textproduct: NWS Phoenix

UPDATE...Updated 06z Aviation Discussion

KEY MESSAGES

- A Flood Watch remains in effect for portions of south-central and eastern Arizona through this evening.

- Shower and thunderstorm activity will persist for parts of the region, mainly from the Phoenix metro and eastward, through early tonight before eventually tapering off.

- A few strong to marginally severe thunderstorms are possible, with the main hazards being large hail and damaging winds.

- A followup weather system Saturday night into Sunday night will result in a period of prolonged light to moderate rainfall. Additional flooding concerns will exits, therefore, a Flood Watch will also be in effect during this timeframe for central and eastern Arizona.

SHORT TERM /TODAY THROUGH SATURDAY/

After morning warm air advection help to set off scattered showers and thunderstorms across portions of the region, another round of convective activity can be expected this afternoon and evening, mainly from the Phoenix metro and eastward. SPC mesoanalysis reveals a fairly tight corridor of MLCAPE 500-1500 J/kg stretching from western Pima County up toward Las Vegas. This axis will be the focal point for the next instance of convective initiation, and with a very supportive synoptic setup in place and 0-6km bulk shear profiles hovering near 30-40 kt, anything that does form could be maintained for a decent period of time. With this combination of instability and shear, some storms could become strong to severe, potentially producing large hail and strong winds. Although PWATs are not as high as what we typically see during Monsoon season, hovering around 1.0-1.3" according to mesoanalysis, they still sit close to 250% of normal for this time of year. Any stronger storms that can persist and continually tap into this relatively moist environment will have the potential of producing heavy rainfall that could lead to areas of flooding. The flooding potential may be exacerbated by training convection over the same areas and the fact that soil moistures remain elevated from multiple rounds of rainfall over the past several weeks. Ergo, a Flood Watch will remain in effect for portions of central and eastern Arizona until this evening. When all is said an done today, it is not out of the question that some areas could see upwards of an inch to an inch and half, with locally higher amounts.

The shortwave that is aiding in all this rainfall development will eventually push off to the east by this evening/tonight, resulting in a noticeable downtrend in activity for Saturday. Some lingering, scattered showers may persist over upslope/higher terrain areas of central AZ, as southwesterly flow remain elevated on Saturday ahead of the broader troughing off the West Coast.

LONG TERM /SUNDAY THROUGH THURSDAY/

By late Saturday evening, conditions begin to align for a longer duration and more widespread rainfall event through Sunday. The aforementioned subtropical jet streak is shown by global guidance contracting and cyclonically curving around the base of the broader troughing off the West Coast, as it begins to progress eastward. Strong dynamical ascent mechanisms and a prolonged period of southwesterly upslope flow over the Central AZ high terrain will align with excellent, deep moisture through the atmospheric column. Impressively high PWATs are still forecast, in excess of 1.00-1.25" and near the maximum of climatology for early November. NAEFS/ENS mean IVT values are consistent with an inland penetrating atmospheric river and are also, unsurprisingly, near climatological maximum values for the time of year (peaking around 300-600 kg/(ms)). However, midlevel lapse rates will be poor and thus instability will be very limited, with MUCAPEs likely never exceeding 200-300 J/kg. We expect showers to blossom first over the high terrain northeast of Phoenix sometime in the evening Saturday before filling in across most of south-central Arizona around or just after midnight. Showers will stay somewhat scattered through the overnight hours before becoming widespread over much of Arizona by mid morning Sunday. As the main shortwave begins to track through southern California later Sunday morning, a fairly solid shield of light to moderate rain should move through southern California and then through Arizona Sunday afternoon. Upslope showers should persist over portions of south-central Arizona into the high terrain into Sunday evening, but the activity should be gradually winding down.

Expected rainfall rates on Sunday are not all that high, but rain totals will add up over time as this event will last longer than our typical weather events. Based on the record moisture levels, strong forcing, but little instability rainfall rates will generally be in the 0.1-0.2"/hr range in the Phoenix area to as high as 0.2-0.3"/hr in the high terrain north and east of the Phoenix area. Fairly steady light to moderate rainfall is expected from just before sunrise through the afternoon in the aforementioned areas with rainfall totals averaging 1-1.5" in Phoenix to 2-3" over the higher terrain. Believe it our not, the NBM is also suggesting the possibility (25%) of rainfall totals over 2" in the Phoenix to as much as 3-4" in the high terrain. Much lower rainfall totals of 0.25- 0.5" are expected over the lower deserts of southwest Arizona and southeast California due to some 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 has been issued for a large area of south-central AZ, and the eastern AZ high terrain starting overnight Saturday night 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.

Dry air will begin to filter into the region from the northwest later on Sunday, overspreading the bulk of the area by Monday morning. We may have some lingering showers across the eastern 1/3rd of Arizona Sunday night into Monday morning, but additional rainfall amounts should be fairly light. The rest of Monday will bring decreasing clouds as the drier air works into the region while another follow-on shortwave trough moves southeastward into northern New Mexico. Temperatures for both Sunday and Monday will be fairly cool with highs as low as the lower 70s for the Phoenix area to maybe the mid to upper 70s across the western lower deserts. Upper level ridging is then forecast to mostly take over across the Desert Southwest going into the middle part of next week, but temperatures will be fairly slow to rise with readings staying below normal.

AVIATION

Updated at 0530Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; SHRA activity and low MVFR to IFR CIGs/VIS starting Saturday evening will be the main aviation weather issues throughout the TAF period. In the meantime, the TSRA activity that has affected portions of the area through the early evening hours has now dissipated with mostly SCT cloud cover with bases as low as 4-5 kft prevailing through Saturday afternoon. Some hi-res guidance brings in additional SHRA activity at times through Saturday afternoon, however, confidence is low of this scenario materializing. Much better confidence of more widespread SHRA activity moving in Saturday evening with CIGs/VIS rapidly deteriorating to MVFR to at times IFR conditions. Winds will generally maintain an easterly component throughout the TAF period with light speeds generally under 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH: SHRA activity and sub VFR CIGs/VIS starting Saturday evening will be the main aviation weather issues throughout the TAF period. At KIPL, winds will generally maintain a westerly component through the overnight period with mostly variable conditions expected early Saturday morning. At KBLH, winds will generally fluctuate between the southeast to south with a period of light variability expected during the morning and afternoon hours. SCT-BKN high- level cloud decks will persist through the overnight period before low to mid-level cloud coverage increases during the day Saturday. More widespread light SHRA activity will move in by the late afternoon/early evening hours with CIGs/VIS rapidly deteriorating to sub VFR conditions.

FIRE WEATHER

Showers and thunderstorms will impact portions of the eastern districts today with heavy rainfall likely in some areas, followed by a lull in rain chances on Saturday. Minimum humidities will range between 30-70% areawide today and Saturday and improve further on Sunday, following excellent overnight recoveries. After the lull on saturday, chances for rain increase again Saturday night followed by widespread light to moderate rainfall on Sunday areawide. 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.

PSR WATCHES/WARNINGS/ADVISORIES

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

Dense Fog Advisory until midnight MST tonight for AZZ562.

CA...None.