textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion.

KEY MESSAGES

- Bands of light to moderate rain with isolated thunderstorms will continue through the rest of this morning for much of Arizona before gradually shifting more over eastern Arizona this afternoon. - Flood Watches are in effect through early evening for most of Arizona with the peak of the heaviest rainfall occurring this morning. - Below normal temperatures are expected to persist across much of Arizona through the rest of the work week, while near normal readings return to the western lower deserts starting Wednesday.

- Dry conditions will prevail from Thursday through early next week as high pressure and gradual warming takes hold across the region.

SHORT TERM /TODAY AND TONIGHT/

Active weather will continue for at least the first half of today as an unseasonably strong Pacific low pressure system moves eastward through the Desert Southwest. The low center is now located across southern California prompting strong forced ascent through much of Arizona. Lower level moisture advection yesterday pushed mixing ratios upwards of 10-12 g/kg with the best moisture across far eastern Arizona. However, drier air aloft has also begun to work its way in from the southwest as it wraps in around the low center. This drier air may limit rainfall rates some, but all in all will not prevent much of the area from receiving decent rainfall through the rest of this morning.

Current objective analysis shows the best moisture pooling out ahead of the low with PWATs of around 1.5" across far south- central Arizona. Modestly strong mid-level southwesterly flow will continue to provide upslope ascent ahead of the low, while forced ascent aloft, which is currently maximized over southwest Arizona, gradually shifts eastward through this afternoon. Scattered showers and thunderstorms have developed across southern and central Arizona since midnight with rainfall rates close to two inches per hour for the stronger convective cells. However, due to a 30-35 kt steering flow, the storm residence time over one location will not be very long and training of cells will be needed for heavy rainfall amounts. A few of the thunderstorms are also becoming more discrete and potentially strong enough to bring strong winds to the surface. Although we can't rule out strong to near severe winds with the strongest storms, it is quite unlikely due to the very limited (250-500 J/kg) CAPE. The biggest threat for the rest of this morning will continue to be periods of moderate to heavy rainfall with training of storms bringing localized rainfall amounts of 1-2" of over a few hour period. Flood Watches are now in effect for much of south-central and eastern Arizona and last through early evening.

CAMs show these scattered showers and thunderstorms eventually forming a somewhat broken line by sunrise as they slowly transition from west to east. The peak of the rainfall for the Phoenix metro is likely to fall between 4 AM and 8 AM this morning before shifting mostly east of the Phoenix area later this morning. The highest rainfall amounts are expected to be focused across the higher terrain northeast and east of Phoenix from around sunrise through noon. The band of showers and thunderstorms is likely to fill in more by around sunrise as it begins to move into eastern Arizona before winding down late morning into the early afternoon. The threat for flash flooding will also peak later this morning with the bulk of shower and thunderstorm activity shifting out of our area this afternoon.

As the low center moves into central Arizona this afternoon and evening, we are likely to have additional wrap-around showers and thunderstorms but mostly focused over the higher terrain between Flagstaff and Phoenix. We should have a few of these storms move into northern La Paz, northern Maricopa, and eventually into Gila County this evening but CAMs mostly show isolated to at most scattered coverage. It seems likely the CAMs are underplaying this potential, so it would not be surprising to have another round of rainfall this evening across south-central Arizona. Lingering chances for some showers and maybe some embedded storms will remain possible over the eastern Arizona high terrain into the overnight hours tonight before the low eventually ejects into New Mexico by sunrise Wednesday morning.

LONG TERM /WEDNESDAY THROUGH MONDAY/

Wednesday will be a transition day as the Pacific low gradually exits the region. Some morning scattered showers are likely to still be across the far eastern Arizona high terrain, but any activity should quickly wind down in the afternoon. Elsewhere on Wednesday, sunny skies and warmer temperatures are forecast for southeast California and southwest Arizona while the south- central Arizona lower deserts will see some lingering clouds and well below normal temperatures.

Model guidance shows upper level ridging fully moving into our region on Thursday and likely staying put through the upcoming weekend. This will lead to gradual drying and mostly clear skies while temperatures slowly rise closer to the normal range for the south-central Arizona lower deserts. The western lower deserts may reach 100 degrees again starting Friday, but for the eastern half of Arizona some reinforcing cooler air from the east Friday through Saturday should keep daytime highs at or just above 90 degrees through Saturday. This cooler easterly flow may even continue into early next week with temperatures in the Phoenix area staying right near seasonal normals for early October.

AVIATION

Updated at 1130Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main weather concerns through this morning will include periods of SHRA/TSRA, lower CIGs, and erratic wind directions. Latest area radars indicate a batch of moderate to heavy SHRA/TSRA progressing ewd into the Phoenix Metro. This band will likely last through 15Z-16Z before clearing east of all TAF sites. Expect MVFR/IFR vsby with cigs falling to around 4-5 kft AGL in the heaviest showers this morning with gusty N-NW winds developing for a brief period before switching back out of the E/NE as SHRA moves east of terminals during the mid-morning. Guidance suggests directions quickly shifting to westerly by early this afternoon. We will likely hold onto 5 kft BKN cigs through this evening. After sunset, lower cigs will dissipate while winds weaken, then obtain the traditional nocturnal east component by 03Z-04Z.

Southeast California/Southwest Arizona including KIPL and KBLH: The main aviation weather concerns through the forecast period will be gusty NW winds and potentially another batch of SHRA/VCSH at KBLH this afternoon. Rain has cleared east of the region, however a few showers will be possible later this afternoon on the back side of a departing weather system. These showers will likely be brief and may impact KBLH. Winds at both terminals will veer out of the W-NW and become gusty throughout the day. Periodic gusts around 25 kts will be possible at both terminals before winds subside after sunset.

FIRE WEATHER

Widespread showers and isolated thunderstorms will affect much of the area early today before mainly affecting the Arizona high terrain this afternoon and evening. Moderate to heavy rainfall is likely to impact the Arizona high terrain through at least early afternoon. Humidities will begin to trend lower across the western districts later today, but more so on Wednesday. Elevated humidities will persist across the eastern districts through at least Thursday. Winds will be breezy to locally windy at times today before becoming light again on Wednesday. Drier conditions and near to slightly above normal temperatures are forecast for later this week and through the weekend.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch through this evening for AZZ534-537>563.

CA...None.