textproduct: NWS Phoenix

UPDATE

Updated 06Z Aviation Discussion.

KEY MESSAGES

- Dry and tranquil weather conditions will continue through the middle of the week with temperatures within a few degrees of daily normals.

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

- There will also be a possibility of embedded thunderstorms with heavier rainfall which could lead to localized flash flooding Friday and Sunday.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Latest mid-lvl wv imagery and 500 mb streamline analysis reveals a trough of low pressure located over the Pac NW along with a weak shortwave progressing through the Intermountain West. This disturbance will result in a few high clouds passing through N AZ this afternoon, otherwise dry and tranquil weather conditions are anticipated across our forecast area during the next 24 hrs. Under mostly clear skies, temperatures will remain within a degree or two of seasonal normals with highs reaching the mid 80s across the lower deserts and the mid to upper 70s for the higher elevations. Due to calm winds and dry conditions overnight, cool overnight low temperatures in the mid 50s to around 60 degrees can be expected Tuesday morning.

Quiet weather conditions will continue into the middle of the week under quasi-zonal westerly flow aloft. Dry air aloft will help keep skies clear to mostly clear through Tuesday before a slight shift in the upper level winds out of the southwest helps to draw more abundant high clouds into our region. Overall there will be very little variation in 500 mb heights which will result in daytime highs topping out in the mid 80s across the lower deserts. Overnight lows will also fall within a normal range in the mid 50s to lower 60s through Wednesday.

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 0547Z.

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

No aviation weather concerns under mostly clear skies expected through Tuesday. The wind pattern will continue to exhibit typical diurnal tendencies with speeds mostly aob 6 kts. There will also be extended periods of light variability to even calm conditions. A period of light southerly crosswinds is likely early Tuesday afternoon before the westerly transition by the late afternoon hours.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather concerns under mostly clear skies expected through Tuesday. Winds will favor diurnal patterns with light speeds, mostly below 8 kts, at both terminals. W winds are expected through sunrise before shifting toward a S-SE direction by late morning and then back W in the evening/overnight hours. KBLH will see a NW wind component through most of Tuesday morning and then shift S by the afternoon. Both terminals will also experience periods of light variability to even calm conditions.

FIRE WEATHER

Warm and dry conditions will transpire through much of this week before a much cooler and wetter pattern sets up Friday into this weekend. Minimum humidity levels will continue to fall, reaching 20-35% today and 15-30% Tuesday before gradual moisture improvement arrives mid-week. 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.