textproduct: NWS Phoenix

KEY MESSAGES

- A Pacific low pressure system will bring multiple rounds of showers and thunderstorms, mainly across central and eastern Arizona, Wednesday and Thursday.

- A Flood Watch is in effect across much of south-central and eastern Arizona from Wednesday afternoon through Thursday evening for periods of moderate to heavy rainfall leading to flash flooding.

- Quieter weather is expected beginning Friday and lasting through the upcoming weekend.

SHORT TERM /Today through Thursday/

A very active period is shaping up during the middle portion of the week as a strong Pacific low pressure system affects the Desert Southwest, bringing multiple rounds of showers and thunderstorms. Latest water vapor satellite imagery as well as objective analysis show the aforementioned low pressure system centered off the northern CA coast this morning. The low pressure system is expected to continue to dive southward and set up off the southern CA coast by this evening. In the meantime, an upper- level shortwave that was responsible in sparking scattered convection yesterday across the southeastern half of AZ continues to lift away from the region with weak subsidence overhead. Therefore, conditions for this afternoon should remain relatively quiet with dry conditions prevailing across the region; the only exception being near the vicinity of the White Mountains where the combination of sufficient mid-level moisture and orographic lift will likely spark a few afternoon showers and thunderstorms. With upper-level heights rising ahead of the digging Pacific low, afternoon high temperatures today will be a few degrees above normal as many of the lower desert communities top out around 90 degrees.

As the Pacific low positions itself off the southern CA coast later this evening, the flow aloft will become established more out of the south. This will result robust moisture advection with PWAT values by early Wednesday morning expected to approach 250-300% of normal and will be near record levels for mid-October. There will also be a shortwave out ahead of the main Pacific trough that will traverse the region early Wednesday morning, which may spark areas of showers, especially across the higher terrain areas. Otherwise, the main period of impactful weather is likely to occur from Wednesday afternoon through Thursday. Latest guidance continues to show moderate instability building across much of south-central and eastern AZ Wednesday with MLCAPE values approaching 1000 J/KG. This amount of instability combined with abundant moisture will yield a favorable environment for thunderstorms to produce heavy rainfall rates in excess of 1-2"/hr with training of activity likely to occur and thus heightening the potential for flash flooding. Latest hi- res guidance shows activity developing Wednesday afternoon across higher terrain areas east of Phoenix before activity develops into the south-central AZ lower deserts heading into Wednesday evening and continuing into the overnight and morning hours of Thursday as the main upper-level jet streak energy approaches, resulting in strong divergence aloft. The period between Wednesday evening through Thursday afternoon will likely be the timeframe with the greatest threat for widespread heavy rainfall and thus flash flooding. Latest WPC QPF depicts total rainfall amounts of 1.50-2.50" across the foothills and higher terrain areas north and east of Phoenix while across the lower deserts, average rainfall amounts will remain under an inch. However, even across the lower deserts, especially with any training of activity, there will be the potential for areas to receive locally higher rainfall amounts in excess of 1". In coordination with the neighboring offices, a Flood Watch has been issued across much of south-central and eastern AZ.

In addition to the heavy rainfall and flash flood threat, another threat to watch out for will be severe weather. Given the moderate levels of instability in place combined with strong 0-6 km vertical shear in excess of 50 kts, the thunderstorms that do develop will likely have some organization to them with the potential to acquire supercellular characteristics. In fact, the Storm Prediction Center has highlighted much of the higher terrain areas east of Phoenix in a slight risk (Level 2 out 5) for severe weather for Wednesday, with a marginal risk (Level 1 out 5) across the rest of south-central AZ, including the Phoenix metro area. The thunderstorms that do develop will be capable of producing damaging wind gusts, large hail, and even an isolated tornado given that there will be about 10-20 kts of low-level shear. The threat of severe weather will also exist into Thursday as well, especially Thursday morning, as the instability and shear parameters will still be sufficient.

As the upper trough gradually pushes inland through the southern Great Basin during the day on Thursday, strong dry air advection will gradually spread from west to east with rain chances ending late Thursday afternoon across the south-central AZ lower deserts and likely take until late Thursday night into early Friday morning for rain chances to end across the higher terrain areas.

With the main trough axis still remaining far enough west on Wednesday, afternoon high temperatures will still be near normal as readings top out in the mid to upper 80s. As the main trough axis pushes inland through the southern Great Basin on Thursday, temperatures will be noticeably cooler with afternoon highs topping out in the low 80s across the south-central AZ lower deserts to low to mid 80s across the western deserts. Depending on the amount of cloud cover and rainfall activity, there is the potential that temperatures in south-central AZ turn out to be cooler than what is currently forecasted.

LONG TERM /Friday through Monday/

Much quieter weather is expected starting Friday with much drier air in place behind the departing trough. Another follow-on shortwave is likely to dive in from the northwest late Friday into Saturday with guidance indicating that this shortwave will become a cutoff low over AZ as it detaches from the main jet stream energy. At this time, this feature is likely to be inconsequential given that there will be an abundance of dry air in place limiting any additional rainfall potential. Under the general troughing influence, afternoon high temperatures through the upcoming weekend will remain below normal. Coolest day is expected on Friday when much of the lower deserts struggle to reach 80 degrees for afternoon highs.

AVIATION

Updated at 1825Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary aviation weather concern will be the potential for showers and thunderstorms Wednesday. Guidance continue to suggest the development of showers and a few isolated thunderstorms between 06Z-12Z overnight tonight. Uncertainty in this activity impacting the terminals remains, with PROB30s in place. The primary impacts with this would be briefly lowered visibilities and erratic wind directions. Expect SCT-BKN cloud bases to lower to around 6-8 kft and may stick around throughout the morning and afternoon Wednesday. Thunderstorm chances increase at the end of the period with some potentially becoming strong to severe.

Outside of any thunderstorm impacts, winds will favor a southerly crosswind component this afternoon, becoming light and variable this evening through the overnight hours. Speeds will mostly remain aob 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns are anticipated through the next 24 hours. Winds at KIPL will favor the S-SE, while directions at KBLH favor the S-SW. Speeds will mostly remain aob 10 kts with light and variable winds occurring throughout the period. Cloud bases, becoming SCT-BKN at times, will remain aoa 7 kft.

FIRE WEATHER

Wind speeds today will generally remain light with limited afternoon gustiness before increasing towards the middle of the week ahead of an approaching weather system. Mainly dry conditions will prevail this afternoon before rain chances increase substantially Wednesday and Thursday with significant rainfall amounts, in excess of 1.00", likely across the higher terrain areas of the eastern districts. MinRHs today will range between 30-50% before increasing above 50-60% across the eastern districts Wednesday and Thursday along with good to excellent overnight recoveries. Temperatures will be in a near normal range tomorrow before falling into a below normal category by the end of the week and lasting through the weekend.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch from Wednesday morning through Thursday evening for AZZ545-547-552-556>558-560>563.

Flood Watch from Wednesday evening through Thursday afternoon for AZZ534-537-538-540>544-546-548>551-553>555.

CA...None.