High and Low Pressure at the Same Time? - HUH?

I often stress that you cannot really understand weather unless you look at it in 3 dimensions. (You can even understand it more if you use 4 dimensions and include change over time.).  So tonight and tomorrow high pressure is moving into the tristate on the weather map but we are staying cloudy.

It is easy to explain if you look at the atmosphere in 3d there is high pressure near the surface and low pressure aloft.

The video explains it.

A Chill On The Way With Pockets of Frost

I have pockets of frost in the forecast this weekend, especially Sunday morning.

Valleys are particularly vulnerable. As cool air drains into the valleys and pools there it can get cold enough early in the Fall season for patchy, light frost.

Car windshields and car metal surfaces, because they cool so effectively,y are often first to get a coating of frost.  Again in valleys this may happen first but even slight low spots in the suburbs can have enough cool air pooling to cause frost early on the windshield.

But because it is early in the season, stored heat is a big factor and I do NOT have a killing frost in the area this weekend.

The video below explains the topic more.

Frost on the Pumpkin This Weekend? Maybe In Spots

Every year about this time we begin to put pockets of frost in the forecast.  And this year the first chance of frost is this weekend.

I am NOT forecasting a killing frost but in valleys, exposed metal and glass surfaces could see just a hint of frost.

The video above explains the cool weekend ahead.

It Is All About The Journey (From Canada)

Coming our way is an air mass and that air mass will change during its trip to the tristate.


First an air mass (sometimes written airmass) is a big blob of air that is  fairly homogeneous. It developed over a geographic region and the characteristics of that region were inherited by the air of the air mass.


It is a simple concept. For example: air that sits over the cold Canadian plains in winter is cold and fairly dry because the air developed over the cold continent. Air that sits over the Caribbean Sea is warm and humid.


The air masses that affect the continental U.S. and their source regions are shown below.

More info available at http://www.shorstmeyer.com/msj/geo165/airmass.html

 "m" means maritime or an air mass that formed over water and is fairly moist. "c" means continental and a continental air mass is dry because it formed over land. "T" means tropical, "P" means polar and "A" means arctic and you already know which are warmer and which are colder than the ground they move over on their way to the tristate.


Here is where the journey comes in. As soon as the air mass begins to move it encounters ground that is either warmer or colder than the air. The farther it moves the more it is modified and by the time it gets here air that brought central Canada low temperatures in the teens will bring Cincinnati low temperatures in the 30s.


This weekend the center of a continental polar (cP) air mass will arrive. It is from the western and central Canadian Plains and it is colder that the air we now have here in the Cincinnati Tristate. So get ready for morning lows in the 30s in spots Saturday morning and in  many locations Sunday morning.  Of course valleys will be colder and there we may see the faint glimmer of frost on exposed metal and glass surfaces.

Is A Frosty Morning Near?



 After that perpetually pesky perturbation called a cutoff low finally is kicked out of here we will see a cool airmass come out way. The cool air is from the northwestern Canadian plains and we are looking at low temperatures dipping into the low 40s over the weekend. that means rural valleys will fall into the mid 30's with the slightest tinge of light frost possible on exposed metal surfaces.

But we had a very warm summer and only a small bit of cool weather so far.  In the next couple weeks the ground, the trees, buildings will gradually cool and once enough heat is lost and an airmass from Canada that is sufficiently cool comes our way we will have our first frosty morning.

That Cutoff Low Part Deux!

Courtesy weather.cod.edu

The satellite image above is what we call a water vapor image.  It literally shows you the humidity in the middle atmosphere. Darker shades show drier air and you can usually assume that means the air is sinking there. Brighter shades indicate greater amounts of humidity and generally rising air.

If you look closely you can see the low pressure system that is forming and not every place in theow has rising air.

As the low wobbles around over the Tristate this weekend when you encounter an area where the air is rising above your location clouds will increase and there is a chance for a shower.  Sinking air means that the sky may clear a bit and rain will end.

