Beware The 50s/30s Forecasts

There are many, many people who look at the model output statistics from the primary American global model, the Global Forecast System (GFS). These products give you highs and lows out to seven days for points all around the nation. You can see them (for Birmingham) here:

http://www.nws.noaa.gov/cgi-bin/mos/getmex.pl?sta=KBHM

By design, the GFS MOS skews toward climatology in the day 4 through 7 period, meaning highs in the 50s and lows in the 30s this time of the year. Rarely does it pick up on major weather changes, which is why often you need to toss those numbers out the window. Sure, there are days when we DO have highs in the 50s and lows in the 30s. But, in a high amplitude pattern like this, with a tap to the Yukon, forecasting 50s and 30s at the longer range can be a disaster.

Last Thursday (December 9) the GFS MOS was spitting out the usual 50s and 30s for Alabama, valid tomorrow (Tuesday December 14). To be exact, the GFS MOS product last Thursday has a high of 50 and a low of 30 for Birmingham on Tuesday December 14 (I had to dig through the archives to find it).

Looks like the actual high tomorrow will be close to 39, and the early morning low close to 23... a good TEN degree bust.

Lets look back at last Thursday's actual forecasts.

Here is a quote from our blog on Thursday December 9 (check it out on the calendar to see it yourself:

"Next Tuesday, I don't think we will reach 40. By Wednesday morning, lows in the 20 to 25 degree range should be common."

So, you see, it is a GOOD thing to depart from the GFS MOS 50s/30s thing somtimes if you want a GOOD forecast. I was not cold enough on the morning lows.

Lets give the local NWS office some credit... they were not as bold as us, but at least they lowered the GFS numbers as well last Thursday. Here was their state forecast from December 9:

STATE FORECAST FOR ALABAMA
NATIONAL WEATHER SERVICE BIRMINGHAM AL
500 PM CST THU DEC 09 2004

.TONIGHT...MOSTLY CLOUDY. A SLIGHT CHANCE OF SHOWERS. PATCHY FOG
AFTER MIDNIGHT. LOWS IN THE LOWER TO MID 50S.
.FRIDAY...PARTLY CLOUDY. A SLIGHT CHANCE OF SHOWERS...MAINLY NORTH.
HIGHS IN THE LOWER 50S TO MID 60S.
.FRIDAY NIGHT...MOSTLY CLOUDY. A SLIGHT CHANCE OF SHOWERS...MAINLY
NORTH. LOWS IN THE UPPER 30S.
.SATURDAY...PARTLY CLOUDY. HIGHS 48 TO 55.
.SUNDAY...PARTLY CLOUDY. LOWS 30 TO 40. HIGHS 50 TO 60.
.MONDAY...PARTLY CLOUDY. LOWS 38 TO 42. HIGHS 48 TO 60.
.TUESDAY...MOSTLY SUNNY. LOWS 28 TO 35. HIGHS 45 TO 55.


What I don't understand is how some weather operations blindly buy into the 50s/30s stuff beyond the third day. Amazing.

And, please let me say I will be dead wrong at times when I depart from the beloved GFS MOS, but my object here is to be CORRECT, not to throw out some watered down forecast that drifts back to the 50s and 30s every time.

Have I vented enough? I love this blog, because you can click on any calendar day and go back to see what we ACTUALLY SAID. I do get tired of being blamed for bad forecasts thrown out by other people. Sure, we will be wrong at times, but at least we are accountable and stand by our products.

Thanks for taking the time to read this rant. Now, back to the regular blog material, and enjoy the cold weather! Headed out to Mountain Brook to speak to the kids at Crestline Elementary school, and then back in the office where I will knock out the afternoon video update.