Tucson, AZ 85718 Weather

Synopsis

An unsettled weather pattern continues with strong to severe thunderstorms possible today. Main threats will be heavy rain, damaging winds, large hail, and potentially a tornado. This threat will be greatest in Graham, Greenlee, and Cochise counties, though an isolated storm can't be ruled out in Pinal, eastern Pima, or Santa Cruz county. Temperatures lower to below normal Friday into next week.

Discussion

A rare October severe setup is in place across eastern and northern portions of the state today as a deep upper level trough and associated jet streak sit over southern California into Arizona. A northward draw of unseasonably moist air along with a strong lower to mid level wind field will produce an environment conducive to organized supercells with threats of very heavy rainfall rates, damaging wind gusts, large hail, the potential for a tornado.

Scattered thunderstorms are expected to develop across mainly eastern areas including eastern Pinal and Graham, Greenlee, and Cochise (mainly northern Cochise) this afternoon, though this threat may extend in an isolated sense as far west as eastern Pima or Santa Cruz counties. Instability is more than adequate today with modeled MLCAPE values of 1000-2000 J/kg, thanks to widespread upper 50s to lower 60s surface dew point and afternoon insolation. Additionally, impressively strong deep layer shear on the order of 35-50 knots will provide ample support for rotating supercells, while low level shear to around 15 knots in the eastern three counties will provide a tornado threat. In this CAPE/shear environment, discrete supercells will likely produce hail to around 2 inches in diameter and wind gusts to 70 mph, along with the threat of a tornado. While the tornado threat is non-zero anywhere a storm forms, the environment in Graham/Greenlee does have a bit of a stronger wind field for a tornado, with this field extending south into Cochise county. This parameter space distribution is covered by today's Tornado Watch.

With thunderstorm coverage greatest further north, closer to the upper low and associated jet streak, storm collisions and clustering in Graham and Greenlee counties will be likely. Any thunderstorm clustering may reduce the hail threat, however the flash flooding threat then becomes more concerning. Precipitable water content of 1-1.3 inches combining with the likely intense storms will produce periods of very heavy rainfall rates. Storm motions will be fast but any clustering or training will produce a flash flooding threat.

Convection will exit to the east through this evening, moving fully into New Mexico by around 9-11 PM MST. A Tornado Watch is in effect until 8 PM with the Flash Flood Watch until 10 PM. The threats will diminish from west to east.

The upper low will meander east through the region Friday into the weekend. High temperatures will be 5-8 degrees below normal across southeastern Arizona with a welcome drying trend to the surface dew points. Moisture will linger in far eastern areas near the New Mexico border to provide a chance of a shower or thunderstorm in the coming afternoons.

Overall forecast temperatures remain below into mid-week next week. Sub-freezing morning lows look likely in the White Mountains and Hannagan Meadow Sunday and Monday mornings.

Aviation

Valid through 24/00Z.

Isolated to scattered thunderstorms expected mainly 22/20Z through 23/03Z, with coverage greatest east of a KE77-KTUS-KOLS line through the New Mexico border. Any thunderstorm may produce severe hail, wind gusts to 50 kts, and brief very heavy rainfall. Otherwise ceilings mainly 5k-8k ft. Surface winds outside thunderstorms south-southwest 15-18 kts south of KTUS, especially KOLS- KDUG with gusts to 25 kts. Winds 8-14 kts to the north. Winds become light and terrain driven after 23/03Z. Surface winds southwest 12-18 kts Friday afternoon mainly KOLS- KALK- KDUG, with winds under 12 kts elsewhere.

Fire Weather

Temperatures near normal today, then cooler than normal Friday into this weekend. Scattered showers and thunderstorms today as a weather system moves into the state. Wind speeds mostly less than 15 mph this week except for today with southwest winds around 15 to 20 mph. Minimum relative humidity remains above 25-40+ percent in the valleys through the week.