August 24th: Low Grade Monsoon Pattern Through Sunday; Excessive Heat Returns This Weekend

Synopsis

Some moisture and instability will be in place for a chance of scattered afternoon and evening showers and thunderstorms mainly over the sky islands and near the International Border in southeastern AZ today. Starting tomorrow and persisting into the weekend, the 500mb anticyclone will reconsolidate over the Southwest which will lead to rising mid level heights and therefore hotter temperatures this weekend. Enough moisture and instability will remain for a chance of showers and storms across southeastern Arizona, mainly over the higher terrain and near the International Border Friday through Sunday. 

Current Conditions

A mid level low, associated with former tropical storm Harold, is currently situated over northeastern Arizona, and the mid level anticyclone is centered over the Southern Plains via SPC 500mb analysis this morning. 

SPC 500mb analysis at 17:00z this morning.

This mid level low will become an embedded shortwave on the northwest side of the ridge and lift briskly northward throughout the day today. Meanwhile, mid level flow across southern Arizona is very weak which was also indicated from the 12z TUS sounding. 

12z TUS sounding courtesy of the SPC.

Still plenty of moisture with 1.60 in of pwat and a 62 degF surface dew point, but instability is limited with only a couple hundred J/kg of CAPE. 

Today's Forecast

Synoptics

Arizona will be in a synoptic transition period today as the mid level low lifts northward and the mid level anticyclone begins to build westward. Mid level flow turns southwesterly by this afternoon which will lead to drier air advecting into the region in the mid and upper levels. Also, weak southwesterly winds are not a very ideal steering pattern for monsoonal storms. 

12z GFS 500mb absolute vorticity for today at 5:00PM MST.

Moisture and Thermodynamics

As mentioned above, drier, southwesterly flow aloft will generate a bit of drying by this afternoon with the 12z WRF-HRRR forecasting pwats to decrease to between 1.25 and 1.50 in by early this afternoon across the state. However, moisture will still be sufficient enough for storms to develop.

12z WRF-HRRR total precipitable water for today at 2:00 PM MST.

Meanwhile, the 12z and 15z WRF-HRRR runs are under forecasting the surface dew points again with the 12z forecasting a surface dew point of 54 degF in Tucson at 11:00 AM MST and as of 10:54 AM MST, surface obs reporting dew points in the low 60s across the Tucson vicinity. The 12z KTUS WRF-HRRR sounding forecasts CAPE to reach around 300 J/kg by late this afternoon which am thinking is a slight underestimation. The surface dew point will likely continue to decrease into this afternoon, but I don't think it will reach the low 50s as WRF is forecasting. Therefore, am expecting CAPE to most likely reach around 500 to 700 J/kg today. Still, mid and upper level drying will likely lead to a slight bit of subsidence above 500mb by this afternoon and evening which is reflected in the model sounding as shown below. 

12z KTUS WRF-HRRR sounding for 4:00 PM MST.

What to Expect

Moisture and instability today will just be sufficient enough for scattered showers and thunderstorms across southeastern Arizona, mainly confined to the sky islands and near the International Border. The 12z and 15z WRF-HRRR advertise a few cells to develop over the Catalinas and Rincons this afternoon where differential heating will generate just enough lift to produce a few cells over those areas. 

12z WRF-HRRR simulated reflectivity for today at 12:00 PM MST.

12z WRF-HRRR simulated reflectivity for today at 1:00 PM MST.

CAMs are in agreement that storm activity will remain outside the Tucson metro this afternoon and evening. Considering the weak, southwesterly steering flow, dry air advection aloft, and marginal instability, this is a very likely scenario (however the chance is nonzero in Tucson). 

Temps will remain relatively on the mild side, especially in southeastern AZ where temps will likely remain below 100 degF this afternoon. The lower deserts between Phoenix and Yuma will be between 100 and 105 degF.

12z WRF-HRRR 2-meter temps for 3:00 PM MST this afternoon.

Friday through Sunday

Starting tomorrow the 500mb high will begin to reconsolidate over the southwest, increasing temps as well as maintaining a low grade monsoon pattern. The high doesn't appear to get too strong with the center only at 592 to 594 dm this weekend. This is thanks to an upper level trough moving into the Pacific Northwest which will squash the ridge and prevent it from building northward. 

12z GFS 500mb absolute vorticity for tomorrow at 5:00 PM MST.

12z GFS 500mb absolute vorticity for Saturday at 5:00 PM MST.

12z GFS 500mb absolute vorticity for Sunday at 5:00 PM MST.

Some moisture will still be present however with the GFS forecasting pwats hanging around 1.25 in to 1.50 in throughout this period as shown below. 

12z GFS total precipitable water for tomorrow at 5:00 PM MST.

12z GFS total precipitable water for Saturday at 5:00 PM MST.

12z GFS total precipitable water for Sunday at 5:00 PM MST.

There should be sufficient moisture and just enough instability for a chance of afternoon and evening storms across southeastern Arizona, mainly over the high terrain and near the International Border. 

The main focus will be high temperatures with a return of excessive heat likely starting tomorrow and persisting into the weekend.  The hottest temps will be in the lower deserts between Phoenix and Yuma where temps could reach between 110 and 113 degF. Tucson will likely remain below the 110 degF mark with models forecasting temps to remain between 102 and 106 degF in Tucson Friday through Sunday. Below is the 12z WRF-GFS high temperature forecast for this period. 

12z WRF-GFS 2-meter temps for tomorrow at 3:00 PM MST.

12z WRF-GFS 2-meter temps for Saturday at 2:00 PM MST.

12z WRF-GFS 2-meter temps for Sunday at 3:00 PM MST.