Friday, August 15, 2014

Yesterday - A Strange Day

Storms developed quite early yesterday morning and tended to drift toward the north, while redeveloping toward the southwest. The CAPE yesterday was impressive but winds were extremely light in the lower half of troposphere.  Plot of CG flashes through 1 am MST this morning (above) shows storms across all of Pima County with most activity central and west portions. There is a narrow, short swath of CGs in metro Tucson. that extended from the University of Arizona campus north-northeastward across the house here.

Below is the Computer Science webcam view to the north at 1 pm. I noticed a narrow line of updraft bases extending from just southwest of here off toward campus at a bit before 1 pm. I had had lunch and napped for awhile. When I got up a bit before 2 pm it was very wet outside and the rain gauge held 0.91" - so I slept right through the biggest thunderstorm rain of the year. Ugh. Atmo recorded 0.60" from this short line of storms.

Storms produced abundant anvil cloud which covered most of southeast Arizona by late afternoon, although the MesoWest rainfall reports through 7 am this morning (above) indicate that most storm cores stayed at higher elevations.. Apparently several storms were severe, although none were reported to NWS or SPC. Cells near Casa Grande apparently produced a short-lived tornado - see chaser photos at                    
and radar echoes south of Ajo were very strong for a while.

Ron Holle sent the visible image below from 6 pm MST - there is a distinct outflow boundary moving over the northeast corner of the GoC, and if one looks closely there is cloud of dust behind the outflow also moving out over GoC.

Today remains somewhat similar to yesterday, although there are more clouds across easten half of Pima County this morning. There are already isolated storm echoes over southeastern Arizona. A large area of Pima County and southern Arizona continues to have PW of 1 1/2 inches or more (TWC morning sounding had 1.73"). The skewT plot of the sounding (below) indicates substantial CAPE again this morning, with light and variable winds continuing below 400 mb. So no steering winds and anvils streaming off a bit more to the east today.

At 500 mb there are two anticyclone circulation centers within the larger high over the Southwest. One is along the California/Mexico border and the other is east of El Paso. Between these centers there is a weak trough-line over southeastern Arizona. The NAM forecasts this feature to shift a bit eastward this afternoon, allowing steering-level winds to become northerly to northeasterly. It is indeed a complicated mess of a pattern.

The model forecasts are a mixed bag. The new NAM this morning continues to forecast scattered storms and heavy, mountain rains for much of southeast Ariaona. The early WRF forecasts from Atmo indicate a downturn with isolated to scattered storms for central and east Pima County. The early WRF-NAM appears too dry again today, although CAPE forecast for this afternoon exceeds 1300 m2/s2. So the exact reasons for the diminished storm activity in the WRF are not obvious.

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