Computer Models (JB-Jan 30, 2007) “Models are not data. They are tools used to help us, or hinder us, in finding a solution with the actual data we have, which is the observation of the current weather and the knowledge of past weather. The latest data is the ob that comes in, not the model guessing at the future. Yes, that is old school meteorology and, yes, it is why I value the past more than the future, for no one KNOWS the future, but someone did KNOW the past to record it.”

Other Factors: Bias of Forecasting Models “The GFS (Global Forecast Index) is a U.S. based model that can extend out 384 hours or to about Christmas Eve. My years of knowledge with this model tell me a few things. 1. It is good with pattern changes and can see the colder/warmer changes that far out. It is easier to predict an air mass 1500 miles wide vs. snowstorms. Air masses do not have tracks to follow, they are just too big. They are huge systems of air that can be predicted weeks in advance now days. This is why I rely more on the GFS for air mass changes vs. storms.

Other Factors: Bias of Forecasting Models 2. Storms can disappear 2-4 days out before they hit. They are on the maps 5-10 days out but seem to hit this "blind" spot at 2-4 days out. The near miss coastal snow last week was a perfect example. 3. The GFS tries to develop too many storms on the extended that never really happen. It is a weakness of this model and you just have to weed out the wanna be storms.” --Rob Guarino (December 8, 2006) GFS Data for Forecasting GFS Data for Forecasting

Computer Models The average model error at 72 hours is about 110 miles and the best model in terms of track is the gfs/eta (sorry it was that last winter) compromise, it beat the Ukie, the ECMWF, etc. draw a 110 mile radius around the low and watch how the weather can change. - Rainshadow-

GFS Bias (DT in January 2007) “The operational GFS model is driving far two much energy down the West Coast of Canada into the US and is PHASING the two streams into a deep long wave trough. The model is doing this because the classical INFMAOUS COLD AIR BAIS that many of us know about with regard to the operational GFS.”

GFS Bias (DT in January 2007) I made this a critical point in my Winter forecast --- -that during these months of the moderate El Niño we would see the GFS constantly over develop West Coast troughs by over phasing the two streams and making large long wave trough on the West Coast which in turn develop a very strong Ridge over the southeast US. And sure enough that is exactly what we have been seeing over the last several days/ runs.

GFS Bias (DT in January 2007) BUT since many folks over the eastern US are not familiar with the GFS cold bias when it shows up on the West Coast it seems that a lot of folks and even some forecasters have forgotten the OPGFS cold bias. Yes over the Eastern US the op GFS suppress storms off the southeast coast in the cold season because it has two much northern stream action and not enough energy in southern stream. Everybody's is familiar with THAT concept. However on the West Coast the same problem exists but since over the last several years all we have seen is a perpetual RIDGE over the West Coast very few forecasters and hobbyists are familiar with the cold bias when it does show up on the WEST coast.”

Agreement Among Computer Models - DT- One of the things that forecasters and whether hobbyists really need to keep in mind is that all the big events....all the really significant major or Historic snowstorms in the Northeast US -- from the VA NC state line to southern Maine and east of the Ohio river always show up on the medium-range models days and days ahead of time. (February 8, 2007)

Agreement Among Computer Models - DT- …This was the case for example with the March 1993 superstorm the Jan 1996 blizzard... DEC FEB Presidents' Day2 HECS.... and FEB Sometimes there is the occasional surprise such as the January 25, 2000 event... but even that case proves the rule because that event sticks out to people's minds since most of the major snowstorm events over the last 14 years have in fact been very well forecasted. (February 8, 2007)

Compile Information on What Can Go Wrong First -DT- “As many of you know my method to use the art / science / philosophy of skepticism during major events whether they are severe cold... major snowstorms or hurricanes. The ideal or goal here is to have increasing forecast confidence as event gets closer. It is very easy for me now to commit to huge amounts of snow based upon the midday runs of the WED operational Canadian GFS and European models. There is real possibility that somebody in Middle Atlantic states is going to see well over a foot of snow. But rather than swinging for the fences right now.. then begin to worry as the data shows something going wrong on say the the Friday's runs or Sunday's runs.... let us lay out the problems or "obstacles" we have to face before we can lock in the big snowstorm…

Compile Information on What Can Go Wrong First -DT- …in this way as these "obstacles" are knocked out and we see less of " what can go wrong" we become increasingly confident of the forecast as event draws near. On the other hand if you swing for the long ball now and something changes well you can have a serious problem with your forecast confidence.”

