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Spotting the dry mid levels

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On 3/22/2018 at 7:14 AM, mweisenfeld said:


How does one see the water content, both actual and forecast, of the mid levels on the models?


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There are several ways to evaluate forecast moisture content in the mid-levels in the models. I'll take 700mb as a baseline here.

One of the more simpler ways would be to look at a relative humidity forecast graphic from a model. Commonly used models, like the NAM and GFS have 700mb relative humidity forecasts that can be readily viewed from Pivotal Weather: http://www.pivotalweather.com/model.php?m=nam&p=700rh

A completely saturated 700mb relative humidity percentage would be 100. A lower relative humidity percentage represents less moisture content. Limiting factors for this method include only getting a snapshot at 700mb, as it doesn't tell you how deep a moist or dry layer in the mid-levels is. Alone, it also does not tell you the quantity of water vapor, which might be more useful in a theoretical sense.

Another method is to look at a forecast sounding or Skew-T diagram. Depending on how familiar you are with reading a Skew-T, in the images referenced below (actual short-term forecasts for parts of the northeastern U.S.), the green line represents the dew-point and the red line is the temperature. If the two lines overlap each other, relative humidity is near or at 100%, representing a moist section (or layer) in a sounding. The farther apart the lines are, the drier the air is at a given level. With Skew-Ts, since you are seeing a model forecast for a large column of the atmosphere, you can assess the depth of any moist and/or dry layers:

Alternatively, you could also evaluate 700mb mixing ratios, if you need or want to find a more precise amount of moisture. Plymouth State has model forecast graphics in which you can choose mixing ratio of a specific level, like 700mb for mid-levels. The output will be in grams per kilogram. http://vortex.plymouth.edu/myo/fx/ctrmap.html

Evaluating observed/actual moisture in the mid-levels is a bit more difficult. You could wait for 00z and 12z soundings and either look at Skew-Ts or review text data to evaluate mid-level moisture. The University of Wyoming allows you to pull text data from soundings, in which you should be able to review the relative humidity and mixing ratios at several levels within the mid-levels: http://weather.uwyo.edu/upperair/sounding.html 

The downside with the latter method is that you will only get two Skew-Ts (usually) per day and the distance between balloon launch sites might mean that you don't get a representative reading for a specific area that you may be interested in.

Hopefully this helps.


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