textproduct: NWS Phoenix

UPDATE

13Z Aviation Discussion

KEY MESSAGES

- Scattered showers and thunderstorms will be possible again this afternoon and evening, with activity mainly focused over the foothills and high terrain features.

- After drying out temporarily this weekend, rain chances will return to the region late Sunday into Monday as tropical moisture gets pulled northward into the Desert Southwest.

- Near to slightly above normal temperatures will persist through the weekend before noticeably cooling early next week.

SHORT TERM /Today through Friday/

Storm coverage was much lower than high-res models indicated last night which was mainly due to lower instability and a stronger cap in place from extensive cloud cover over southcentral AZ. Latest wv satellite imagery and area radars still show abundant high clouds streaming into AZ with showers and thunderstorms percolating over southern portions of the state. All of this activity is developing ahead of a coastal low pressure system located west of the Baja California (not related to the hurricanes much farther south). As we head through this morning, several lobes of vorticity will progress northward out of MX and provide a focus of lift for additional showers and storms this afternoon into this evening.

It is difficult to pinpoint exactly where these vortmaxes will end up moving as some models focus convective activity over the western deserts this afternoon while others bring storms into central AZ. Either way, there will likely be another round of widely scattered convection this afternoon, especially over the terrain features of southwest AZ and the foothills and higher terrain areas NE of Phoenix. There is still around a 20-30% chance that storms form over the lower deserts, but this will be highly dependent on if activity across the higher terrain can congeal enough to produce strong outflows which could generate additional convection. Instability will not be as high as yesterday (SBCAPE < 1000 J/kg) but there still could be a strong storm or two which could produce locally gusty winds and patchy blowing dust. The coastal low will weaken and lift northward on Friday which will cause upper-lvl winds to shift out of the west and bring much drier air into our region. Coincidentally, PoPs will decrease below 10% across the entire region on Friday.

The presence of the upper-low over the next couple of days promoting near seasonal 500 mb hghts, coupled with periods of extensive high clouds will help keep temperatures near normal across southcentral AZ. Highs across the lower deserts of central AZ will likely remain in the mid to upper 90s both today and Friday. Temperatures along and west of the Colorado River valley will be slightly warmer with highs reaching the lower 100s. Overall, the HeatRisk should remain in the minor category with only very small pockets of moderate.

LONG TERM /Saturday through Wednesday/

Dry westerly flow is expected to persist through at least Saturday with large scale troughing remaining over the Pac NW while the subtropical ridge slowly shifts southwestward from east TX into the interior of MX. Due to the presence of the ridge to our east, 500 mb hghts are expected to peak around 586-588 dam Saturday afternoon. This will result in highs topping out between 100-105 degrees across the lower deserts (4-6 degrees above normal) under mostly sunny skies and zero chance of rain. From Sunday into early next week, there does look to be a significant degree of cooling as tropical moisture arrives and rain chances increase. Highs will likely fall below 100 degrees across the lower deserts on Sunday and continue to cool to the upper 80s and lower 90s Monday through Wednesday.

Our attention then turns to the tropical Pacific Sunday into early next week where potential moisture from two weakening hurricanes (Polo and Odalys) will come into play. There is still a high degree of uncertainty as to a) how far north the residual moisture will be shed out ahead of these decaying storms, and b) where the plume(s) of moisture will set up. Another major question mark will be just how deep and progressive a developing trough of low pressure will be over the Intermountain West. All of these factors will have to line up perfectly for our forecast area to get significant rainfall. Both the GEFS and EPS are telling a different story with varying degrees of moisture and timing and intensity of said trough which is really complicating things. The latest NHC track for Hurricane Polo shows the storm making landfall along the coast of the Baja California by Monday which if aligned with the passage of the aforementioned trough would draw a large plume of moisture into interior MX and and much of AZ. NBM PoPs have already increased to around 20-30% with an initial band of moisture on Sunday, but with the main lift arriving on Monday, PoPs are now widespread 30-60% across our forecast area. The remnant tropical moisture may linger into Tuesday before fully exiting the region on Wednesday. It is also worth pointing out that some ensemble members are showing significant rainfall totals in excess of 1-2" across southcentral AZ Monday-Tuesday but there is still significant spread between members. Therefore, we will likely have to wait until this weekend for things to become more clear.

AVIATION

Updated at 1245Z.

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

Showers and thunderstorms during the pre-dawn and dawn hours have popped up across portions of the region but were not captured well in prior forecast guidance. Due to very low forecast confidence, metro TAFs may need to be updated for the next few hours until convection begins to diminish. Sudden wind shifts and perhaps some brief reductions in surface VIS will be the main operation concerns.

Another round of SHRA/TSRA are possible later today, but overall forecast confidence in this potential and timing is low. Isolated to scattered showers may develop in/move into the metro from the south later Thursday morning/midday. This is reflected with VCSH in the TAFs in the morning. SHRA/TSRA may then develop in the metro later in the afternoon-evening timeframe, but chances will be higher outside the metro area. Activity around the region will be capable of producing several outflow boundaries which may cause erratic wind behavior and outflow collisions could lead to new development. Winds tonight through Thursday morning will continue to favor an E/SE component with light speeds. Confidence in winds then degrades from there with winds potentially influenced by showers and/or cloud cover through the afternoon and evening. Thick high cirrus cigs and brief periods of cigs as low as 8-10K ft AGL will continue through the TAF period.

Southeast California/Southwest Arizona including KIPL and KBLH:

There is a low chance (10%) for VCTS/TS at KBLH Thursday afternoon. Otherwise, no weather issues are expected through Thursday evening under increasing high cirrus decks. Confidence in a specific wind direction at any given time is low given the weak pressure gradient. Extended periods of light and variable winds will be common.

FIRE WEATHER

Widely scattered showers and thunderstorms will be possible again this afternoon and early evening, mainly focused over the higher terrain of the eastern districts. Localized gusty and erratic winds will be possible in the vicinity of any storm that develops. Rain chances will decrease Friday into this weekend before much better chances for wetting rainfall arrives early next week. MinRHs will range from 25-35% today with a gradual decrease to around 15-25% on region-wide Saturday. Significant moisture return is expected across the eastern districts by Monday which will keep RH values above 40% with excellent overnight recovery. Winds will generally follow diurnal patterns with speeds around 10-15 mph with typical afternoon gusts in the upper teens.

PSR WATCHES/WARNINGS/ADVISORIES

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