Humidity on the Way Back

The animation above shows why there will not be much rain through the 4th of July weekend even though the humidity comes back with a vengence.

The weather has been dominated the past few days by a Canadian air mass.  In winter this type of air mass can be quite cold but in summer the air is only a bit cooler while the change everyone notices is the  lower humidity.  So with temperatures in the mid 80s, sunshine and lower humidity summer Canadian air masses bring enjoyable summer weather.

Surface humidity alone is not enough to get widespread rain.  For a big rain event in the FOX19 viewing area you need surface moisture plus moist air above that.  Meteorologists talk about a deep layer of moisture and measure it with something called precipitable water.  If all the moisture in a column of air above a point rains out the total depth that fell is precipitable water.

Now back to the animation. For a deep layer of moisture and a large value of precipitable water we need air flow from a moisture source like the Gulf of Mexico and the Carribbean Sea.  But our air flow in the middle and  upper atmosphere through the 4th  is from the west and therefore quite dry because any Pacific moisture falls out as the air rises over  the mountains of the west and there is no other moisture source as the air travels towards Cincinnati.

So my forecast calls for the air above the surface layer to be dry and that will give us only scattered showers mainly Sunday as a front comes through the FOX19 viewing area.

Thunderstorm Rotation

In my previous blog I talked about supercell structure and mentioned that most of the rotation that causes tornadoes is imported. I promised more on that today so here it is.

Just how rotation is imported into a thunderstom is no longer a big mystery.

First wind shear in the environment around the thunderstorms causes some rotation in the storm itself, but evidence is increasingly indicating that to get a big tornado thunderstorm rotation has to be enhanced by additional rotation that is imported into the storm from the surrounding environment. then tilted to nearly vertical and stretched into a narrower and faster rotating vortex.

In general there are two sources of this rotation:

1. Vertical wind speed shear - slower winds speeds at the surface and faster winds aloft impart rotation as shown in the video below. The image below the video shows what it would look like for a wide area around the thunderstorm as rotation is generated and imported.
2. The interaction of the cold pool beneath the thunderstorm and the warm, moist inflow. Shown below in the third graphic element. You have likely experienced that cool rush of air just before the rain starts. That is the arrival of the rain cooled, cold air pool.
   The gust of wind as it arrives is called the gust front.



Video showing generation of rotation by vertical wind  speed

shear in a thunderstorm environment.

Rotation from vertical wind speed shear being imported into a thunderstorm.



How rotation is generated by the interaction of the cold pool and warm, moist inflow.

Once the rotation is imported into the storm it is stretched vertically in the intense rotating updraft. This makes the vortex narrower and causes it to rotate faster much like when an ice skater pulls her/his arms close to the body and spins faster. When rotation that is spread over a wide area is concentrated over a smaller area the rate of rotation increases. This is called conservation of angular momentum.

The Structure of Supercell Thunderstorms

With the invention of radar our knowledge of thunderstorms (called convective storms by meteorologists) began to take big leaps forward.  When doppler radar came along we could take a better look inside the storms and we began to piece together  the puzzle of how tornadoes form.

Starting in the 1880s with John P. Finley there is a rich history in the U.S. of tornado research. Before that there were scattered efforts to understand tornadoes, water spouts and other vorticies, even one by Ben Franklin. Through the years early years up to the time of Ted Fujita most of the research was done using debris patterns. 

On April 9, 1953 for the first time a hook echo was documented on a radar screen and associated with a tornado near Champaign, IL.

The image at left is the hook of a strong tornadic supercell near Tuscaloosa, AL this year. Hook echoes are a sign of "spin up" in a thunderstorm and the possibility of a tornado. The hook shows dry air being pulled into the circulation and wrapped around the tornado.

 


Schematic of a hook echo as seen on a radar screen

 
At left is a schematic explaining the hook echo and how it is related to the supercell thunderstorm.

The tornado is found where the  circular dashed arrows are at the junction of the the mini-cold front and the stationary front. At the top of the storm the updraft core is offset from the tornado because the rotating updraft core is tilted.

This will make more sense once you look at the next diagrams.

The diagram/photograph below shows a typical supercell with a tornado. What is most important is that each supercell is different but each shares many typical features. When storm chasing we look for those  common characteristics to determine the strength of the storm and the location of the tornado.

The top of the intense rotating updraft penetrates into the stratosphere. The anvil stops at the top of the troposphere and the updraft is tilted.

A tilted updraft means that rain falls ahead of the tornado and not back down through the updraft. If the intense rain fell through the updraft the billions of rain drops would work against the rising air and the storm would not live long enough to spawn a tornado.

The final diagram puts it all together. Rain cooled air rushes out the front of the storm (forward flank downdraft). That air forms a mini cold front and the top of the cool air is often revealed by a shelf cloud. The cool rush of air helps lift and tilt air that is rotating in horizontal tubes ahead of the storm. That rotation is pulled into the updraft and contributes to the rotation of the tornado.

