Severe Saturday?

Twin Cities forecasters are talking about the possibility of severe storms on Saturday, which means Twins fans and U2 fans can't rest easy. Weigh in with your thoughts here.

How Legitimate Are All These Heat Index Readings?

How Reliable Are Weather Stations?
With higher and higher heat index reports coming in from parts of the state (mostly western MN) like competing bids at a Sotheby auction, we wondered how reliable such readings might be. We asked a National Weather Service meteorologist for an opinion on the legitimacy of such readings and how often the NWS might review or approve weather stations beyond the larger ones such as the Minneapolis/St. Paul airport. Here's what he had to say:

The first-order stations with ASOS (Automated Surface Observing System) instruments (such as the MSP airport) are rigorously monitored and maintained by NWS technicians. They undergo at least routine maintenance, and when errors are detected non-routine maintenance is performed. As for all the other stations out there -- FAA, AWOS (Automated Weather Observing System), MNDOT system, etc. -- I am not aware of the maintenance they undergo, but based on my experience I believe it is less rigorous than the NWS maintained stations.

If a record high dew point is observed in the state (which may have happened I believe) there will be a much closer look done, likely by the state climate office consulting with the local NWS office and possibly some other folks at national climate centers.

Why Isn't Wind Factored Into the Heat Index Calculation?
If you're like us (which is to say weather enthusiasts with very little scientific knowledge), you might wonder why wind conditions don't factor into the heat index. At some level, it would seem that wind -- even a warm, humid wind -- would slow the effect of heat on the body. Of course a scientific explanation trumps a misinformed hunch. Thanks to Nick Benson (@Ottergoose on Twitter), we received a well-reasoned explanation:

The heat index doesn't take wind into effect because heat loss at high temperatures comes from perspiration, not convection. Only air cooler than you can actually cool you down; sweating only works if the air has capacity to absorb your perspiration. Think of air as a mop; if it's saturated, the floor will stay damp no matter how quickly you scrub it. Likewise, saturated air blowing over you will have a minimal impact on absorbing perspiration (cooling).

And so ends a Simply Science version of TMF.

Will Wednesday Provide the Century Icing to the Heat Wave Cake?

Tuesday's 119 heat index

OK, "icing" may not be the right word, but you get the drift. Will Wednesday delivery the coup de grâce for this record-setting heat wave? The majority of local forecasters are predicting that Wednesday will indeed be the hottest of the current heat wave. (The national forecasters, not so much.) As of late afternoon on Tuesday, here's how the weathercasters saw Wednesday:

WCCO: 100
KSTP: 100
FOX: 97
KARE: 100
NWS: 97
Weather Channel: 96
Accuweather: 96
Intellicast: 95
Star Tribune: 99

Widely Predicted Heat Wave Finally Arrives in Earnest. Eighties or 90s Next Weekend?

Heat wave update as of 4 p.m.

Until today, the heat wave that was so universally predicted for so many days didn't pack the punch we were led to believe. Today, the forecasts came true. For the first time in many days, the forecast high temperature and heat indexes were reached, and even exceeded. By late afternoon, the heat index of 112 in Minneapolis was the highest of any major city in the U.S. Take a bow, y'all.

Here's a summary of how the heat wave is "performing" against forecasts made on Friday night.

Here's a summary of the over (under) prediction of the heat wave to date for the period since Friday night. At this juncture (and assuming the 98-degree high for Monday holds), KARE has the best forecast record for the period measured; Fox, the worst.

80s or 90s for Next Weekend?
Finally, in another test of the extended outlook forecasting ability of the weather mavens in our midst, here's next weekend's forecast temperatures as of last night's (Sunday night) evening forecasts. (Note: KARE does not provide specific forecasts beyond day 5. The "90s" information was provided anecdotally.)