DT's RULES FOR MEDIUM / EXTENDED FORECASTING RULES
RULE 1 CONSENSUS RULE:
The MODEL CONSENSUS RULE says when 1 model stands out from what most of the other models are showing the CONSENSUS is often right. . But this rule has limitations... in complicated synopic situations the CONSENSUS Breaks down. But as a a general run in Most cases when 9 models that say A followed by B but 1 model says C followed by D... the consensus of the 9 models are far more likely to be right than 1 Model. .
Often MR ( medium range) ensemble and SR (Short Range) Ensemble data -- such as individual 500 MB plots and /or the Spaghetti plots should be viewed to see if whewre the TRUE consensus lies. .
RULE 2 FLIP FLOP RULE:
The FLIP FLOP RULE is useful whenever a model shows a significant changebe it at the surface 500 MB... or 300 MB -- from what it was showing OR from what the Consensus is. The Model that shows a significant change OR has a solution that is particularly different from the Consensus will be referred to as a FLIP FLOP .
The BIGGER the change from then previous model run... the LESS likely to is to be right and the more likely it is to be a "hiccup" sometimes known as a Bad Model run. A surprisingly high amount of time the Operational Run of the GFS is at odds with its own ensemble and may have only 1 or 2 (if that) of the ensemble members supporting the Operational run.
RULE 3-- THE RULE OF O.M.S - Other Model Support
This is a corallary" to the CONSENSUS rule. Sometimes the Model which is far away from the CONSENSUS (called the Model Outlier) turns out to be correct! This rule --RULE OF O.M.S. has proven time after time to be a excellent criteria in determining whether the Model which is Model Outlier is a "Bad run" of the model OR whether the Model Outlier is "on to something". Most of the short and long range Models these days are of sufficent quality and skill so that when 1 model comes out with a seemingly whacky or "way out there" solution IF that solution is valid then the other models will begin to shift towards the Model outlier solution shortly.
EXAMPLE The 5 DEC 2002 Northeast US snowstorm. Most of the SR data was showing the heaviest snow occurring over VA MD lower PA and lower NJ... and Just clipping NYC Long island and coastal CT/ southeast Mass. The 18z run of the DEC 3 ETA showed a significant shift in the northward track of the Precipitation shield and the strongest Vertical Velocities (VVs)... with the 850 Low tracking into WVA and bringing all rain into DCA PHL and NYC.. However the other 18z Models-- the MM5 and GFS-- did NOT show that and therefore the 18z ETA DEC3 idea was discarded. AT 00z DEC the ETA shifted back to the south....
On DEC 4 the 18z ETA again shifted the precipitation shield much further northward and that trend was strongly supported by the 18Z GFS and MM5.
RULE 4- STAY WITH THE CONSISTENT MODEL
When the overall pattern has been a mild and dry one Models that show major Low pressure development should be viewed with a great deal of skepticism...unless the Overall pattern changes FIRST. For example in the winter months... during the event of a strong southeast US Ridge... System that models develop into Major events that appear to be bringing cold and snow/ice or rain often ended up being often much weaker and /or displaced to the north by at least a few hundred miles.
By the same token... Models that shows a large trough coming into the Eastern Pacific or Western US without removing a blocking pattern over Canada or without the negative NAO flipping to Positive Phase... OFTEN over warm the atmosphere... may take western US Lows (short waves) into the Upper Plains and or Midwest/ Great Lakes.
In the MR period (Day 3 to day 7) during Stable patterns t is often useful to use the NEC performance scores to see which is the HOT model. Link
RULE 5- RULE OF MODEL TRENDS - unstable trends
This is especially true at 48 hours and beyond with synoptically unstable patterns as well as Tropical cyclone forecasting. MODEL TRENDS are more important than the what the Model is actually depicting. For example suppose Model "A" is the M.O. (Model OUTLIER) and the majority or consensus depict some other solution in most cases the consensus model group is often correct. BUT
When the model consensus or majority cluster begins to breakdown WATCH THE SHIFT TOWARDS the Model A or M.O. solution. Incremental changes towards the M.O. solution will often lead a forecaster to make incremental changes in the forecast usually because of concerns over model uncertainty and consistentency issues. This is often a mistake. Once there is a discernible shift towards the Model A / M.O. solution it is often wise to make large changes in the forecast that is reflective of the Model A OUTLIER solution.
