October 17th, 2024: Windy and Cooler with Chances of Mountain Snow and Valley Rain on Friday

 Synopsis

A mid level trough will dig southward across the Intermountain West today, and become anchored over Arizona as a closed low by tomorrow. This feature is expected to provide the state with cooler temperatures, windy conditions, and light precipitation chances. The majority of precipitation is forecast to occur over the higher terrain in Northern Arizona, but a brief period of showers and isolated thunderstorms are possible in Phoenix and Tucson tomorrow afternoon. Precipitation accumulations are forecast to be light with a trace to 0.10 inch in the lower deserts and between 0.25 and 0.50 inches over the higher terrain with locally higher amounts possible along the southwestern facing slopes of the Mogollon Rim and White Mountains due to orographic enhancement. Snow levels will remain mainly above 7000 feet with a dusting to 6 inches of accumulated snow possible with the highest amounts over the tallest peaks of the Rim and White Mountains. 

Current Conditions

As of 1PM MST, SPC 500mb analysis displays a broad trough across the Intermountain West, and a high amplitude ridge across the Eastern half of CONUS. 

SPC 500mb analysis as of 1PM MST this afternoon.

Meanwhile, surface observations in Arizona this afternoon showing temperatures in the 80s across the lower deserts and in the 60s to 70s over the higher terrain. 

Surface observations as of 1:50PM MST courtesy of the NWS.

Southwesterly winds are also gradually increasing in magnitude as the aforementioned trough digs southward strengthening the pressure gradient across the state.

Tomorrow's Forecast

Synoptics/Dynamics

 The mid level trough to our northwest is forecast to rapidly dig southward throughout the day and into the evening hours. By tomorrow, this trough will become more of a closed low and anchor itself over Arizona.

12z GFS 500mb absolute vorticity valid for tomorrow at 11AM MST courtesy of Tropical Tidbits.

This closed low is expected to weaken throughout the day tomorrow as models forecast 500mb heights in the core of the low to rise likely due to vortex shrinking (from increasing terrain). However, differential cyclonic vorticity advection will provide enough synoptic scale ascent across the region to generate periods of clouds and precipitation.

Moisture and Instability

As is typically the case with mid latitude trough/upper level lows during this this time of year in Arizona, the dynamics are robust but they're relatively moisture-starved. However, the UA WRF forecasts precipitable water in the 0.75 to 1 inch range in Phoenix and Tucson tomorrow which is near the 90th percentile based on SPC TUS sounding climatology. 

12z WRF-HRRR precipitable water valid for 8AM MST tomorrow.

Most of this moisture is forecast to be confined below 600mb as indicated by the 12z WRF-HRRR KTUS and KPHX model soundings shown below.

12z KPHX and KTUS WRF-HRRR model soundings valid for tomorrow.

The source of this low level moisture is forecast to be from the Gulf of California as indicated by the 2-meter dew point forecast from the 12z WRF-HRRR as shown below.

12z WRF-HRRR 2-meter dew points valid for 8AM MST tomorrow.

Interestingly, sea surface temperature analysis in the Gulf of California indicates SSTs in the 80s which is several degrees above normal for this time of year. This is likely attributed to the anomalously hot temperatures the past several weeks across the Southwest.

CDAS Sea Surface Temperatures valid for 12z 10/17 courtesy of Tropical Tidbits.

Anyway, instability will be quite limited mainly due to a strong subsidence inversion near 600mb induced by mid level dry air being transported from the west. UA WRF model soundings indicating maximum CAPE on the order of 100 to 300 J/kg across the region tomorrow. Even though this amount of CAPE is quite minimal, it still should be enough to promote a chance for an isolated thunderstorm or two.

Timing and Impacts

Overall, this is expected to be a relatively low grade event, but any precipitation during this time of year is always welcomed. The timing of precipitation will be primarily dependent on a quasi-cold front/dry line moving from northwest to southeast where the maximum in surface convergence will be coincident with ascent induced by differential CVA. Current UA WRF guidance suggest the best window of opportunity for precipitation in Maricopa (including Phoenix) and Pinal Counties to occur is during the morning hours as shown below.

12z WRF-HRRR simulated reflectivity valid for tomorrow at 6AM MST.

12z WRF-HRRR simulated reflectivity valid for tomorrow at 10AM MST.

This band of precipitation is forecast to shift gradually southeastward with the best chance in Pima County (including Tucson) during the middle to late afternoon hours.

12z WRF-HRRR simulated reflectivity valid for tomorrow at 4PM MST.

 

The window of opportunity for precipitation in the lower deserts will be very brief with minimal precipitation accumulations expected. In fact, current UA WRF precip estimates range from nothing to 0.10 inches in the lower deserts, between 0.25 and 0.50 inches over the higher terrain of Northern Arizona with locally higher amounts possible along the southwestern facing slopes of the Rim and White Mountains due to orographic enhancement.

12z WRF-HRRR 48-hour precipitation forecast valid for 12z 10/17 to 12z 10/19.

Snow levels will be on the higher side (mainly above 7000 feet) with the UA WRF forecasting between a dusting to 6 inches with the highest amounts expected over the tallest peaks of the Rim and White Mountains.

12z WRF-HRRR 48-hour snowfall forecast valid from 12z 10/17 through 12z 10/19.

It will be windy tomorrow as well especially during the morning and early afternoon hours and ahead of the cold front/dry line with southwesterly winds forecast in the 15 to 25 knot range in the lower deserts and between 20 and 30 knots over the higher terrain with locally stronger winds possible.

12z WRF-HRRR 10-meter wind speeds valid for 11AM MST tomorrow.

Lastly, we finally get a real cool down with high temperatures expected to be in the 70s across the lower deserts.

12z WRF-HRRR 2-meter temperatures valid for tomorrow at 3PM MST.

Overnight temperatures will somewhat chilly across the region with models forecasting low temps in the mid 40s to low 50s across the lower deserts.

12z WRF-HRRR 2-meter temps valid for Saturday at 5AM MST.

Some of the higher valley locations in Cochise County could see low temperatures near freezing as well.

Weekend Forecast

The aforementioned closed low is forecast to remain anchored over Arizona Saturday and continuing to weaken throughout the day. 500mb heights will remain below normal leading to high temps in the 70s across the lower deserts, and a chance of scattered showers across the higher terrain of Northern Arizona. By Sunday, the closed low is expected to lift northward into the Plains, and high pressure aloft will rebuild into the region allowing high temperatures to return to near normal.