textproduct: NWS Phoenix

UPDATE

Updated 00Z Aviation Discussion.

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 this evening through the overnight period as heavy rainfall could result in localized amounts in excess of 1-2 inches and flash flooding.

- Quieter weather is expected by the middle of the week before potentially another more active period materializes by the end of the week into next weekend.

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

SHORT TERM /TODAY THROUGH MONDAY/

Current objective analysis shows the subtropical ridge centered well east of the area over the southern Plains while a broad trough encompasses much of the western 1/3 of the CONUS. This pattern configuration is resulting the flow to be out of the south- southwest, aiding with maintaining elevated moisture levels across the region as the latest SPC mesoanalysis shows PWATs of 1.5"+ across much of the AZ lower deserts and between 1.1-1.5" across the higher terrain areas along with 10-12 g/kg of low-level mixing ratios. The elevated moisture combined with 500 mb temperatures cooling to between -7C to -8C will result MLCAPE values later this afternoon into early this evening to peak at around 1500 J/KG. The favorable thermodynamic environment combined with better ascent due to increasing upper-level divergence from a jet streak moving through the Great Basin will result in favorable conditions for the development of widespread storm activity across much of south- central AZ, especially from early this evening into the overnight period.

Latest hi-res guidance, similar to yesterday, shows storms developing across the AZ High Country as well as southeast AZ by the early to mid afternoon hours. The activity across southeast AZ will eventually send an outflow northward that will likely spark additional storms into Pinal and Maricopa Counties with activity affecting the Phoenix area heading this evening. Latest forecast soundings for late this afternoon into early evening show a nearly uncapped atmosphere once again and thus it will take just a single outflow to result in convective initiation across the lower deserts. Initially with the storms, strong downburst wind gusts and areas of blowing dust in the dust prone areas will be the primary hazards as DCAPE values will be high once again, peaking at 1500 J/KG before heavy rainfall and flash flooding eventually become the more primary hazard. Given the potential for more heavy rainfall that could produce localized amounts of 1-2" and thus lead to flash flooding, a Flood Watch is in effect for much of south-central AZ starting early this evening and continuing through the overnight period. There is a potential that storms may last into the overnight period as some of the hi-res guidance is showing given that an upper-level shortwave trough will be approaching from the northwest that may aid in sustaining the convective activity for a longer period of time with heavy rainfall/flash flooding being the primary hazard.

The forecast for Monday remains a bit uncertain as everything will be more conditional as to what happens later today into the overnight period and how fast the atmosphere can recover. If the atmosphere is able to recover fast enough where decent instability develops, then another round of enhanced shower and thunderstorm activity will be likely once again as the upper-level shortwave will be positioned to the north-northwest over southern Nevada and Utah, providing good upper-level ascent across much of central AZ. Latest forecast soundings show very favorable thermodynamic profiles once again with MLCAPE values exceeding 1000 J/KG along with a uncapped atmosphere, supporting the potential for some robust thunderstorms. However, this is assuming that the atmosphere is able to fully recover from the activity that is likely to materialize later today through the overnight period.

With the subtropical ridge now positioned well east of the area, temperatures this afternoon will be noticeably cooler as readings top out in the low to mid 100s across the south-central AZ lower deserts and in the mid to upper 100s across the western deserts. Even cooler temperatures are forecast for Monday, especially across south-central AZ, where more abundant cloud cover and potential rainfall may limit afternoon highs to just the middle to upper 90s while warmer readings in the mid 100s are expected for the western 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 2315Z.

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

Much uncertainty with the current TAF forecasts as another round of thunderstorms, gusty outflow winds, and blowing dust expected for later this evening. Delayed thunderstorm potential at TAF sites by a few hours this forecast update. Thunderstorms over SE AZ expected to become widespread early this evening and push a strong S/SE outflow north into the valley TAF sites. Best timing would suggest approximately 04z-09z with the initial outflow gusty winds (>30 kts) as well as scattered thunderstorms developing along this outflow and lowered visibilities in blowing dust. Even after this initial wave of storms, there could be lingering convection overnight which could impact overall wind trends as well, straying from the typical diurnal trends. Please stay tuned for future updates.

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. 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 this evneing. Skies will be mostly clear with FEW mid and high clouds moving into the region 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 through late tonight for AZZ534-537-538-540>563.

CA...None.