Moisture origin?

[Jared Farrer]

I was wondering were the moisture comes from? I mean naturally most of it in the general areas of the great plains and eastward most of it comes from the gulf? Due to the Rockies most of the moisture from the pacific is sapt out of the air mass as it moves eastward, right?

I guess that is my question. And maybe Jeff and others would chime in on this.

Where does the mositure come from?

 

[Mike Peregrine]

Due to the Rockies most of the moisture from the pacific is sapt out of the air mass as it moves eastward, right?

The Rockies do tend to leach the moisture coming off the Pacific. But there are times when the air over the plains does get Pacific moisture to work with from more to the south, near the Baja peninsula. Storms can sweep it up from there and pull it toward the plains. But more often, as you mentioned, the moisture is pulled up from the Gulf. As a system moves in from the west, moisture advects northward from the Gulf over the plains. Later in the summer, we get additional moisture from crops and foliage, which is at its peak by that point. When we get moisture in the air from this source, it's known as evapotranspiration. We also get a degree of moisture from other natural sources ... rivers, lakes, etc.

 

[Terry Tyler]

there are a couple different places that moisture comes from...

there is gulf moisture, that comes from the gulf, and there is monsoon moisture that comes up southwest from the pacific...the moisture channels, correct me if im wrong are just huge masses of water vapor that are moved around by synoptic weather forces...

for example, consider the lake-effect snow from the great lakes...the wind blows the water vapor off the warmer water that is making the vapor...the 'moisture' just comes from water that is being turned into vapor form...

i may not be answering correctly, but that is how i see it...

 

[Oscar van der Velde]

Besides advection of moisture, local generation is a significant factor. A lot of moisture comes from the ground and is transferred into the boundary layer by evaporation and transpiration from plants. This can range between 1-10 mm/day and is diffused over the boundary layer. If we take 5 mm evaporation and 2000 m boundary layer depth = 5 kg of water distributed over 2000 cubic meters, average column mixing ratios could increase daily by 2.5 g/kg. That is a dewpoint increase of 2-3 Kelvin (4°F) per day. Of course, it is not constant but depends on soil and vegetation, insolation, mixing and relative humidity of the air.
It is a major issue in numerical models, e.g. it is commonly observed (in Europe) that the GFS model in midsummer situations with weak winds and strong insolation creates too high dewpoints, especially later in the runs.

Oscar

 

[Brian Stertz]

I think when it comes down to good severe wx airmasses, the origins are off the C/E Gulf and Caribbean and into the Plains as the LLJ rockets northward. As mentioned by Mike P., the always favorable evapotranspiration can make things interesting for clustered severe wx events in late spring into early summer. Also, there are some cases where moisture sneaks along the Mexican Gulf Coast up into the Edwards Plateau. Then settling into W.Texas and E. New Mexico...ahead of the dryline with some interesting narrow tongues of deep moisture poking northward from the Del Rio TX area.

 

[Jared Farrer]

I was reading on this subject, and found alot of what I was already thinking. The Rockies do sap alot of the Pacific moisture out of the westerlies. If you look at the terrain just east of the rockies it is pretty apparent. It's desert! I think that speaks for itself. I knew all this to be true. I was just wondering how much. Some mositure does get through obviously but its miniscule. The same thing happens to countries north of the Himalayas. The Himalayas sap all the moisture out of the Indian mositure plumes, causing the monsoons in some places.

I always run into people who clam they know EVERYTHING about this and that. LOL. Like one guy swears he knows everything about winter weather. The guy calls rain, moisture? And swears that alot of Indiana's moisture comes from Oregan from the Pacific. as I explained to him that moisture is water vapor, water in gaseous form. Not liquid. LOL. I tried to explain this and the rockies to him but he got upset and ended the conversation. I was just tring to be nice? I just get amused these days by all this!
Anyway thats my take on it, and perhaps someone with better knowledge will speak up.

 

[Jared Farrer]

Besides advection of moisture, local generation is a significant factor. A lot of moisture comes from the ground and is transferred into the boundary layer by evaporation and transpiration from plants. This can range between 1-10 mm/day and is diffused over the boundary layer. If we take 5 mm evaporation and 2000 m boundary layer depth = 5 kg of water distributed over 2000 cubic meters, average column mixing ratios could increase daily by 2.5 g/kg. That is a dewpoint increase of 2-3 Kelvin (4°F) per day. Of course, it is not constant but depends on soil and vegetation, insolation, mixing and relative humidity of the air.
It is a major issue in numerical models, e.g. it is commonly observed (in Europe) that the GFS model in midsummer situations with weak winds and strong insolation creates too high dewpoints, especially later in the runs.

Oscar

Your absolutly right! Evapotranspiration plays a hugh factor in mositure content. Places like the corn belt see alot of this in the summer months. One of my theories on the why storms are so grungy nine times out of ten, where I live is this very fact. I live in Indiana, and our storms are perculiar. I also think that some moisture sources bring different effects to the slate. Like pollution, and things of this nature. So it stands to reason that transpiration i.e. moisture from the ground and namely plants, give off a different type of moisture. i.e. maybe it's a little polluted and what not. So far I havent heard any reasons why our storms are a little different. So if you want to post some reasons or theories that would be great.

