Over many years of being a weather hobbyist and prior to having complete weather data available on the Internet, it was never clear to me what happens when a predicted cold front and its associated thunderstorms “falls apart” before reaching us.
So what happened to today’s cold front and the expected thunderstorms?
Basically, the front lost its “upper air support”. But what does that mean?
Usually when a strong cold front approaches, the upper air “heights” typically represented by the 500 mb lines (blue lines on the map below) move southward, meaning a reduction in height and a resulting vertical upward motion in the atmosphere. This supports rain and thunderstorms.
However, the most recent models today show these 500 mb height lines not moving south but instead, halting and then moving northward after midnight.
This causes sinking motion and downward vertical movement. Precipitation and thunderstorms can’t form.
That’s what’s happening tonight.
Why did the models get this wrong on previous days? Nobody knows but there is a strong tropical system off the southeastern coast and it’s been my observation over many years that when tropical systems are present, the models just don’t do that well.
Over the past year and a half, I’ve been directly downloading weather model “grib” (gridded binaries) data from NOAA for these forecasts instead of relying on the adequate, but limited data freely available from university sites on the Internet.
Over the past day I’ve been redoing my scripts/programs to have them run properly starting at 2 AM Saturday night.
To give you an idea of how late some of these weather models become available in Eastern Daylight Saving Time, here’s a few of the changes I need to make:
NAM model 9:38 PM EST —> 10:38 PM EDT
GFS model 10:38 PM EST —> 11:38 PM EDT (first 24 hour forecast data)
HIRESW Models 10:10 PM EST —> 11:10 EDT
HIREF ENSEMBLE: 11:19 EST. —> 12: 19 AM EDT
CMC GDPS 11: 45 PM EST —> 12: 45 AM EDT
ICON Model 11:21 PM EST —> 12: 21 AM EDT
To those times, add 5-10 minutes to download the data, time to review the data and you’re well into the wee hours of the morning.
Even the hourly HRRR, RAP and NBM models require several hours to incorporate the changes in the upper air measurements (also done an hour later.)
So when you hear “check back at 11” for the weather on the TV during Eastern Daylight Saving time, you’re not going to get an updated forecast on new data. It’s true for the next day forecast and even more so for the five day forecasts.
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.
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.