Modelcasting versus Forecasting: Why you can’t just use models

Modelcasting has become a big issue in the meteorological world.   Cell phone apps, websites, and even some forecasters simply look at what each individual run of the model shows them and makes that their forecast.  This has led to large changes in some forecasts and led many of you to ask us why the forecast in your local area has gone from rain, to snow, back to rain and then back to snow 22 times in the past 5 days only for the sun to be shining the day of.

The truth is, just as our knowledge of what the weather will do is imperfect, a computer’s knowledge of what the weather will do is just as imperfect.  A human forecaster can use logic to look at a map and realize that something doesn’t make sense.   A computer has rigid equations and programs and follows them to the letter.  It doesn’t matter to the computer if its showing the worst storm ever known to man, if that’s what the programs and equations say goes there, then the computer puts it there.

As trained forecasters, meteorologists are supposed to know better.  We know the model has biases, that the model has errors.  We are well aware that the model is just downright bad at forecasting certain storm evolutions.  This brings me to newly formed Tropical Depression 8, probably better known to many of you as Investigation area 99L.  A little while back in the comment section on an article about then Tropical Storm Franklin, I was asked about TD8 and how the Euro model was showing a stronger system than other models.   I explained then that the GFS was very weak in forecasting the type of system that 99L was and continues to be, with even the 06Z run this morning continuing to show barely any development of TD8 as we’ll see down below.

Reality against the Machine

The image above is the model forecast for early this morning.   The current NHC data shows that TD 8 has a minimum central pressure of 1011 mb while this model data shows a minimum pressure of 1014 mb.  As you can see from the image, it also doesn’t show much convection.

This is where a meteorologist needs to know their models and how those models do and don’t work.  We here at Firsthand have been looking at this system for Tropical development for days and first mentioned that the system might develop in this area in an article back on August 8th.  We did this despite models like the GFS not showing any development because we know of the models weakness and added value to the model based on our own meteorological knowledge.  This added value by the meteorologist is essential in any forecast.  We head to the image below for another example.


This image, which is the model forecast for Monday afternoon, shows no major closed circulation.  Based on the previous image, this image would even indicate that TD 8 is weakening.  However, the forecast from the National Hurricane Center doesn’t call for weakening.  The forecast expects that TD 8 will have been a Tropical Storm with winds between 50 and 60 mph.  Do you see that Tropical Storm on this image?   I certainly don’t.  Modelcasting this storm would be a disaster if you happened to be on a cruise ship in that area.

Finally,  this image shows what’s supposed to be a tropical system on Tuesday the 14th.  The forecast during this time period calls for a strengthening Tropical Storm with the NHC forecast heading up to 65 mph and nearing hurricane strength.   That type of forecast simply isn’t here on this model and it wouldn’t be on any app that uses this model for it’s forecast.

This same logic seen here with this forecast for TD 8 can be applied to many other scenarios.  Whether its the development of a winter storm, a major outbreak of severe weather, or even something as simple as the morning temperature, The data that goes into and comes out of computer models allows us a much better idea of what the weather is going to do, but that data and the programming that goes into creating those models is only as good as our understanding of that data and programming.

Modelcasting with a computer can solve a lot more equations, but if the equation itself isn’t perfect then the answer to it isn’t going to be perfect either.   The difference is that meteorologists who use the data as a tool, instead of just modelcasting and outright using the data, can understand what when the tool isn’t giving you the right answer.


Robert Millette

Tropical Storm Watches Issued


Tropical Storm Watch Issued for North Carolina

The Tropical Atlantic has become very active with 2 Tropical Depressions developing in the last day close to the Southeast Coastline, one near Florida and the other threatening North Carolina.  Hurricane Gaston also quickly intensified into a category 3 Hurricane but remains no threat to land.  NOAA aircraft are scheduled to investigate both Tropical Depressions today.

Major Hurricane Gaston


Gaston remains a well organized hurricane and current satellite images indicate that the eye remains quite distinct with deep convection around it.  The upper-level outflow is well  established both to the west and the east of the system providing good outflow at the top of the system.  Maximum sustained winds are 120 MPH and the minimum central pressure has dropped to 957 millibars as Gaston continues to strengthen slightly.