It isnt just that we get more HP supercell type structures, its more than that. LPs are somewhat rare in Indiana, of course because we dont get dry line interactions here. LP's are synonymous with the dry line, and with the western plains. Our profiles often are missing that dry intrusion that occures in the low to mid levels, in the plains. I guess the great lakes might laso play a part in the equation?

Anyway psot your thoughts guys!

 

[Gene Moore]

As a subset of this thread I'm going to take issue with the classic assumption that the rockies dry out the environment over the plains. That mechanism can be overwhelmed by a system with a large amount of Pacific moisture; such as a cut off low that sits off Baja for a week before tracking east. Also midlevel monsoon flow regimes often do make it past the peaks of the rockies and get out onto the plains. This is most obvious beginning in late June through August. During this time when surface through 700 Mb temperatures are often too hot for the cap to break this extra midlevel moist feed provides an extra kick.

Generally it's accepted that for severe weather low level moisture is good and midlevel moisture is bad. For the most part I tend to hold with this concept, especially during the early spring months. This is, once the cap breaks I want my parcel to see nothing but cold bone dry air all the way up. That said, I tend to loosen up on this rule during the late season into the summer months when hot dry mixing can use a little help. If we look at a typical stalled frontal situation in July often we can find the only storms on the plains can be traced back to a Baja monsoon feed off the Pacific/ frontal system intersection. This type of knowledge is better for general convective forecasting, not just severe weather.

 

[Mikey Gribble]

I don't understand what you mean by your "storms are different" Jared. Wind shear has a lot to do with the type of supercell you get too.

 

[Jared Farrer]

I don't understand what you mean by your "storms are different" Jared. Wind shear has a lot to do with the type of supercell you get too.

Yes, it does. Here in the east IN and the surrounding areas do see alot more HP type storm structures. Now this is due in part to the amount of moisture the great lakes region normally has. In the plains there is often a dry intrusion in the middle levels. That can have an impact on supercell structures. In the east we do not typically get this dry air slot. Now that is just one aspect. The other is that our storms are typically more grungy tan in the plains. e.g. Say your on a storm in east KS thta just produced a tornado okay. Now say your about 2-3 miles east of it, with a clear line of sight. Now would you be able to see this tornado, giving its more of a classic setup. The answer most likely would be yes, right. Now same exact deal only now your in central Indiana. Would you be able to see the tornado, the answer is barely.

Now why is that? Its not rain wrapped, their might be a little rain and hail in between you and it. The reason is in IN the storms are grungier than out west. Be it pollution, pollen LOL, or what have you. The stout moisture profile doesnt help, I'm sure. IN also has more tornadoes being produced by squall line type segments than anywhere else too. April 11th 2007 for example. Nice setup, great little sup out ahead of an advancing squall line. The sup produces a EF0-EF1 tornado then not long after the line behind it produces a EF2 tornado. Supercell structures embedded in a squall line, very common for IN.

I hope that explains it a little better!

P.S. Storm structures are also hard to pick out as well. It like looking at a nice clear picture of you mother for example. Then you come back a year later and its all dusty. To use a somewhat eccentric analogy! Most storms here are just dirty looking!

 

[Terrence Cook]

For good storms in Indiana you need a strong mechanism for moisture advection, assuming the LLJ is sending the moisture from the gulf... once you get storms to fire the LLJ doesn't just turn off, your moisture is still piling up (and with no dry air intrusion anywhere near IN) the storms are going to have very low LCL's limiting visibility and a view of the storm structure. So the atmosphere in general will be grungy/hazy and look to be overloaded with precipitation and/or moisture.

Another reason is that IN only gets isolated supercells once in a blue moon, or about twice a year...

I don't think that it's anymore complex than that, but i'm probably missing something.

 

[Jared Farrer]

First off, I was talking in the vertical i.e. dry intrusion in the vertical. I was not talking in the horizontal. Below is a skew-t from May 4th 2007 around the greensburg area. It is meant as an example. You can see that starting around 800 mb drying occures. Thats normally were you would want the dry intrusion, right around 750mb. It helps prevent water loading, as well helps in the downdraft velocitys. Low LCLs as we all know is quite common on good chase days. Without them means high based storms and all hope of nadors goes out the window. The LLJ is also pretty coomon. It also picks up steam at night due to many mechanisms. Their are two types of LLJs maybe more, the nocturnal LLJ, and the cyclone induced LLJ.


Now I guess I am confusing people, I am sorry. I began talking about how much pacific moisture plays a role in the plains. Then I switched to why east of the mississippi storms are commonly grungy? And I mentioned pollution. For those of you familiar with nucleation, which is the idea behind cloud seeding and silver iodide. It happens everywhere but it seems to be alot more common east of the mississippi. I wanted to see if anyone had any theorys on the matter. Anyway whatever it is, it is not a result of the jets or high based or low based storms. I am not talking about supercell mode, as some seem to think. And If it was as simple as that then it would be alot more common!
Anyway I had to rewrite this twice now because ST accidently erased it. LOL. Sorry if I confused anyone!