textproduct: NWS Phoenix

UPDATE...Updated 00z Aviation Discussion

KEY MESSAGES

- Temperatures will cool across much of Arizona dropping to below normal levels while southeast California maintains near normal readings.

- Showers and thunderstorms will become more widespread across eastern and central Arizona through Friday with the potential for heavy rainfall and localized flash flooding.

- A decrease in thunderstorm coverage across lower elevations will arrive this weekend with the better chances favoring Arizona high terrain locations.

SHORT TERM /Today through Friday/

Early afternoon satellite imagery depicts several small MCVs traversing the region adding ascent to an already favorable underlying synoptic pattern of strong divergence aloft atop deepening moisture profiles. In fact, 12-15 g/kg low level mixing ratios have spread across much of the CWA supporting scattered showers ahead of many of these vorticity features. On the other hand, this thicker moisture has negatively affected insolation over much of the region yielding limited instability with notable inhibition still present. This evolution has added complexity and greater uncertainty to the forecast over the next 36-48 hours as most modeling systems have not fully captured the extent of cloud cover and ongoing showers. One consequence of this allows for the cancellation of the Extreme Heat warning over the western CWA as clouds and rain have negated intense heating and associated heat impacts.

Despite all the convectively detrimental features across the local forecast area, there are a few regional features which could eventually encourage storms over south-central Arizona tonight into Friday morning. Intense ascent between 2 well defined easterly waves (one over central Baja and the other over west Texas) feeding on 1500+ J/kg MLCape across northeast Sonora has fostered explosive convective development with evidence of a congealing MCS already this afternoon. High resolution modeling capturing this complex suggests outflow and vorticity advection forcing new development over southern Arizona by early evening. While outflow will not be as strong as previous days, the combined effect with synoptic energy importation may bring activity towards the Phoenix metro late evening. Concurrently, a shortwave near the four corners captured in NE flow was tapping an unstable airmass over northern Arizona, and also heading towards south-central Arizona tonight.

As a result, there is growing evidence that multiple MCVs and plume of higher theta-e congeal somewhere around Pinal/eastern Maricopa counties overnight as discussed in an alternative forecast outcome yesterday. While MLCape/DCape will be substantially lower during the overnight, MUCape around 500-750 J/kg will still be present albeit with deteriorating midlevel lapse rates. Nevertheless, as mixing ratios reach 15 g/kg and total column PWATs exceed 2.00", any amount of focused ascent should sufficient for scattered showers and thunderstorms. Very efficient rainfall rates with the potential for warm rain processes could yield at least 2-3"/hr given the moisture profiles, and this scenario matches many historical late night/early morning events that have produced significant flooding around the Phoenix metro. The current Flood Watch may need local extensions through Friday morning once area of storms become established overnight. Otherwise, deep convection should eventually exhaust elevated instability by Friday afternoon as atmospheric profiles become moist adiabatic. While a few lingering showers may persist through the afternoon, there is very little evidence of renewed convection during the afternoon/evening given the expected expansive thick cloud cover left over from the morning activity.

LONG TERM /Saturday through Wednesday/

Although some uncertainty remains, guidance continues to trend drier for this weekend, particularly the GEFS. This drier air is expected to advect in from the south southwest starting Friday morning and continue through most of Saturday, lowering PWATs to as low as 1.4- 1.5", but the Euro maintains a more optimistic 1.7-1.9". The easterly wave is also forecast to weaken while it briefly shifts back toward central Texas. The combination of the expected drying, very little instability, and weaker upper level support is likely to limit shower and thunderstorm potential on Saturday. As of now, much of the lower deserts should stay quiet on Saturday, while scattered showers and storms are much more likely over the Arizona high terrain. Temperatures will begin to recover starting Saturday, but daytime highs are still likely to fall short of 100 degrees in the Phoenix area.

Starting Sunday into early next week, the subtropical ridge is likely to become more elongated from the Great Basin to the Lower MS River Valley. The remnants of the easterly wave or even a newly developed disturbance may also become more of a player for monsoon activity by around Monday or Tuesday as moisture remains plenty adequate for convection. An increase in easterly upper level winds across Texas may eventually stretch into the Desert Southwest by late Monday into Tuesday which could help drive monsoon activity throughout portions of the lower deserts again. NBM PoPs still favor the Arizona high terrain through the period, but it seems likely the lower deserts will see another day or two of active weather.

Forecast temperatures for next week show a gradual upward trend, despite fairly high model spread. Lower desert daytime highs may again reach 100 degrees in the Phoenix area by Monday and eventually back to near normal during the latter half of next week. Locations across southeast California and southwest Arizona are likely to maintain near normal temperatures throughout next week.

AVIATION

Updated at 2345Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Heavy rainfall and thunderstorm impacts along with some gusty winds will be the main aviation weather issues through early Friday. In the meantime, through at least 03-04z, mostly tranquil weather will prevail with westerly winds aob 10 kts. Thereafter, a complex of thunderstorms over southeast AZ will gradually shift northwestward with a gusty southeasterly outflow moving in after 04z with additional shower and thunderstorm activity likely to erupt along and behind the outflow. Main uncertainty in the forecast lies in the general location of the heaviest activity as it could be east of the terminals or directly over the terminals. PROB30 group for thunderstorms and thus MVFR visibilities remain in effect given the uncertainty in the general location of the heaviest activity with several amendments to the TAFs likely through the evening and overnight as confidence grows. VCSH will likely continue through at least Friday morning with easterly winds persisting potentially all day Friday with no westerly shift. SCT-BKN cloud bases aoa 10 kft will be common throughout the period with the potential for bases to lower as low as 6-7 kft in shower and thunderstorm activity, especially during the overnight period.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours under passing mid to high cloud decks. Winds will generally remain out of the southeast at KIPL and out of the south at KBLH with overall speeds between 8-15 kts.

FIRE WEATHER

Active monsoon weather with elevated moisture levels will continue through the rest of the week. Shower and thunderstorm chances will increase over the eastern districts today with only isolated chances over the western districts. Localized heavy rainfall is expected today into early Friday across the eastern districts with some potential for gusty outflow winds late this afternoon and early evening. MinRHs will be 35-55% for the eastern districts through Saturday to 20-35% across the western districts. Outside of potential thunderstorm outflows, winds for the eastern districts will tend to be light and somewhat favor diurnal trends. Winds will predominately favor the south and be breezy at times across the western districts.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch through late tonight for AZZ534-537>563.

CA...None.