textproduct: NWS Phoenix

KEY MESSAGES

- The combination of abundant moisture and a weather system moving into the region will help to bring several rounds of showers and thunderstorms across south-central and eastern Arizona through Wednesday.

- The main forecast concern will be heavy rainfall leading to flash flooding with flood watches in effect from late tonight through Wednesday evening across much of central and eastern Arizona.

- Shower and thunderstorm chances will greatly diminish by Thursday and generally come to an end starting Friday with the introduction of much drier air.

- Near normal temperatures, briefly dropping to below normal levels across south-central AZ on Wednesday, can be expected through the latter portion of the week before warming to slightly above normal levels by the weekend.

SHORT TERM /Today through Wednesday/

A potentially very active period of weather is likely to materialize, especially late tonight through the day on Wednesday, across much of south-central AZ as several rounds of showers and thunderstorms with moderate to heavy rainfall leading to flash flooding are likely. Latest objective analysis shows the subtropical high centered over the southeastern states with an upper-level trough over the Pacific Northwest. This overall pattern configuration is resulting south to south-southwest over Arizona with more westerly flow from the lower Colorado River Valley westward through southern California, helping to bring in drier air across these areas. However, with the south to south-southwest situated across much AZ, it is continuing to bring a constant plume of subtropical moisture with PWATs across much of the south-central AZ lower deserts above 1.5".

The combination of strong daytime heating and ample low-level moisture will drive MLCAPE values of 1000-1500 J/KG this afternoon with hi-res guidance indicating isolated to widely scattered storms developing across the higher terrain areas of eastern and southeastern AZ. Across the lower deserts of south-central AZ, there is also the potential of a few storms very late this afternoon through early this evening, especially across any terrain escarpments given the near zero convective inhibition that is forecast to be absent. Given the ample moisture in place, heavy rainfall leading to localized flooding will be the primary threat. However, strong downburst wind gusts will also be possible given that DCAPE values are still projected to be sufficient high, near 1000 J/KG.

The main show is then forecast to materialize late tonight through Wednesday as moisture increases further along with the forcing for ascent. There will be a couple of features aloft that will provide the forcing with a mid-level shortwave over the southern Baja area expected to move overhead from the mid-morning to late afternoon hours on Wednesday along with an upper-level jet streak from the Pacific trough setting up along the Great Basin with the region expected to be located along the right entrance region, providing ample divergence aloft. The increased ascent combined with the ample moisture will result in widespread showers and thunderstorms, with the latest hi-res guidance indicating activity starting near to just after midnight with several rounds of activity into Wednesday evening. With projected PWATs expected to peak between 1.8-2.0" along with low-level mixing ratios as high as 14 g/kg and MUCAPE values in excess of 1000 J/KG, the overall environment will be favorable for thunderstorms to produce heavy rainfall rates in excess of 1-2"/hr and thus heighten the flash flood potential. Both the GEFS and EPS mean QPF show average amounts of 0.5-1.0" over the Phoenix area to over 1" over much of the higher terrain north and east of Phoenix. However, locally higher amounts are likely in the most persistent bands, with some of the HREF members showing amounts in excess of 3-4". Therefore, given the more widespread coverage along with the potentially very heavy rainfall amounts for portions of the area, a Flood Watch has been issued from late tonight through Wednesday evening for all of the south-central AZ lower deserts and the higher terrain areas east. Shower activity may persist across the region into the overnight period, however, activity should be lighter in nature given that most of the instability should be exhausted and the overall coverage will be diminishing as the upper-level ascent shifts away.

Afternoon high temperatures today will generally be right around normal, with readings in the upper 90s to around 100 degrees across the south-central AZ lower deserts to lower 100s across the western deserts. With the abundant cloud cover and the rounds of rainfall activity forecast, much cooler temperatures are anticipated for Wednesday across south-central AZ with the potential that many areas struggle to hit 90 degrees for afternoon highs. Across the western deserts, afternoon temperatures with more sunshine should top out the upper 90s to low 100s.

LONG TERM /Thursday through Monday/

Drier air will gradually spread eastward on Thursday, with lower desert rain chances mostly coming to an end. However, lower level moisture will still remain in place likely allowing another round of showers and isolated thunderstorms on Thursday across the eastern Arizona high terrain. Given the drying aloft on Thursday, the threat for heavy rainfall is expected to be much lower than Wednesday. The synoptic flow is forecast to shift more out of the west northwest late this week into the weekend and this will help to bring a continuous flow of drier air through the region. By Friday, PoPs are at most 10-15% in eastern Gila County to near 0% in the Phoenix area. Lower level moisture should also eventually get scoured out with surface dew points dropping into the mid 30s to lower 40s across the western lower deserts to the upper 40s/lower 50s in the Phoenix area starting Saturday.

Temperatures will mostly be an afterthought for the next several days with highs likely failing to reach 100 degrees in the Phoenix area through Thursday to right around normal for the western deserts. Once the drier air moves through the region later this week, temperatures will begin to warm with lower desert highs climbing up to around 105 degrees by Saturday.

AVIATION

Updated at 1818Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main impact through the TAF period will be SHRA/TSRA and associated impacts. Chances are low this evening as models are not in the best agreement at this time, so a PROB30 remains in the TAFs at this time. Confidence is better compared to previous packages for Wednesday morning, with models in good agreement that VCSH will start just past midnight, then increasing coverage and widespread TSRA impacts to the terminals by 11Z. The longevity of significant operational impacts from TSRA activity may vary, with periods of VCSH likely between redeveloping storms over the metro heading into the afternoon and evening hours. During any TSRA activity cloud bases between 5-6kft will be likely. Winds should remain westerly through this evening switching out of the east for the overnight hours with speeds mostly aob 10 kts. By Wednesday morning winds will continue out of the east with speeds picking up into the low teens with periods of gusts near 20kts.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather concerns will exist through the TAF period under mostly clear skies. KIPL will maintain a E-SE direction before eventually going VRB Wednesday morning. KBLH will continue out of the south, with gusts between 20-25 kts expected to continue through this evening, then relaxing.

FIRE WEATHER

Periods of scattered showers and thunderstorms are expected through Wednesday across the eastern districts supporting a heavy rainfall and flood threat. Moisture levels will remain elevated except for southeast California. Expect minimum humidity levels to still fall into a 15-25% range for the western districts while the eastern districts will only dip to 35-50% through Thursday. Winds will favor the south to southeast over much of the area through Wednesday with gusts of 20-30 mph at times across southwest Arizona. By Thursday into Friday, much drier air is forecast to spread eastward across the region, eventually ending rain chances for all but the far eastern Arizona high terrain along with lowering humidities.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch from midnight MST tonight through Wednesday evening for AZZ534-537>563.

CA...None.