Category Archives: Commentary


Updated with graphic below.

This week’s weather is expected to be influenced by a flat to ridge-shaped jet flow.

I know the TV weather people like to talk about the jet stream as something that has a mind of its own. They talk about changes in the jet stream as something that just changes, without any apparent reason.

Frankly, I don’t think that is all that accurate nor is it conceptually useful.

What makes more sense to me is that the jet stream winds flow confined to the three dimensional valley created between the bulges of the two major air masses. The position of the jet stream and its configuration takes on the shape of the edge of the air masses and the valley between them.

GEFS forecast jet wind speed and position (300 mb level) for this Thursday. Air mass 1 and Air mass 2 have their own circulations. Between the two air masses bulges is a valley where the the jet stream flows. Conceptually, it makes more sense to think of the jet flow shape and position as influenced by the size of the relative air masses and their individual shape. Speed ups (“jet streaks”) and slow downs in the jet flow are result of constrictions and widening between the air masses. I have intentionally drawn air mass 1 smaller than the tropical air mass 2.

Localized increases in speed of the jet stream (called ‘jet streaks’) are related to narrowing of the valleys between the air masses, just as a garden hose nozzle creates a water jet of increased velocity.

(Indeed, the the name jet stream has little to do with jet airplanes. The stream of air comprising the jet stream displays the physics of “jets” in the field of fluid mechanics, as in the garden hose reference above.)

The movements, size, temperature and densities of the main air masses on the spinning earth determine the changes in the jet wind shape and position. While there certainly is a feedback mechanism where the jet flow reciprocally affects the air mass shape, conceptually, I feel this leads to a poorer grasp of the main determinants of jet stream position and shape.

So why am I talking about this today?

Well, the position of the jet flow this week with its northern position (for December) and bulge northward is the result of the the tropical warm air mass (2 above) being larger than the current colder denser polar air mass to the north (1 above.) Simplistically, there isn’t a larger or dense enough cold air mass to displace the very large warm tropical air mass to the south.

Why is the tropical air mass so warm? Well maybe the ocean surface temperatures, warmed by the atmosphere are maintaining their heat.

Global Sea Surface Temperature anomalies. Things are looking pretty warm out there! (Click on image for a larger view.)

People talk about climate change and global warming. This is one of the ways it manifests itself.

Eventually, very dense cold air to our north will accumulate enough that it will bulge downward and push the jet stream position to our south. How the cold air slides down and its shape will determine the new valley position and the new jet stream configuration.

There is some evidence that a pattern change may be setting up, with cold air pooling in far northwestern Canada by Christmas eve. Following a very mild upcoming Christmas weekend, the final week of this year and the first week in January may be positioning itself for a pattern change

ICON model forecast for this Friday. Cold air pooling in Canada suggests a movement southeastward, pushing the jet into a more winter-like position by the very end of December. (Click on image for a larger view.)


Why is it raining now at 10AM? I thought it was going to start in the afternoon?

That’s a great question. I have no answer.

Every model from last night (RAP, HRRR, NAM, HIRESW, NAM-NEST, GFS, etc) had the rain starting about 1-3 PM. Even more incredible is that the new models from this morning 12z (8AM EDT) were still holding off the showers until the afternoon here.

Current Radar 10:20 AM EDT—

NEXRAD radar with Satellite Water Vapor superimposed. (Click on image for a larger view.)

Here’s the latest NAM model which is representative of all the NOAA and Canadian model forecasts for today—

This morning’s NAM model forecast for 10 AM, still showing the showers about 2 hours to our west. This model, which was run only two hours ago, was representative of all the US models from this morning and last night. (Click on image for a larger view.)

Interestingly, I took a look at the 2 AM German ICON model forecast for this morning. Not a bad forecast ! Would I have used its forecast instead of all the NOAA models. Not likely—

ICON model 06z rain and cloud forecast for 10 AM today. Clearly it did something right. (Click on image for a larger view.)


The National Weather Service/NOAA creates a weather prediction model that is a statistical composite of many different models— it’s called the “NBM” or “National Blend of Models”.  The NBM is created and updated hourly by NOAA and is the product of about 8-9 years of development.   It’s current version, 4.0, became operational this past December 2020.

The NBM is a sophisticated and advanced attempt to combine  the best forecasts of numerous weather models including multiple U.S. short/long range/ensemble models, several Canadian models/ensemble models and the European model/ensemble models.  It also combines various “model output statistics” (“MOS”) and hourly “LAMP” forecasts.

It attempts to achieve its aim by re-examining the forecasts of the models hourly and comparing those forecasts to actual measurements adjusted to actual measured conditions six hours earlier.   From the comparisons to actual conditions, it statistically weights the forecast going forward to the best performing model(s) six hours earlier.

The NBM ingests different model runs and forecast statistics at scheduled times during the day and creates a variety of forecast products, including maximum/minimum forecast temperatures and precipitation forecasts.

The NBM is the model I’ve been using for my high temperature forecasts.

I’ve noticed something for awhile, recently confirmed today by an official notice.   The NBM high temperature forecasts (“TMAX”) are too low,  and the error is higher with higher expected TMAX temperatures.

(I’ve been including the “sd”(standard deviation) in my forecasts for months.   I’ve noticed that the actual TMAX is often a degree or so higher than the NBM predicted value PLUS its standard deviation.)  

Today, the NBM predicted TMAX for Blue Bell was 49º with an sd of 4.4º  The actual TMAX was 53º.

So the actual TMAX  was = TMAX + sd!

The NOAA statistical modeling group today released a report today.  This graphic captures the problem of the cold bias in the NBM max temp forecasts.

NBM TMAX Bias  (Click on image for a larger view.)


With this disclosure, I’ll be getting rid of my inclusion of the standard deviation in my forecasts (as had been my practice these past six months) and instead will simply be adding the sd to the forecast high temp going forward…and hoping for the best.

With the recent disclosure that the precipitation statistics are also biased too low, we can hope for an early  release of  NBM version 4.1 or NBM v.5.  (Actually, the precipitation problem is being addressed with an update in March.)