textproduct: NWS Phoenix

KEY MESSAGES

- Chances for showers and thunderstorms will continue through early this evening, mainly west of Phoenix, with Flood Watches in effect.

- Quiet weather under mostly clear skies is expected for Tuesday and Wednesday before potentially another more active period of weather materializes by the end of the week into the weekend.

- Near to slightly below normal temperatures are expected throughout much of the week.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Latest water vapor satellite imagery shows a well-defined upper- level shortwave over southern Nevada and is the main feature that drove the widespread rainfall activity that was observed across much of south-central AZ throughout the morning hours. Rainfall intensity has been on the light to locally moderate side and thus rainfall accumulations for the most part have remained under 0.3", with some localized areas observing amounts up to 0.5". Most of this activity has moved east of Phoenix and will be ending in Gila County during the early afternoon hours. The overworked atmosphere from the morning activity will essentially end the potential for more organized convective activity for the rest of today for much of south-central AZ. The main focus for convection for this afternoon will shift west of Phoenix, across portions of La Paz into northern Yuma County as well as western Maricopa County as ample heating will cause MLCAPE values to peak at 1500-2000 J/KG, which is more than sufficient for strong thunderstorms to develop. Although strong downburst wind gusts will be possible, the primary hazard will be heavy rainfall leading to flash flooding with localized amounts in excess of 1-2" possible, especially with any training cells. As the upper-level shortwave lifts northeastward into Utah by this evening, much of the upper-level support will be lifting to the north as well and thus activity should diminish by the early evening hours with quieter conditions expected during the late evening and overnight hours tonight.

Quieter weather is expected Tuesday and Wednesday as significantly drier air due to west-southwest mid-level flow will overspread the region, especially in the mid to upper levels with strong subsidence aloft. A few isolated storms will still be possible Tuesday afternoon, mainly across the terrain escarpments given that boundary level moisture will still be sufficient with mixing ratios above 10 g/kg. However, by Wednesday, mixing ratios will have dropped to less than 10 g/kg with little to no instability expected and thus storm chances should be completely eliminated across much of the CWA.

Due to the abundant cloud cover and rainfall this morning, afternoon high temperatures across much of south-central AZ will likely struggle to reach 90 degrees while significantly warmer highs due to more sunshine can be expected across the western deserts as readings top out in the mid to upper 100s. Temperatures Tuesday and Wednesday, with much more sunshine, should rebound into the low 100s across the south-central AZ lower deserts and top out into the mid 100s across the western deserts, with these readings near to a few degrees below normal for early September.

LONG TERM /THURSDAY THROUGH SUNDAY/

Forecast concern is increasing for the potential of tropical moisture surging northward into the region as early as Thursday night into Friday, but more so for the coming weekend. NHC shows a tropical disturbance currently near 10N and 107W with a 90% chance of cyclone formation this week and likely tracking northward off the coast of Baja by Friday into the weekend. There is still a lot of uncertainty with this potential tropical system, but model guidance has been coming to a bit better consensus the last several model runs. Roughly 90% of the GEFS and EPS members now show PWATs increasing to 1.8-2.2" over portions of our area by Friday or Saturday. Given this likely increase in moisture, PoPs have gone up starting Friday and continue through the weekend. It will likely be at least 2 or 3 more days until we have a good idea on what would likely happen late this week. Getting tropical moisture into our region is definitely not unusual, but many times when it happens we struggle to build instability due to extensive cloudiness and poor lapse rates. However, it seems quite likely a Pacific trough will be situated along or just off the West Coast which may provide for enough upper level forced ascent to overcome the likely lackluster instability. We will continue to watch this throughout the week as it very well could lead to another round of heavy rainfall at some point.

AVIATION

Updated at 1740Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Lingering SHRA/VCSH conditions and low confidence on wind directions later this afternoon into the early evening are the only weather issues anticipated through the TAF period under midlevel CIGs. The area of showers focused towards the east of the Phoenix airspace will continue to shift north/east through the next few hours, ending SHRA/VCSH conditions by the early afternoon. Winds will continue to favor easterly directions and likely will trend out of the southeast to south through the afternoon. Strong to severe TS are likely to form in La Paz, Yuma, and perhaps evening Western Maricopa County this afternoon, however, chances for TS in the vicinity of the terminals is too low for mention in the TAF (below 20%) and this activity should remain well west of the terminals. By tomorrow, winds will return to more light and diurnal tendencies under mostly clear skies.

Southeast California/Southwest Arizona including KIPL and KBLH:

The primary concern at the Southeast CA terminals will be the potential for distant TS outflow winds to cause an abrupt E/SE wind shift and higher gusts early this evening, as well as a low chance (~10%) for TS within the vicinity of KBLH mid to late afternoon. Winds will continue to favor SE to S at both terminals, then shifting SW during the evening. Speeds at KBLH will remain mostly above 10 kts with gusts into the low 20s during the daytime, then relaxing AOB 10 kts overnight. Occasional SW/W gusts 20-25 kts are possible at KIPL this evening. Skies will remain clear to mostly clear.

FIRE WEATHER

Lingering light showers will persist across the far eastern districts through early this afternoon with scattered showers and storms across portions of southwest Arizona through early this evening. Drier conditions will move in beginning Tuesday with rain chances mostly ending through midweek. Expect MinRHs of 15-25% across the western districts to 35-50% over the eastern districts today before falling back to 15-25% areawide by Wednesday. Temperatures will range from around normal over western districts to slightly below normal over the eastern districts.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch until 8 PM MST this evening for AZZ531-533-534-538- 539-559.

CA...None.