Excessive Heat Watch; 100 in metro Thursday?

I hope the A/C is tuned up and ready to go!

Some headlines for the blog today:

-Excessive Heat Watch issued for metro Thursday:

Heat indices over 100 likely...

-Temperature Forecast Techniques:

Just how do meteorologists come up with a "number?"

-Isolated Overnight T-Storm?

Warm front may spark a rumbler overnight.

-1st named storm in Atlantic Hurricane Season:

Tropical Storm Arlene gears up off Mexican Coast.

The heat is on for Thursday:

Get ready for another heat blast.

A strong ridge of high pressure is building overhead Thursday. You'll start to feel the dew point creeping up today, and by Thursday, a full blown "hot front" will push temps to near 100 degrees.

An Excessive Heat Watch is posted by Twin Cities NWS. It will likely be upgraded to a warning Thursday.

URGENT - WEATHER MESSAGE

NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN

414 AM CDT WED JUN 29 2011

...HOT AND HUMID CONDITIONS EXPECTED THURSDAY...

.A HOT AND MUGGY AIR MASS WILL MOVE INTO THE AREA EARLY ON

THURSDAY AND CONTINUE THROUGH FRIDAY MORNING. HIGH TEMPERATURES IN

THE LOWER TO MID 90S ALONG WITH SURGING DEWPOINTS EXPECTED TO

CLIMB WELL INTO THE 70S WILL CREATE HEAT INDEX VALUES OVER 100

DEGREES THURSDAY AFTERNOON AND EARLY EVENING. OVERNIGHT LOW

TEMPERATURES WILL ONLY DROP TO THE MID AND UPPER 70S THURSDAY

NIGHT. THE INNER DOWNTOWN AREAS OF THE METRO WILL BE THE MOST

SUSCEPTIBLE TO THE HOT AND HUMID CONDITIONS.

MNZ060-062-291715-

/O.NEW.KMPX.EH.A.0001.110630T1700Z-110701T1200Z/

HENNEPIN-RAMSEY-

INCLUDING THE CITIES OF...MINNEAPOLIS...ST. PAUL

414 AM CDT WED JUN 29 2011

...EXCESSIVE HEAT WATCH IN EFFECT FROM THURSDAY AFTERNOON THROUGH

FRIDAY MORNING...

THE NATIONAL WEATHER SERVICE IN TWIN CITIES/CHANHASSEN HAS ISSUED

AN EXCESSIVE HEAT WATCH...WHICH IS IN EFFECT FROM THURSDAY

AFTERNOON THROUGH FRIDAY MORNING.

* TEMPERATURE...HIGHS IN THE 90S THURSDAY WITH A HEAT INDEX FROM

100 TO 106.

* IMPACTS...THESE HOT AND HUMID CONDITIONS WILL LEAD TO A

HEIGHTENED RISK OF HEAT RELATED STRESS AND ILLNESSES. THIS WILL

BE ESPECIALLY TRUE FOR THE CENTRAL URBANIZED AREAS OF THE TWIN

CITIES METRO.

How high will it go?

Depending on what you look at, you can get a temperature in the 95 to 100 degree range in most of southern Minnesota by late Thursday afternoon. Combine a dew point in the mid 70s, and heat index could reach 100 to 106 degrees!

Temperature forecast techniques:

Just how do meteorologists come up with tomorrow's high temp? Where does that "number" come from anyway??

There are various forecast techniques used to calculate and predict temperatures.

One old tried and true method is "temperature computation." This is basically what forecast models do these days, but we used to do it by hand back in the "old school" days.

You start with the latest actual temperature...let's say the overnight low was 62 degrees

From 62 degrees, you add the average "diurnal variation" (difference between the average high & low temp on a given day) for the day. This time of year that's 21 degrees for the metro.

So 62 + 21 = 83 degrees if everything is "average."

Now you apply various correction factors. These may include

-Warm or cold advection (warmer or colder air moving in)

-Sun/clouds & sun angle

-Wind

-Precipitation

-Snow cover

-Soil moisture

-Residual heat/cold

-Miscellaneous factors such as smoke, haze etc.

-Lake effect in Great Lakes

After you apply the correction factors (specifics stored deep in the Weather Lab vault) you come up with a number. If you did it right, you should be pretty close to the forecast high or low temp.

850 millibar "mix down" technique:

Another way to shortcut a forecast high temperature is what's called the 850 millibar mix down technique. Basically you use the air temp at 850 millibars (around 5,000 feet above ground level) and "mix it down" to the surface. As the air descends it warms adiabatically. The temperature at 850 mb can be a great indicator of the potential surface temp on a given day.

