Summary
Surface moisture remains similar to yesterday, with a Flash Flood Watch in effect for areas east and south of Tucson, and a potential for heavy rain and flooding across southern Arizona. Atmospheric conditions are very tropical, and weather models indicate high moisture and instability, especially in Phoenix and Tucson.
Forecasts suggest slow-moving storms that could struggle to propagate away from the higher terrain due to weak 0-6 km mean winds and limited Convective Available Potential Energy (CAPE) in the low deserts. This causes variable rainfall amounts in the forecasts, creating uncertainty about exactly where the heaviest rain will occur. Some forecasts predict significant precipitation (>0.5″), raising the risk of flash flooding in metro and surrounding areas, while more recent forecasts show less rainfall (<0.25″).
Tomorrow’s setup is similar, though it depends on whether the atmosphere gets worked over by today’s activity. Longer-range forecasts show a warming trend through the week with decreasing thunderstorm chances.
Yesterday’s Weather
Thunderstorms developed over the Mogollon Rim and Southeastern Arizona and moved west toward the valleys of Tucson and Phoenix, as well as the low deserts, but stopped as these areas had very little Convective Available Potential Energy (CAPE) in the boundary layer. The eastern third of New Mexico had fewer storms as mid-level dry air has been pulled into the area by the flow that used to be the polar low in Texas.
The upper-level wind flow has strengthened as the remnant showers from last night’s storms have moved further west. These showers are in the mid-level of the atmosphere and are not using surface-based CAPE.
The visible picture below shows a band of clouds and light showers stretching from Kingman in the north to Phoenix, Casa Grande, Tucson, south to Nogales, and east to Sierra Vista/Douglas. Clearing is occurring to the east of the clouds (Cochise, Graham, and Greenlee counties north to the Mogollon Rim), and this will allow the cleared areas to get full heating from the sun today.
Below is the HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right). The observed precipitation data shows storms were mostly in the mountains from the Grand Canyon south to the Mogollon Rim, eastward to the White Mountains, then south across Graham and Cochise counties. The forecast precipitation is in good agreement with the observed precipitation except for an underestimate in Cochise County. A spotter in eastern Cochise County has received 4.52 inches since Friday.
The polar low over Texas has dissipated, with some energy from this low helping to form a new Tropical Upper Tropospheric Trough (TUTT). The TUTT low over Baja California is in the same location. Former Tropical Storm Elida has been sheared apart by strong upper-level winds, and only a low-level circulation remains and will eventually spin down to an open wave. Other new features are a polar low in southern Canada and a tropical depression (TD) in the Gulf of Mexico.
Analysis
Surface moisture is not much different than yesterday. A Flash Flood Watch has been issued for areas east and south of Tucson, but Pima County may also be affected if storms are a little further west than forecast. The atmosphere remains very tropical, and any storm can produce heavy rain and flooding.
The HRRR-UA WRF 12Z initialization (left) and observed sounding (right) for Phoenix at 5 am are below. The soundings are similar, and there are decent winds in the mid-levels that can help push the morning showers along. Precipitable water remains high, and if the clearing continues as forecast, we have a better chance of getting to the convective temperature.
At 250mb, the only location and wind flow that is accurate is the location of the high-pressure in northern Utah and the new TUTT low in the northern part of the Gulf of California. South of the International border, everything depends on the Chihuahua upper-air sounding, so the high-pressure circulation has been inferred rather than determined by observations, so we can’t put good confidence in it. There are too many other ways to draw this analysis without more data.
At 500mb, the monsoon high is over northwest Colorado with 10 to 15 knots of east winds underneath the ridge. Temperatures in Arizona are not too warm at –5°C to –6°C, with some colder spots associated with last night’s storms.
At 700mb, plenty of moisture is available in the lower elevations of Arizona, while it’s drier on the Colorado Plateau.
Today
The 12Z HRRR-UA WRF forecast soundings for this afternoon show potential for heavy rain in Phoenix and Tucson. The atmosphere remains very wet; there’s enough CAPE for storm development on the mountain, and the wind shear is good, meaning inflows shouldn’t be disturbed by outflows. However, 0-6 km mean wind is only a few knots toward the west in Phoenix and south in Tucson, which shows storm propagation is slow albeit in the right direction for storms to move toward the respective metro areas.
The HRRR-UA WRF forecast radar reflectivity from the 12Z (below, left) and the 15Z (below, right) forecasts present a conundrum. Both forecasts are similar, except the 15Z forecast does not bring the heaviest rain into either Tucson or Phoenix. In the 15Z forecast, the storms struggle to propagate into the valleys, probably due to weak winds and limited CAPE in the valley.
The total accumulated precipitation forecast through 5 am is below: HRRR-UA WRF 12Z (left) and 15Z (right). The 12Z forecast for Tucson shows totals >0.5” for the metro area and >1.5” in central Pima County, which would result in flash flooding south and west of I-10. The 15Z forecast has less precipitation in these areas, <0.25”. The 12Z forecasts over an inch (>1”) in parts of Phoenix, while the 15Z shows much less. The 12Z relies on downstream development, which is inherently uncertain, so the 15Z is probably the more likely scenario, though we should be prepared for the possibility of the 12Z forecast.
Tomorrow
The 12Z HRRR-UA WRF forecast soundings for tomorrow are similar to today, though what happens will be dependent on what happens today (typically a down-day follows and up-day). High precipitable water values, good mid-level winds, and for Phoenix, the mean wind is stronger and still in a favorable direction.
Storms form in the eastern section of Arizona and move west, mainly in the mountains as cooling in the deserts from today’s storms could limit activity in the low deserts.
Outlook
Winds aloft become more from the southeast with much less precipitation in both Arizona and New Mexico as indicated in the radar forecast below.
Global model ensemble forecasts show a warming trend through Saturday, with lower chances of thunderstorms.
Discussion written by Pat Holbrook and Patrick Bunn