Category Archives: Commentary

Commentary

FORECAST POST-MORTEM

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We had about 2.5 inches snow just northwest of the city.  I had predicted zero to 1 around here, so what happened?

First, it was always what I called a boundary conditions forecast, since the forecast parameters for snow vs. sleet were so on the borderline of different outcomes.   I wrestled with the forecast for days.

Here’s what I got wrong- temperatures at two critical levels of the atmosphere below 7000 feet were forecast to stay at or below freezing.  I ignored these temperatures, since the other critical level of the atmosphere, “the thickness level 1000-500 mb”, (a measure of the average temperature below 18,000 feet) has always served me well and it was expected to warm up.  It didn’t.

It turns out the earlier NAM’s predicted 500 mb thickness level was wrong.   It had been forecast earlier in the day to rise to 5440 meters by 7 PM.  (Too warm for snow usually.)

However,  the 7 PM  NAM initial conditions showed the thickness level to be 5420 and not increasing.  (Snow vs rain occurs if  the thickness level is at or below 5400 in our area.)    So the mid levels of the atmosphere never warmed as expected.

This “critical thickness” level error was made by both the GFS and NAM models.   Since the “predicted thickness” level was so close to the “critical thickness”,  any error was going to blow the forecast.

Indeed, the NWS had to issue a Winter Storm Warning last night as late as 6:30 PM.  So they were also caught by surprise.

That wasn’t all that went wrong with the models forecast-  All models, including the National Blend of Models had the storm intensifying and lasting through at least midnight.  The NBM had the chance of precip of near 100%  at midnight in PHL.  But the storm was a faster mover, didn’t develop as expected, and was long out of here by 10 PM!

So that’s what happened — Parameters on the borderline weren’t accurately predicted and the storm speed and development wasn’t modeled properly!

Hey, bad modeling by engineers designing  bridges causes them collapse;  this bad forecast is just going to just melt away today!

Commentary

Cecily Tynan doesn’t know what an “Omega block” is

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On today’s 6PM weather segment on Channel 6, Cecily Tynan showed a graphic of an upper trough expected to influence this weekend’s weather and incorrectly labeled and described it as an “Omega Block”.

An Omega Block is the result of high pressure and the jet stream takes the shape of the Greek letter Omega. What she showed is a plain old upper trough.

Correctly identifying an Omega Block is one of the ABCs of meteorology.

 

 

Commentary

Inconsistent Weather Forecasts- Commentary

I’ve been looking at the weather for years.  I can’t remember a previous period of time where the major models have been so inconsistent with forecasts.   The major models, while often showing some disagreement with various features, seem to be in less agreement over the past month or so.

More importantly, the models themselves seem to change with their own forecasts from day to day.  Two examples this week- based on the forecast from the weekend, Tuesday was expected to be mild, but it wasn’t until Monday that it became apparent that temps would be near 60.  Thursday was supposed to be much colder at the start, now  not so until later in the day. What looked like snow is now a rain forecast.

Basically, this season, you can’t count on the models for a specific scenario beyond 2, maybe 3 days in advance.

I’m not sure why this is the case.  I have been reading that temperatures in the Arctic circle are very mild this year, with the cold pools of air being further south, in Siberia for example.  Not the norm. I suspect this is throwing off the models quite a bit.

Either way, weather forecasting so far this cold season has been more of a challenge than usual.