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tarheelwx

CAD "ALWAYS" Overperforms

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It  has been stated many, many times on this board that  "CAD Always Overperforms" and the models always play catch up.  I never really agreed with this as I know there have been many, many instances where we were borderline and nothing happened, or the event just didn't impress or even live up to some of the models. Today's storm was a perfect example - most places struggled to get down to the temp that the models were forecasting and this ended up being a nuisance event at best.  Sure the sleet and snow on the front end was a bit better than expected, but that wasn't really related to CAD.   

I think what this boils down to is that people tend to remember the really great CAD storms that overperform, but those that come up short are lost in our memory forever.  It's kind of like a great game that is won on a last second shot involving 2 great teams, vs. a game between 2 decent teams where the final result is 5-10 points. 

Feel free to chime in on this.  I wasn't sure wherre to put it so mods feel free to place it elsewhere.

TW

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 "CAD Always Overperforms" Yeah I agree and have never subscribed to that logic. It's just way to broad a statement.  I think each situation is different as the mechanisms that scrub the cold out and the length of time the cold air has been in the CAD area are different from storm to storm.

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Every situation is different. If you're strictly looking at if the temp is below or above temp, then yes it's overstated. But, if you're looking at the lasting power of a CAD then it does seem to over-perform. There has been many bust to the colder side when CAD has been forecasted to erode.    

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FWIW the CAD did overperform with this event in SW Va. We stayed snow through the entire main precip shield. Short range models almost universally had us switching to heavy ZR around 2am - never happened. Ended up with 3.5-4" of cement instead of 1" of snow and ice crust.

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19 minutes ago, FallsLake said:

Every situation is different. If you're strictly looking at if the temp is below or above temp, then yes it's overstated. But, if you're looking at the lasting power of a CAD then it does seem to over-perform. There has been many bust to the colder side when CAD has been forecasted to erode.    

Technically speaking I believe for mine and your area this event was accurately forecasted and did not. 

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42 minutes ago, frazdaddy said:

Technically speaking I believe for mine and your area this event was accurately forecasted and did not. 

Yep, temps and freezing rain coverage this morning matched a lot of the model output (..for our area).    

 

So here's the next test for CAD over-performance (...lets see what it does on Thursday):

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 325 PM Tuesday... The CAD will slowly erode on Thursday. But, residual affects will last much of the day with a light steering flow. Areas of fog, mist, and drizzle will continue through at least late morning. Highs generally upper 40s and 50s. Much of the guidance still show 70+, but this seems way over done unless it clears out much more quickly and abruptly than currently expected. So, we can go with some 60s for a compromise.

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12 minutes ago, FallsLake said:

Yep, temps and freezing rain coverage this morning matched a lot of the model output (..for our area).    

 

So here's the next test for CAD over-performance (...lets see what it does on Thursday):

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 325 PM Tuesday... The CAD will slowly erode on Thursday. But, residual affects will last much of the day with a light steering flow. Areas of fog, mist, and drizzle will continue through at least late morning. Highs generally upper 40s and 50s. Much of the guidance still show 70+, but this seems way over done unless it clears out much more quickly and abruptly than currently expected. So, we can go with some 60s for a compromise.

Usually here in the triad I dont know if you call it over-performance but it is usually under-forecasted.   This one really fooled me.  Any time the HP is not locked in place our CAD just fades away at showtime and this was the case here in the piedmont.  When I saw E winds last night instead of NE I pretty much knew it was game/set/match.  Never even made it to freezing here but we got a slight slushy covering last evening due to about 2 hours of wet snow/sleet mix.

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1 minute ago, CentralNC said:

Usually here in the triad I dont know if you call it over-performance but it is usually under-forecasted.   This one really fooled me.  Any time the HP is not locked in place our CAD just fades away at showtime and this was the case here in the piedmont.  When I saw E winds last night instead of NE I pretty much knew it was game/set/match.  Never even made it to freezing here but we got a slight slushy covering last evening due to about 2 hours of wet snow/sleet mix.

Yeah, length of time in the battle zone for the cold air directly affects residual CAD. I made an analogy in another thread about unplugging your freezer and hoping for ice, might work if you don't leave the freezer door open.

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CAD also has big implications to the forecast in regards to severe weather in the Spring. Over the years I’ve seen the SPC really struggle with their outlooks because of an “overperforming” CAD, particularly in the Piedmont of NC. 

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2 hours ago, frazdaddy said:

Yeah, length of time in the battle zone for the cold air directly affects residual CAD. I made an analogy in another thread about unplugging your freezer and hoping for ice, might work if you don't leave the freezer door open.

CAD always overperforms

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Never got below 35 here yesterday, that’s about what models were showing, and the few sleet pellets were from the very dry air. This , as well as, the other CADs this year , have not overperformed down here, at all! A few were warmer than modeled!

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CAD typically over performs in regards to global model predictions in the mid-range. This event was no different if you go back and look at the prior 5 day lead runs. 