What we now know is there are two extremes to what we call tornadoes.  At the smaller, less intense and short-lived end are leading edge type funnels. These are often called "gustnadoes" because they form on that cool rush of air you have felt. That rotating air that is being imported into the thunderstorm can spin-up enough to cause an EF0 or EF1 and maybe occasionally an EF2 tornado that is narrow and on the ground only a short time.

At the other extreme, the 500 pound gorilla end of the spectrum, are mesocyclone tornadoes at the base of the rotating updraft. Meteorologists are convinced that when rotating air is brought into the thunderstorm it reinforces the rotation of the updraft and leads to an EF3, EF4 or EF5 tornado.

I will have more on the import of rotation in tomorrow's blog.

Why The Delay in the Warm Up?

This past week we have been talking about a warm up, a return to the hot and stickies with sultry afternoons in the low 90s and plenty of humidity to go with it. Originally we thought it would arrive Sunday but now we have delayed the return of a 90° high temperatures a week, until the end of the 7-Day forecast.

The way we usually explain a big summer warm up at jet stream level is shown in the top video. A strong low pushes far southward in the west.  Meteorologists say it "digs".  Downwind the reaction is for hot humid air to surge northward and invade the Cincinnati Tristate region. Eventually the low heads east and the hot air "sloshes" back to the west and we cool.

I like to compare this action/reaction pair to a water bed. Sit on one end and it sinks, the reaction is for the other end to rise.  To be sure this is a far from perfect analogy.

Reality is much more complicated than the situation in the top video.  This week there have been multiple lows pushing into the west. When a new arrival tries to dig it has to overcome the low already over the western United States which is also trying to dig southward.  The result is that the low arriving from the Pacific ejects the low already in place. Meteorologists call the newly arriving low a "kicker" because it forces the low in the west to move east. Take a look at how this works in the 2nd video.



So we warm up more slowly because each of the ejected lows stops the jet stream in the east from moving northward like it does in the simple case shown in the first video.The result is the hot air stays south.

The second video also shows what models are indicating for the 4th of July.  One of the ejected lows stalls then sinks southward from Canada bring cool to chilly, potentially wet weather for the 4th.

Don't worry many, many things can change in the model runs before the 4th gets here.

Just Where Does That Sticky Air Come From Anyway?

I have been advertising a break from the stickies the past few days.  In yesterday's article I explained it using the concept of air masses.  Yesterday the models (all of them) kept the humidity down until Wednesday next week.

Not just a break was on the way, but a 5 day break.  Then today the flip-flop kicks in and the models have changed their minds. (For the stickler-scientists among us I know weather computer models do not have minds just equations.)

The break will be only two days with a nice Sunday as the transition day to warmer, more humid air.

OOPS! spoke too soon - the models have shifted a bit more bringing rain in on Sunday. It is still the transition day but it could be wet as a warm front moves into the area and only slowly heads northeast.

Fronts and air masses explain it all but it may just be easier to show where the air comes from.  A trajectory model calculates the movement of a parcel of air.  So if I enter the latitude and longitude of Cincinnati and tell it I want to know where Monday's air in Cincinnati is today it spits out the map below.

Air that sits right now off the western tip of Cuba will be inhaled by your lungs Monday. That air is not far from Havana where the heat index this afternoon was 98°.

Many things can change in a forecast 5 days in the future. If cloud cover prevails and rain falls it will still be humid but the heat index not so high.

Cooler and Less Humid for the Weekend ... Thanks Canada!

The hot and humid weather is going away for a while giving us a needed break from the summer combo of heat and humidity as Canadian air invades the tristate.

Just what do we mean by Canadian Air, or Gulf of Mexico Air, or the bane of Cincinnati winters, Arctic air? Those names identify what meteorologists call a source region and any air that sits over a source region becomes an air mass like Canadian Air.

Technically Canadian air is a continental polar (cP) air mass. Air masses are named for the type of source region, continental or maritime which is the same as dry or moist sources and  whether they are warm or cold (tropical, polar, arctic). When an air mass comes from a source region it brings the characteristics of that source region with it, just slightly modified.

Here are the air masses that affect Cincinnati:

• cA - continental arctic - bitter cold from the North Pole
• cP - continental Polar - cold in winter, cooler and less humid in summer
• mT - maritime tropical - think "Dog Days"
• mP - maritime polar - it if gets here it comes from the Pacific Ocean and dries out coming over the Rocky Mountains and is a pleasant winter air mass.
• cT - continental tropical - in summer we occasionally get hot and NOT humid - it comes from Northern Mexico, New Mexico, Arizona and Texas. It is fairly rare in Cincinnati

This map will help you make more sense of it.



Source regions of air masses that affect North America

 Continental polar (cP) air in winter is colder that cP air in summer. The source region, the Canadian plains is obviously colder in winter. In summer the main change is that the humidity drops and the haze clears with warm noticeably lower humidity it gives Cincinnati a nice summer day.

In summer it is never long before the maritime (humid) tropical air (mT) returns.

Rain ... Rain ... Go Away!

