18z 12/24 model disco

[Reale WX]

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?

Not jumping around, just strengthening along the thermal gradient over VA NC and SC coast. Hour 48 by itsellf is wierd because it has two little lows but nothing overly strange

[SP]

After a quick glance it appears to have continued a western (NW) trend by about 30 miles or so? I look forward to seeing the 00z after 7 fishes, scotch, cigar and playing santa. Not in that particular order either. I liked Wes' comment on the MA....

Cheers!

[Dalfy]

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 agree with this, thanks for the answer and explanation! Therefore I really dont think this run should be thrown out as other 18z runs seem to be hinting at the same idea as well.. Thus it seems that any developements within the past 3 hours dont counter the solution the 12z models showed.

[baroclinic_instability]

I am not trying to spam what is going on here, but read post 38. This is what it will eventually come down to, and this is why models keep flipping back and forth between a glancing blow and a massive bomb off of NE. Also note the 12Z GFS should not be discarded like a piece of trash. The 0Z showed the potential as well, so it isn't a fluky run guys...

If folks don't key into this storm and how it develops near the GOM, some folks may be caught a bit by surprise.

[dtk]

This run will be slightly better than 12z which was much better than anything before.

Correct me if Im wrong.. is the 18z NAM data run off of 12z gfs data (which was much further west and may have had data problems). This could obviously be a problem

This is wrong.

[dtk]

Sorry, I should have said for the initial conditions. It does use the gfs for boundary conditions.....however, keep in mind the fact that the nam domain is pretty big for a regional model.

[snowalot]

Just wanted to say TY to all the meterologists who take their time to post on these forums. I certainly have learned alot. I am sure many others appreciate it as well. This storm is certainly one that things can be learned from as it has generated its fair share of discussions. Might be even more interesting because of all that has gone on than one that is a more definitive miss or hit.

[yoda]

18z RGEM is starting... should be interesting to see if it decides to come west a bit and look like the NAM or GFS...

[usedtobe]

so where are all the Mets to tell us this storm is impossible due to the position of the western ridge on the leeward side of the rockies and how there must initialization errors as the Euro/RGEM/GEM/NOGAPS/Ukie are all east? I assume that is still the case?

Well for my area it still is, for your area, you always had the chance. Good luck with your snow.

[ctsnowlover]

I suspect they will all head west now...phasing is likely and the Euro was correct 2 days ago but phased too early and thus hit the MA harder...suspect this is a NYC north type phase....

[LocoAko]

I am not trying to spam what is going on here, but read post 38. This is what it will eventually come down to, and this is why models keep flipping back and forth between a glancing blow and a massive bomb off of NE. Also note the 12Z GFS should not be discarded like a piece of trash. The 0Z showed the potential as well, so it isn't a fluky run guys...

If folks don't key into this storm and how it develops near the GOM, some folks may be caught a bit by surprise.

Great post, and I wish there were more like it!! I tried my best to get through it - got the general jist of it without having taken Dynamics or Synoptic yet. Nice use of equations combined with graphics.

[ctsnowlover]

Well for my area it still is, for your area, you always had the chance. Good luck with your snow.

Thanks, I deleted the post as I do not want to appear contentious. I enjoy yours and many Mets posts, just a certain one galls me...I hope the MA gets in on the action.

[snowalot]

I thought this was interesting from HPC for their afternoon snow discussion for the lower Mid-Atlantic. They have discounted not only the 12Z GFS but the 18Z NAM as well.

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD

355 PM EST FRI DEC 24 2010

VALID 00Z SAT DEC 25 2010 - 00Z TUE DEC 28 2010

DAYS 1-3...

...OH VLY/APLCHNS...

AREA OF SNOWFALL IN THE MIDWEST WILL CONTINUE EWD THRU THE OH VLY

THIS EVENING/OVERNIGHT AHEAD OF A DIGGING/DEVELOPING ERN CONUS

TROF. PVA/MID LVL FRONTOGENESIS/HT FALLS COMBINED WITH A MODEST

PLUME OF MSTR WILL SUSTAIN LIGHT SNOW EVENT FROM IL/IN TO THE

APLCHNS. MODELS HAVE COME TO GREAT AGREEMENT HERE MASS

FIELD-WISE... ALTHO THE NAM IS WETTER THAN THE CONSENSUS. BASED

ON OBSERVED SNOWFALL... A GENERAL 2-3 INCHES WILL BE ANTICIPATED

IN THE PLAINS/VLYS... BUT MORE IN THE FOOTHILLS/APLCHNS WITH

UPSLOPE ENHANCEMENT. ONCE THE WAVE SLIDES EAST... ALL BUT THE

TRRN WILL SEE PRECIP END. NW FLOW BEHIND THE WAVE COMBINED WITH

FLOW FROM DEVELOPING COASTAL STORM WILL PROMOTE A LONG-LIVED

UPSLOPE EVENT TO HIR TRRN OF KY/WV... ALTHO THE OVERALL MSTR WILL

BE LIMITED... SO ONLY 3-6 ADDITIONAL INCHES FCST LCLY.

