Wednesday, January 13, 2010

No 11F Low

The National Weather Service is currently predicting a low of 11F for Bedford, MA and a low of 12F for Natick, MA. I find these forecasts hard to believe. Why? Because around 2pm today, the dew point shot up to 11F in Bedford and 13F in Natick and the dew point is currently 15F in Bedford and 17F in Natick. Yes, it is certainly possible that the dew point could drop overnight. But, I think more likely is that the jet stream shift that was due arrived a bit earlier than expected and the NWS has not used the sudden change in dew point as an indicator. Based on recent experience, late evening dew point has served as a lower bound for overnight temperature, so I'd be very surprised to see the NWS predictions come true. I'd bet that the overnight low will be around 20F in Natick and 17-18F in Bedford.

Update (1/14): I was wrong. The dew points dropped significantly this morning and temperatures fell along with them, hitting 14F in Natick and 9F in Bedford.

Tuesday, January 5, 2010

Suspicious Forecast

I was surprised and a tad suspicious when, as of 1/4/10 12pm, the overnight low for 1/4-5 forcast by NWS for Central Middlesex County was 5-10F. At that point, the dew point was around 20F. I realize the dew point can rise/fall quickly overnight, but the predicted 15F degree drop seemed unusually large, especially considering that we weren't expecting any large shifts in pressure. My suspicions were partially confirmed when, later that day, the NWS updated its overnight low forecast to 15F. But, when I saw a temp of 24F and a dew point of 17F shortly before going to bed at 10pm, I was still suspicious whether we'd really make it down to 15F.

The next morning I awoke to find that the overnight low was 22.4F and that I wasn't alone---the main airport in my prediction region, KBED, recorded a low of 25F. Though, to the NWS's credit, another nearby airport in a nearby county, KOWD, recorded a low of 16F. Still, I'm surprised the NWS prediction could be ~15F off less than 24 hours in advance, especially when daily variations in temperature are typically no more than 15 degrees! Here's hoping the NWS prediction programs noticed this anomaly...