METAR 101 and Why We're Having Record Heat

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There was some confusion today when it comes to the temperature at Little Rock.  Was it 107 or 108 degrees for the high temperature?  John Robinson with the National Weather Service office in North Little Rock explains in this email he sent me.

"Automated observations taken by ASOS (Automated Service Observing System) at airports such as Little Rock contain a coded group near the end, called a T Group.

The T Group begins with the letter T and contains 8 digits, which represent the current temperature and dewpoint with an accuracy to one-tenth of a degree Celsius.  For example, consider the following observation from Little Rock:
KLIT 282053Z 25009KT 10SM CLR 42/12 A2997 RMK AO2 SLP146 T04170122 58017
The T Group, T04170122, means that the current temperature is +41.7C and the current dewpoint is +12.2C.

The temperature conversion used by ASOS then takes the 41.7C temperature and converts this to 107F.  This is done this way not only by ASOS but also by the program that produces the NWS' Hourly Weather Roundup, which is seen on the Internet each hour.  Apparently, some programs outside the NWS do not use the T group in converting temperatures from C to F.  Instead, they use the 2-digit temperature group, in whole degrees C, seen in the main part of the observation.  This would be "42" in the observation above.  Using the 2-digit group with whole degrees is not as exact as using the 3-digit group with degrees and tenths.

So, again, the 41.7C is converted to Fahrenheit and becomes 107F.  If the 42 in the main part of the observation is converted instead, the 42C would equate to 108F, but that's not the point at which the conversion is supposed to be done.  If you do the raw conversion of C to F, 41.7C comes out to 107.1F, so it's not even getting close to 108F.  Of course, just because it was 41.7C on the hourly observation, that doesn't necessarily mean that it didn't get a tad higher between hours.  If, for example, it actually topped out at 41.9C, that would come out to 107.4F.

Every 6 hours, ASOS puts a 1 Group and a 2 Group at the end of the observation.  The 1 Group is the exact temperature in Celsius (degrees and tenths) that was the high for the 6-hour period that is ending, and the 2 Group shows the low.  So, let's consider this observation from Little Rock: 

KLIT 282353Z 18010KT 10SM CLR 39/14 A2994 RMK AO2 SLP138 T03890144 10417 20383 56008

The 1 Group here is 10417, which says the high for the day was 41.7C, which equates to 107.1F.

Before the NWS sends out the Climate Summary with the statistics for that day, the ASOS is called and it reports what the high and low, in Fahrenheit, have been so far that day.  At the end of the day, the high and low for the 24-hour period will go into the official record.

Some smaller airports have automated observing systems called AWOS (Automated Weather Observing System).  Typically, these observations do not contain T Groups, 1 Groups, and 2 Groups.  Thus, there is no choice but to use the 2-digit Celsius temperature in the main part of the observation as the current temperature, and convert it to Fahrenheit.  The high and low have to be determined by taking the highest and lowest temperatures appearing on any observation and using these as the high and low for the day, since there is no way to tell whether a high or low for the day might have occurred in-between transmitted observations."

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Anyone who said they forecast 107 degrees for the high temperature today isn't telling you the truth.  With that said, the forecast did call for 102-103 degrees today.  That's not too far off, but when you're talking about extremes, people notice the difference.

Anyway, what caused it?  It was the perfect combination of what I call the "Triple Ingredients To Top 100" plus one other very important factor which made it get out of hand.  I'll explain.  Normally in summer here are the 3 ingredients to hit 100....

1. Strong high pressure
2. Dry ground
3. Ample sunshine

We had all of that Thursday plus one other factor which caused temperatures to reach levels never seen before in June weather history for Little Rock and other locations.... DRY AIR!

The presence of dry air allows the temperature to cool rapidly and efficiently at night, but it heats rapidly during the day.  Usually during an Arkansas summer, we have thick and high humidity levels.  Water vapor is the number 1 greenhouse gas in our atmosphere.  High levels of humidity will prevent temperatures from varying too much compared to when dry air is in place.  Heat gets trapped by water vapor and it's not allowed to enter or escape as efficiently compared to the drier air.  I hope this makes sense.

However, when the humidity levels are high, it feels even hotter due to the inability for your body to cool itself.  Evaporating moisture off your body is more difficult because the air surrounding you is already full of moisture.  When dry air is in place, the evaporation rate is much higher and you will not sweat as much.  This can lead to evaporational cooling.  That's what we experienced Thursday.  Dewpoint values were in the 50s and with readings around 107, it felt like 105 degrees.  There was an actual cooling effect due to the dry air.

Now when dewpoints rise, the temperature may not reach the extreme levels it did Thursday, but since your body can't cool itself properly, the "feels like" temperature exceeds the actual temperature.

Over the next few days, the humidity levels will begin to increase some and this will take temperatures down very slowly, but it will feel hotter because of humidity levels will increase.  The National Weather Service has already issued a heat advisory for Friday and Saturday to account for this.

Someone asked why we didn't have thunderstorms develop.  The dry air didn't allow for that and there was a very strong cap in place meaning the air couldn't rise enough to build clouds or even storms.

As moisture levels do increase, the chance for isolated, stray showers may increase as well, but the coverage will be extremely small.  In other words, the drought continues for at least the next week and probably beyond.

Here are the daily record highs over the next 2 days.  They will be challenged.

Friday June 29th... 105 (1988)
Saturday June 30th... 102 (1931)

In the upper levels, strong high pressure is in control.  The jet stream and cooler air is locked up to the north.  This is very typical in the summer

The visible satellite shortly after 4 PM shows virtually NO cloud cover bringing us tons of sun.  Another ingredient for super hot weather.

The dry ground is also taking its toll.  The suns energy is used in heating the air since the soil moisture is almost gone.  The map above shows many areas now in an "extreme" drought.

This is from the National Weather Service office in NLR.  Check out the widespread misery.

Here's a preliminary list of highs today from the NWS office in North Little Rock.