...LWR MID-ATLC...

PRECIP WILL SPREAD INTO THE REGION LATE CHRISTMAS DAY AHEAD OF THE

APPROACHING DYNAMIC TROF... WHILE THE SFC FTR DEVELOPS IN THE ERN

GULF AND CROSSES FL. TRANSITION TO WET SNOW MIX SHOULD OCCUR

OVERNIGHT THEN CHANGE TO ALL SNOW AS CAA TAKES OVER ONCE THE LOW

ACCELERATES NWD OFF THE COAST. QUESTION REMAINS HOW MUCH PRECIP

WILL BE LEFT TO FALL ONCE THE THERMALS BECOME SUPPORTIVE OF SNOW.

CONSENSUS IS EMERGING DISCOUNTING THE 12Z GFS AND 18Z NAM... THAT

TAKES THE LOW FARTHER OFFSHORE AND BRINGS PERHAPS 1-2 INCHES TO

ERN/CNTRL NC AND EXTREME SE VA.

...NEW ENGLAND...

HPC CHOSE TO DEFER TO CONTINUITY HERE AFTER SEEING SPREAD OF 12Z

GUIDANCE. REGARDING SFC LOW OFF THE COAST... 12Z GFS/18Z NAM AND

SEVERAL GFS ENSEMBLE MEMBERS JUMPED WEST WHILE REMAINING HELD

SERVE OR TRENDED EAST. H5 EVOLUTION IS NEARLY AS

DISPARATELY-HANDLED AS AT THE SFC... WITH THE GFS AGAIN WRAPPING

IT TIGHTER AND FARTHER WEST. THERE WILL BE PLENTY OF DYNAMIC

ENERGY AS THE TROF GOES NEG TILT SUNDAY... AND THE DEEP LOW

OFFSHORE WILL PROMOTE A BURST OF HVY SNOWFALL TO PORTION OF NEW

ENG IN THE THE PREFERRED SCENARIO. STILL THINK CAPE COD AND NRN

COAST ME WILL SEE HEAVIEST AMTS WITH TIGHT ACCUM GRADIENT INLAND.

BLEND INCORPORATED 06Z GFS/00Z ECMWF REFLECTING PREFERENCE LOW

TRACK. STILL LOW CONFIDENCE HERE.

...WEST...

DEEP TROUGH IN THE ERN GULF OF ALASKA ROTATES A SIGNIFICANT

SHORTWAVE INLAND ALONG THE WEST COAST FROM NRN CALIF TO THE

CANADIAN BORDER ON SATURDAY/SUNDAY. WITH THE DEEP ONSHORE FLOW

DIRECTED PRIMARILY AT THE CASCADES AND SISKIYOUS...THE SYSTEM

SPLITS WITH SNOW LEVELS IN THE CASCADES FALLING TO 3000-4000FT MSL

AND SATURDAY AFTERNOON/EVENING THROUGH SUNDAY MORNING...THE SIERRA

AND WINDWARD FOOTHILLS RECEIVE ANOTHER ROUND OF MODERATE TO HEAVY

SNOWFALL AS CRITICAL TEMPERATURES FALL TO ELEVATIONS IN THE

5000-6000FT MSL RANGE. USED A BLEND OF THE EC/NAM/GFS THERMAL

PROFILES AND HPC QPFS ALONG THE WEST COAST...TO GENERATE THE

SIGNIFICANT SNOWFALL GRAPHICS.

[ctsnowlover]

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.

May be the best post ever here....

[baroclinic_instability]

Great post, and I wish there were more like it!! I tried my best to get through it - got the general jist of it without having taken Dynamics or Synoptic yet. Nice use of equations combined with graphics.

No problem. Also not sure why some folks are randomly throwing guidance out without analyzing this more. 0Z GFS must need to be thrown out as well then.

[nwp]

Nice to see you posting over here dtk. I get this same question often as well. After years of working in NWP, I have developed some thick skin...as I am sure you have too.

Sorry, I should have said for the initial conditions. It does use the gfs for boundary conditions.....however, keep in mind the fact that the nam domain is pretty big for a regional model.

