Thursday, July 19, 2018

Yesterday Mostly Down Day

Yesterday was not nearly as active as I thought it would be. Tried to believe the NAM versions of WRF forecasts yesterday, since they fit better with what I thought would happen (strong thunderstorms impacting parts of the metro area). However, Mike L pointed out in his discussion of the morning forecasts that the NAM versions were too wet and thus probably too active during afternoon. So my trying to second-guess the various model forecasts went down the tubes. More below on what went wrong yesterday. View above is of storms well north of the Catalinas yesterday afternoon.

The only sites in the Pima County ALERT network that had rainfall were in the far southwestern corner, with one gauge near Tubac recording over an inch. The plot of detected CG flashes below (from Atmo and Vaisala - for 24-hours ending at 1:00 am MST this morning) shows that most areas of southeast Arizona and southwest New Mexico were very suppressed yesterday afternoon.

Shown here are yesterday's 12 UTC and 00 UTC soundings from TWC. The morning sounding had very large CAPE and was moist to 500 mb. I expected mountain storms to develop with this profile.

However, the evening sounding (below) shows a much different situation. During the day warming and drying occurred, due to subsidence, from 300 mb down to 500 mb. There was also some drying from 500 to 700 mb, but its harder to infer the reasons for this. However, the BL situation was not good for storm development - the temperature was nearly well-mixed to 700 mb but moisture was well-mixed only to about 800 mb. The mixing ratio decreased steadily from 800 mb to just above 700 mb. It seems to me that this was due to dry air mixing downward in a layer of wind shear and possible net subsidence. No storms on the Catalinas or Rincons, but storms did develop off the Santa Ritas - close but no cigar.

Today's situation is quite challenging once again. The morning sounding (above) has a bit less PW but CAPE is very high. The WRF forecasts from 00 and 06 UTC last evening and night were all very consistent - they forecast storms to our east and also south. Very strong storms make it westward  to the Rincons but then crash rapidly. Storms are not forecast for low elevations. Interestingly, the Rincon storms do not send an outflow across the metro area (in the model forecasts) but, rather, a strong outflow pushes back eastward as the storms fall apart.

The forecast skew-T below for TWC (from the 06 UTC WRF-GFS) essentially forecasts a repeat of yesterday's shallow BL with well-mixed moisture, overlain by a deep well-mixed layer in temperature but which again has mixing ratio decreasing with height. Another deadly-suppressive thermodynamic structure, if the model forecast verifies well.

No second guessing this morning and will just observe whatever happens this afternoon.


  1. Up near Flagstaff yesterday, a cell parked over the San Francisco peaks and dropped what appears to be snow!
    Image at:

  2. More likely it is hail and/or graupel.

    Here's a view of the storm taken from Mormon Lake:

    1. David thanks for sharing that photo, I shared that suspicion that it wasn't snow. One can dream!