Wednesday, June 29, 2016

Early Morning Storms

Please see previous post concerning timing of damaging winds during Sunday's microburst.

So much going on that it's hard to keep up. There were widespread rains and thunderstorms during the night and early morning hours - considerably greater than official forecast, and perhaps wider coverage than forecast by the WRF model runs yesterday - which did pick up the early activity nicely.

The forecast that was still up at the NWS website at 5:40 am MST this morning is shown above for TUS, indicating 20% POPs overnight. Actually, overnight 85% of the ALERT sites measured 0.04" or more rainfall. There were a number of sites over an inch and a site in the Catalinas had nearly two inches. Here at the house we had 0.01" during an early afternoon shower yesterday and then an additional 0.29" after midnight.

A forecast like this, with high POPs for six consecutive 12-hour periods is unlikely to verify well, because of the strong mesoscale impacts that storms in one period have on the next period.

The composite radar image below is from 5:27 am this morning. The two plots below that are for CG flash density ending at 6:45 am - the first is for past 6-hours and the bottom panel is for past 12-hours (from and Vaisala).

Today is a very serious forecast challenge because of the nighttime storms. The morning upper-air sounding plot for TWC on campus is shown above. There has been substantial cooling below 700 mb. The sounding also exhibits an "onion" shaped profile above 850 mb, due to the mesoscale downdrafts that followed the early morning rain. This type sounding is often associated with the infamous Tucson donut hole during the afternoon. The visible image below is from 6:45 am MST and shows the extensive cloudiness from southern Arizona all the way south to Sinaloa - there also appear to be a couple of embedded cyclonic circulations. I would note that the cloudiness seems to moving toward the west to southwest, and clearing may come in from the east later this morning.

The 06 UTC forecast from the WRF-NAM at Atmo seems to have problems with its forecast of the early rains being too conservative. The forecast temperatures for 5:00 am appeared to be 10 to 15 F too warm over southeast Arizona. Thus, the afternoon heating and forecast of widespread storms may be too extreme. Very important to check Mike's discussion later today. 

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