Bob, the major source of low-level moisture in the Plains if from the Gulf of Mexico. In other words, the Gulf is the main contributor to signficant low-level moisture in the Plais. The size and temperature of the Gulf is often the reason why we see dewpoins in the 60s (March/April) and 70s (May onward) in the Plains. The fact that it is a large body of water results in not only a large area of high-moisture low-level air, but it also means that air usually has a significant residence time over the Gulf, which results in both high Tds and, usually, a deep moisture profile. It is true that there are other sources of low-level moisture, including that contribution from evapotranspiration (plants releasing water vapor as a result of photosynthesis), but most of the time, evaporation off the Gulf gives us "the juice", so to say. While it is true that there is evaporation off other bodies of water (such as Lake Texoma or other large lakes or wetlands), the effects are often local and shallowl. Evapotranspiration, however, can be a significant contributor at times, mostly in the "corn belt" area of the upper Midwest come June/July/August, which is largely the cause for the upper-70s and 80s dewpoints.
As we saw this year, with the repeated continental air intrusions into the Gulf, scouring out the good moisture, we weren't really able to get strong moisture into the Plains for much of the season. This is further evidence of the importance of the GoM. Without the moisture from the Gulf, we were stuck with Tds not much into the 60s (more often upper-50s), which is rather paltry in terms of climatology. I think there is a strong connection between the lack of good moisture in the Plains this year, and the relatively poor chasing so far. It should be noted that the upper-air pattern has not been very conducive for tornadic events for most of the spring so far, but the lack of moisture and the upper-air pattern are strongly connected in this case.
Originally posted by Bob Hartig
On a day with high Tds, does it really matter where the humidity originates?
The gist of it is that you can't really get high Tds (at least on the meso- or synoptic scale) without the Gulf. I wouldnt' consider 50s and maybe low-60s to be "high Tds". This time of year, "Gulf moisture" generally means Tds in the upper-60s and into the 70s. Lakes and wetlands are usually too small to have a larger-scale contribution and the water vapor contribution from them is usually pretty shallow (very near the surface). Additionally, since most lakes in the US are considerably cooler than the Gulf, the equilibrium Td is considerably lower as well.
