textproduct: NWS Phoenix

KEY MESSAGES

- Another large Pacific weather disturbance will bring the next opportunity for rain Tuesday and Wednesday with additional chances likely arriving later in the week.

- Temperatures this week will hover below the seasonal normal with readings around 10 degrees below normal during the middle of the week.

SHORT TERM /Today through Wednesday/

The North America flow pattern will favor increased amplification and partial blocking over the next 72 hours with mean troughing favored over the western Conus. Early afternoon WV imagery shows this pattern already coming to fruition given deep negative height anomalies along the northern California coast and the development of an intense meridional jet on the western periphery of this wave. This jet core combined with vorticity rotating into the deepening trough base well offshore should ensure the upstream coastal California wave continues to dig southeast while detaching from the northern stream jet flow. This partially cutoff circulation should wobble over southern California through Wednesday with several vorticity centers within the larger gyre producing forced ascent while tapping an anomalous southerly stream of higher PWATs.

All modeling suites continue to exhibit excellent consistency with respect to the initial evolution of features this evening and overnight resulting in elevated rainfall chances through south- central Arizona. While still well removed from the vorticity centers and cold core of the upstream system, the formation of a strong midlevel jet is already underway across southern Arizona downstream of incoming height falls. Satellite and radar already depict the classic signs of elevated warm advection near the international border, and confidence is excellent that this moist isentropic upglide around the 300K layer will expand and lift through the central CWA this evening and overnight. While MUCape is limited to only around 100 J/kg with marginal lapse rates, a few embedded elevated storms are possible with the upglide and strengthening ascent. However, most locations should only experience scattered showers

As dynamics increase Tuesday with a notable vorticity lobe pivoting towards the lower Colorado River valley, a steep cold front will surge towards through SE California towards the Colorado River with pronounced southerly flow being maintained in the warm sector over south-central AZ. While some measure of drying should surge into SE California, vorticity forced ascent combined with frontal convergence within a moist environment over Arizona appear vary favorable for waves of showers and embedded storms throughout much of the day. Given the approaching cold core and dynamic lift, lapse rates should steepen while boundary layer mixing ratios near 7 g/kg are advected poleward. Pockets of MLCape near 500 J/kg seem a reasonable forecast near peak heating resulting in isolated instances of heavy rainfall and small hail given freezing levels falling below 7K ft. All told, HREF mean rainfall amounts through Tuesday night of 0.10-0.50" across lower elevations seems appropriate with non-isolated higher amounts around 1.00" likely relegated to the foothill upslope communities north of Phoenix.

The slow forward progression to this cutoff low adds additional uncertainty to the forecast Wednesday as the low level cold front and moisture plume shifts towards eastern Arizona, yet the upper cold core and energy only advance into SE California Wednesday afternoon. Additional showers are likely (better than a 60% chance) through much of the eastern CWA though forecast soundings indicate little remaining instability with nearly moist, adiabatic atmospheric profiles. As a result, the most likely outcome is extensive thick clouds throughout the day with several showers lifting along the cold front and tied to sheared vorticity within the cyclonic jet corridor. Other than the rainfall chances, unusually cool temperatures will be notable with locations around the Phoenix metro struggling to even reach 65F (daily normals are around 75F and the last time KPHX failed to reach 65F was March 14th). Showers should gradually abate Wednesday evening as the cutoff finally starts ejecting northeast in response to another strong Pacific wave diving along the California coast.

LONG TERM /Thursday through Sunday/

The forecast for later this week and through the weekend is much less certain as guidance is unsure of the track of a third weather system. Thursday is likely to be a mostly dry day with only some lingering light shower chances across eastern Arizona as the second system will be exiting to the east. However, by Friday and Saturday rain chances may again spread over the region as yet another fairly potent Pacific trough tracks southward along the coast of California. Over the past couple of days, guidance has been trending toward keeping the low more to our west, potentially far enough to the west to not bring widespread rain chances. However, it may eventually swing through our region by around Sunday or Monday. Forecast PoPs and rainfall amounts for this third event are still very uncertain and changes are expected. If the low center does manage to stay completely to our west and south (20-30% probability), we may not see much additional precipitation, but it seems more likely it will at some point swing through the Desert Southwest. We should hopefully have a better idea what will happen during the middle part of this week.

AVIATION

Updated at 1800Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concerns will be the presence of VFR CIGs during much of the period along with periods of VCSH and SHRA that could result in reduced VIS. Multiple windows of rainfall are expected with the first most likely occurring between 02-06Z this evening. The next rounds look to be from 12Z onward but VCSHs through much, if not the entire night is conceivable. In terms of sub VFR conditions, the most likely timeframe would be with that second round of rainfall, but confidence is still too low at this time to include any prevailing MVFR CIGS or VIS in the TAFs. The most likely sites to see these sub-optimal conditions would be KSDL and KDVT where TEMPO groups have been introduced to address these higher probabilities. Other than perhaps a brief switch to the SSW/SW this evening associated with the first round of showers, winds should favor and E/SE component throughout the forecast window.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. S/SE winds at each terminal with eventually switch to the S/SW later in the period with speeds remaining generally light (aob 10 kts). VCSH activity cannot be ruled out during the late evening into overnight timeframe, but confidence is too low to include any mention of rainfall impacting the terminals at this time. BKN-OVC skies will be common through Tuesday morning with the lowest bases around 7-8k ft.

FIRE WEATHER

After a brief respite to the active weather pattern during the first half of today, rain chances will increase later today with wetting rainfall chances returning by Tuesday morning. Periods of showers and a few isolated thunderstorms will affect much of the eastern districts on Tuesday with much more limited activity across the western districts. Below normal temperatures and elevated moisture will remain in place keeping MinRHs between 40-70%, while overnight recoveries will be good to excellent at 60-100%. Locally gusty winds upwards of 20 mph will be possible at times, though weaker wind speeds will be more common through the period. The active weather with rain chances persisting across the eastern districts should continue through Wednesday before a brief break is seen on Thursday. Another weather system may eventually affect the area by the weekend as temperatures mostly remain below normal and humidities stay elevated.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.