textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Temperatures around 3 to 8 degrees above daily normal will result in widespread Moderate Heat Risk with isolated pockets of Major Heat Risk at times, through the middle of the week.

- Chances for showers and thunderstorms remain in the forecast for at least one more day, with the best potential focused over foothill and high terrain locations, though lower desert storms cannot be ruled out.

- An eastern Pacific are of low pressure will help to dry the region out while also providing cooler temperatures during the back half of the week and into the weekend.

SHORT TERM /Today through Tuesday/

Current upper-air analysis reveals a slightly more refined pattern compared to this time 24 hours ago, but multiple different systems remain in close proximity to the Desert Southwest. The interplay between the westward progressing sub-tropical ridge and a weak eastern Pacific trough continues, helping to keep moisture levels around 150-200% of normal for this time of year, which translates to PWATs between 1.0-1.4". This still remains a decent ways off from what we see during the Monsoon Season (which officially begins today), but it has been sufficient enough to spark some isolated to scattered convection across portions of our forecast area. In fact, some folks across eastern portions of our forecast area might have been woken up by some early morning thunderstorm, and isolated cells cannot be ruled out for this area even through sunrise. With this moisture likely not going anywhere for another 24-36 hours, another round of convective activity is expected this afternoon and evening, with perhaps even more coverage for lower desert areas and the Phoenix metro.

Even though moisture levels are not increasing, our rainfall chances are actually a bit higher (20-40%) compared to yesterday's PoPs. CAMs are in agreement with these higher chances as they show better shower and thunderstorm coverage, but why is this the case if moisture is nearly the same? The answer likely lies within the evolution of the upper-level pattern. Guidance points toward increasing flow aloft across central and eastern Arizona, creating mass speed divergence over the region, setting up a favorable environment for updraft maintenance. Orographic lift will likely be the triggering mechanism, but the synoptic lift will allow for storms that do pop to live longer. As flow aloft increase, it will also switch from more westerly to northwesterly, which may also aid in seeing better rainfall coverage across lower desert areas. Potential high terrain outflows will be more aligned with mean flow, which in turn could create a more advantageous setup for these boundaries to descend to the lower elevations depending on where cells initiate. It is highly conditional, but even the Phoenix metro may see some rainfall out of this. Besides gusty outflow winds, the main impacts with potential convection will be occasional lightning and areas of blowing dust, the latter being dependent on if we can get lower desert storms over dust prone areas.

We begin to slowly dry out starting Tuesday, but lingering moisture will still hover close to 150-175% of normal. This slight drop, with the addition of subsidence aloft thanks to greater influence of broad ridging, will tamp down further rain potential in the near term. Chances for parts of our area are not zero, there are CAMs that do hint at isolated activity, but any rainfall may occur will be heavily based on orographic interactions.

In terms of temperatures during this timeframe, it will basically status quo from what we saw this past weekend across the region. Afternoon readings will be around 3-8 degrees above normal for the middle of June, which equates to observations ranging from 105-112 for lower desert areas. Unfortunately overnight lows will not offer much in the way of relief, especially around population centers, as elevated moisture, combined with the Urban Heat Island, will keep readings mostly in the 80s. The NBM has actually been off my several degrees the past few nights at Sky Harbor, where we saw our first 90 degree low of the year yesterday morning, which was 4 degrees above forecasts. The one potential avenue for some cooler than expected readings will come from the above mentioned rainfall/outflows this evening, but that is not a guarantee. Nonetheless, this mix of mild overnight, and hot daytime temperatures will result in widespread Moderate, to even pockets of Major HeatRisk. Proper heat precautions should be used if plans take you outside for extended periods during the next few days.

LONG TERM /Wednesday through Sunday/

The remainder of the week looks generally unexciting, at least for our area, as dry air filters in from Wednesday onward. There could be just enough left over moisture to squeeze out some showers over southeast Arizona or high terrain areas to our north and east, but the majority of activity should be focused over Tucson and Flagstaff CWAs. Something a bit peculiar is the presence of PoPs and QPF for the Phoenix metro on Friday, even though PWATs will be falling closer toward near and below normal for June. Moisture at those levels would likely not even be enough to get high terrain storms, let alone lower desert rainfall. This could be just an artifact of the ingested WPC rainfall forecast that gets adjusted with time, but we would be hard pressed to get rainfall on Friday with dry air rushing in. Who knows, maybe if we can get enough synoptic lift with an approaching eastern Pacific trough to interact with stubborn moisture, some rain chances cannot be completely ruled out, but as of now, I lean more toward the drier solution.

Speaking of that trough, its migration closer to the Desert Southwest by the latter half of the week will help us "cool" off a bit as temperatures fall closer to normal, with perhaps some areas even seeing below normal readings by the weekend. Now, this does not mean we get a break from the triple digits, but at least daily forecasted highs fall closer to 99-108. Not a massive amount of relief, but enough to back regional HeatRisk down to the lower end of the Moderate category. Heat precautions will still need to remain in place regardless. Lower moisture with those cooler temps will also allow for some more comfortable overnight lows in the upper 60s to near 80 degrees. Something that will have to monitored is the potential for elevated fire weather conditions as this feature will increase winds regionwide. Strength of gusts is still uncertain at this time, but with very dry air expected to be in place, it would not take much wind to increase fire danger.

AVIATION

Updated at 1000Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Thunderstorm activity and associated strong winds late Monday afternoon/early evening will be the main aviation weather concerns. Winds will generally follow diurnal tendencies through the early afternoon. Southeast winds across all the terminals will linger for the next several hours, gradually decreasing and eventually shifting out of the W/NW by 18-19Z. During the afternoon, thunderstorm activity developing over the higher terrain areas to the north will likely drift southward and attempt to reach the Phoenix area between late afternoon/early evening. Even though a handful of Hi-Res models show an increase in TS activity, there is high uncertainty in the overall coverage and timing. Therefore, a PROB30 has been maintained for all sites between 15/23Z to 16/03Z. Gusty, erratic winds, likely favoring a northerly component at first, will be the primary hazard. If thunderstorms develop nearby any of the terminals, there is a 50-70% chance of gusts exceeding 35 mph. FEW-SCT mid-level cloud decks will be common with occasional BKN coverage during convective activity.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns will exist through the next 24 hours under FEW mid to high cloud decks. Winds will follow a nearly identical pattern to the last 24 hours, varying between SE during the day and SW/W during the evening at KIPL with some westerly gusts to around 20 kts possible between 03-07Z, and S to SW winds at KBLH generally remaining between 6-12 kts with afternoon gusts to around 20 kts.

FIRE WEATHER

Elevated moisture levels will result in isolated to scattered thunderstorms once again this afternoon and evening, mainly for higher terrain areas, that will be capable of producing gusty and erratic winds and dry lightning. Outside of any thunderstorm outflows, winds will be diurnally driven with the typical afternoon and evening upslope/upvalley breeziness between 15-25 mph. Enhanced moisture will bring some benefit as MinRHs range between 15-20% for the majority of the region, with the exception being SE California where readings will be closer to 10%. For those areas with better moisture, the overnight period will offer at least some modest relief as MaxRH readings rise to 30-50% each night. Those drier spots further west however will only see values increase to around 25%. Moisture scours out during the back half off the week, which will decrease dry thunderstorm potential, but drop RHs closer to 10-15% during the afternoons.

PSR WATCHES/WARNINGS/ADVISORIES

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