Phoenix, Arizona

18Z Aviation Discussion.

• A followup weather system tonight through Sunday night will result in a prolonged, widespread rain event for portions of the region, leading to gradual rises in area washes, creeks, streams, and rivers at a minimum.
• Since rain will fall on already saturated soils, particularly for locations from Phoenix north and east into the Central Arizona high terrain, runoff will be very efficient and lead to flooding concerns. Thus, a Flood Watch is in effect for much of Central and Eastern Arizona tonight through Sunday night.
• Conditions will quickly dry early this upcoming work week, along with daytime temperatures slowly warming closer to, albeit still below, daily normals during the middle of the week.

Lingering elevated instability, persistent southerly to southwesterly upslope flow, and some modest 850 mb convergence continues to trigger spotty convective showers to the east and north of Phoenix early this morning, despite the area falling under subsidence on the backside of a quick-moving shortwave as it ejects northeastward over the Four Corners. On the backside of this feature, some modest midlevel drying has also been realized, with RAP analyzed PWATs dropping by 0.1-0.2" compared to Saturday afternoon and partial/temporary clearing of cloud decks seen in longwave IR satellite imagery (evidence of only a couple shallow, near-saturated layers). This has set the stage for a lull in rain chances/shower activity through the daytime hours today, although areas of spotty light to moderate showers are likely to continue generating throughout the day in these upslope favored areas, or where transient pockets of weak instability are released by increasing upper level ascent. Daytime highs in the middle to upper 70s will be common across the lower deserts today, a few degrees below normal for Halloween. It would be a good idea to wrap up trick or treating and other outdoor activities on the earlier side this evening, as conditions begin to align for a longer duration and more widespread rainfall event across the region.

A subtropical jet streak which stretched from near Hawaii to Baja California has begun to cyclonically curve as it phases with an impulse from higher latitudes and a broad closed low off the Washington Coast. As these features phase together and begin to progress inland along the West Coast, the cyclonically curved subtropical jet will pull abundant, deep moisture into the Desert Southwest. NAEFS/ENS mean PWATs will surge in excess of climatological maxima for this time of year tonight into Sunday, between 1.00-1.50" over Southern AZ, and forecast IVT values are advertised at upwards of 250-750 kg/(ms), consistent with an inland penetrating atmospheric river. Ahead of this system, a period of moist isentropic ascent is likely to take place late today into Sunday morning as almost the entire tropospheric column becomes saturated. This will result in an initial shield of precipitation developing/filling in across the area and pushing northeastward this evening, affecting much of the area by 7-9 PM MST. Scattered to numerous showers may also begin to fill in as the nose of the subtropical jet punches into the Desert Southwest late this afternoon into the evening. While scattered/embedded thunderstorms cannot be ruled out through the duration of this event, instability will be quite a limiting factor. REFS mean MUCAPEs, backed up by GFS bufr soundings, will peak generally between 200-400 J/kg, and these pockets of instability will be transient and relatively short lived. Midlevel lapse rates will also be quite poor at between 4.5-6 C/km. If it weren`t for the poor lapse rates and low instability, the very high shear environment would cause concern for some organized convective modes. As it stands, the main concern with any scattered/embedded convective elements late today through Sunday night will be locally heavier rainfall and possibly some lightning. The convective showers could also help mix higher momentum air down to the surface and result in locally strong wind gusts.

Throughout this entire event, southwesterly upslope/orographic enhancement will be a source for higher rain rates and result in the most persistent areas of precipitation. As a result, rainfall totals will show a strong gradient towards the central AZ high terrain, north and east of Phoenix, and drop off quickly into the lower deserts. NBM/WPC QPF over Phoenix seems to be biased a bit high and will rely on likely the training of convective elements. Average forecast rain totals for the Phoenix area range between 0.75-1.75", again, with a strong gradient towards the north and east, and totals over the foothills/higher terrain should range between 2-4" and locally higher over the most prominent terrain features. Much lower rainfall totals of 0.25-0.5" are expected over the lower deserts of southwest Arizona and southeast California due to rain- shadowing effects and the lack of upslope forcing.

