on average) doesn't make it an initialization error, for sure] were over the Midwest, or within the NAM domain, so the GFS lateral boundary conditions shouldn't be causing any problems

I thought you said that it was the assimilation of data @ initialization on the 12z gfs run that caused problems so shouoldnt the nam be unaffected since it uses different parameters? Also I thought the nam only used GFS data on lateral boundary conditions not on other things?

[A-L-E-X]

Good example there. This storm can surprise folks if they aren't keeping close eye. The 18Z NAM is not a joke, nor is the 12Z GFS.

And it does look like there is an increased amt of convection over Texas. Looks more like an el nino STJ lol.

[CTWeatherFreak]

18z NAM tucked just inside or over the 40/70 benchmark. Probably needs to be tossed because of GFS errors but interesting.

Ryan, is there any way to know now 'polluted' this 18z nam run is? Does it take the gfs init verbatim? Does it not reinit at all?

NM... just read Chris87's explanation, and from the sound of it, this 18z nam run can stand on its own sweet self