I was planning on a quick piece on the summer solstice but as Mother Nature would have it and as you know, once again it rained. Through 9PM of the 21st of June our total rainfall since January 1st is 39.40". It is not even the end of the first half of the year and we are within  3.30" of our normal annual total. Our average annual rainfall is 42.72"

Forecasting rainfall amounts, making a quantitative precipitation forecast, (a QPF to the meteorologist) is much easier now than before computer and models. There are really only 4 primary factors that determine how much rain will fall.

• The precipitable water of the atmosphere above a location
• The amount of moisture imported by winds blowing into a region
• and one you have heard a great deal about - the amount of lift - how fast the air is forced to rise.
• How quickly the area of upward motion is moving over an area.

Precipitable water is how much water is in a vertical column above a spot.  A total of 2" of precipitable water means that if all the moisture fell out as rain rain gages would record 2". Precipitable water is a guideline number telling us how primed the atmosphere is.

Moisture transport into an area explains the following radar image from the National Weather Service NEXRAD Doppler Radar on June 21, 2011. It is the estimate of rain totals during a stormy, wet  first afternoon of summer.

The precipitable water that afternoon was 1.7" - 2.0" but many areas got 2"-3" of rain and parts of Clermont County from Bethel southwestward received 4" to 5".

Moisture transported from a far away as the Caribbean streams northward into the Ohio Valley and is drawn into the updrafts of thunderstorms. In all, as long as there is an updraft moisture continues to be concentrated in the storms and fall out as rain.

Of course a slow moving storm can drop more rain on a given spot than a fast mover.

Luckily thunderstorms work against themselves by stabilizing the atmosphere. As some of the rain evaporates the air is cooled becoming more dense and harder to lift eventually weakening the storm.

Welcome to Astronomical Summer 2011.

The Slow Moving Storm Track

It is not just on Friday that I get questions about weekend weather it is actually everyday. But on Friday that dim distant speck of light at the far end of the tunnel we call the work week is blazing with expectations. Friday is the day I have to take a firm stand and declare what, when, how long and how much. The goal - no surprises.

The news is not so good for the tristate this weekend. It looks like the jet will not ease as far north as I expected and under the jet a series of disturbances will bring showers and thunderstorms occasionally.

The map below shows the situation.

The main steering current which is the same as the storm track, is shown with the broad white arrow. Out west east of the Rocky Mountains disturbances will form and follow the steering currents. That arrow will waver north and south a bit this weekend so occasionally a cluster of thunderstorms will come our way while others will miss us.

The yellow arrow shows where the northward bulge has begun causing a surge of hot air into Kansas and Nebraska. Eventually the bulge will follow the yellow arrow all the way to the Great Lakes and Canada.

The reason the entire pattern will be slow to reach southern Canada, eventually resulting in an invasion of the hot, stickies and high temperatures in the 90s, is the two lows at the top of the map. Like a well matched set of gears they are rotating and staying in place. Until those lows move nothing else will move much.

It looks like by dawn Monday the eastern low will move enough to take the storm track north, drying us out and leaving behind the haze, heat and humidity of summer.

Clearing The Haze

Haze, it is something everyone can identify but at the same time only a few have an idea what it truly is. Haze is not fog and haze is not smoke, it is something else.  It almost always occurs in the Cincinnati tri-state region in summer when the air is humid. 

There are places in the world with tons of humidity and almost no haze, the shores of the Red Sea and Persian Gulf  for example. The air there is generally very clean with little in the air from both the natural world and human activity.

A good example where both humidity and naturally occurring substances create haze, remember it is not smoke, it is the Smoky Mountains. That naturally "smoky" look is haze caused when humidity interacts with terpenes. Terpenes are released by conifer trees and react with water to create particles that clump into aggregates large enough to scatter all colors of light resulting the the whitish/bluish haze seen all along the Appalachian Mountains. 

Terpene is where the word turpentine is derived and terpenes give your Christmas tree that great smell. Citrus fruits also owe their characteristic smell to terpenes.

In Cincinnati the story is not quite as "natural" when we get haze.  The industrialized Ohio River Valley has many substances emitted to the air that react with water vapor to clump into particles that are big enough to equally scatter all colors of light. "Big enough" is still microscopic but to rays of sunlight what is too small for us to see is large and in the way as the ray streaks through the atmosphere.

Haze is in the forecast for Sunday, Monday and Tuesday. We see it every summer around here and just like the "Dog Days" we all know what the weather will be during the "lazy, hazy days of summer".

By the way meteorological summer is June, July and August.

Questioning Normal

After the rough winter of 2009-2010 then the drought of 2010 followed by a second tough winter in 2010-2011 the inhabitants of the Cincinnati region were ready for an gentle spring in 2011.

What they got was the second wettest month on record, and when rainfall was totaled for the 31-day period ending 05.03.2011 - it turned out to be the wettest 31 days in the history of the region with 15.96" of rain.

What I got was questions. Questions about climate change, river flooding, drought following wet periods, severe storms and global warming.

It seems the atmosphere works in mysterious ways but in fact most general principles the govern the atmosphere and how it works are fairly straightforward.  

In the next few months I hope to help you figure out how weather works.