RULE 6 RULE OF THE SEASONAL BIAS:
SEASONAL BIAS is the most often over looked aspect to the MR forecast. The rule drought begets drought´ is a form of this rule. In a drought go with the NWP that shows the least amount of rain UNTIL there is some sort of major pattern change. In a dry winter that has not featured a lot of storms lean toward the weaker smaller development with less QPF scenarios.
EXAMPLE 1 The winter of 2000 2001 along the East coast was a classic case for Winter with respect to the issue of Seasonal Trends. In the MR (MEDIUM RANGE) period system after system appeared to be taking tracks that favored the Middle Atlantic region over the Northeast and New England. Throughout the winter season most of the MR models consistently showed systems that APPEARED to be mainly Middle Atlantic events in the day 4 to day 8 range. YET as the period in question got closer, the tracks would shift with each subsequent model run or cycle. For example on 6 FEB 2001 there was a moderate / significant snowstorm in New England. Yet the MR models showed the 500 MB as well as 850 and surface Lows originally forecasted to propagate across the VA/NC border... then with each new model run... showed a constant shift north. In fact the 500 Low would pass across DC/ lower MD.
The seasonal bias may have also played a part in the 1-3 MARCH 2001 super hype snowstorm in the Middle Atlantic and Northeast. Early on the MR models depicted heavy snowfall threat in the Middle Atlantic and less of snow threat for the Northeast . When in fact the New England region was the area that one area that saw the Heavy snowfall.
EXAMPLE 2 Hurricane season 2002; Of the 8 named Tropical cyclones only 1 formed in the MDR -Main development Region of the tropics. 7 of the 8 formed north of 20 degrees N latitude. Yet there were numerous T.W that various Models would develop in TC in the tropical Atlantic or Caribbean.
RULE #7 GFS BIAS
on the East Coast;
On the East Coast; in the MR period when the 00Z GFS/ 12Z GFS is showing pattern or storm development that is "opposite" of the ECMWF 9 times out of 10 go with or lean towards the ECMWF. Note that is especially true when the MRFX has Upper air patterns (at 500 or 250 MB) that do NOT match the 00z or 06z run of the OP-AVN.... and/or match the ECMWF.
The AVN has 2 major problems on the East coast and both of these make this model the model LEAST likely to verify beyond the 72 hr time frame.
6.1 Because of the severe cold bias of the GFS it loves to overdevelop troughs and as a result often takes features in the subtropical Jet and merge / phase that feature with the trough in the PJ hence making the Big storm or Big event.. when no other model is doing so. . Thus there are many well known cases where the OP GFS / OP MRF have developed large Lows well Inland sometimes even as far west as the Ohio valley/ Great lakes only to see these features verify on the East Coast.
6.2 In the event the GFS/ AVN does NOT forecast the Superstorm of the century it OFTEN has the feature in the STJ passing harmlessly out to sea off the GA or SC coast as a 1010 Low.
6.3 As a result of 6.1 and 6.2 Most of the Major events that have affected the East coast in the 20 years or so the AVN/ GFS has been way off and seriously outperformed by the most of the other MR models
6.4. The GFS is the only MR model that TRUNCATES.... HPC has recently upgraded the GFS... to a T254 or 55Km model resolution but this is only to Day 7 / 168 hrs on the 00z run. From 180 hrs to 240 hrs the 00z GFS runs at a T170/ 77 KM model resolution... and from 240 hrs to 384 hrs / Day 16 the 00z GFS runs at T126 / 108 KM resolution.
At the 06z 12z and 18z the GFS runs at the Respectable T254 / 55 KM resolution ONLY TO 84 hrs... AFTER WHICH it truncates at the T170/ 77 KM model resolution... to Day 10 / 240 hrs... and from 240 hrs to 384 hrs / Day 16 the 06z / 12z / 18z GFS runs at T126 / 108 KM resolution.
