Category Archives: Basic Weather Facts


A reader asks—

“If you have time to indulge, would you mind explaining the difference between the ICON and NBM models (the later I believe is a blend of models?), with an eye towards which rain forecast for tomorrow (modest or healthy) is the one you lean towards?”

Good question and I’ll do my best to [try] to keep it simple and straightforward.

First, I should say the latest NBM model has moved towards the ICON model with over 1.25 “ rain (ICON model- 1.6”) for Friday afternoon through Saturday morning. mid-afternoon, then cloudy. Sunday cloudy with light rain again towards evening. Clearing Monday early morning.

Complete forecast on Friday.


The NBM is the “National Blend of Models”, a blend of many different models that is run hourly by NOAA. It’s an increasingly accurate composite model that statistically evaluates numerous other model’s inputs, determines their biases and errors, and comes up with a better forecast. For temperature forecasts alone, it’s pretty complex—

Statistical step evaluation for temperature (MAE= Mean Absolute Error)  Source: NOAA


Numerous models are part of the blend.  While it is called the “National Blend of Models”, the NBM now incorporates the European Models, Canadian Models and the Australian models.  It includes all of the NOAA models- GFS, GEFS, NAM, NAM-NEST HRRR, HREF, SREF, HIRESW, RAP, GFS-MOS, NAM-MOS, EDKMOS, etc.

On an hourly basis, the NBM starts with the hourly HRRR model , adds each of these other listed models above on a regular schedule, and compares their forecasts to something called URMA, which is an error-corrected measurement of actual observed conditions six hours previously.  It basically sees which model is doing better from actual measurements six hours earlier and weights that towards a better composite forecast.  

The NBM model is available as a standard model in “grib” format and also available as a text file.

If you want to see the latest NBM forecast in text version you can click here for Philadelphia and Blue Bell.

ICON Model

The ICON model is a German global (and regional) weather numerical prediction model from Deutscher Wetterdienst.  It is run 4x daily, every 6 hours. What’s interesting about the ICON is its unique attempt to reduce computational errors by slicing/dicing up the atmosphere into icosahedrons (pyramids) instead of spherical cubes, which is what the new GFS-FV3 uses.   FV3= is Finite Volume Cube).

All numeric weather prediction models divide up the atmosphere three- dimensionally. The geometry of each model, and the number of vertical levels and the horizontal size (resolution)  determines its computational load and the amount of computational error created.   (Many of the equations for weather prediction become infinite series calculations that require cut off.  Successful geometric calculation can reduce the cut off error at the top of the troposphere. )

I’ve already gotten too technical, but the ICON is what is called a “non-hydrostatic model”, meaning it’s good at small scale vertical motions associated with thunderstorms and precipitation.

Why do I use the ICON model?  It seems to do very well with temperatures, cloud cover and high precipitation events.  I’ve been surprised how good it is. When the NBM shows high statistical spread (uncertainty), I lean towards the ICON forecast if it’s within the NBM spread. 

It’s been a challenge for me to learn how to download the ICON and convert it.  Since it’s icosahedral in geometry, it takes one of my old iMacs, used as a dedicated server, over an hour to re-interpolate the model into the standard latitude and longitude format and reduce its file size to something manageable.   And that’s the time needed to download and convert only a few parameters!

I don’t know why the ICON is not included in the NBM and I’m not aware of any research papers comparing the ICON to other models.  But I’m sure they’re out there somewhere on Google.   


Recently, NOAA updated the global forecast weather model (GFS) to version 16. This version had numerous enhancements including an upgrade in cloud physics and an increase in the number of vertical levels of the atmosphere for which weather parameters are calculated.

Another fascinating upgrade is planned for next week.

The version 16.1 upgrade will incorporate a significant improvement in the accuracy of various three dimensional atmospheric measurements (such as humidity, temperature, pressure) that go into the “initialization” — the starting point for each GFS model run.

Incredibly, the new atmospheric data will come from the radio signal of GPS satellites!

Basically, the signal transmitted by GPS satellites will be picked up by other low orbit satellites as the GPS signal travels through the atmosphere.

The technique, known as radio occultation, detects changes in the GPS radio signal as it passes through different atmospheric thicknesses and angles. The changes in the signal reveal the physical measurements of temperatures, pressure, humidity to higher precision..

This improvement in data input is expected improve GFS model forecast accuracy.

More information about radio occultation can be found here.


I spend all too much time searching and viewing weather information sites on the Internet.

Today I stumbled upon an impressive weather web site (from our very own for anyone wishing to learn the basics of atmospheric science and weather forecasting.

Great for for kids.. and adults too! Hey, I learned (and re-learned) a few things!

It’s from the National Weather Service- NOAA, and it’s called “JETSTREAM” —

It’s a very comprehensive, extremely well-designed site that’s structured as a comprehensive introductory course to atmospheric science, somewhere between high school and college level. I think even younger kids into science would enjoy it.

It beautifully explains and illustrates things like jet streams, vorticity, upper air charts and weather. It’s the real deal and it’s basic weather science information as it should be presented. I think it could inspire more than a few budding meteorologists. I would have loved to have had access to this as a kid.

You might want to check it out…or share this with someone.