The animation below compares the cutoff low to a wagon wheel.  The spokes represent areas with lift and in between the spokes the air is sinking.  When a spoke moves over you showers develop because the air is rising.  This creates periods of rain and cloud cover interspersed with some clearing and dry weather.

Now if the low wobbles, or the spokes are curved or winds in the atmosphere change the shape of the low the pattern becomes complitaced and harder to forecast.

Take a look at the video for more.

Cutoff Low Watch!



The Cutoff Low that will be with us for a few days.


A low pressure system is an area of rotating rising air.  Hurricanes, tornadoes, dust devils, waterspouts and those red  "L"s you see on the weather map are all low pressure systems. All lows  rotate counter-clockwise in the northern hemisphere (clockwise south of the equator). To be completely accurate a few small scale lows like tornadoes, dust devils and water spouts rotate clockwise in the northern hemisphere (counter-clockwise southern hemisphere) because of special circumstances.

In tornadoes the air rises violently and in an EF5 the upward velocity may be has fast as 160 mph or more. In the cut off low that will affect us for a few days the air rises much more gradually a realistic figure is half a mile per hour.  Because the cut off low covers thousands of square miles much more energy is involved than a powerful tornado.

Even though the upward motion is relatively slow it is sufficient to create clouds and rain, but with only restricted amounts of moisture and the gradual lift showers are scattered and no heavy rain falls.

The showers are most likely during the warme4s part of the day when the upward motion is enhanced by the rising air warmed by the surface t hat was heated by the sun.

Phil Pugh Asks .... What Happened?

Steve: I am curious. Yesterday the 10 day forecast had virtually no chance of rain for the next 9 days. Today that same forecast has 5 of 9 days with 30% or greater chance of rain. What happened to make such a dramatic change in such brief period of time? I would find such explanations an interesting addition to the weather forecast. You could have the “Inside the forecast” segment and the “What happened we didn’t expect” segment!

Well Phil ... here is the answer ....




Courtesy weather.cod.edu

 

At jet stream level a "cutoff" low is going to form above us.  It is called a cutoff low because it becomes isolated from the main branches of the jet stream and because there is no wind to push it along it wobbles around over an area for days sometimes.

Because it has a core of air colder than the surrounding area the atmosphere under it is unstable and showers pop up mostly during afternoons. And because it is a low pressure system air within it is rising creating the cloud cover that can lead to showers.

But because the amount of moisture is limited and the low is fairly weak the rains are generally light, brief and scattered.

Now .... how did the forecast change? 

Models generally do a good job upstairs in the atmosphere. The chart above is from about 18,000 feet above sea level and shows the "cutoff" low Sunday at 2PM.  Sometimes the model takes a while to evaluate how the jet stream interacts with the Rocky Mountains. 

In this case models this past weekend showed the forecast weather pattern to be what meteorologists call a "progressive" pattern. Systems move in and move out fairly quickly.

Once the models got a handle on the amount of energy at jet stream level and how the energy would interact with the mountains the forecast changed from a "progressive pattern" to a "closed" or cutoff low. All cutoff lows are closed lows but some closed lows can be embedded in the jet stream flow and move quickly so are not "cutoff lows".

Because they move only slowly the forecast went from a couple chances of rain in 10 days to the possibility of a shower nearly every day.

A Tale of Two Jet Streams

The northern jet is the dominant jet in this animation, courtesy of U.S. taxpayers and NOAA's Air Resources Laboratory. The highest wind speeds (>=70 kts) are shaded in blue and the southern jet shows up in the last few frames.

What you see happening here is called a "cutoff low" by meteorologists. Literally closed lines (contours) form which encirlce a pool of cooler airupstairs. Cool air upstairs also indicates low pressure, in this case betweent 35,000' - 40,000' above sea level. The cutoff low basically spins in place or as I often say treads water above us and does not go very far very f ast becasue it is isolated (i.e. cuttoff from) the main jet stream flow.

