SOUNDING ANALYSES WHEN DATA ARE FROM UNRELIABLE RRS SIPPICAN SONDES
Prepared 4:00 pm MST on July 6, 2007
I think that the Tucson NWS upper-air soundings from yesterday afternoon and from this morning illustrate several problems that forecasters, and other users, will have with the data from the unreliable Sippican sondes being used operationally by the NWS RRS program.
First, let's examine the 5 pm MST (0000 UTC)
Tucson sounding from yesterday - see Fig. 1. This sounding was taken on
a very hot afternoon, about two hours before severe thunderstorms moved
into the Tucson metro area.
Fig 1 -- 5 pm TUS Sounding for 05-July-07 (000 UTC 06-July -07)
Things of importance:
The analysis and forecasting dilemma is obvious. Since the Sippican sondes tend to measure Td (RH) too low, we have no idea what the actual BL moisture structure was prior to the thunderstorms and macrobursts.
Fig 2 -- 5:00 am. morning TUS Sounding for 06-July-07 -- 1200 UTC
Given the unreliable character of the Sippican moisture measurements, it is not clear how the forecaster, or other users, can actually diagnose what is the real potential for convective storms. The morning forecast discussion from the Tucson NWS Office, based to some degree on this sounding, concluded that:
"KTUS 12Z SOUNDING ANDHAS BEEN AT MID LEVELS WITH SEVERAL DEGREES OF WARMING IN THE500-400MB LAYER. THAT SHOULD COOL A BIT AS AN INVERTED TROUGH OVER NTX TRIES TO EITHER MIGRATE W INTACT...OR SHEARS TOWARD US IN THERATHER FAST BUT DIFFLUENT FLOW ALOFT. WE ALSO HAD A 40KT NE SPEEDMAX AT 500MB THIS MORNING WHICH IS GOING TO HELP IN THE SHEARDEPARTMENT THIS AFTERNOON/EVENING. BUT THE UPPER 40/LOW 50 DEWPOINTSWE HAVE RIGHT NOW ARE NOT GOING TO YIELD MUCH CAPE. SO UNLESS WE SEEDEWPOINTS REVERSE COURSE OR AT LEAST HOLD WHERE THEY ARE...I DOUBT WE`LL SEE MORE COVERAGE THAN WHAT WE HAD YESTERDAY."
Interestingly, here at the house, there have been thunderstorms and occasional showers since about 2 pm. Updraft bases appear to be at about 600 mb and an intense downburst appears to be in progress a couple of miles to the east. I suspect that the morning sounding was dry and that there has also been some additional increase due to advection. This combines to make this afternoon much more convectively active than yesterday.
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