textproduct: NWS Phoenix

UPDATE

Updated Aviation

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 resulting in flood watches from late tonight through Wednesday evening across much of central and eastern Arizona.

- Shower and thunderstorm chances will greatly diminish by Thursday, coming 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 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 2320Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Lower confidence forecast through Wednesday afternoon, however the threat exists of a longer duration, high impact TS event Wednesday morning into early afternoon. In the meantime, south winds may turn westerly mid evening, however confidence is low and a 150v240 type wind may prevail. The best odds are for SHRA/TSRA to remain just outside the KPHX airspace through the evening with less than a 20% chance of direct operational impacts before the overnight period. Trends in modeling suggests SHRA/TSRA not moving into the PHX area until after sunrise, though uncertainty on exact timing and location of TS impacts is high.

At this time, will highlight the 15-19Z time frame as the most likely for significant operational impacts. In addition to direct TSRA sweeping through the PHX airspace, instances of MVFR (and possible brief IFR) flight categories are possible in the form of vsby and cigs. An initial period of strong wind gusts and BLDU is also possible, though precise details and chance of occurrence at any given terminal precludes inclusion in this TAF package. SHRA may linger well into the afternoon, and depending on the time of arrival of TSRA, a prolonged and more intense TS event may persist into the afternoon. The most likely outcome is for TS to leave the PHX area by mid afternoon with residual thick midlevel cigs and lack of instability for further development the remainder of the day. Should morning activity miss the PHX metro or fail to materialize, late afternoon and early evening SHRA/TSRA would become more likely.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather issues will exist through Wednesday evening under mostly clear skies. Confidence is very good that KIPL winds will maintain a SE direction with periods of nearly calm conditions, while KBLH winds will continue southerly with afternoon gusts 20-25 kts.

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.