Gaston has not moved very little during the last several hours and should remain generally stationary overnight and Monday. Gaston remains in weak steering currents caused by a blocking mid-level ridge to its northwest.  A trough that is currently over eastern Canada is expected to dampen by the time it nears Gaston, but it should be strong enough to erode the ridge and allow the hurricane to become embedded in the mid-latitude westerlies. This pattern change should result in Gaston’s turning east-northeastward continuing in that direction through the remainder of the forecast period.

The atmospheric conditions suggest that Gaston could maintain its strength for the next day or so, however, given the expected slow motion of the cyclone there is some chance that cold water upwelling would counteract that.  Beyond that time, the hurricane is likely to encounter an environment of increasing shear, drier air, and cooler water. Given these expected conditions, Gaston should begin to weaken on Monday.

Tropical Depression 8, Tropical Storm risk for North Carolina

Tropical Storm Watches have been issued for the coast of North Carolina from Cape Lookout to Oregon Inlet.


Satellite imagery shows that Tropical Depression Eight is currently comprised of a swirl of low-level clouds accompanied by minimal shower activity.  This structure is due to the impacts of 20-25 kt
of southeasterly vertical wind shear and abundant mid- to upper-level dry air seen in water vapor imagery. Maximum sustained winds are currently 35 MPH and the minimum central pressure is 1010 millibars.
The initial motion is West to Northwest at 10 MPH.  For the next 48 hours, the depression is expected to move west-northwestward to northwestward toward a weakness in the subtropical ridge near the North Carolina coast.  After that time, a mid-latitude shortwave trough moving through the northeastern United States is forecast to erode the ridge and cause the cyclone to recurve Northeastward into the westerlies.  The track guidance is in good agreement with this scenario, and the new track forecast lies near the consensus models through 48 hours which would bring the storm within 35 nautical miles of Cape Hatteras.

Wind shear is expected to decrease during the next 48 hours and  depression 8 is expected to move into a more moist environment.  Based on this, the intensity guidance is showing
strengthening as the system approaches the coast of North Carolina. The intensity forecast also shows some strengthening, but it is on the low side of the guidance envelope due to uncertainty about
whether the environment will become as favorable as the models are suggesting.   Depression 8 is expected to recurve but with such a small distance between its expected location and the coast landfall as a tropical system is certainly not out of the question.

Tropical Depression 9

Depression 9

Flight-level wind data from an earlier NOAA reconnaissance mission along with WSR-88D Doppler radar data from Key West indicate that the depression had been moving southwestward.  However, the most recent radar data and nearby surface observations suggest that the cyclone has now turned toward the west. The last reliable wind data from the NOAA WP-3 recon aircraft supported an intensity of 35 MPH, and that intensity is being maintained for this advisory given that the radar and satellite signatures haven’t improved. The central pressure of 1007 mb is based on a reliable observation from ship WMKN, located just north of the center.

The initial motion estimate is to the West at 9 MPH. Now that deep convection has waned, the system has turned westward and this motion is expected to continue for the next 24 hours or so. This short term motion is supported by NOAA recon dropsonde data, which indicated that 500 mb heights were 10-20 meters higher over the southeastern Gulf of Mexico than what the global models have been forecasting. After that time, the global and regional models are in surprisingly good agreement on the cyclone slowing down and turning toward the west-northwest and then northward in the 36- to 48-hour periods as the depression moves around the western periphery of a narrow subtropical ridge that is expected to be located over South Florida. By 72 hours and beyond, the tropical cyclone is forecast to lift out and accelerate to the northeast towards Western Florida coast.  The current forecast Track brings the system on shore North of Tampa.

Strong vertical shear that has been inhibiting this system for the past week is expected to gradually subside to less than 15 MPH in 18-24 hours, which should allow for more organized convection
to develop. However, the southerly low-level inflow will still be disrupted by the terrain of western Cuba.  By 36 hours and beyond, the depression will moving over SSTs greater than 30C and the light vertical wind shear is expected to back around from a northerly to a southwesterly direction, which usually favors more significant intensification. However, there is  lot of dry air in the region north of Key West and this will play a factor in preventing rapid intesnfication of this system.

Robert Millette

Staff Meteorologist

Firsthand Weather