Heat Storm: 100 degrees tomorrow? Why dew point matters

50/50 shot at 100 degrees tomorrow for metro southern Minnesota

95 to 100 degrees likely for the Twin Cities by 5pm Wednesday

“Thermal Ridge” – hottest plume of air right over the Twin Cities from 2-6pm tomorrow

7 days at or above 90 degrees in the metro so far in 2012

3 more days possible Wednesday-Friday?

14 long term average number or 90 degree days in a Twin Cities summer

77 degrees GFS dew point forecast at 10pm Wednesday evening

82 degrees – highest dew point ever recorded at MSP Airport

(Set July 19th, 2011 at 3 & 4pm)

88 degrees – record dew point for Minnesota set last July 19th at Morehead

Sticky season – several days of 70+ dew points next 2 weeks?

Debby Downer – T.S. Debby dousing Florida & Georgia with heavy rains

68 heat.png

Source: Twin Cities NWS

Heat Storm Ahead:

Here it comes.

An intense “thermal ridge” is building and forecast to push over Minnesota Wednesday afternoon. The heat plume should set up overhead between 2pm and 6pm, right at “max tem time.” (The time of day when solar heating produces the highest temps of the day)

Depending on which model parameters you look at, Wednesday’s high temperatures in southern Minnesota should range from the mid 90s to over 100 degrees.

68 temps.PNG

Source: Iowa State University

One tried and true technique we used back at “Weather Command” in Chicago back in the day is to use the forecast 850 millibar temps to calculate expected high temps for the following day. The NAM model is forecast +28 to +30 degrees C at about 5,000 feet tomorrow afternoon. If you mix that air down to the surface, you can yield a high temperature between 100 and 105 degrees!

It’s possible we may not quite get there tomorrow, but I think 95 to 100 degrees is likely, and there’s a 50/50 shot temps will reach 100 degrees in southern Minnesota tomorrow.

Fire up the AC.

By the way, it’s a good idea to check your outside AC unit to see that the metal grates are clean. Cotton seed and other debris can coat the cooling plates, and this reduces your AC performance. My HVAC guy says a light brushing or careful spray of water can remove the debris and improve your AC performance on hot sticky days. Be careful not to damage the thin metal plates or get water into the AC unit.

Now there’s some “news you can use.”

It’s not the heat; It’s the dew point:

58 degrees. Remeber that dew point number.

58 degrees is about the dew point threshold where comfort is equal to the actual air temperatures.

When the dew point rises above 58 degrees it makes the air feel hotter as your body is less able to cool off through evaporation. Below 58 degrees and it feels cooler than the actual air temp.

You can see this effect on the heat index chart below.

69 hx dews.PNG

Source: NWS/OK Climate Survey

Here’s another way to gauge how you will feel at different dew point temps.

40s – crisp, almost fall like

50s – comfortable, good sleeping weather

60s – getting sticky

70s – tropical, sweaty, very uncomfortable

80s – oppressive, unbearable

Dew points are expected to reach into the 70s Wednesday afternoon & evening. Get ready for a free sauna.

68 dewps.PNG

Source: Iowa State University

One counter intuitive factor with high dew pints is they can actually hold air temps down. It takes more energy to heat and cool moist air, and sometimes high dew points can keep air temperatures a few degrees lower than you might see with drier air. (This is one reason desert temps get so hot.) It’s no bargain though for your body. The “feels like” or heat index temps mover higher with the dew point. Ugh.

Brief “Heat Storm”

At least the barbaric heat won’t last.

A weak “cool front” really a “dry front” will push through Thursday. That will ease temps about 5-10 degrees, but knock dew points down into the upper 50s to near 60 degrees Thursday & Friday. Temps will still push 90 Thursday & Friday, but you’ll feel a lot more comfortable with the lower dew points.

Debby dumps on Florida & Georgia:

Talk about taking your sweet time.

Tropical Storm Debby is still lollygagging over northern Florida today, The system is moving at a walk, a forward speed of a mere 3mph.

68 debby nhc track.gif

Source: NOAA/NHC

This is why some flooding and torrential rainfall totals of up to 25″ in some areas may occur before Debby moves out over the open Atlantic. Models are split on Debby’s eventual strength once it crosses into the warmer waters of the Gulf Stream.

68 debby ntensity.png

Source: NOAA/http://derecho.math.uwm.edu/models/al042012_inten.png


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