Bad RRS Data Impacts Short Term Forecasts

Postmortem on Last Evening

First – the event last evening illustrates the problems forecasters can get into with bad data from the RRS soundings. From the 8:30 pm TUS NWS Forecast Discussion:

THUNDERSTORMS THATFIRED NEAR THE MOGOLLON RIM/WHITE MOUNTAINS HAVE PROPELLED ANOUTFLOW BOUNDARY SOUTHWESTWARD...WITH NEW STORMS FIRING ALONG ITACROSS GRAHAM AND COCHISE COUNTIES. THESE STORMS HAVE REMAINED BELOWSEVERE LIMITS...ALTHOUGH CERTAINLY HAVE SEEN PLENTY OF LIGHTNING ANDOUTFLOW WINDS OF 40-45 MPH WITH THE STRONGEST STORMS...ALONG WITHAREAS OF BLOWING DUST. DECENT COMPLEX OF STORMS IN WESTERN COCHISECOUNTY IS PUSHING OUTFLOW INTO THE TUCSON METRO AREA/EASTERN PIMACOUNTY THIS EVENING. RUC SUGGESTS THIS COMPLEX WILL STAY PRIMARILYSOUTHEAST OF TUCSON. 00Z KTWC SOUNDING SOMEWHAT DRIER WITH A PW DOWN TO NEAR ONE INCH.

However Mike Leuthold wrote last evening about the apparently benign TWC 17 July 00Z sounding (see SPC plot and diagnostics above):

"You have to 'fix' it. I used nsharp to bring up the PW to near the GPS value. Then, I saw about 500J/Kg ML CAPE and about 800 surface based CAPE."

The sounding last evening was too dry, as is often the case with RRS sonde data, and GPS indicated actual precipitable water about 1.3 inches – a critical flaw in the data since the missing moisture was obviously in the boundary layer, affecting CAPE and the local evolution of the spectacular lightning storms. Pat Holbrook had correctly called for an “up” day or early evening yesterday morning.

The storms of interest for Tucson developed late afternoon from the flanks of Mt. Graham southward. The storms moved south and west producing two very distinct outflows that moved southwestward across all of southeast Arizona. One area of storms moved southward into Mexico as a long-lived MCS and a second area broke to the west along I-10 with the northern outflow. These storms moved into Tucson metro area around 9 pm producing frequent, vivid lightning and areas of brief TRW+. This cluster of storms dissipated rapidly after 10 pm. As the outflow moved across the metro area it apparently triggered new development along the slopes of the Catalinas, so that visually the storms seemed to be moving northward, even as radar indicated they were moving to the southwest. Quite a show!

As for rainfall, 32 of the 93 ALERT gauges had measurable rainfall after 7 pm while none had any rainfall during the day. Maximum amount was 0.79” at Rancho del Lago near Vail, on the flanks of the Rincons. Here at house we had 0.38” which was most welcome; the lightning was a better show than the Fourth of July!