{"id":2620,"date":"2025-09-10T17:46:13","date_gmt":"2025-09-10T17:46:13","guid":{"rendered":"https:\/\/discussions.weather.arizona.edu\/?p=2620"},"modified":"2025-09-10T17:46:13","modified_gmt":"2025-09-10T17:46:13","slug":"monsoon-moisture-tapped-by-an-approaching-low-pressure-system-from-the-west-risk-of-rain-in-the-region-today-through-friday-with-the-best-chances-for-tucson-metro-today-and-tomorrow-september-10th","status":"publish","type":"post","link":"https:\/\/discussions.weather.arizona.edu\/?p=2620","title":{"rendered":"Monsoon moisture tapped by an approaching low-pressure system from the west; risk of rain in the region today through Friday, with the best chances for Tucson metro today and tomorrow: September 10th, 2025"},"content":{"rendered":"\n

Yesterday’s Weather<\/h2>\n\n\n\n

Yesterday, moisture flowed from Mexico into southeast Arizona then northward into west central New Mexico and then eastward from there, as indicate by the green annotated line on the GEOS19 visible satellite image below. The thunderstorms, represented by the lightning flashes, have the cirrus from the storms blowing eastward by strong westerly winds. <\/p>\n\n\n\n

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GOES19 visible satellite images from September 9th from 4:30pm, College of DuPage<\/figcaption><\/figure>\n\n\n\n

Shown below are the 24-hour accumulated precipitation forecast from yesterday\u2019s HRRR-UA WRF 12Z (left) and equivalent observations from the MRMS system (right). Although the model forecasted the location of the storms in New Mexico well, it was too light with actual precipitation totals. The likely reason for this is a lack of upper air observations along the path of the monsoon moisture flowing in from northern Mexico, Sonora\/Chihuahua. \u00a0<\/p>\n\n\n\n

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HRRR-UA WRF 12Z 24-hour precipitation accumulation forecast from yesterday (left) versus observed 24-hour precipitation accumulation from MRMS multi-sensor data (right).<\/figcaption><\/figure>\n\n\n\n

Analysis<\/h2>\n\n\n\n

Surface dewpoint temperatures from this morning show areas of 50\u00b0F or higher are contained to the southern part of Arizona and eastern New Mexico.<\/p>\n\n\n\n

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Surface weather station observations for Arizona and New Mexico at 9:15am, NWS<\/a><\/figcaption><\/figure>\n\n\n\n

The observed sounding from Phoenix at 5am this morning has three inversions as indicated by the red annotated arrows (below, right). The one nearest the surface may be a result of a very low-level increase in moisture. The second one at about 720mb is is located where the mid-level clouds\/moisture are moving in from the south. The one furthest from the surface is at the top of the boundary layer, located at about 600mb.\u00a0On the HRRR-UA WRF 12Z initialization (below, left), the precipitable water is similar to the observation at 23mm (0.9 inches). The top of the boundary layer is 50mb lower on the observed sounding and the forecast initialization smoothed out the increase in moisture at that level.<\/p>\n\n\n\n

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HRRR-UA WRF 12Z initialization at 5am this morning (left), observed sounding from Phoenix at 5am this morning (right).<\/figcaption><\/figure>\n\n\n\n

At 250mb, eastern Arizona is in a broad area of divergence associated with the trough and ridge pattern, to our respective north and south. The wind speed is 75 knots in western Arizona decreasing to 50 knots along the same streamline in eastern Arizona. This pattern shifts a little to the east tomorrow as winds become more from the southwest as the trough moves into northern Nevada.\u00a0<\/p>\n\n\n

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The low-pressure trough is over northern California at 500mb, with the high-pressure ridge in Mexico and Texas east of El Paso. Flow is from the south to southwest across the\u00a0region with wind speeds at 15 knots in southeast Arizona and most of New Mexico, increasing to almost 50 knots in northwest Arizona. The wind pattern does not change much over the next few days, even as the low-pressure drops south into central California and the ridge moves eastward into the Abilene area of Texas.\u00a0<\/p>\n\n\n

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At 700mb, the moisture axis extends from the central Gulf of California northward into southeast Arizona then eastward into New Mexico. Dewpoint temperatures are near 2\u00b0C in southeast Arizona, but over the next few days will increase due to advection of moisture in Mexico.\u00a0<\/p>\n\n\n

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Today<\/h2>\n\n\n\n

The HRRR-UA WRF 12Z forecast sounding\u00a0for Tucson 3pm (left) and Nogales at 1:30pm (right) show no Convective Inhibition (CIN) and plenty of Convective Available Potential Energy (CAPE) for storms to develop this afternoon and evening.<\/p>\n\n\n\n

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HRRR-UA WRF 12Z forecast sounding for Tucson at 3pm (left) and Nogales (right) at 1:30pm this afternoon.<\/figcaption><\/figure>\n\n\n\n

The simulated radar reflectivity and 10-meter wind speed forecast from HRRR-UA WRF 12Z is shown below. The model forecasts the western edge of the storms to move west about 20 miles by evening. Microburst winds of 40mph or more will be associated with the storms along with blowing dust.\u00a0Strong winds are forecast to approach the eastern edge of the Phoenix area in the evening, around 9pm. \u00a0<\/p>\n\n\n\n