Quieter day in wake of Mesoscale Vortex

 Overview:

Convection surrounding Tucson to the south, east, and northwest last sent sent out a series of weak outflow boundaries which intersected over portions of eastern and central Pima counties resulting is some relatively short lived showers and thunderstorms largely to the west of the Tucson metro area. This left a us once again with considerable cloud cover overnight. This morning, visible satellite imagery as well as mesoscale analysis indicates the presence of a convectively driven mesoscale vortex (MCV) west of Tucson that is propagating north/northwest ward. While global model intitializations fail to capture this feature, mesoscale intitializations do indicate an MCV although there is some disagreement in the satellite imagery and the models as to the exact placement of this feature. The MCV will serve as a complication to today's forecast, both in terms of how persistent cloudiness will be today, as well as its impact on areas favored for ascent and descent. Ascent ahead of this feature as it propagates northwestward will likely result in enhancement of convective development over the Mogollon Rim. In the greater Tucson area, while moisture and instability will likely remain in place, subsidence in the wake of the MCV may suppress activity until late in the day, and even then likely confined to the higher terrain of southern Cochise county. I expect slightly more coverage of convection locally tomorrow particularly in the surrounding higher terrain south and east of Tucson as more abundant sunshine should provide for greater destabilization.

Last Night:

I am including a long loop of the radar imagery from last night largely because I find it interesting. In the loop, you can see outflow boundaries propagating westward, southward, and northward intersecting over eastern and central Pima county, including over the Tucson mentor area.

Radar Loop Ending at 8 AM MST 

(weather.cod.edu)

The interacting outflow boundaries led to an overnight blow up of convection over central Pima county. Near the end of the loop, you can also spot evidence of the aforementioned MCV, working its way northward over the Tucson area. The MCV appears to have originated in as a result of the large cluster of convection that occurred over Sonora, Mexico.

This Morning:

The sounding from Tucson this morning continues to show conditions similar to those from the past couple of mornings.

12Z KTUS Sounding

(spc.noaa.gov)

Namely, we continue to have fairly well saturated conditions with a moist adiabatic lapse rate above 500 hPa. Steeper lapse rates from 750-500 hPa along with still high surface dewpoints supports decent CAPE values of 1000 J/kg. Precipitable water values are still extremely high for this time of year with values of 1.8". While this would certainly support excessive rainfall rates should convection occur, intitation given the impacts of the MCV (see below) will be problematic.

The visible satellite picture from this morning clearly shows the MCV propagating north-northwestward over the Tucson area.

Visible Satellite Loop Ending at 10 AM MST 

(weather.cod.edu)

This vortex is also apparent in the 500 hPa mesoanalysis from this morning

500 hPa mesoanalysis valid at 10 AM MST 

(spc.noaa.gov)

As well as the 500 hPa Vorticity field from the HRRR intitialized UAWRF

500 hPa Vorticity for the UAWRF 12Z HRRR initialization valid at 10 AM MST 

(weather.arizona.edu)

Impact of the MCV:

This region of Vorticty should act to enhance ascent in the direction of travel, leaving subsidence in its wake.

500 hPa Vorticity from the UAWRF 12Z HRRR initialization valid at 1:30 PM MST 

(weather.arizona.edu)

By 1:30 MST as the system has moved into Maricopa county, we see evidence of convection developing along the Rim in the noise in that has developed in the vorticity field. Meanwhile, evidence of subsidence in the wake of this feature can be seen in the forecast sounding 

KTUS sounding UAWRF 12Z HRRR initialization valid at 7 PM MST 

(weather.arizona.edu)

valid for 7 PM today. At 600 hPa, there are indications of a subsidence inversion, although this can be overcome with sufficient surface heating.Note that CAPE values may be underdone as surface dewpoints continue to be be forecast to drop into the mid 50's by afternoon, which has been a consistent trend in the models that has not been realized.


The total precipitation figure valid at midnight shows the expected concentration of activity along the rim

Total Precipitation from the UAWRF 12Z HRRR initialization valid at midnight 

(weather.arizona.edu)

and is consistent in both the 12 and 15Z intitializians of the UAWRF as well as being supported by the 12Z HRRR ensemble forecast which shows less that a 10% chance of heavy rainfall in the Tucson metro. However I think we should take note of the fact that the ensemble max indicates that the chance of heavy storms in the Catilnas is small but not 0%. Given the large PWTR and the expected under forecast instability and convection that does develop should produce good downpours.

HRRR ensemble max precip and prop of > 1" valid at 5 PM MST

(spc.noaa.gov)

Tomorrow:

With the MCV out of the way, still high values of PWTR, and a typical 500 hPa monsoon pattern, I expect convective coverage to increase over the region tomorrow.

The 500 hPa pattern  shows the center of the upper-level high situated over New Mexico tomorrow, with broad southernly flow over Arizona.

HRRR ensemble mean 500 hPa flow valid at 1 PM MST

(SPC.noaa.gov)

The relative lack of clouds as shown in the forecast infrared image below

Outgoing Long wave radiation from UAWRF 12Z HRRR initialization

(atmo.arizona.edu)

should allow for plenty of destabilization with values approach 1000 J/kg in the immediate metro area, but exceeding that threshold in Santa Cruz county.

CAPE from the UAWRF 12Z HRRR initialization valid at midnight 

(weather.arizona.edu)

In addition to CAPE, as PWTR values start to fall back due to mid and upper-level drying, we should see a return of the possibility of stronger wind gusts with downdraft CAPE values exceeding 1800 J/kg.

KTUS forecast sounding from the UAWRF 12Z HRRR initialization valid at 2 PM MST

(weather.arizona.edu)

However, in the absence of a convergence boundary or other feature to help initiate

 convection,

10m wind from the UAWRF 12Z HRRR initialization valid at 2 PM MST

(weather.arizona.edu)

we will mostly be relying on terrain to trigger storms.