textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- Dry and tranquil weather conditions will continue through the middle of the week with temperatures remaining near the daily normals.

- An unsettled weather pattern will return late in the week and persist through the weekend with the best rain chances on Friday and Sunday.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Quiet weather conditions have settled into the region under quasi- zonal westerly flow aloft. A dry air mass now in place will keep skies clear to mostly clear through Tuesday, while temperatures rise a bit under the modestly higher heights aloft. Daytime highs are expected to reach into the mid 80s across the lower deserts starting today and last through the middle part of the week despite increasing clouds starting Wednesday. Overnight lows will also fall within the normal range through Wednesday with lower desert readings mostly between 55-60 degrees.

LONG TERM /THURSDAY THROUGH SUNDAY/

The weather pattern is expected to shift late this week with a deep trough getting carved out over the Pacific Northwest and southwest to off the coast of California by Thursday. The polar jet associated with the northern stream system is expected to dip farther southward off the West Coast, while a stronger subtropical jet across the central Pacific shifts eastward toward Baja. Although these jet branches are not expected to phase together going into Friday, there is likely to be some combined energy reaching the Western U.S.

As the trough sets up to our west on Wednesday into Thursday, high level moisture will stream into our region, followed by increasing southerly moist flow in the lower levels Thursday night into Friday. Guidance is in very good agreement southerly low level moist advection will combine with some top-down saturation in the upper levels on Friday across southern and central Arizona. PWATs are shown to quickly rise to just above 200% of normal by Friday morning, while low level mixing ratios rise to a respectable 8-9 g/kg. This decent moisture is also expected to coincide with modest low level upslope forcing and quite strong mid and upper level ascent within the left exit region of the subtropical jet. Although we are still four days out, confidence is quite good in shower activity blossoming at some point Thursday night/Friday morning across south-central Arizona and lasting through much of Friday. Guidance also shows modest MUCAPE of 250-500 J/kg on Friday which should allow for some isolated to scattered weak thunderstorms. Shear values look to be on the low side, so at this point we are not anticipating any strong storms. Forecast QPF from Thursday night through Friday night currently averages around 0.50" across the Phoenix area to upwards of 0.75-1.00" in the higher terrain north of Phoenix. The higher end of guidance is at least double that, so it's not out of the question to have to worry about some minor flooding issues Friday. WPC has also included this area in a Slight Risk on their Day 5 ERO.

The Friday weather event will not be the last period of rain expected over the next week as another likely even stronger system is forecast to track east northeastward toward southern California and northern Baja on Saturday before moving through our region on Sunday. There is much more uncertainty with this second weather system compared to the first as there are notable timing and evolution differences. There are also some differences with the potential phasing of the polar and subtropical jet streams which if it occurs would likely result in a decent low pressure system developing within the base of the large scale trough. Models at least agree that there will be as much if not better moisture then the Friday system and stronger mid-level and frontal ascent. Given the uncertainty, QPF amounts are still quite volatile to any big changes in the strength of the system and track. However, so far ensemble mean QPF amounts indicate overall higher average amounts compared to Friday's event, as well as more widespread coverage. Rain chances could also extend into Monday, especially across eastern portions of Arizona.

Temperatures will take a hit later this week and through the weekend with NBM forecast highs dropping into a 75-80 degree range Friday and Saturday and even lower starting Sunday. The coolest air so far this fall should move in with the Sunday weather system as highs may fall to as the upper 60s to lower 70s for Sunday and Monday.

AVIATION

Updated at 1715Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No weather issues will exist through Tuesday afternoon under a few passing high cirrus decks. Confidence is excellent that a light east, or nearly calm wind will transition to W/SW by mid afternoon, then revert back to east overnight. Extended periods of a light and variable component will be common. A period of southerly cross runway winds is likely Tuesday afternoon, albeit with speeds under 8kt.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather issues will exist through Tuesday afternoon under clear skies. Winds will be light and variable for the majority of the time frame with a trend for a light nocturnal west component at KIPL and north trajectory at KBLH.

FIRE WEATHER

Weak high pressure will remain over the region early this week allowing for gradual drying and temperatures within the normal range. Minimum humidity levels will continue to fall, reaching 20- 35% today and 15-30% Tuesday. Overnight recoveries will remain good at 40-65%. Winds will remain light each day favoring typical diurnal patterns. A shift in the weather pattern late this week and into next weekend should bring about cooler, wetter, and breezier conditions.

PSR WATCHES/WARNINGS/ADVISORIES

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