Dan's Wild Wild Science Journal

Pressure pattern at around 18,000 feet (in colour bands) and at the surface (in black lines) for today. The the big high pressure ridge over the East is clearly visible.

For those of you in the Midwestern and Southern parts of America you are likely wondering what the heck happened to fall! Temperatures here in Huntsville in Alabama are running 15 degrees above normal in the afternoon!

This heat wave actually stretches from Oklahoma all the way into Illinois, into Virginia and deep into Old Dixie. It’s really the same heat wave that existed June through August and brought many cities the hottest summer on record.

Same Old Heat Wave

The only difference now is that the days are over an hour shorter and the sun is not as high in the sky. So instead of 106° we see 96°F. Instead of a dew point of 77° we are getting some drier air lingering behind the very weak cool fronts that have managed to break through the upper level ridge. The dew point her ein Huntsville  now is 59°F.

By early next week the pattern changes dramatically with a trough in the east. This means much cooler air and even some desperately needed rain. (Courtesy Unisys weather ECM Model.)

Good news is coming.

The long range numerical weather models are all signaling a pattern change to cooler weather starting this weekend. The strong upper level (heat) ridge will shift to the West and allow the Pacific NW to dry out and see some sun. The East will finally feel some real fall weather.

Right On cue

Click image to read the NRDC report on summer temp. records. Some amazing numbers here.

Meteorological Autumn began back on September first, but most people still use the equinox to denote fall. That occurs at 3:09 GMT Thursday morning. That’s 10:09 PM Wednesday evening (Central U.S. summer time.)

The NRDC has compiled some incredible statistics of the summer heat that are worth taking a look at. You cannot blame any one event or even a brutally hot summer on climate change, but this past summer had some very suspicious patterns to the heat. Patterns that are exactly what you would expect as a result of increasing greenhouse gases.

You can certainly rule out the sun. It has been unusually quiet (see the previous post). You can also rule out  El Nino, since the La Nina has been rapidly developing and this acts to cool the planet.

Colder than normal water is already appearing in the Equatorial Pacific (NOAA)

One of the reasons for the forecasts of a an active hurricane season is the predictions that La Nina would develop this summer. Remember that La Nina is an ocean circulation phase that brings unusually cold water to the surface of the Tropical Pacific.

These are the temperature probabilities for a normal/above/below normal autumn (September through Novermber). Click for full resolution image.

Well, it has started and most of the ocean atmosphere models are predicting a moderate to strong event.

This has more impact than just the number of hurricanes. NOAA revised their 2010 hurricane prediction down somewhat today but they are still predicting a 70% chance of 4-6 major hurricanes (Earlier typo said 406- that would be exceptionally bad ). La Nina is a big factor in that.

The colder tropical waters cause a shift in the jet stream. This makes it possible to make a decent prediction of the fall and winter ahead. Here in the Southeastern USA we tend to have a dry fall with normal to above normal  temps. The winter tends to be drier than  normal and sometimes a bit on the mild side.

What about where you live?
The chart at the right shows the probabilities for temperatures during a La Nina event. The image is for the period from September through November.

Take a look at the Northeast USA and Eastern Canada. The chart shows that during a La Nina year the odds are VERY low that the autumn will be below normal. The odds are higher that it will be near normal and the odds are higher still that it will be a warm autumn.

This does not mean that it will not be colder than normal. It just means that in the past this happened rarely.

You can see more of these graphics for your part of the world at the IRI website here. Precipitation maps are also available.

Duck and cover! The new U.S. Record hailstone. Image from the NWS Aberdeen SD/NOAA.

If you are a weather nut there are certain records that you know off the top of your head. No need to look them up. So I knew the record books listed the largest hailstone ever measured as the one from Coffeyville, Kansas that fell in 1970. That stone weighed in at a whopping 757 grams  (That’s 1.67 pounds for you metrically challenged folks).

Nancy Knight (Top expert on hailstones) of the National Centers For Atmospheric Research holds the previous record holder, the Coffeyville KS hailstone. Diameter 14.4 cm.

Then came the severe thunderstorm that hit Vivian, South Dakota on 23 July, 2010. Les Scott saw his yard covered with HUGE hailstones and saved the biggest he could find. A power outage caused it to melt a bit but he thought it might be a good idea to let the local NWS office know about it.

They of course were very interested and came out and measured and weighed it. The Warning Coordination Meteorologist for the Aberdeen, SD  NWS then contacted the NOAA Climate Extremes Committee. They have declared that the long standing Coffeyville Kansas ice chunk from the sky is no longer number one!

The official weight of the Vivian hailstone was recorded at 879 grams or 1.93 pounds! That’s a new record! The stone also broke the record diameter of a hailstone. The previous record was 7 inches from a stone that fell in Aurora, Nebraska in 2003.

Digital rainfall estimation off of the Doppler Radar in Little Rock, Arkansas. Nearly 20cm (8 inches) of rain fell overnight in the Ouachita National Forest.

