Monday, September 21, 2015

TD-16E Heads Toward Southeast Arizona

Plot of CG flashes through 12 UTC above (from Vaisala and Atmo) shows that thunderstorm activity has so far remained well south of the border. However, large complex with very cold cloud tops (IR image below, also for 12 UTC) is moving northward toward southeastern Arizona.

Composite radar display from KEMX radar (NWS TUS) above is for 5:33 am MST. Rain has just inched into the far south edge of the ALERT network, with one site (Arivaca) reporting 0.04" through 5:00 am.

I grabbed two radar images from College of DuPage's radar page (more and better products there than at the NWS local web page) to illustrate some problems with tropical rain events, which produce much rain at lower levels through warm-cloud processes. Above is the 1242 UTC 0.5 degree (base) scan showing the huge problems with terrain blockage across much of the southern half of the radar domain. Below is the 1237 UTC echo-top product, which shows a patch of echo southwest of radar with echo tops to 20 to 30 thousand ft MSL - quite a bit of this echo area is within the terrain-blocked region.

Above is the morning forecast for TD-16E from NHC - which indicates the weakening depression may move right across eastern Pima County.

As Mike mentioned yesterday afternoon, the models continue to forecast considerably different outcomes for this event. These two forecasts are from the 06 UTC runs of the WRF model at Atmo and show forecast rainfall through 5:00 am MST tomorrow morning.. Both the NAM (above) and GFS (below) versions now are forecasting the max rain area to occur over or just east of the Tucson area. The NAM version has a much more broad event that impacts all of southern Arizona and southwestern New Mexico. But, the GFS forecasts a much heavier band of rainfall across southeast Arizona, with less rainfall to both the west and east. We will have to switch into observing mode and watch how the details evolve today.

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