Marine Weather and Tide Forecast for Rillito, AZ

UPDATE

The mid-morning satellite picture shows generally clear skies with few to scattered cirrus moving out of Gulf of California convection to the west. As previously discussed the overhead high pressure center should keep thunderstorm chances to a minimum today, if not mostly zero. Surface dew points have risen this morning at Nogales and at several other stations through Sells, though this moisture increase is likely fairly shallow and should mix out through the day. The strong subsidence aloft from the high pressure center as well as the shallow surface moisture should limit thunderstorms today, but an isolated storm or two can't be ruled out as indicated by several 12Z high-resolution models.

DISCUSSION
Clear skies across southeast Arizona this morning as strong high pressure aloft over western New Mexico is the dominant feature of our weather pattern. Moisture levels are below normal across the region, but most notably from Tucson eastward with PWAT values 0.7" to 0.9" at this time, with surface dewpoints mostly in the 40s to lower 50s. A bit more moisture west of Tucson where PWATs range from 1 to 1.2 inches. Storm activity will struggle once again today as the strong high nudging west to the AZ/NM border results in broad subsidence (with dry air aloft). Can't rule out a storm or two mainly to the south to southwest of Tucson from Santa Cruz into south central Pima County where moisture and instability levels are more supportive and the subsidence under the high isn't as pronounced. The hottest temperatures of this extended heat stretch are expected today, with the current forecast of 109 degrees in Tucson. Wouldn't be surprised if it ends up slightly higher (110 or 111) if stronger mixing of drier air aloft occurs dropping surface dewpoints aiding surface heating.

While there won't be much change with the mid/upper level high on Thursday, centered near or just west of the AZ/NM border, lower level moisture will start to be on the increase. This will begin to occur late tonight/Thursday morning as a likely first modest push of moisture occurs courtesy of outflow from convection in Sonora. This will set the stage for a few more storms (mostly 10 to 20 percent chance) on Thursday with the focus still on locales south to southwest of Tucson. Wind will be the primary concern with any storms on Thursday.

From Friday through Monday, we will quickly transition into a much more active monsoon pattern. During this time, the mid/upper level high will be on the move eastward into a more favorable position over New Mexico by Saturday, with the stage being set for broader deep layer southeast to southerly flow. This will result in sustained above normal moisture with 110 to 150 percent of normal PWAT values. Troughing over the NW part of the state could also provide a little bit of additional lift especially Sunday/Monday.
Bottom line is this will translate into storm chances in the 20 to 50 percent range Friday increasing to 30 to 60 percent Saturday through Monday. As is usual for the monsoon, we will likely have a more 'up' day or two but it's too far out to really get a good feel for the most active days. As deeper moisture moves in, we'll also be adding increased flash flooding potential in addition to wind threats. Thereafter, back to a more typical lower to mid to start the month of September but still considerable uncertainty that far out.

The Extreme Heat Warning remains in effect through Saturday.
Admittedly, temperatures on Saturday are expected to drop a few degrees compared to today through Friday. We'll continue to monitor that for any later adjustments in the timing of the Extreme Heat Warning. By Sunday into early next week, temperatures drop to more seasonable levels.



AVIATION
Valid through 27/12Z. SKC-SCT clouds mainly 10-13k ft AGL through the forecast period with slight chance of -SHRA/-TSRA for points near and west of KOLS between 26/21Z and 27/03Z. Surface winds mainly light and terrain driven with occasional afternoon gusts around 15 kts Aviation discussion not updated for TAF amendments.

FIRE WEATHER
Very isolated thunderstorms (10 percent or less chance) for most locales today. Storm chances nudge higher to around 10 to 30 percent Thursday, with a more active pattern taking hold as early as Friday but especially this weekend through Monday (30 to 60 percent daily chance). Temperatures will remain well above normal through most of the week with a cooling trend this weekend into early next week. Minimum relative humidity values mainly 10 to 20 percent in the valleys and 20 to 30 percent in the mountains through Friday. Minimum RH values increase to 20 to 40 percent starting Saturday in valleys and 30 to 50 percent in the mountains. Winds will generally remain less than 15 mph and terrain driven through the week.

TWC WATCHES/WARNINGS/ADVISORIES
Extreme Heat Warning until 8 PM MST Saturday for AZZ501>509.