textproduct: NWS Phoenix

UPDATE

Updated 18Z Aviation Discussion.

KEY MESSAGES

- Rain chances return to portions of Eastern Arizona starting today, with most activity focused over foothill and high terrain areas.

- Best chances to see lower desert rainfall will occur on Wednesday, but otherwise, most of the activity should remain to the north and east of the Phoenix metro through the end of the work week.

- Near to slightly above normal temperatures will persist over the next several days.

SHORT TERM /Today through Wednesday/

Current RAP analysis reveals shortwave ridging over the Rockies as flanking waves spin near the Great Lakes and Great Basin. The latter of these disturbances will be the main weather focus over the Desert Southwest through at least the middle portion of the week, helping to renew rain chances during the waning days of monsoon season. Even though southwesterly flow will persist, which typically translate to rather benign conditions across the region, the Pacific low will be strong/deep enough to help pull sub-tropical moisture upwards into eastern Arizona today, with PWATs increasing toward 150- 175% of normal. Combine this moisture with positive vorticity advection and a developing upper-level jet streak, and you get an environment favorable for shower and isolated thunderstorm development. However, with a sharp east-west cutoff of sufficient moisture expected, most, if not all, rainfall activity will be confined to the easternmost third of Arizona, and therefore, coverage over our area will be focused primarily over Gila County. With this setup and better synoptic support, rounds of showers and isolated of storms will be possible from mid- morning into the early nighttime hours as the environment will not be solely reliant on surface heating and orographic influences, which typically limit warm season activity to the afternoon and evening hours. Due to the potential to see multiple instances of rain over the same areas, minor flooding will be the main concern, especially if a specific locations experiences several heavier cells.

One forecast aspect that will need to monitored later this evening will be the potential for areas of blowing dust in lower deserts of Pinal and Maricopa Counties due to gusty southeasterly outflow winds from distant showers and storms. Hi-res data is highlighting this potential and with a dry boundary layer in place, DCAPE values will near or exceed 1000 J/kg, making this outcome not so far-fetched, even though this is not your typical blowing dust signal. However, dust generation will be heavily dependent on if strong enough convection can become established. At the very minimum, the southeasterly flow will help increase boundary layer moisture Tuesday night, which could set the stage for further activity on Wednesday.

Speaking of Wednesday, the southwestern wave is shown splitting in two separate areas of energy, with one area being ejected toward the Northern Plains, while another area lingers off the Baja Coast. The latter area will tilt regional flow more out of the south and perhaps even slightly southeasterly, helping to nudge moisture further west over the lower deserts. This, along with the previously mentioned low-level moisture increase, has caused a shift in the forecast compared to 24 hours ago, with the NBM now indicating the better potential for rainfall over South- central Arizona will now be on Wednesday as opposed to Thursday. Though the moisture may still be in place, with the low now significantly weaker, and the aforementioned jet streak migrating further north, the synoptic setup becomes much less favorable to see rainfall outside of the typical enhanced terrain areas. One wildcard though is the projected presence of a 700mb wave moving up out of Northern Mexico Wednesday morning. Current forecast tracks have this kink progressing near the Arizona/New Mexico border, which would likely provide better support for rainfall once again for areas further east of here. With that being said, any subtle changes to trough evolution could yield shower and thunderstorms for lower desert areas, which is likely why 20-30% PoPs are indicated for Phoenix as the rainfall potential cannot be completely ignored.

Even will that, temperatures have not been mentioned. Highs across the region will not vary too much compared to what we have seen the past few days with readings running near to slightly above normal. Lower deserts can expected MaxTs around 93-102 degrees while higher elevation locations will experience temps closer to the lower 80s to lower 90s.

LONG TERM /Thursday through Monday/

Heading into the back half of the week, the influence from the previously-mentioned low will continue to diminish, but sufficient- enough moisture for further rain chances will remain in place, at least through Thursday. NBM/WPC continue to paint Eastern an Northern Arizona in decent PoPs (30-60%), including our portion of Gila County, thanks to orographic influences, but synoptic support will be all but gone by this point, so desert chances drop back to generally below 20%, but not all the way zero. PWATs 100-150% of normal are likely why non-zero PoPs are shown for the lower elevations, but unless you find yourself around enhanced terrain features such as the Kofas or Table Top Wilderness, another rainfall should not be counted on at this point. Gradual drying begin on Saturday, but it may not remain that way for too long as we may start to see fringe moisture associated with Hurricane Polo by as early as the weekend. Forecast confidence regarding where this tropical cyclone will progress in the longer term is low, and there will likely be many forecast changes regarding it in the coming day, but its potential influence will be monitored regardless over through the remainder of the week.

In term of temperatures, much like during the short term, not much fluctuation in day-to-day readings is expected to occur. Daily values will hover right around to just above normal, which translates to lower desert readings ranging between 94-105 degrees through the weekend. Overnight lows will be slightly more elevated compared to where they typically are for late September, likely an effect of lingering moisture and cloud cover (at least for some spots), with readings between 70-82 degrees. Meanwhile higher elevation conditions will be quite comfortable as most locations should be in the 80s, with perhaps a few spots touching the lower 90s. Lows for these spots in the upper 50s to lower 70s will be common.

AVIATION

Updated at 1733Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Primary aviation impact will be the potential for VCSH/VCTS this afternoon/evening, and gusty outflows. A westerly shift this afternoon looks unlikely with a brief period of S-SSW cross winds before shifting back out out the SE for the remainder of the TAF period. Under 10% chances remains for SHRA/TSRA activity to develop in the metro, with activity expected to remain the the south and east of the terminals. The the main SE'rely outflow from the previous package has been shifted back a few hours now expected just past midnight tonight, however a few gusty periods may still develop this evening ahead of the main outflow. Gusts will likely be near 20kts with low confidence of BLDU or reduced VIS. FEW-SCT clouds will be common through the afternoon with lowest bases around 8-10 kft AGL. There will also be increasing high clouds (around 25 kft) through the period and should become BKN by this evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation impacts are anticipated through the next 24 hours with mostly clear skies. Winds will favor W/NW at both KIPL and KBLH, with wind speeds mostly staying below 10 kts and periods of light and variable winds.

FIRE WEATHER

Near to slightly above normal temperatures will prevail over the next several days, with minimal day-to-day fluctuations. Daily rain chances return for the high terrain through the end of the workweek with lower desert chances focused on Wednesday. MinRHs between 15-30% will be common across the region through the start of the weekend, with even higher readings (35-50%) for the far eastern districts. MaxRHs will range from fair to good over the western and central districts, to good and excellent for the eastern districts into the weekend. Drier air by the weekend will lower RHs slightly, but they should remain above critical fire weather thresholds. Outside of any thunderstorm related outflows, winds will follow familiar diurnal trends with typical terrain induced afternoon gusts around 15-25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.