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 Friday through Sunday resulting in much cooler temperatures and enhanced chances for widespread rainfall.

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

SHORT TERM /Today through Thursday/

Semi quasi-zonal aloft continues over the region today and will shift out of the southwest starting Thursday. Both directions will keep calm weather, dry conditions, and near normal temperatures over the region today and Thursday, while the next weather system is at the doorstep of the region and is expected to impact the region as early as Friday morning. Until then, as previously mentioned, quiet weather is expected. Heights aloft are near the NAEFS climatological average for late October, resulting in near to slightly above afternoon highs, with today's highs in the mid 80s, and tomorrow the western lower deserts will remain in the mid 80s, while south- central AZ will be 1-2 degrees warmer, in the mid to upper 80s. Ahead of the aforementioned weather system, PWATs will begin to gradually increase as subtropical moisture will begin to be advected northward into the region, increasing PWATs from 100-130% of normal to to 200-220% of normal by Thursday night. This will lead to increasing in cloud coverage over the course of Thursday. This increase in cloud coverage in combination of the warmer temperatures will lead to Friday overnight and morning's low temperatures 5-7 degrees above normal.

LONG TERM /Friday through Tuesday/

Forecast confidence is increasing for two separate weather events, Friday through Sunday, both of which could bring impactful rain amounts to portions of central Arizona. The increasing moisture on Thursday into Friday is expected to peak during the daytime hours Friday with PWATs reaching 225-250% of normals and low level mixing ratios of 8-9 g/kg, centered over Maricopa and Pinal Counties. Forecast soundings show the moisture will be quite deep, extending fairly evenly throughout the column with modest elevated MUCAPE up to around 500 J/kg. Strong jet forced ascent is also seen moving over the area on Friday combining with the modest southerly low level upslope ascent.

Showers are expected to begin developing a few hours after midnight Thursday night before increasing in coverage and intensity by around sunrise Friday. Both the GEFS and EPS show QPF maxima centered from Phoenix to around the Tonto NF with amounts averaging 0.50-0.75". Considering the available instability and steep mid-level lapse rates, we should expect scattered thunderstorms too, mostly embedded with the shower activity. General rainfall rates are likely to mostly fall within a 0.05-0.20" range, but it would not be surprising to see very localized rates of around 1" per hour within any thunderstorms. These higher rainfall rates should pose a minor flood threat on Friday, mainly affecting area washes and the typical unbridged low water crossings. There is potential for higher rainfall amounts with the NBM 75th percentile showing amounts between 1.50-2.00" centered over the foothills just north of Phoenix. A Slight Risk for excessive rainfall remains on the updated WPC ERO and based on the model run-to-run consistency, Flood Watches are likely to be needed for at least north-central and northeast Maricopa County. The shower and thunderstorm activity should also extend eastward into Gila County later on Friday before quickly winding down Friday evening as we lose upper level support.

The large scale trough will remain positioned along and into the Western U.S. into Saturday with the main low center near the Pacific Northwest, but another strong shortwave diving into the base of the trough is expected to help bring another low developing off the coast of California Saturday night into Sunday. Saturday is expected to be mostly a dry day, but there will still likely be some isolated upslope type showers mostly focused over the high terrain north and east of Phoenix. However, this will just be the calm in between weather systems with the second one moving in starting late Saturday night expected to be stronger and more impactful.

By Saturday evening, guidance shows a strong atmospheric river (AR) starting to make its way into northern Baja and far southern California. By Sunday morning, the full force of this AR is expected to be directed at southern Arizona and northern Sonora Mexico with PWATs increasing to 275-325% of normal, or right near record levels for our area this time of year. Compared to model runs 24 hours ago, the latest depicts even better moisture and a stronger system. Upper level forced ascent should begin to increase over our area by Saturday night, coinciding with increasing low and mid level upslope south southwesterly flow.

A large area of light to moderate rain is likely to develop ahead of the approaching weather system late Saturday night into early Sunday affecting the bulk of the region, particularly central and eastern Arizona. Although model uncertainty is decreasing, there is still a good deal of uncertainty in expected rainfall amounts. This looks to be a longer duration light to moderate rain event with limited instability keeping rainfall rates under 0.50" per hour and even lower across the lower deserts. However, because this may end up easily being a 6-12 hour rain event, the rainfall could quickly add up, especially into the higher terrain northeast of Phoenix where upslope forcing will be maximized. The latest NBM QPF shows a wide range of potential amounts with averages around 1" in the Phoenix area to upwards of 1.50-2.00" over the high terrain northeast of Phoenix. The lower end of the forecast shows amounts closer to 0.50" and 1.00-1.50" respectively, but the higher end potential is more like 2-3". 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 portions of central and eastern Arizona, but by mid to late Monday morning, 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 1725Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; and Southeast California/Southwest Arizona including KIPL and KBLH:

No major weather issues will exist through Thursday afternoon under periods of increasing thick cirrus decks. Wind behavior will be similar to the past 24 hours including an extended period of mid/late afternoon south cross runway winds (140v240) around the Phoenix metro before briefly becoming NW in the evening. Confidence in a full westerly wind shift at the Phoenix area terminals is low. SE California aerodromes should continue with a general diurnal S/SE component and nocturnal W/NW component. Extended periods of nearly calm conditions will be common throughout the region.

FIRE WEATHER

Dry conditions with near normal temperatures will persist through Thursday under increasing cloud cover. Minimum humidity levels through Thursday will fall into a 20-40% range following fair to good overnight recovery of 40-60%. Starting Friday. and through the weekend, MinRH's will increase substantially with widespread 50-80%, due to several days of unsettled weather, bringing widespread rainfall, and cooler temperatures. Otherwise, winds will remain unseasonably light with minimal upslope breeziness.

PSR WATCHES/WARNINGS/ADVISORIES

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