textproduct: NWS Phoenix

KEY MESSAGES

- Active weather will persist through Monday with periods of showers and thunderstorms, some potentially producing strong winds and heavy rainfall across south-central Arizona.

- A Flood Watch is in effect for south-central Arizona for Sunday evening into Sunday night as heavy rainfall could result in localized amounts in excess of 1-2 inches and flash flooding.

- Near to slightly below normal temperatures are expected for the first part of this week before returning to slightly above normal during the latter part of the week as drier conditions settle over the region.

SHORT TERM /TODAY THROUGH MONDAY/

The atmosphere will remain well primed for additional rounds of showers and thunderstorms today and Monday with heavy rainfall a continued concern across south-central and eastern Arizona. Current objective analysis shows the subtropical ridge now positioned across the Southern Plains, but still extending into southeast Arizona. A Pacific trough centered over the Pacific Northwest is slowly making headway through the Western U.S. with the southern fringes of the trough now moving into northwest Arizona and southern California. Moisture levels did improve a bit on Saturday, but we are still fighting against some drier air aloft getting pulled in from the southwest. Very strong heating on Saturday definitely helped to raise instability levels and much of that remains, except for far eastern Arizona where widespread rainfall occurred.

South southwesterly flow today into Monday should improve lower level moisture across most of Arizona, but the drier air aloft will continue to stream into southern California into western Arizona. Low level mixing ratios through Monday are forecast to be between 10-12 g/kg, just enough to worry about a heavy rainfall threat due to the forecast MUCAPE of 1000-1750 J/kg across the area. Yesterday's heavy rainfall only affected higher terrain areas as there just wasn't enough lift to get storms to fire early enough over the lower deserts, but that should change today.

Guidance shows a strengthening upper level shortwave moving into central California later today, while a secondary jet max will be positioned to our north/northeast. The shortwave to our west is not expected to help until at the earliest Sunday night, but the jet max to our north will at least position central and eastern Arizona in a favorable area of ascent. Assuming near optimal heating conditions today, at least over south-central Arizona, forecast MUCAPEs should rise to around 1500 J/kg by late afternoon providing plenty of instability for storms to develop. Forecast soundings also show very little CIN during the latter half of the afternoon which should even allow for some scattered storms to initiate over the lower deserts. CAMs, especially the 06Z HRRR, show widespread showers and storms developing late in the afternoon across Pima, Pinal and Gila Counties before spreading north and northwestward through much of Maricopa County in the evening. A few storms may become strong enough to drop 50+ mph winds with western Pinal and southern Maricopa Counties likely being the best area for the strongest winds and dust to occur.

Given the amount of instability and moisture levels reaching above normal for late August, we are anticipating some areas will receive heavy rainfall this evening into early tonight. Another Flood Watch has been issued for Maricopa, Pinal, and Gila Counties for this evening into tonight with localized 1-2 inch amounts quite possible focused across western Pinal through central Maricopa County this evening and then over northern Maricopa County during the early overnight hours. We also can't rule out the possibility of storms lasting longer Sunday night as the shortwave trough will be getting closer to western Arizona, but eventually we should exhaust most of the available instability.

Monday's forecast remains more uncertain as it seems likely today/tonight will be very busy resulting in a somewhat work-over atmosphere going into Monday morning. However, guidance still shows ample moisture on Monday with even better moisture in the mid levels compared to today. Cloud cover will likely be an issue for at least eastern Arizona limiting heating and instability, but there should be decent heating and recovery over south-central Arizona. The track of the shortwave trough takes it more into southern Nevada into Utah during the first half of Monday putting northern Arizona more in the prime area better shear and stronger storms. Despite the shortwave brushing past to the northwest, we should again fall within a fairly favorable area of ascent over central Arizona due to the position of the jet max over western Utah/northwest Arizona. Monday's storm potential is expected to be more conditional and more scattered compared to today, but it would not be surprising to see a few strong storms and localized heavy rainfall again. In fact, the forecast sounding for Monday in Phoenix looks nearly identical to today.

Temperatures are finally starting to cool off with the exit of the ridge and the increased rain activity. Highs today are expected to top out at around 105 degrees in the Phoenix area to 106-110 degrees across the western deserts. Monday should be the 'coolest' day of the week with the NBM calling for highs in the mid to upper 90s for Phoenix to right near normal for the western lower deserts.

LONG TERM /TUESDAY THROUGH SATURDAY/

A much quieter period of weather is anticipated starting Tuesday as the flow turns more out of the west southwest ushering much drier air through the rest of the area. Forecast PWATs are expected to drop to just below normal on Tuesday, likely staying on the lower side through around Thursday. NBM PoPs drop off to mostly below 10% starting Tuesday and carrying through at least Wednesday. Temperatures will also be a bit slow to warm back up as the trough will still have some influence over our region. Forecast highs return back into the normal range by Wednesday, while overnight lows will be a more comfortable upper 70s to low 80s for the lower deserts.

High model uncertainty becomes an issue late week into next weekend as models are currently favoring a tropical cyclone to move northwestward off the coast of Baja and potentially even getting close to California early during the following week. It is impossible to know at this point where this TC will go, but it at least seems fairly likely it will help to direct moisture northward into our region by next weekend. Assuming this occurs, we may eventually return to busy monsoon weather.

AVIATION

Updated at 0955Z.

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

The main aviation concern this forecast period will be another round of thunderstorms, gusty outflow winds, and blowing dust starting potentially as early as late this afternoon and lasting through much of tonight. Until then, VFR conditions with a typical diurnal wind pattern is anticipated. Winds will start off light out of the E-SE this morning before gradually shifting out of the WSW this afternoon. Intermittent gusts into the teens to around 20 kts can be expected by the late afternoon period.

Heading into this evening, an active period of weather is likely with TS and BLDU becoming the main hazards. Latest model guidance suggests TS will develop to the south of the terminals and gradually build into the Phoenix Metro along a northward progressing outflow. Some models do develop convection earlier today (possibly as early as 22Z-00Z) and thus timing may need to be adjusted in later issuances. For now, the best timing of arrival of the outflow and subsequent TS looks to be around 01Z at KIWA and 02Z at the remaining TAF sites. Strong S-SE winds >30 kts will likely accompany the outflow along with vsbys around 2-4 SM in BLDU. Periods of VCTS/TSRA can also be anticipated and will last for several hours. Another round of SHRA may also develop around the 06-10Z timeframe. SCT mid/high cloud decks will be common through the period, with the lowest CIGs forecasted at 10kft during thunderstorm activity.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns are expected at the SE California terminals. At KIPL, winds will fluctuate between westerly and southeasterly with westerly winds increasing again by this evening. At KBLH, winds will fluctuate between southerly and southwesterly through the TAF period. Speeds will be elevated (10-15 kt) most of the period with some gusts of 20-25kt during the afternoon and early evening. Skies will be mostly clear with FEW-SCT mid and high clouds moving into the region by this evening.

FIRE WEATHER

Elevated moisture and rain chances will persist across the eastern districts through Monday with gusty winds, heavy rainfall, and localized flooding still a threat. The western districts will generally stay dry with at most a few isolated showers and storms. Expect MinRHs of 15-20% across the western districts to 25-40% over the eastern districts before drying out again beginning Tuesday. Temperatures will range from around normal over western districts to slightly below normal over the eastern districts.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch from this evening through late tonight for AZZ534- 537-538-540>563. CA...None.