Even if the average forecast rainfall totals come to fruition on Sunday, the bulk of the rain will runoff as it will be falling on already saturated soils. In turn, another Flood Watch will be in effect for South-Central AZ starting tonight and lasting into early Monday morning. During this time we anticipate the potential for many of the washes filling or overflowing, and flooding in some of the larger creeks and rivers as all of the expected rain works through the water basins.

Rain chances will quickly deteriorate by late Sunday morning for Southeast CA and by the late afternoon for much of Southwest AZ as the winds aloft abruptly shift westerly, then northwesterly behind the trough axis. Moisture will quickly be scoured out by this, except in the lower levels for South-Central AZ. The aforementioned closed low off the Washington Coast will serve as a quick followup behind the subtropical jet and initial trough axis, bringing a brief period of WAA and continued upslope flow for more westward-facing slopes to the east of Phoenix through the overnight hours into Monday morning. Activity will gradually trail off as even the lower level moisture is scoured out, and flow shifts northwesterly through essentially the entire column, shutting off any upslope component.

Through the middle of the upcoming work week, following the departure of this dynamic weekend system, we can expect dry conditions, and transient upper level ridging, resulting in temperatures slowly warming closer to seasonal levels (despite daytime highs likely remaining a few degrees below the daily normals). By late week, ensembles diverge considerably with respect to another upper low/positively tilted shortwave trough possibly impacting the region. Some solutions show quasi-zonal flow taking shape over the Southwest US after the transient ridging shifts east of the area, while others show a shortwave disturbance capable of diving well south of the region and resulting in a brief moisture return. Regardless of the outcome, fairly benign conditions seem to be the most likely outcome. The trajectory of the late week system is not favorable for rain chances, its residence time too short for better quality moisture to be pulled into the area, and if it remains an open wave, its positive tilt does not bode well for providing upper level support for precipitation. NBM PoPs become introduced to the eastern AZ high terrain on the order of 15-25% Friday as a result, but we should not put too much stock in this, as the forecast remains highly uncertain at this time.

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

No major aviation concerns are expect through late this afternoon under SCT to at times BKN cloud decks around 5-6K ft. There could be a few isolated showers that develop but will likely not impact the terminals directly. Winds will favor an E-SE direction through this evening with speeds remaining aob 10 kts.

Considerable aviation weather impacts are expected to arrive starting as early as 00Z-01Z this evening and lasting through at least Sunday morning, with long duration SHRA and IFR-MVFR CIG & VIS. A few terminals may near alternate minimums and even airport minimums due to the low CIG & VIS by early Sunday morning. Confidence in sub-VFR conditions is high with cloud decks falling to MVFR around 03Z-06Z tonight and likely reaching IFR flight categories starting around 08Z-10Z with arrival of heavier bands of SHRA. The low flight categories may persist through Sunday afternoon. Winds will predominately be easterly for much of the period, however a westerly shift will be possible closer to sunrise as the heaviest rain bands progress through the region.

Southeast California/Southwest Arizona including KIPL and KBLH

Aviation concerns will remain low through late this afternoon with light winds and gradually increasing mid-lvl clouds are expected at both terminals. Significant aviation weather impacts can be expected starting tonight into Sunday morning with a prolonged period of SHRA and sub-VFR CIG/VIS. VCSH/-SHRA is forecast to start later this evening with conditions rapidly degrading below VFR starting around midnight tonight. MVFR CIG/VIS is favored, however periods of IFR may develop at times Sunday morning, mainly at KBLH.

Spotty showers and possibly a few thunderstorms will impact primarily the higher terrain of the eastern districts today before rain chances increase areawide tonight. Widespread wetting rainfall is expected across the area tonight through Sunday night. Minimum humidities will range between 30-70% areawide today and and improve further on Sunday, following excellent overnight recoveries. Outside of any thunderstorms, winds will remain generally between 10-20 mph and favor southeasterly to southerly directions through the weekend. Conditions will gradually dry out next week as the unsettled pattern departs and temperatures moderate closer to seasonal normals, however, humidities will remain above 25% areawide, and temperatures will still remain in a slightly below normal category.

AZ

Flood Watch from this evening through late Sunday night for AZZ534-537>563.

CA

None.