There are many similarities to the flash flood that hit Montgomery County, Arkansas last night and the Big Thompson Canyon flood in Colorado in 1976. Both were National Forest campgrounds near streams. 145 campers died on 31 July in 1976 in the Colorado flood. The death toll now stands at 16 in Arkansas.  Some are still missing.

IR satellite image at 12:45 AM Friday showing the MCS over SW Arkansas. The heavy rain lasted for nearly 5 hours. The dark purple indicates clouds with a temp. of -70C. Clouds that cold would be very high (Over 50,000 feet.)

What Caused It

The flood was caused by what meteorologists call an MCS.

MCS stands for Mesoscale Convective System. In other words, a big cluster of thunderstorms. These systems can produce heavy rains as thunderstorms redevelop over the same place for hours. Last night the winds aloft that steer the storms were very light and this contributed to the heavy rainfall.

The MCS was kicked off by a very slow moving upper level low that has been drifting northeast across Texas for the past few days. It moved into Arkansas last night. A flood  gauge near the campground showed a rise in the river of 2.4 meters in one hour! The total rainfall is nearly 20cm or around 7.5 inches of rain.

It’s Happened Before

I was working on-air in Tulsa, Oklahoma during one of these events.  It was Memorial Day weekend in 1984. That night is burned in my memory. That flash flood killed 14 people.

These events are why you should always have a weather radio with you when camping. There are some excellent and inexpensive models available now.

Stream discharge from a river gauge near the campground. (USGS)

Budget Cuts May Impair Warnings

Another part to this story is the flood gauges on rivers. Most of these are put in place by the USGS. Budget cuts are forcing the removal of many of them.  This means there will be fewer gauges that forecasters can rely on to issue short fused warnings. Some gauges with many years of data will no longer be maintained.

Here is a stream gauge near the site of the deadly flood. The stream discharge triples in just a few hours!

The public is always worried about getting hit by a tornado when in reality the main weather killers in America are lightning and flash floods.

A Climate Connection?

One last thing.

If someone tells you this was caused by climate change they are wrong. If someone tells you this was NOT caused by climate change they are also wrong. The atmosphere is holding more water vapor now than it did 40 years ago. What you can say is that as the planet gets warmer, we will likely see heavier and more frequent extreme rainfall events.

Like this one.

Would it have happened anyhow? Possibly. No one can say for sure about any one event.

Dan

From NOAA- The El Nino is fading away in the Pacific but Atlantic sea surface temps. are near record levels. Both of these factors argue for an active hurricane season in the Atlantic.

Hurricane season begins June 1st and NOAA’s hurricane experts will release their 2010 season forecast Friday 28 May. It’s not much of a secret among meteorologists what it will be.

Likely BAD.

Actually Daffy, it's HURRICANE Season. (Warner Bros.)

Forecasting the number of hurricanes we will see each year is a very tricky project. There is some skill but changes in the sea surface temperatures and in the upper level wind shear during the summer months make any forecast iffy.

So, that said here is why NOAA will forecast an increased number of hurricanes this year.

THE AMO:

The Atlantic Multi-Decadal Oscillation is an ocean atmosphere oscillation that runs in 10-30 year cycles. It causes warmer than normal waters in the main development region and favorable winds for more hurricanes. (On top of changes caused by increasing greenhouse gases). The AMO cycle has been favorable for above average hurricane seasons since the mid 1990′s.

Models are forecasting El Nino to quickly die and an increasing possibility of a La Nina. La Nina's cause more hurricanes. Just the opposite of El Nino's.

EL NINO:

Last winter’s El Nino is now rapidly disappearing. The warmer than normal waters in the Equatorial Pacific have almost disappeared and virtually all of the models are forecasting a return to either neutral or even La Nina conditions by the end of summer.

El Nino’s in the Pacific produce a weather pattern in the Atlantic that causes increased wind shear. This usually means fewer hurricanes. La Nina’s produce lower wind shear in the Atlantic and are associated with more active seasons.

SEA SURFACE TEMPS:

The oceans worldwide are the warmest ever measured right now. There are very strong indications this is due to the increasing greenhouse gases in the atmosphere. The record warmth in the Atlantic is likely caused the AMO discussed above and the Arctic Oscillation.

This is the same pattern of atmospheric pressure that was responsible for the cold and snowy winter in the UK and the Eastern USA. It also causes warm waters to spread over the main development region for hurricanes in the Atlantic.

You add up all of this and you get very good odds for an active season. Keep in mind that the Arctic Oscillation may change and sea water temps could cool. El Nino may hang on longer than expected too. If this happens we could see a normal season, or even below.

The odds however say otherwise.

Dan

PS There is some fascinating new science in one of the great questions of meteorology. Will we see more or fewer hurricanes in the warmer World of the late 21st century?? I will have that post soon. I’m still reading the papers…