textproduct: NWS Phoenix

UPDATE

00Z Aviation Discussion.

KEY MESSAGES

- A strong low pressure system will support multiple rounds of showers and thunderstorms, some of which may be strong to severe through Thursday afternoon across much of central and eastern Arizona.

- Moderate to heavy rainfall with totals over one inch may result in areas of flash flooding across south-central Arizona with a focus on upslope, terrain areas.

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

- Near normal temperatures will cool to below normal levels by the latter half of the week and continuing through early next week.

SHORT TERM /Today through Thursday/

A high impact weather event continues to unfold across much of central and eastern Arizona, as an anomalously deep low pressure system along the southern California coast sends multiple shortwaves and favorable jet forced ascent into the region intercepting a very moist airmass. Objective analysis indicates a plume of total column PWATS in excess of 1.25" focused into central Arizona with low level mixing ratios 8-10 g/kg, and combined with steepening midlevel lapse rates (~7 C/km in the H7-H5 layer) will result in MLCape values nearing or exceeding 1000 J/kg by this afternoon. Juxtaposed with these thermodynamic profiles, deep layer 0-6km shear will easily exceed 50kt supporting supercellular structure/splitting cells in more robust thunderstorm updrafts. Special 18Z KPSR sounding highlights an even higher instability potential should sufficient heating continue supporting a severe potential with large hail and damaging the primary threats. Interestingly, however, the latest HREF and REFS output is less enthusiastic regarding the spatial coverage of storms across lower elevations until later this evening, and it remains to be seen if the ingest of updated thermodynamic profiles changes short term model output. Nevertheless, there is still modest support for terrain encouraged ascent spurring rapid updraft development around the Phoenix metro with better confidence of thunderstorm clusters to the north and east of the city through the evening.

Confidence in thunderstorm development and coverage across south- central Arizona grows markedly overnight through Thursday morning as the aforementioned California low edges inland placing the eastern CWA within a right rear jet entrance region providing a prolonged period of upper divergence and ascent. The nocturnal tendency will be for increasing H9-H8 southerly winds, and within a unidirectional wind profile, reduce Corfidi vectors encouraging training echoes. Moist orographic upslope will also aid in persistent ascent over the same areas, and with PWATS solidly holding or exceeding 1.50" overnight, the flash flood threat will be heightened heading into Thursday morning. A notable handful of high resolution models support a conceptual idea and historical precedent in similar scenarios of storms aligned through northern Maricopa County through the morning hours before shifting into Gila County during the afternoon. The magnitude of elevated instability combined with abundant moisture profiles will yield a favorable environment for rainfall rates in excess of 1-2"/hr.

As the upper trough lifts inland towards the Great Basin Thursday, supporting jet energy will exit the region while low level winds veer helping erode boundary layer moisture, driving a frontal boundary and lower theta-e airmass into lower elevations and eliminating instability. However, moist upslope will persist across the eastern CWA with lingering instability juxtaposed with strong deep layer shear supporting the continued threat for severe weather. In fact, numerous HREF and REFS members depict very impressive 2-5km updraft helicity signatures with notable 0-1km shear indicative of rotating updrafts in the stronger storms through the afternoon. Subjectively, the synoptic setup and model output looks more impressive for portions of southern Gila County tomorrow than much of the forecast area currently. Thus, will amplify the severe threat in the eastern CWA during the afternoon before activity rapidly exits the area in the evening.

LONG TERM /Friday through Tuesday/

With strong dry air advection in place behind the departing trough, weather conditions will turn much quieter beginning 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 2350Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concerns through Thursday morning continue to be periods of VCTS/TS which could result in gusty winds, IFR/MVFR vsbys due to heavy rainfall, lightning, and hail. There will be a lull in thunderstorm activity for several hours after 01Z this evening, however storm chances will ramp up again late tonight. The most likely period of time to see thunderstorms overnight will be generally after midnight through 7 AM Thursday morning. Winds will likely shift back out of the east this evening and remain easterly through the period, although thunderstorms could cause wind directions to fluctuate to northerly or westerly briefly. CIGs are expected to remain around 5-7K ft AGL and then may dip to 3-5K ft early Thursday morning as showers continue to progress through the metro area.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns are anticipated through the forecast period. Winds will favor a southerly component at KBLH and fluctuate between southeasterly and westerly at KIPL. Gust around 20-25 kts can be expected at KBLH through early this evening. FEW to SCT clouds, mostly aoa 6-7 kft, will be common through this evening.

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 through Thursday evening for AZZ545-547-552-556>558- 560>563.

Flood Watch through Thursday afternoon for AZZ534-537-538- 540>544-546-548>551-553>555.

CA...None.