And because it is a low pressure system air is rising inside the cutoff low and that lift generates showers and clouds. So under a cutoff low or a "slow low" as some call it, for several days showers are possible along with cloud cover and cool-ish weather.

The video below has more.

The Official Temperature May Be A Lie!

Well, calling it a lie is a bit too strong, but I got your attention.  Often on nights dominated by high pressure to our northwest the Cincinnati tristate's official temperature, observed at CVG, is warmer t han the surrounding area and not representative. 



The terrain around Cincinnati, many broad uplands and many deep, steep valleys.



It is because the warm air over the densly populated part of the tristate is transported by the northeasterly winds to the airport and the airport as a result stays warmer than would be expected without the northeasterly winds.

So on a clear night valleys get much cooler than uplands because of cold air drainage and uplands in the path of the warm plume from the urban heat island stay warmer than uplands not in the path of  the warm plume.  The transport of warm air by the wind is called by meteorologists warm air advection.

The video has more.

Cool then Warm Again .... And So It Goes Again and Again and ...



Cool Air to the north courtesy Weather.cod.edu

As  the map above shows there is a big mass of cool air to our north making its way southward.

So, is this the beginningof a long cold winter?  The answer is no.

The reason we will warm back into the 80s is discussed in the video below.

Valleys are Cool!

Topography of the Tristate with the pre-glacial course of the Ohio River. Cold air drains into the valleys and they are often colded and  frostier in the morning.

Cincinnati and the surrounding countryside north of the Ohio river is a region of valleys, not really hills. The last glacier this far south reached the present day position of the Ohio River and the loose sediment  bulldozed ahead of the wall of ice filled old valleys and mostly leveled things out.

The Ohio River before the glacier followed the white arrows across Norwood, north into Butler County then southwest along what is now partly the Great Miami River Valley and partly the White Water River Valley.

The fairly loose material eroded into today's steep sided stream valleys of the modern tristate.  South of current Ohio River the landscape was left untouched by the glacier and the hill sides in Kentucky are steeper and the valleys deeper.

Now to get to the weather part of this post cold air drains into the those valleys and often they are much colder than the uplands.  In fact it is not uncommon to have temperatures in Morrow, OH along the Little Miami River 20° colder than the level areas high above the valley.

That leads to earlier frosts, more frequent fogs and sometimes icy weather in the valley when higher elevations are above freezing and getting rain.

There is more in the video.

COOOOOOOOLER!

It is the time of the year that I try to forget about ragweed and long for frost.  The earliest the official temperature has dropped to 32° is the 28th of September but the average date is mid October.

When the longer clear nights of Autumn allow the temperature to drop into the mid and upper 40s in most areas, rural valleys can see freezing. Cold air is more dense than air that is warmer and as in contact with the ground is cooled by the ground it flows down hill. 

Any bicycle rider, motorcyclist or owner of a convertible can tell you in the dips along the road and in the deeper valleys it is colder. That is called cold air drainage and the more dense cooler drains down hill and pools in low spots and valleys. So when it makes the mid 40s in the suburbs it is not unusual for rural valleys to get close to freezing.

As you can see on the map above blue is on the way. Of course the blue is the cooler air that gets here this week.

For specifics on the forecast take a look at the video below.

Taking A Break from the Tropical Traffic Jam and Talkin' Winter

The winter of 2010 - 2011 was a tough one caused primarily by the La Niña pattern in the equatorial Pacific Ocean.  Water colder than normal is characteristic of La Niña winters.  The cold water is the result of a pattern change in the global atmosphere and a cause of the northern branch of the jet stream being dominant.

Last winter the southern jet was also very active bringing about the clash of warm moist air from the tropical Pacific with cold, polar air from northern Canada. The clash took place for the most part right over the tristate.

I am not going out on a limb yet, but the elements are there for another fairly rough winter. The La Niña pattern is being restablished over the equatorial Pacific with temperature anomalies (i.e. departures from normal) in the negative indicating cooler than average water. The blue areas below are the cooler areas.

