Category Archives: Commentary

SO WHAT’S WITH THE (UN)FORECAST SNOW?

So what’s with the snow falling (which was not forecast)?   That’s a great question.  Here’s the short answer—

I wasn’t even looking for the possibility of snow today.   But as soon as I saw the snow, I went back to the high resolution NAM NEST model and saw that it was forecast to 32º or below at key levels of the lower atmosphere with plenty of precipitation occurring after 11 AM.

NAMNEST model 32º F (isotherm) temperatures at 6000 and 3000 feet above sea level (lavender and violet lines) and simulated radar (blue/green shading)

Add to this strong vertical motion and heavy precipitation with dynamic cooling and we got the brief period of snow.

I wish I had forecast that possibility, but I skipped over the thermal profiles this weekend, thinking snow wasn’t a consideration.

As for the professional forecasters and the talking heads on TV,  I don’t know what their excuse is.

Anyhow, this is what makes the weather so interesting.   The forecast might have been off, but at least we know why.

Put another way, often we blame the models for being wrong.  This time, the wrong forecast was due to the forecasters.

THERE’S GOT TO BE SOMETHING GOOD ABOUT STANDARD TIME

This Sunday morning,  Eastern Daylight Saving Time changes to Eastern Standard Time — the early morning hours get lighter (at least initially), the afternoons gets darker earlier, and the daylight hours that are already shortened seem even shorter.

What could possibly be good about this?

Well, there’s at least one good thing —  the weather models (which are run on Universal Time”UTC” (previously called Greenwich Mean Time “GMT”) are available an hour earlier! On the east coast, this makes a giant difference.

Every evening, the National Center for Environmental Prediction begins incorporating the latest weather data measurements including weather balloon radiosonde data at 00 UTC, which is 7 PM Eastern Standard Time and 8 PM Daylight Saving Time.

The incredible amount of data to ingest and the number of calculations needed for models to output takes 2 – 4 hours to complete, even on super computers.

Since last March (the beginning of DST), every meteorologist, TV station, etc. on the east coast has had to make their evening  and 11 o’clock broadcasts without the latest GFS model data, simply because the first 24 hours of GFS forecast data hadn’t become available until 11:38 PM EDT — after the 11 o’clock news was over.

With the change to Eastern Standard Time, the GFS now becomes available at 10:38 PM EST— plenty of time to update the weather on the 11 o’clock news.

Similarly, the NAM whose 24 hour forecast data became available about 9:57 PM EDT will now become available at 8:57 PM EST.

For those interested, the exact model time availability can be found here.

For amateurs who enjoy weather forecasting and for those who do it professionally, this one hour difference makes all the difference in the world.

It’s a small conciliation for the shorter days and the colder weather, but it’s something. 🙂

Over the years, I have talked about this on previous blog posts—here,  and here.


BTW, the GFS is showing a frigid outbreak for the time period around Nov 12th.

TODAY’S UNEXPECTED RAIN IN PHILADELPHIA

I find it fascinating that most of today’s models didn’t predict the rain that developed in a narrow, persistent band over Philadelphia. Interestingly, little rain developed at the shore to this point.

Radar 4:30 PM Friday, courtesy of weathertap.com

Going back to last night’s models, the WRF-NMMB and to a lesser extent the WRF-ARW were the only models to predict this rain last night.   However, the same models run this morning didn’t show it as much and the GFS, NAM really were off about this.

The unexpected rain was the result of some vorticity and vertical lift in the mid levels of the atmosphere, captured on this afternoon’s 2PM RAP model analysis—

RAP Model 2 PM Friday Analysis showing lift and vorticity at the 500 mb level (~18,000 feet)

I don’t know why, but as I mentioned last week, hurricanes seem to throw off the model forecasts, often in big ways.

This rain is expected to clear out later today.  The weekend looks to be beautiful with sunny skies and comfortable temperatures in the 70s.  I’ll do a brief update later this evening.

 

SO WHAT HAPPENED TO THE STORMS?

So what happened to the storms and flash flooding expected for today?

Clearly, the models didn’t do a very good job here.  On Sunday, the models overwhelming predicted a severe weather event.  They continued to do this on Monday, but by Monday evening there was a lack of agreement, with some models showing the insignificant showers we ended up with.

By this morning, the severe weather parameters had become very unimpressive and I posted that change this morning.  Still, the Rapid Refresh model (RAP) available at 9:35 AM showed a line of storms about 4 PM.

While at work, the rest of this morning’s models became available between 10:15 and 12:40.   The NAM NEST, WRF-AWF, WRF-NMMB, RAP, HRRR and HRDPS are the models I look at for thunderstorms. (Hey, it’s almost a full-time job, but it’s still a hobby.)

By this morning’s model run, the models had really backed off considerably with even the rain, shunting anything developing to our south.   Too late to make an updated web announcement, while at work.

Interestingly, they were still talking about severe thunderstorms on the radio while driving home at 6:30 PM.

It’s always a tough call to cancel the call for severe weather when even the slight possibility could endanger people if it occurs.

Anyhow, the “elusive” search for the model that’s always correct is elusive for a reason.

Looking back, even the GFS, a large-scale model, did better yesterday than some of the high resolution models.  And if I had to hang my hat on a model yesterday, the Canadian HRDPS probably called it the best at the earliest time.  But it’s not always right…