The Ecmwf uses a 25 KM resolution through day 7... the Navy uses a 60 KM through day 6 at both runs... the UKMET runs at 60KM at 00z and 12z runs... Canadian approx. 85 KM through day 10.
The AVN has 2 major problems on the east coast and both of these make this model the model LEAST likely to verify beyond the 72 hr time frame.
In August 1998 Hurricane Bonnie the AVN strongly and consistently showed that Bonnie would pass EAST of Hatteras when in fact Bonnie went over eastern NC and through Norfolk area as a Borderline Cat 1 hurricane.
In the 24 -25 January 2000 SECS ( significant east coast snowstorm) that pounded the Middle Atlantic with 10-20 inches of snow.. the AVN was furthest EAST of any of the MR or SR Models . It is my assertion that the AVN east bias was the PRIMARY reason why so many Middle Atlantic Forecasters both Govt and private Blindly assumed the 24-25 January 2000 Low was headed out to sea EVEN right up until 3PM forecast discussions 24 January
6-8 January 1996 Blizzard: Once more the AVN was by far and away the weakest with the Jan 1996 Noreaster. The ECMWF from Day 7 consistently forecasted a major Noreaster / Blizzard for the East coast and was quickly followed within the next model run by the UKMET...Nogaps... and Canadian. The AVN / MRF utterly missed this system and never at any time showed the a SECL (significant east coast low) from day 10 through 24 hrs before the snow began at Washington / Baltimore and Philly) I will present the actual Model data from Jan 1996 shortly.
The 11-13 March 1993 Superstorm: Day 7 the ECMWF showed a 975 MB Low along the Immediate East Coast... The UKMET 968 over AOO... with the Nogaps and other Models showing a similar deep SECL. The old MRF at least this time did show a strong system (986 MB over RDU) but it was consistently too far inland and weakest of all the MR models. I will present the actual Model data from March 1993 shortly. When the event moved within 72 h and 60n hrs the AVN suddenly developed a perfect 968 MB Bomb over SBY MD...
The 8 -12 February 1994: This was actually TWO events that came during one of the coldest winters in the Midwest / Great lakes and Northeast in the last 100 years. The AVN consistently mishandled the SPLIT FLOW pattern and displayed surface lows/ short waves taking a much further north track... into the Ohio valley over an over developed 500 MB ridge in the southeast which the AVN/ MRF Models showed bringing Mild air / rain to the Northeast & Middle Atlantic. The 2nd low 10-11 FEB 1994 which brought Crippling and Historic ice storm from RDU to RIC to DC to PHL and Heavy snow in New England... was depicted by the 72 and 60 hr AVN as Rain with temps in the 40s and 50s (850 MB temps +5) as the main Low would go through Ohio and into western PA/ NY. However the ECMWF UKMET and Canadian model consistently and correctly showed the systems sliding east... downplaying the Southeast Ridge and bring Much colder snowier / ice forecasts. I will present some of the Maps from February 1994 shortly.
29-30 December NJ / NYC/ CT snowstorm. Correctly forecasted in the MR by the ECMWF UKMET Nogaps Canadian (GGEM) the MRF AVN played a game of amazing flip flop. The 00z morning runs of the MRF from Day 8 would show a SECL off the coast with significant snow amounts over the Northeast only to see EVERY run of the 12z AVN either completely remove the Low or Place it out by Bermuda. HPC obsessed and focused solely on the AVN NEVER forecasted a SECL from Day 7 right through 48hrs. In fact one of the less skilled forecasters at HPC Gleefully taunted how the snow crows would be wrong and this perspective was based solely on the 12z AVN... as the forecaster unwisely ignored the ECMWF UKMET NOGAPS ETAXX GGEM SEF and strong ensemble support.
It is MY contention and feel free to disagree that because of model Truncation problems... the cold bias and over phasing bias the GFS/ AVN/ MRF has missed most of the major events over the eastern US in the last 20 years or so. ONCE the event moves within the 84 hour time frame the superior model resolution of the AVN / GFS and lack of model resolution truncates gives the model a lot more creditability.
RULE #7 DOUBLE EE RULE
When the 60 - 84 Hour ETA matches the ECMWF day 3-4 at surface and 500 MB that is a pretty good winning hand. Take it to the bank.