This technique can work very well on windy sunny days in spring and summer.

Thursday's forecast 850 millibar temps of around 29 degrees Celsius would yield a surface temp of 100 to 105 degrees if everything worked out just right! Relatively high dew points may slow the heat burst, because it takes more energy to heat moist air.

Either way it's going to feel oppressively ht Thursday.

This is Amazon Jungle heat folks.

Rumbler ovenight?

As the heat gurgles north tonight, there is a chance that isolated T-Storms could develop in southern Minnesota and western Wisconsin. Don't be shocked if you hear thunder and rain in the wee hours of Thursday morning.

Arlene kicks off Atlantic Season:

Tropical Strom Arlene is the 1st named storm of the Atlantic Season.

Arlene is forecast to track westward into Mexico Thursday. The primary threat is heavy rainfall.

The details from NHC:

BULLETIN

TROPICAL STORM ARLENE INTERMEDIATE ADVISORY NUMBER 3A

NWS NATIONAL HURRICANE CENTER MIAMI FL AL012011

700 AM CDT WED JUN 29 2011

...ARLENE EXPECTED TO MAKE LANDFALL IN NORTHEASTERN MEXICO EARLY

TOMORROW...

SUMMARY OF 700 AM CDT...1200 UTC...INFORMATION

----------------------------------------------

LOCATION...21.8N 95.2W

ABOUT 175 MI...280 KM E OF TAMPICO MEXICO

MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H

PRESENT MOVEMENT...WNW OR 300 DEGREES AT 8 MPH...13 KM/H

MINIMUM CENTRAL PRESSURE...1002 MB...29.59 INCHES

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...

* THE COAST OF NORTHEASTERN MEXICO FROM BARRA DE NAUTLA NORTHWARD TO BAHIA ALGODONES

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE

EXPECTED SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN 24

HOURS.

DISCUSSION AND 48-HOUR OUTLOOK

------------------------------

AT 700 AM CDT...1200 UTC...THE CENTER OF TROPICAL STORM ARLENE WAS

LOCATED NEAR LATITUDE 21.8 NORTH...LONGITUDE 95.2 WEST. ARLENE IS

MOVING TOWARD THE WEST-NORTHWEST NEAR 8 MPH...13 KM/H. A TURN

TOWARD THE WEST IS FORECAST LATER TODAY. ON THE FORECAST TRACK...

ARLENE IS EXPECTED TO MAKE LANDFALL ALONG THE NORTHEASTERN COAST OF

MEXICO WITHIN THE WARNING AREA EARLY ON THURSDAY.

MAXIMUM SUSTAINED WINDS ARE NEAR 40 MPH...65 KM/H...WITH HIGHER

GUSTS. SOME STRENGTHENING IS FORECAST UNTIL LANDFALL ON THURSDAY.

TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP TO 115 MILES...185 KM

FROM THE CENTER.

ESTIMATED MINIMUM CENTRAL PRESSURE IS 1002 MB...29.59 INCHES.

HAZARDS AFFECTING LAND

----------------------

RAINFALL...ARLENE IS EXPECTED TO PRODUCE TOTAL RAINFALL

ACCUMULATIONS OF 4 TO 8 INCHES OVER THE MEXICAN STATES OF

TAMAULIPAS...VERACRUZ...AND EASTERN SAN LUIS POTOSI...WITH POSSIBLE

ISOLATED MAXIMUM AMOUNTS OF 15 INCHES OVER MOUNTAINOUS TERRAIN.

THESE RAINS COULD CAUSE LIFE-THREATENING FLASH FLOODS AND

MUDSLIDES.

WIND...TROPICAL STORM CONDITIONS ARE EXPECTED TO REACH THE COAST

WITHIN THE WARNING AREA BY THIS EVENING...WHICH COULD MAKE OUTSIDE

PREPARATIONS DIFFICULT OR DANGEROUS.

STORM SURGE...A STORM SURGE WILL RAISE WATER LEVELS BY AS MUCH AS 1

TO 2 FEET ABOVE NORMAL TIDE LEVELS ALONG THE IMMEDIATE COAST NEAR

AND TO THE NORTH OF WHERE THE CENTER MAKES LANDFALL. NEAR THE

COAST...THE SURGE COULD BE ACCOMPANIED BY LARGE AND DESTRUCTIVE

WAVES.

NEXT ADVISORY

-------------

NEXT COMPLETE ADVISORY...1000 AM CDT.

$$

FORECASTER PASCH/BEVEN

PH