textproduct: NWS Phoenix

KEY MESSAGES

- Tranquil weather conditions will continue through Thursday with temperatures within a few degrees of daily normals.

- An unsettled weather pattern will return to the region late in the week resulting in much cooler temperatures and enhanced chances for widespread rainfall through early next week.

- Embedded thunderstorms and heavier rainfall may affect some areas resulting in localized minor flooding, primarily Friday and Sunday.

SHORT TERM /Today through Thursday/

A distinctive split flow pattern remains across the country with progressive, low amplitude features poised to gradually buckle towards a more blocked pattern over the next week. Broad northern stream troughing extends from the Pacific NW into the plains early this afternoon with a pronounced southern stream jet approaching the southern California coast incorporating a deep negative height anomaly near 33N 140W. For the remainder of the week, the slow amplification phase will allow the northern stream troughing over the Pacific NW to partially absorb the enhanced energy in the southern stream forcing highly anomalous u-component jet energy and a notable atmospheric river towards the Southwest. This scenario is very reflective of a mature El Nino phase and its atmospheric influence towards lower latitude shortwave intensification and moisture transport.

Over the next 48 hours, quasi-zonal midlevel flow across the forecast area will steadily become more cyclonic with the approach of the aforementioned southern stream troughing. Other than a marked increase in mid/upper level moisture profiles and thickening cloud cover, pressure falls along the coast will allow backing low level winds aiding in the advection of a plume of a higher theta-e airmass from northern Mexico into south-central Arizona, and setting the stage for increased rainfall chances into the medium range (beyond Thursday). Otherwise, H5 heights in a 575-579dm range will hold nearly steady, and narrow multi-model ensemble output yields excellent confidence of temperatures within a few degrees of daily normals.

LONG TERM /Friday through Tuesday/

Friday is still looking like a fairly wet day across the bulk of central Arizona as moisture levels rise to near 200-250% of normal under strong low level moisture advection, while simultaneously situated under strong difluence aloft in the left front quadrant of the incoming near record strength (for this time of year) subtropical jet. Guidance continues to heavily favor showers and isolated thunderstorms developing Friday morning centered over south- central Arizona, then expanding north and east throughout the day. Rainfall rates may not be particularly heavy with the bulk falling at less than 0.25"/hour, however embedded heavier showers and/or thunderstorms with rates 0.50-1.00"/hour may be possible.

GEFS and EPS ensemble mean QPF for the Friday event shows an average of around 0.25"-0.75" from Phoenix and into the higher terrain north and northeast of Phoenix to 0.10-0.25" over much of the rest of southern and central Arizona. The higher end of guidance (NBM 75% percentile) still shows potential for heavy rainfall with amounts of up 1.00"-1.50" across south-central Arizona, albeit more tied to convective elements, and likely not widespread. As a result, WPC's ERO continues to depict a Slight Risk for excessive rainfall on Friday. Given the propensity for these higher end NBM QPF probabilities to subjectively be overzealous in mid range forecasts over lower elevations, the ensemble means would be more favored over larger area. Nevertheless, some higher end amounts of over 1" should be expected in more convectively dominated areas, but these type of totals are likely to be fairly localized.

As one portion of the subtropical jet and embedded shortwave lifts northeast late Friday, the best ascent will shift out of Arizona leaving lesser rainfall chances, and tied more towards isolated upslope driven shower activity Friday night and Saturday. Models continue to highlight a distinct break in rainfall Saturday before the unsettled pattern returns Saturday night and Sunday with an atmospheric river approaching the region. The majority of modeling suggests the brunt of this expected modest atmospheric river impacting areas across southern Arizona and northern Sonora. While the overall trend in ensembles continues to depict mean troughing advancing through the SW Conus, uncertainty remains quite high regarding some very important details regarding the track, strength, phasing, and orientation of the southern stream shortwave trough yielding substantial differences in the amount of moisture that may be thrust inland by the atmospheric river.

Large differences in output continue to be exhibited among each iteration of operational and ensemble members with the key being the ability for deep southwest flow at lower latitudes to become juxtaposed with the atmospheric river in order for moisture to breach the terrain gap in the Baja peninsula. Given the continued wide range in solutions, it may be a couple more days to reach a better consensus. Overall, both GEFS and EPS members continue to show wide ranges from nearly no accumulations across lower elevations to well over 2 inches. Realistically, the GEFS mean 0.25- 0.50" for south-central Arizona looks to be a bit too low considering the potential amount of moisture, but the EPS mean 0.75- 1.50" may be a bit too high and a symptom of grid spacing bleeding greater high terrain amounts into lower elevations. Regardless, the bulk of the rain should fall throughout the day Sunday (which could be a rare "wash-out" day for some locations), but as stated above, how much total rain will fall is still quite uncertain. Considering this will be the second rain event within 3 days, any additional moderate to heavy rain could compound any flooding issues that occur Friday. Rain chances may linger through Sunday night across eastern Arizona, but by Monday, much of the area should be drying out as the system is quickly pushes east of the area.

Temperatures during this unsettled weather pattern will fall below normal starting Friday with highs dropping to between 75F-80F before bottoming out closer to 70F highs for Sunday and/or Monday. It would not be out of the question for some lower desert communities stuck in prolonged rain Sunday to not even breach the 70F threshold. NBM guidance shows fairly large forecast temperature spread for the first half of next week as the large scale troughing may linger near the region with depressed H5 heights for a couple more days. If this happens, highs will very slowly creep upward through the low to mid 70s, whereas if the ridge quickly takes over, highs closer to 80F would be more common by next Tuesday or Wednesday.

AVIATION

Updated at 1720Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns are anticipated through the forecast period under clear skies. Winds will continue to follow diurnal patterns, however an extended period of southerly winds can be expected this afternoon before the full westerly shift occurs. Winds will return out of the E-SE late tonight. Speeds should remain aob 6 kts at all terminals.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns are expected at the SE California terminals through the forecast period. Winds will fluctuate between SE and W at KIPL and remain predominantly S to SW at KBLH. Speeds at both sites will remain around 7 kts or less. Skies will be clear through this evening before SCT high cirrus begins to move into the region later tonight.

FIRE WEATHER

Warm and dry conditions will persist through at least the middle part of the week before a much cooler and wetter pattern sets up Friday into this weekend. Minimum humidity levels will bottom out today at 15-30% Tuesday before gradual moisture improvement arrives starting Wednesday. Overnight recoveries will remain good at 40-65%. Winds will remain light each day favoring typical diurnal patterns and minimal upslope breeziness.

PSR WATCHES/WARNINGS/ADVISORIES

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