Thursday, August 16, 2018

Done In By Yesterday's Morning Showers

Yesterday was nice example of  how convective feed backs (clouds and low-level cooling) can mess up forecasts in the very short term of the 12 to 24 hour time window. The 12 UTC WRF RR that I saw was very close, but others were considerably off as was NWS official forecast. The above ALERT map is for 24-hours ending at 7:00 am MST this morning. Rain amounts were generally quite light (0.05" total here at house) and occurred mostly before noon yesterday.

Plot of CG flash density (below for 24-hours ending at 7:15 am - from and Vaisala) shows most storms stayed over high terrain along the Borderlands, with some activity on the Rincons, and one lonely flash in metro during the morning showers.

Another morning with heavy cloud cover and some sunrise showers to northwest and north. View above from 7:00 am. The visible satellite image below (from 7:15 am) shows the clouds and also that Cochise County and northeastern Sonora are mostly clear - obviously another difficult forecast day.

The 500 mb pattern is extremely chaotic, with several weak troughs wandering around within the weakened large-scale ridge. An anticyclone center seems to be near Houston (the 250 mb anticyclone is centered over northern Sonora to our south). Model forecasts want to shift the upper-tropospheric anticyclone northward over us during day today.

Sounding above for TWC at 12 UTC is from NCAR RAP. The winds aloft are light and variable up to 500 mb and then southerly to southwesterly above. The PW is extremely high - just a bit below 2 inches! If I modify the sounding above for afternoon conditions, I estimate a convective cloud base a bit above 600 mb, some capping above and only a sliver of CAPE. The sounding up at Phoenix has both a better wind profile and much more CAPE likely this afternoon - unless the heavy cloud cover moves overhead.

The NAM versions of the WRF (06 and 12 UTC) forecast considerable thunderstorm activity over Pima County this afternoon. The 06 UTC GFS version forecasts only a bit of thunderstorm activity and puts Tucson right in another donut hole. However, the TWC soundings from that GFS forecast (below valid at 5:00 pm) seem to indicate storms right in the metro - so some aspects of GFS seem inconsistent.

Official forecast from NWS for TUS is 50% chance of precipitation today and 30% tonight. I'll wait for Mike's much more detailed discussion of the various models and their initializations, and also wait and watch again.

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