Notice the large cool water pools north and south of the equator and the area right along the equator. When the water is cooler than normal in these regions less energy is transferred to the atmosphere and the southern branch of the jet is weaker than normal. The northern branch dominates guiding cold air into the Ohio Valley. At the same time mositure arrives from the Pacific causing a winter wetter than normal.

If enough cold air sneaks into the pattern it can be a white winter instead of wet.

Typically the La Niña or El Niño pattern does not set up until January. When it did last year it brought a snowy February. The La Niña dominated the spring resulting in the wettest 31 day period in Cincinnati history, ending May 3 with 15.96" of rain.

The video below has more.

Lingering Lee

What is left of TS Lee

The remnants of Lee can be found in two places. One part along the east coast attached to a front moving northeastward and away from us and another part right over top of us shown in the water vapor image above.  The part that is above us is detached from the jet stream and stuck in a tropical traffic jam. It can do nothing but wobble around above us for a while. As a result we have the chance of showers each day into the weekend.

Once Maria gets close the remains of Lee get absorbed and dissipate. With Maria out of our way cool air will plunge southward next week. The video has more.

Hyperactive Tropics Means The Hot Stuff is Gone and May Not Return

Take a look at the satellite pics (courtesy of the U.S. Navy, Naval Research lab, Monterey, CA) and you will see a great deal of tropical activity. First is Hurricane Katia. She is heading towards the east coast but is expected to turn back out to sea before getting to the U.S.

In the Bay of Campeche and behind katia are two disturbed areas that probably will not develop any further. Tropical Depression 14 will like become Hurricane maria. Then from the Cape Verde region of coastal West Africa across the continent disturbances are lined up coming from the Asian/Indian Monsoon region.  It is going to be busy for a while in the tropics.

As the tropical systems develop and move into the middle latitudes weather systems here slow until the tropical system is out of the way. Progress can then resume.

Because the remnants of TS Lee will be over us this week and not move away until Katia moves into the North Atlantic we will not warm much this week. Then the approach of what will likely be Hurricane maria will slow any warming trend.

In short it is too late for a 100° high temperature and probably too late for a 90° high temperature.

Tropical Traffic Jam 2011 Part II

As of this posting Katia is a Category 3 hurricane with 140 mph wind gusts.  You can also see the cloud cover associated with the remnants of Tropical Storm Lee and a stalled front along the east coast.  The new, large cluster of thunderstorms over the SE Atlantic is getting organized and will likely become Hurricane Maria in the next couple days.

How a tropical system like TS Lee transitions to a middle latitude system is only poorly understood.  The primary source of energy for the lows and fronts you see on a weather map is derived from the jet stream systems that form where the cold polar air meets the warmer midlatitude and tropical air.

Tropical systems are completely different their primary source of energy is the heat released when moisture in muggy tropical air condenses.  Condensation is a secondary source of energy for middle latitude systems. 

We do know that the process is gradual and "tropical traffic jams" occur when a tropical storm or hurricane approace because middle latitude systems do not readily combine with tropical systems.

The video below the satellite graphic explains how this particular tropical traffic jam is going to work.

3 Days and Counting - Fireworks Ahead! - A Fly in the Ointment?

Viewer Submitted Pix of the 2010 Fireworks

Now we can get more specific about Sunday evening and the fireworks forecast. I have had Monday, Tuesday, Wednesday and most of Thursday to look at the long-term model. At 4 runs per day that is 16 model runs. And the trend is for more rain, a larger area of rain and most of the rain falling Sunday evening.

But do not dispair - there are many ways the weather can go in our favor. Scattered showers mean many areas with no rain.  Timing may hold up the rain or move it out of here faster and the models may just be off the mark.

In fact after many years of doing this rain is patchy more often than not and there are always breaks in a rain event.

The video shows the model output.