Sunday, September 04, 2016

Comments On NWS Tucson Weather Story Today

Here's another example, from today, of what I don't like about many NWS graphics, cartons, and "Weather Stories". The problem with agencies like NWS playing around in social network stuff and dumbed-down cartoons is that these media are usually too restrictive for needed technical details - what Paul Harvey used to call "The Rest of the Story.". 

The two images used above by TUS NWS this morning are essentially an apples and oranges mix of information regarding moisture in the atmosphere. The satellite-derived water vapor image (no units or color scale shown) is a display that provides a visualization of RH in the upper part of the troposphere - often times dry air at upper-levels lies above very moist air in the lower troposphere (obviously not so today with a morning sounding observation of PW of only about 11 mm). Key point here is that the water vapor images convey essentially no information about moisture or PW in the lower troposphere. The graphic on right is a model forecast (from which model?) of PW for some unknown time next Wednesday (no color bar shown to quantify what the colors mean). The bulk of PW in the atmosphere occurs below 500 mb. 

So, this weather story displays mostly a non-quantitative, "changing of the colors" from red to green over Arizona and New Mexico.

Here are two more quantitative examples. Above is an image from CIMSS at University of Wisconsin for one of the water vapor radiation channels. The units of this display are in radiative temperature (C). Warmer temperatures indicate drier (but not quantified) RH in upper-troposphere. The three water vapor bands usually used to create the product shown in the weather story have peak sensitivities from just above 500 mb to 300 mb. Warmer temperatures come from lower in the atmosphere and indicate little water vapor above at higher levels - thus warm colors indicate dryness.

The forecast of PW below is from the 12 UTC WRF-GFS today, on the 5.4 km grid, and is valid at 6:00 am MST next Thursday. The units are mm of PW but are displayed using a somewhat different color scale than in the weather story. The huge increase of moisture over southern Arizona is associated with the decaying Hurricane/Tropical Storm Newton (not yet named by NHC).

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