textproduct: NWS Phoenix

UPDATE

Updated 12Z Aviation Discussion.

KEY MESSAGES

- A strong low pressure system is expected to bring multiple rounds of showers and thunderstorms through Thursday across much of central and eastern Arizona.

- Moderate to heavy rainfall activity leading to areas of flash flooding as well as isolated to widely scattered strong to severe thunderstorms leading to damaging wind gusts, large hail, and even a few isolated tornadoes are all possible.

- Drier air filtering into the region by the end of the week will lead to more benign weather conditions during the weekend through at least early next week.

- Near normal temperatures today will cool to below normal levels by the latter half of the week and last through the weekend.

SHORT TERM /Today through Thursday/

A highly impactful weather event is likely to unfold across much of central and eastern AZ through Thursday as a potent low pressure system currently just off the southern CA coast impacts the Desert Southwest. Late last evening, a few robust thunderstorms, likely instigated from a leading shortwave, materialized across portions of south-central AZ. This included a supercell storm that traversed portions of the Phoenix East Valley from Chandler northeastward through Mesa with several reports of large hail. This activity was just an appetizer of what is likely to transpire during the next couple of days.

Latest SPC mesoanalysis shows robust moisture advection well underway as PWATs across south-central AZ were already approaching 1.3-1.5" and will likely go a bit higher, peaking at 1.6-1.7" by this afternoon. This moisture combined with steep mid-level lapse rates will result in MLCAPE values peaking upwards of 1000 J/KG by this afternoon. This amount of instability combined with abundant moisture will yield a favorable environment for thunderstorms capable of producing heavy rainfall rates in excess of 1-2"/hr with a good potential for training of activity and thus heightening the potential for flash flooding. In addition to the flash flood threat, another major threat is severe weather. Given the ample amount of instability expected to be in place combined with very strong 0-6 km shear values in excess of 50 kts, the thunderstorms that develop will likely have some organization to them with the potential to acquire supercellular characteristics. As a result, damaging wind gusts in excess of 60-70 mph, large hail in excess of 1-2" in diameter and even an isolated tornado or two will be threats with any given severe thunderstorm. In fact, given the highly unusual favorable severe weather setup for this part of the country, the Storm Prediction Center has highlighted a portion of north-central AZ, just north of Phoenix in an enhanced risk of severe weather for later today, mainly for very large hail. Even areas outside of the enhanced risk could see significant severe weather.

Latest hi-res guidance highlights activity beginning to develop across the higher terrain areas north and east of Phoenix and across much of the AZ high country by the late morning/early afternoon hours before activity potentially expands into the lower deserts of south-central AZ shortly after. The most widespread activity and likely the greatest threat for flash flooding is forecast to occur from later this evening through the overnight and morning hours of Thursday as the main upper-level jet streak associated with the low pressure system moves over the Great Basin area, putting much of the region in a favorable location for upper-level divergence. The threat for severe weather will also continue into Thursday as instability and shear parameters will still remain more than sufficient. Latest QPF from WPC continues to highlight average total rainfall amounts of 1-2" across the higher terrain areas north and east of Phoenix, while lower amounts of 0.25- 0.75" are forecast for the lower deserts. However, locally higher amounts will be possible in areas that see training of activity. As the upper trough gradually pushes inland through the southern Great Basin during the day on Thursday, strong dry air advection will gradually spread from west to east with rain chances first ending across the south-central AZ lower deserts by the mid to late afternoon and will likely take until late Thursday night into early Friday morning for rain chances to end across the higher terrain areas.

With the main trough axis still to the west of the region, afternoon temperatures will still warm up well into the 80s across much of the lower deserts with some areas across the western deserts touching 90 degrees. As heights aloft decrease with the trough axis pushing inland through the Great Basin on Thursday, temperatures will noticeably cool with many areas only topping out in the low 80s. However, depending on the amount of rain and cloud cover, it is possible that areas in south-central AZ remain cooler than currently forecasted.

LONG TERM /Friday through Tuesday/

With strong dry air advection in place behind the departing trough, weather conditions will turn much quieter beginning on Friday. A piece of energy left behind from departing trough is currently being depicted by the model guidance to separate itself from the main jet energy and briefly morph into a cut-off low over AZ during the weekend. This feature for the most part should be inconsequential, although enough moisture could be present for some showers to materialize over the AZ high terrain on Saturday but for the most part should remain on the light side given the overall lack of instability that will be present. The lower deserts should remain rain-free as there will be too much dry air in place. Under the presence of the low, temperatures will be several degrees below normal during the weekend with afternoon highs struggling to reach 80 degrees across the lower deserts. As the upper-level low exits the region and weak ridging builds overhead early next week, temperatures will rebound to more seasonal levels.

AVIATION

Updated at 1203Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Rounds of isolated to scattered strong thunderstorms with heavy rain leading to sub-VFR VIS, brief strong and erratic wind gusts, isolated large hail, and low CIGs will be possible today through at least Thursday morning. Confidence in timing out best chances RA/TS is moderate, as isolated RA/TS may develop at any time. An increase in activity is expected going into this afternoon and then there may be a break this evening/early tonight before another round develops later tonight-Thursday morning. A downtrend in activity is expected by Thursday evening. Winds will favor a light easterly component through most of the TAF period, with periods of southerly winds during showers and storms. Lowest CIGs today are expected around 5-7K ft AGL and then may dip to 3-5K ft Thursday morning with showers and storms.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the TAF period. Outside of extended periods of VRB conditions, S'rly flow will be favored at KBLH, with some gusts upwards of 20-25 kt during the afternoon hours, while winds at KIPL will take on familiar diurnal trends, flipping between SE and W. FEW-SCT skies will be common through Wednesday evening, with the lowest bases around 6-7 kft.

FIRE WEATHER

A low pressure system will bring very active weather conditions across much of central and eastern AZ through Thursday as multiple rounds of showers and thunderstorms are expected. Significant rainfall activity is likely in some areas, particularly across the higher terrain areas of the eastern districts where rainfall amounts in excess of 1-2" are possible. MinRHs through Thursday will range between 30-40% across the western districts and between 40-70% across the eastern districts along with good to excellent overnight recoveries. Some gusty winds with peak gusts upwards of 25-30 mph are likely this afternoon and once again on Thursday across the western districts. Much quieter conditions are expected starting Friday along with a decrease in humidity values. Near normal temperatures today will cool into a below normal category starting Thursday and lasting through the weekend.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch from 11 AM MST this morning through Thursday evening for AZZ545-547-552-556>558-560>563.

Flood Watch from 2 PM MST this afternoon through Thursday afternoon for AZZ534-537-538-540>544-546-548>551-553>555.

CA...None.