DT on GFS Model Error (January 2007) The GFS has a truncation problem after 180 hours so the operational solution in particular often ends up with goofy solutions from 192 to 384 HOURS. By model truncation I am referring to the problem that the GFS model has in its resolution which is significantly different after 180 hours. From 0 to 180 hrs the operational GFS grid points are 55 km apart... but after 180 hrs the grid point spacing is essentially DOUBLED. Of all the global weather models which deal in the Medium & Long range forecasts ONLY the GFS model has THIS particular problem. There is a long history as to why the folks at EMC / NCEP decided to set up the GFS this way... without going to too much detail EMC believes that running the model four times today at two different resolutions is more useful than running the model twice a day at the same resolution. Of course the folks at EMC are completely long on as but that's not to stop them from continuing to do it nor are they going to do it that their entire premise is bulls*#t.

DT on GFS Model Error (January 2007) Also, all of the models have a particular bias or problem or tendency and good forecasters have to keep these model biases or tendencies in mind. In this particular case the cold bias of the GFS... which is a tendency of the model to overdo the power of the northern branch of the jet stream also known as the polar jet is a well-documented problem with the operational GFS... and. To a lesser degree the GFS ensembles. It is not suddenly a problem which has appeared out of nowhere because you may not have known about it.

Computer Forecast Models (Tier Forecasting – Rob Guarino) TIER FORECASTING TIER I: 00 to 120 hours out. Models are very good and timing and amounts from storms start to take shape. NAM/WRF start to play a BIGGER ROLE. TIER II: hours out. Models get a good handle on the Cold vs. warm but start to evolve the JET and storm tracks. This is a good time slot to start watching for potential storms but not so much the actual track just yet. TIER III: hours out. Models see the pattern and make adjustment on potential temperature changes. Storms will be all part of the magical act. Here one day gone the next.

Computer Forecast Models (GFS Ensembles) “The fundamental concept with ensembles forecasting is to use them in stochastic manner NOT a deterministic one. We have to follow the TREND in the GFS ensembles with each new 0z and 12z run because on the next GFS ensembles and instead of saying only 3 members showing the wave development... we might see 4 and then on the next run it might see 5 and on the next 6 members... well you get the idea…cont.” -DT (January 2007)-

Computer Forecast Models (GFS Ensembles) “… the GFS is going to suppress the feature because the GFS does not know how to handle strong shortwaves in the STJ. This is what we talk about when we referred to the cold bias. It over does the northern branch so strongly that the system ends up getting crushed on the model…cont.” -DT (January 2007)

Computer Forecast Models (GFS Ensembles) …”and remember AFTER 180 HOURS is when the model resolution truncates into the much coarser model grid-- so the cold air bias of the op- GFS really kicks in and we see the southwest shortwave getting crushed as it moves East. Even so we can still see some sort wave a flat one of Low pressure moving off the North Carolina coast at 228 hrs. 06/fp0_228.shtml Therefore it is going to be a BIG mistake to simply to look at the operational GFS runs over the next several days... see the shortwave or surface Low being crushed by the over done polar Jet... and say "it's not going to happen...” -DT (January 2007)- 06/fp0_228.shtml

But… In reference to the February 1-2, 2007 storm…the GFS trended west as the event drew closer. Forecasters anticipated a widespread 2 to 4 inch snow event as a result. The “trend is your friend” rule was in full effect. However, the GFS went back to the east as the event drew even closer. Some assumed it was model error. It was not. The storm was a BUST in that most areas expecting minor snow received only a trace.