The hi-res shorter range models typically do a good job and sometimes over-estimate CAD. There are also certain situations with CAD that are more likely to bust in the short range than others. For example... you can have short-range busts when you have a really dry air-mass in place, or when the crux of an event depends on how long CAD hangs on before eroding.

There are other instances where CAD events generally bust the other way. One example I see is that if I'm relying on a wedge front coming through and funneling Cold air to the upstate after an event has started, this is usually 2 to 4 hours behind what the models show.

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6 minutes ago, mackerel_sky said:

Never got below 35 here yesterday, that’s about what models were showing, and the few sleet pellets were from the very dry air. This , as well as, the other CADs this year , have not overperformed down here, at all! A few were warmer than modeled!

Here is what the GFS showed at a 5 day lead. Does that look accurate to you?

gfs_T2m_seus_24.png

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33 minutes ago, burrel2 said:

Here is what the GFS showed at a 5 day lead. Does that look accurate to you?

gfs_T2m_seus_24.png

That's not bad...it seems to be within a few degrees.  I think the whole "overperform" thing is subjective because it's a nebulous term.  If it was supposed to be 50 and it ended up in the 30s, I'd say that overperformed.  If we were supposed to get all rain, but instead we got mostly rain with a few sleet pellets and wet snowflakes mixed in for a half our.  I'd say that performed as progged.

For a good "overperforming" CAD, where you get much more frozen over a much wider area than the models thought 5 days out, you need the source region really dry and very cold and have the high build in a good 6 hours prior to the precipitation falling...not coincident with or after the bulk of it has fallen.  We need dews in the teens at least to get a good overperformer around here.

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5 hours ago, tarheelwx said:

It  has been stated many, many times on this board that  "CAD Always Overperforms" and the models always play catch up.  I never really agreed with this as I know there have been many, many instances where we were borderline and nothing happened, or the event just didn't impress or even live up to some of the models. Today's storm was a perfect example - most places struggled to get down to the temp that the models were forecasting and this ended up being a nuisance event at best.  Sure the sleet and snow on the front end was a bit better than expected, but that wasn't really related to CAD.   

I think what this boils down to is that people tend to remember the really great CAD storms that overperform, but those that come up short are lost in our memory forever.  It's kind of like a great game that is won on a last second shot involving 2 great teams, vs. a game between 2 decent teams where the final result is 5-10 points. 

Feel free to chime in on this.  I wasn't sure wherre to put it so mods feel free to place it elsewhere.

TW

.Not every cad event over performs but in general temps do normally bottom out a little lower than advertised and it breaks down slower than advertised. However it seems most people seem to judge a cad event solely on the basis of whether people reach 32/get freezing/frozen precip or not. In reality, We have a lot of cad events where we are in fact colder and for longer but few people pay attention if there is no real threat for frozen/freezing precip. But also it might seem like there are less surprises because frankly the models are able to forecast these events better than they used to be.  

Dry air/low dewpoints/wetbulbs are still a problem for the models but this year and the past few really, there has been a lack of any real dry/cold air with these events, including this one..where temps soared into the 40s in va/nc yesterday and little in the way of dry air or really cold air off the surface to make up for it.  We also have had more cases of cold air not being established before the moisture and it's always hard to over perform if that is the case. But the lack of dry air with recent events certainly seems to be a change vs the past....quite often with these wedges wetbulbs would be  below freezing...but recently, and certainly this year, that hasn't been the case. Sort of hard to over perform with freezing/frozen precip when wetbulbs never or barely get below freezing. 

 

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1 hour ago, burrel2 said:

Here is what the GFS showed at a 5 day lead. Does that look accurate to you?

gfs_T2m_seus_24.png

Lookout & yourself are a couple more posters that I really love to read.

You guys are well versed in reading models and knowing the climate in the NE Ga & Western Carolina areas.

I agree with the assessment that each setup will stand or fall on it's own merits.

Obviously there are certain areas more likely to benefit from CAD as opposed to others.

The closer you are to the East Side of the Escarpment the better your situation,

I live just East of Paris Mountain in NE Greenville county and it seems these CAD setups do tend to over perform here, 

But I am in a pretty good location to keep that Damming deeper and a longer period of time.

The further east & south  you go,

You open the door for the dreaded warm nose.

We had about 3 hours maybe 4 of sleet with some snow mixed in here in the Blue ridge, Taylors, & Greer areas.

A few times car tops were whitened.

It definitely over performed based upon my expectations.

 

 

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For this past  Tuesday's event, the  NAM and RGEM had temps getting down to 30 or  even 29 at GSO.  I think it bottomed out at 32 and it was basically a non event.  I beleive the snow/sleet from Monday through Monday evening was not CAD related, but more WAA precip really before the CAD really got established.  That being the case, I really saw the Tuesday event as a solid underperformer.  In other words, several of the models over estimated the CAD.

TW

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Thread title is correct!!! :P

Saw a brief period of 35-45 mph wind gusts late this afternoon 
across the NW Piedmont, peaking at 48 mph at the Piedmont Triad 
Airport. Otherwise, wind gusts have largely under performed, 
especially east of the Triad; likely a residual effect of the late 
erosion of the strong CAD. 

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