Category Archives: Commentary

SO HOW DID MY FORECAST GO BAD?

With high certainty today, I had made a forecast for an immediate switch to sleet and rain.  I blew it.   I feel I have an obligation to explain why the forecast went wrong today.

I’m going to get technical but I’m also going to share some of my secrets for winter weather forecasts in Philadelphia.  (It’s the least I can do to make up for the forecast error!)

First, I prefer the NAM model for precipitation type. I use that model almost exclusively for snow storms but also use the GFS for QPF and other things.

With the NAM, there is something called FOUS data (Forecast Output Statistics).  It’s a form of  tabular data that’s been available for years, even when the NAM was previously called the ETA.

The FOUS data allows a quick and incredibly powerful way of looking at a forecast for a single area, six hours at a time.

Here’s the FOUS data from this morning’s NAM model run for PHL. (I’ll explain the boxed in numbers in a moment)  —

The data is tabular and somewhat coded to allow different data to be shown using two digits in each column.  The rows are 06-60 are six hour increments, starting here at 12Z (7 AM EST.)

Those who read my forecasts know that I put a lot of emphasis on upper atmospheric temperatures vs lower atmospheric temperatures. One way of estimating upper level temperatures is atmospheric “thickness” between near surface and 500 mb (18,000 feet).  It’s called the 500mb thickness. (The Boxed column above.)  It works out that this thickness is an indirect measure of density and temperature in the upper levels.  As the thickness gets higher, the average temperature in the upper atmosphere is higher.

In Philadelphia, at or near sea level, the critical 500 mb thickness  (rain vs snow) (which relates to density and ultimately temperature) is 5400 decimeters.

In FOUS data, the 5 is dropped and the last zero is dropped.   In the FOUS data, 5400 dm , the 500 mb critical thickness would show in this column as 40.

Let’s cut to the chase:   When the thickness is above 5400 (40)  in most cases, it’s usually too warm to snow in the upper atmosphere.  Today it showed 46-55!

I rarely see any exceptions to this, although it did cause a forecast error last season:  We had snow last season, (4 inches) even though the FOUS thickness was 5420 (42)-

February 18 2018 FOUS  We had snow with Thickness above 40

 

So, what happened today?

Here again is today’s FOUS, now with the lower atmosphere temperatures boxed.  The actual temperature columns are at 1000, 800 and 900 mb. (the lower atmosphere.)

Today’s FOUS with temperatures

These temperature numbers are also ‘coded’.  Here, 98 means -2 degrees celsius. So, you can see that the NAM forecast shows the lower levels at or below freezing until sometime late afternoon, (when the last number goes to 04).

So the reason for the poor forecast- with the 500 mb thickness at 46-49, I thought it was a straight-forward forecast of too warm to snow today. (Or to use a common expression, “too big to fail”. LOL).

It works most of the time, but not today. Today, the temperatures in the lower atmosphere called the shots.

I’ve shared how I approach the last grouping of numbers which is a summarized way of viewing the 3D temperature profile- thickness and temperatures and precipitation type.

As I write this note (1:45 PM) , the snow is changing to rain!  I knew it was going to happen eventually.

I guess the NBM is doing well!

HURRICANE FLORENCE COMMENTARY

The flooding and devastation brought by Hurricane Florence continues in the Carolinas.

I wanted to just weigh-in on the forecasting aspect of the storm.  I haven’t heard very much credit given to the real meteorologists, mathematicians, and scientists that developed such remarkable computer models of the earth’s atmosphere.

The computer models did incredibly well with the precipitation forecast and early/late track of this storm and its remnants.   Possibly the best I’ve seen in recent years.

The new GFS-FV3 model, still in trial mode (to replace the current GFS and become operational in January 2019), also did very well.

While the intensity forecasts are generally unreliable, and were off  with this storm, (it luckily hit the coast as a category 1 instead of the previously forecast category 3-4), the rest of the forecast was very impressive.

Thank you to the National Weather Service and their scientists!

 

SO WHAT’S WITH THIS COLD WEATHER?

People are asking me, what’s with this prolonged winter weather and cold?

One thing not talked about recently is the solar cycle.  We are about to enter a solar minimum.   The solar cycle is a regular, periodic change in sunspot number and the mimimum correlates with reduced total solar irradiance.

The solar cycle repeats ever 10-11 years.

The SORCE PROJECT has been measuring the total solar irradiance since 2003.

SORCE project total solar irradiance
SORCE Project Total Solar Irradiance

The previous solar minimum was in 2009.  The minimum has been shown to have a total solar irradiance that is reduced by about 0.5 -1 watt/m2.  Over the surface of the entire earth, that reduction adds up.

Maybe you’ll recall that the summers of 2009 and 2010 were usually cool?  I think the coming summers of 2018 and 2019 might be similarly cool.   (We’ll find out!)

So if it’s a cool summer, remember, it may be the sunspot cycle.

BTW, Here’s a link from my old blog in 2009 talking about the solar cycle.

Clarification 4-14-18: When I say cooler weather, it doesn’t mean we won’t have heat waves.  But it means that there will be fewer days in the 90s and average temperatures for the months of June July and August may be just below average.

SNOWSTORM POST-MORTEM

My previous post included the link for the official snow totals. I always like to evaluate my forecast to improve on future forecasts.

For the immediate PHL area, snowfall was between 7-11 inches and areas  somewhat further north and west, totals approached 15 inches.

With my call of 4-7 inches with an emphasis on “7 most likely”, my forecast fell short.  So what happened?

Official QPF measured at the airport yesterday was 1.06 water, somewhat higher than the 0.87 predicted by the NAM and higher than the GFS. But overall the models did well on QPF. (The actual snow total for just Wednesday was 6.7 inches at PHL airport, according to the NWS.)

My emphasis on solar insolation through clouds lead my forecast astray.  Had it been January, I would have predicted 8-10, which I mentioned in several posts.  So I learned yesterday that heavy snowfall rates trump solar effects, even in late March.

While the models did well on QPF, they did less well on wind. That was lucky for our region.  Temperatures were predicted well.

Mesoscale banding set up northwest of the city and snowfall was heavier in those areas.

I’m ready for Spring!

Final Two day snow totals