Summary

Surface dewpoints show continued moisture across Arizona, with some drying in northwest New Mexico. The monsoon high-pressure system has reestablished itself over Arizona, but atmospheric conditions are complex due to surrounding jet streams and low-pressure systems.

Today’s forecasts indicate that, while there is enough atmospheric instability (CAPE) for mountain thunderstorms, storms are likely to weaken as they move into lower elevations unless there are favorable outflows. Thunderstorm activity is expected in the eastern Pima and Pinal counties and along the Mogollon Rim, with potential for outflow winds up to 40 mph. Precipitation forecasts vary, especially for Tucson, depending on the timing and movement of storms and the dependency on outflows.

Tomorrow, there is a higher risk of heavy rainfall, particularly in Phoenix. Forecasts disagree on the exact amounts; we should know more about precipitation totals tomorrow, but preparing for a strong event is wise. By Friday, storm activity should be focused on the mountains, with lower overall rainfall expected, but the monsoon pattern will remain active into the weekend and next week.

Yesterday’s Weather

A visible satellite imagery animation for yesterday, with radar and lightning strikes overlaid, is below. Although there are many clouds, very few are thunderstorms. The thunderstorms moved southwest, but for the most part, the storms stayed in the mountains. Eastern New Mexico is under the influence of a polar low-pressure system now in Texas, which is bringing some dry air into the region. 

Rainlog gauge data below shows the isolated nature of the storms, with most precipitation falling in the mountains. 

Below is the HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right). Compared to the Rainlog data above, the accumulated precipitation map below shows more precipitation, circled in black. The forecast missed the mountain storms in the red area and appears overly generous in the green-circled area in the valleys of western Pima County. 

A TUTT is over Baja California, in the same relative position as the last couple of days, with Tropical Storm Elida forming in the Pacific Ocean south of the TUTT low. The polar low has dropped south from Arkansas to the Texas Hill Country. Heavy rain and flooding are occurring from the Del Rio to San Antonio area. Dry air has dropped south across most of New Mexico. 

Analysis

Some more drying is indicated in northwest New Mexico, where the only dewpoints < 50°F are. Otherwise, it remains moist across most of Arizona. Flash Flood watches have been issued for Yavapai County and the northern part of Gila.

The HRRR-UA WRF 12Z initialization (left below) and observed sounding (right below) for Phoenix at 5 am are shown below. The only slight difference between the initialization and the observed sounding is that the initialization has too much low-level moisture, causing the mixed-layer and most unstable layer Convective Available Potential Energy (CAPE) to be unrealistically high.  Surface/most unstable CAPE on the observed sounding is not representative of the boundary layer, as there’s a spike in near-surface moisture from irrigation, which affects the CAPE calculation. The “best” CAPE value this morning is the mixed-layer CAPE on the observed sounding, about 400 [J/kg].

At 250 mb, the monsoon high-pressure system has reformed over Arizona but is still being squeezed between the polar jet to the west and the circulation around the polar low-pressure system to the east. The TUTT over Baja is in about the same location as previous days.

The monsoon ridge is near the Four Corners at 500 mb, and the polar low has drifted into southwest Texas. The reflection of the TUTT low is near the Baja; the other circulation indicated farther west is not in upper-air analysis, so likely a model artifact.

At 700 mb, the polar low in Texas has moved into the Del Rio area, with plenty of associated moisture. An easterly wave is indicated by the dashed black line and will move west to the Baja Peninsula tomorrow.

Today

The forecast soundings for Phoenix (left) and Tucson (right) are below, and due to some warming aloft, there is not much CAPE, though plenty of precipitable water. There’s enough CAPE for thunderstorms to form over the mountains, but storms will weaken as they move off the mountains.

Scattered showers and thunderstorms will move into the Tucson area in the early afternoon while other storms develop in the Mogollon Rim. Storms will move southwest off the Mogollon Rim and, by evening, approach Phoenix and Tucson. The heaviest precipitation will still be on the mountains, with lighter totals in the valleys. Storm generation/continuation into lower elevations will mostly depend on outflow winds, which could reach 40mph.

The HRRR-UA WRF 15Z (below, right) accumulated precipitation forecast has backed off compared to the 12Z (below, right). If storms form on Mt Graham and outflows aid lift toward the Tucson valley, then the metro area could get hit, but we’re favoring the 15Z due to the forecast soundings and the conditional nature of the 12Z forecast.

Tomorrow

More CAPE will be available for storms in Phoenix than in Tucson, but the potential for heavy rain is high, as the forecast precipitable water is close to 56mm (2″).  

The HRRR-UA WRF 12Z (below left) and 15Z (below right) forecasts show quite a different solution for Phoenix. The 12Z forecast indicates heavy rain in Phoenix, while the 15Z forecast does not. The difference is that outflow from the storms blows the CAPE out of the Phoenix valley for 15Z. We’ll need to monitor the forecasts tomorrow to see if there’s consistency with either of these forecasts. It will still be an active day for both Tucson and Phoenix tomorrow, and heavy rain is likely under either forecast. However, it’s difficult to determine how much rain (more or less than 0.5″) at this time.

Outlook

On Friday, forecasts indicate storms will be over the mountains, though this will mostly depend on what happens Thursday, whether storms will move into the valley or not. The forecast radar reflectivity shows the activity on Friday afternoon predominantly in Graham and Cochise counties.

Global model ensemble precipitation forecasts indicate lower storm totals after Thursday, while still maintaining the active monsoon period we are in.

Discussion written by Pat Holbrook and Patrick Bunn