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January Medium-Long Range Discussion


Holston_River_Rambler
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I have really enjoyed these threads for the last few years. There is not a lot I can add I am just getting started in this hobby.  Carver I have seen you post these ensembles several times could you tell a novice how to read them. Thanks in advance. 

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11 minutes ago, CDUB_VOL said:

I have really enjoyed these threads for the last few years. There is not a lot I can add I am just getting started in this hobby.  Carver I have seen you post these ensembles several times could you tell a novice how to read them. Thanks in advance. 

Welcome to the forum!  I'm not Carvers, but I have some time and can answer the question. Hope this helps.

Each model run generates individual ensemble members, each of which represents a slightly different possible future evolution of the weather based on small perturbations introduced into the initial conditions. By running the model at different times and with these perturbations, the GFS ensemble provides a more comprehensive picture of the potential range of future weather scenarios and their associated probabilities.

Here's how it works:

  • Base model runs: The GFS model itself is run four times per day, starting at the aforementioned UTC timestamps.
  • Ensemble generation: For each base model run, ensemble members are generated by introducing slight variations in the initial conditions, such as temperature, pressure, and wind speed. These variations represent the uncertainties inherent in the initial data and atmospheric processes.
  • Forecast calculations: Each ensemble member then evolves independently through the model, simulating the atmosphere's future state under slightly different conditions. This creates a range of possible weather forecasts for each forecast lead time.

Therefore, when you access GFS ensemble forecasts, they represent the combined information from multiple model runs and multiple ensemble members, providing a more nuanced picture of the potential future weather than any single forecast could offer.

It's important to remember that even with the ensemble approach, weather forecasts are inherently uncertain, and the range of possibilities represented by the ensemble can increase as the forecast lead time gets longer. Nevertheless, GFS ensembles are a valuable tool for meteorologists and the public alike to understand the potential range of future weather events and make informed decisions.

 

 

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21 minutes ago, CDUB_VOL said:

I have really enjoyed these threads for the last few years. There is not a lot I can add I am just getting started in this hobby.  Carver I have seen you post these ensembles several times could you tell a novice how to read them. Thanks in advance. 

Ensembles are simply a blend, kind of like a music ensemble group made up of a French horn, trombone, baritone, flute, bass, violin, and cello for example.  When I post an ensemble mean, it is several perturbed members (they tweek the 12z run multiple times, and then run the program) averaged together.  Above, I posted the individual members of the snow ensemble. Sometimes the mean can be skewed by one huge, outlier run or there could be multiple perturbed runs which support the mean.  Above, there are several runs that support the original operational(or deterministic run.  That likely means the deterministic is more credible if the ensemble supports it at this range.  The deterministic runs are the GFS and CMC...they have one shot to get it right.  The GEFS and GEPTS are compiled of multiple perturbed runs....At longer ranges say outside of 4-5 days, the ensembles are often better.  Inside of day3-4, we start to rely more on the deterministic runs.  This is important, at day 5, the deterministic is for all intents and purposes just one of the individual ensemble members.  

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3 minutes ago, ShawnEastTN said:

BTW not related to the coming event but Gatlinburg is blaring warning sirens with loud speaker messages asking people to seek higher ground.

emoji991.png Watch this video on Facebook
https://www.facebook.com/share/v/VD8qmva8V7ih8Q5u/?mibextid=4p3i6U

Sent from my Pixel 7 using Tapatalk






 

Sad with more heavy rains coming Friday.  We needed the rain just came with a cost. 

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So the Euro is a mess as was the GFS.  Models are trying to sort out blocking over the top, potential for a coastal low to pop, and strength of the front.   Outside of NW TN, that is just a cold front passage.  That run was a nothing-burger for most.  The stronger hp is almost shunting the system eastward.  I would look for that front to be stronger as we get closer to the event.

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5 minutes ago, weathertree4u2 said:

So, what is everyones guesstimate as to when modeling should start to converge on a solution? Friday? 

Saturday or Sunday.

Seriously, I had the Euro giving me 30 inches 2 days out a couple years ago and ended up with 2 inches. Right now I'd feel fairly confident we're going to get really cold. Models don't often miss that. Any snow amounts between a dusting to 10 inches are on the table across most of the area. I say dusting because this arctic blasts usually squeeze out at least that.

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7 minutes ago, weathertree4u2 said:

So, what is everyones guesstimate as to when modeling should start to converge on a solution? Friday? 

Maybe.  LOL.  The GFS and Euro both weakened the system, but the GEFS and GEPS beefed up.  Let's see what the EPS does.  Probably still an ensemble driven forecast at this range.

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The 12z Euro was MUCH warmer....lost a lot of the cold air - washed out.  I don't know if that is because we have a SLP trying to back the flow along the coast.  However, the Euro was slower almost right off the bat.  But the trend all winter has been slower and weaker for many storms.  Tricky to get a forecast from that trend w/ systems.

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WAAAY too much time left to get attached to any solution.  This is about the time models do indeed get wonky with systems or lose them altogether.  Any scenario is in the table right now.  The only thing I can comfortably say is it’s gonna get chilly.

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3 minutes ago, Silas Lang said:

We usually don't get much ice. Rain or snow for the most part here in the valley. When was the last one? Anyone have the storm thread? And is that a legit possiblity with this air mass? 

I could be wrong, but that looks like an error.  We normally get ice if the cold is already in place and settled into the valley.  Maybe the lower level cold advances that quickly, but I doubt it.  '94 kind of had that happen in Knoxville though.  I don't remember the sequence.  It was a sloppy mess.  The problem with tracking these so far out, there are just wild swings with almost every run.  

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1 minute ago, BNAwx said:

WAAAY too much time left to get attached to any solution.  This is about the time models do indeed get wonky with systems or lose them altogether.  Any scenario is in the table right now.  The only thing I can comfortably say is it’s gonna get chilly.

Yeah the cold definitely seems to be coming regardless. Hope it ain't with a bunch of ice. Would be a mess for days if that was the case. 

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2 minutes ago, BNAwx said:

WAAAY too much time left to get attached to any solution.  This is about the time models do indeed get wonky with systems or lose them altogether.  Any scenario is in the table right now.  The only thing I can comfortably say is it’s gonna get chilly.

We noted yesterday that modeling is infamous for losing storms in the day 5-7 range, and them sometimes getting them back. I suspect this is about to be a Miller A and we are seeing some sort of transition from an anafront to a coastal.  Models kind of have one foot in one camp and one foot in the other.  But yeah, a long way to go.

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3 minutes ago, Silas Lang said:

Yeah the cold definitely seems to be coming regardless. Hope it ain't with a bunch of ice. Would be a mess for days if that was the case. 

I think it was '94(John remembers this winter much better than I do)...it was just nastiness.  We had ice and snow.  The interstates were snarled as it stayed cold long after the storm.  I had water in my apartment as the rooftops were damming water behind ice.  I remember chipping ice from my car.  I shattered, and I thought is was my rear window breaking out.  Just a brutal pattern.  This may have a lot of that in it.

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The EPS (control) is much more aggressive than the ensemble.  Normally, they looks super similar at this range.  The MSLP mean and precip is also more aggressive than 0z.  This might be a case where the operational is much weaker than the ensemble.  

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