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 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. 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. 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.”

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.

Computer Forecast Models (GFS Bias) Let me emphasize again one other point which is a really important one not only for those who are trying to practice and learn synoptic meteorology but also those who are trying to trade energy markets. As I stated in the Winter Forecast in a week to moderate El Nino pattern the GFS in particular has a heavy bias to constantly breaking down any sort of cold pattern in the day and the developing some sort trough on the West Coast. Sometimes this is going to be the correct way to go but a lot of times the operational GFS is going to be the outlier even against its own ensembles. DT

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

Computer Forecast Models “Just like last week when the operational GFS overdid to amounts of arctic air coming down the West Coast and over develop a PIG SE US Ridge that was supposed to develop over the Southeast and delay the arctic front arriving in the east Until Jan It's the same sort of GFS model problem. Only now we are moving back to the more typical cold bias we are all use to with the OP GFS with southeast US Low pressure getting crushed or suppressed.” -DT-