[yoda]

FWIW -- 18z RGEM at 24 hrs

[adelphi_sky]

I thought this was interesting from HPC for their afternoon snow discussion for the lower Mid-Atlantic. They have discounted not only the 12Z GFS but the 18Z NAM as well.

...LWR MID-ATLC...

PRECIP WILL SPREAD INTO THE REGION LATE CHRISTMAS DAY AHEAD OF THE

APPROACHING DYNAMIC TROF... WHILE THE SFC FTR DEVELOPS IN THE ERN

GULF AND CROSSES FL. TRANSITION TO WET SNOW MIX SHOULD OCCUR

OVERNIGHT THEN CHANGE TO ALL SNOW AS CAA TAKES OVER ONCE THE LOW

ACCELERATES NWD OFF THE COAST. QUESTION REMAINS HOW MUCH PRECIP

WILL BE LEFT TO FALL ONCE THE THERMALS BECOME SUPPORTIVE OF SNOW.

CONSENSUS IS EMERGING DISCOUNTING THE 12Z GFS AND 18Z NAM... THAT

TAKES THE LOW FARTHER OFFSHORE AND BRINGS PERHAPS 1-2 INCHES TO

ERN/CNTRL NC AND EXTREME SE VA.

Just as the models are supposed to be within their best range, they are throwing them out. lol Priceless.

[Reale WX]

Sorry, I should have said for the initial conditions. It does use the gfs for boundary conditions.....however, keep in mind the fact that the nam domain is pretty big for a regional model.

Thanks!

[WidreMann]

RGEM looks much better. Don't know how much I trust it as a model, but since it isn't GFS or NAM-based, I would hope that HPC wouldn't throw it out too.

Maybe they are avoiding models that have only trended for one run? It'd be dangerous at this point to assume a more westward track until more of the models are on board and for more than just a run or two.

[JoMo]

No problem. Also not sure why HPC is randomly throwing guidance out without analyzing this more. 0Z GFS must need to be thrown out as well then.

Because the NAM/GFS had 'numerous' initialization errors with the upstream system.

http://www.hpc.ncep.noaa.gov/discussions/pmdhmd.html

[yoda]

18z RGEM at 36 and 48... still a miss

http://www.weatheroffice.gc.ca/data/model_forecast/3294_100.gif -- 36

http://www.weatheroffice.gc.ca/data/model_forecast/3295_100.gif -- 48

[ctsnowlover]

FWIW -- 18z RGEM at 24 hrs

48 hrs looks way better than 60 hr GGEM, must be initialization errors:)

[psuhoffman]

RGEM is a miss verbatim but look at the h5 and h7 it leaves room for the storm to come west. It would not take much.

[baroclinic_instability]

Because the NAM/GFS had 'numerous' initialization errors with the upstream system.

http://www.hpc.ncep....ons/pmdhmd.html

Yeah I know Jomo. I am questioning why they even threw that out. Either way, 0Z GFS might as well be tossed into the bin too..may have had initialization errors too.

[SnowGoose69]

Yeah I know Jomo. I am questioning why they even threw that out. Either way, 0Z GFS might as well be tossed into the bin too..may have had initialization errors too.

My question more than anything is how are they so sure the same issue doesn't occur for the 00Z runs tonight?

[Dr No]

Anything future run that shows snow accumulation in the MA or NE will have "initialization errors". Seems as though many folks have a lot invested in an OTS solution.

[needbiggerboat]

18z gfs rolling

let’s see if it delivers a Red Ryder BB Gun with a compass in the stock, and this thing that tells time

or

a bag o’ glass

[WhiteoutWX]

18z RGEM at 36 and 48... still a miss

http://www.weatherof...st/3294_100.gif -- 36

http://www.weatherof...st/3295_100.gif -- 48

Not all that bad. This whole setup is so razor close for everyone.

I dont remember what it had at 12z, is this a westward shift?

[Max]

Because the NAM/GFS had 'numerous' initialization errors with the upstream system.

http://www.hpc.ncep....ons/pmdhmd.html

They say problems, but do not go into detail about what the problems are.

IMO and this is just from what I'm seeing off of water vapor and spc mesoscale analysis, is that the shortwaves and there are 3 of them are behaving in a way that would allow for a strong phase.

The Southern stream is much stronger than modeled already with a closed 5640dm height line in Texas. All other levels are also stronger. The leading Northern shortwave is speeding up and moving easterly making

more room for the second northern shortwave to dive southward and possibly phase with our southern shortwave. Honestly HPC might just being doing this just because they cannot believe how far west the these two particular models have come west. If the west trend continues with these two models at 0z, than how in the hell will HPC explain that. They will not be able to do it for a 3rd straight time.