textproduct: NWS Phoenix

KEY MESSAGES

- Bands of light rain will be present over the region through the remainder of today into Monday morning before more scattered showers and thunderstorms become possible starting Monday afternoon.

- A Flood Watch has been issued for Monday night and much of Tuesday for higher terrain areas of central and eastern Arizona. Moderate to heavy rainfall may also impact portions of the Arizona lower deserts, but it is expected to be more localized.

- Near to below normal temperatures are expected through the middle part of the week before warming to slightly above normal by next weekend.

SHORT TERM /Today through Sunday/

The Desert Southwest finds itself in the middle of a number of weather regimes with sub-tropical high pressure to our southeast, an approaching Pacific low to the northwest, and Tropical Cyclones Odalys and Polo still lingering over the Eastern Pacific. Due to the position and interactions of the former two systems, enhanced southwesterly flow aloft is currently pulling in sheared moisture away from the center of what remains of Odalys toward our forecast area resulting in considerable cloud cover. Though the aforementioned cold-core is still several hundred miles away, its associated ascent appears to have a far enough reach to lift the tropical moisture and set off some light rain showers over the southwestern CONUS. Bands of rain are likely to be observed through tonight as the current setup will not change much over the next through tonight allowing for more and more moisture to get pumped in. The lack of large scale ascent will limit coverage so not everyone will see showers, and even those who do, it should not amount to much as a decently dry-sub cloud layer is in place. A few hundredths to maybe around a tenth of an inch at most is anticipated.

As we transition into Monday, the Pacific trough will begin to dive to the southeast while Hurricane Polo inches closer to the Baja Peninsula, increasing regional ascent and enhancing moisture profiles even further across the region. This combination will help to produce some heavier bands of rain compared to this afternoon, and if enough surface heating can take place through the day, isolated to scattered thunderstorms will also be possible. The best chances for convective activity will be focused around the Lower Colorado River Valley as model guidance points towards enough clearing taking place to where marginal atmospheric destabilization can take place. The current HREF run indicates SBCAPE values reaching 100-500 J/kg across our western most counties, not all that impressive, but enough to potentially spark some strong to even severe storms given the dynamics in place. Any storms that can form will not be the typical pop-up monsoon storms. With 0-6 km shear, it isn't out of the question that could become sustained supercells capable of producing strong/damaging winds even some small hail.

Elsewhere across South-Central Arizona, continued abundant cloud cover will limit destabilization, but a few rumbles of thunder cannot be completely ruled out due to the more vigorous ascent as low a draws closer along with orographic enhancement. Rain totals will vary greatly from locations to location. Most areas can expect anywhere from a few hundredths up to half an inch, with locally higher amounts in the high terrain north and east of Phoenix along with areas of the western deserts. Isolated instances of flash flooding cannot be completely ruled out for those western areas if strong storms were to be realized.

Tuesday is shaping up to be the most interesting day of the next three, and how things evolve will depend mainly on the locations of the cold-core. Models are still not in full agreement regarding where the center of circulation will be come Tuesday morning, but its eventual location will determine where more rounds of showers and thunderstorms develop. Regardless, the interaction between the lingering tropical moisture and the anomalously strong low will at the very least maintain ongoing activity and/or produce more during the pre-dawn hours. Most, if not all, the rainfall by this point will be focused from the Colorado River on eastward.

Even without daytime heating, hi-res models indicated an increase in instability ahead of sunrise, likely a result of low-level warm air and moisture advection. The WAA alone will increase support for further showers, but the addition of instability will enhance convective potential as well. CAPE values once again will not be overly impressive, hovering around 500 J/kg, but the dynamic environment will be the main reason why some of the convection could be strong to severe as 0-6 km shear over Southwest and South-Central Arizona will be upwards of 60 kt. As mentioned above, the position of the low will be important. If some degree of daytime heating is allowed to take place ahead of the areas of greatest ascent, then thunderstorm coverage may become more widespread than the mid-morning timeframe as further destabilization could occur. This increase in coverage is most likely for areas from Phoenix and eastward, but a slower progression of the low may allow for more storms slightly further west. Abundant moisture and the potential for sustained convection will also increase flooding risk Tuesday morning into the evening. Add orographic enhancement to the mix and the foothill and high terrain areas of Maricopa, Pinal, and Gila Counties will see the the greatest possibility for heavy rainfall. Therefore, a Flash Flood Watch remains in effect from late Monday night through Tuesday evening

LONG TERM /WEDNESDAY THROUGH SATURDAY/

Upper level ridging is forecast to quickly surge in from the west on Wednesday leaving clearing skies and lowering surface moisture through Thursday. Temperatures for the coming week will stay somewhat stable across the western lower deserts, but still on average slightly below normal today through Tuesday. Much cooler temperatures are expected for most of Arizona with highs only in the 80s for the Phoenix area Monday-Wednesday. Once the ridging moves in from the west later this week, temperatures will creep upward again with highs topping 100 degrees across the western deserts by Thursday or Friday and potentially for the Phoenix area by next weekend.

AVIATION

Updated at 1830Z.

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

The main weather concern will be the increasing chance for SHRA and at some points erratic winds. Confidence is low in the exact timing for direct impacts to the terminals, however the general outlook is VCSH later tonight before a thicker band of -SHRA begins to filter in over the metro by tomorrow morning. Latest guidance shows activity lingering through most of the back half of the TAF package, however further analysis will be needed. The remainder of this afternoon can expect diurnal trends, before a NW/NNW direction is expected to develop for the overnight periods, where gusts into a 15- 25kt range cannot be ruled out tonight as a result of nearby shower activity. BKN-OVC cloud decks with bases aoa 10kft are expected to be common through the TAF period, while FEW-SCT lower decks are likely to develop tomorrow morning during showers.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main aviation concern will be the possibility of VCSH/SHRA mostly at KIPL, however KBLH may also be affected. Confidence is low in the exact timing of impacts, but the best timing for VCSH/SHRA to start is near 12Z tomorrow morning. KIPL can expect typical diurnal tendencies fluctuating between SW and SE. At KBLH, any rain in the morning and afternoon is expected to remain to the south and east of the terminal. BKN/OVC high clouds will continue with FEW-SCT mid level clouds.

FIRE WEATHER

Moisture will increase across the area today and Monday with light rain developing this evening followed by locally moderate to heavy rainfall with thunderstorm chances later Monday into Tuesday. The bulk of the expected rainfall will favor the eastern districts. Min RHs today will dip to 20-30% this afternoon before improving to 40-60% over much of the area for Monday and Tuesday. Winds through Monday will generally follow the typical diurnal tendencies, but with a tendency to favor the south. A front moving through the area on Tuesday will turn winds out of the west northwest with gusty winds of 20-30 mph possible across the western districts. Drier conditions and near to slightly above normal temperatures are forecast for later this week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch from late Monday night through Tuesday evening for AZZ534-537-541-542-545-547-549-552-555>558-560>563.

CA...None.