textproduct: NWS Phoenix

KEY MESSAGES

- Hot temperatures continue this week resulting in areas of major Heat Risk and Extreme Heat Warnings through Thursday across lower desert locations.

- Isolated thunderstorms over higher terrain areas of eastern Arizona will send gusty outflow winds towards lower elevations of south-central Arizona the next couple days.

- The weather pattern will become very favorable for moisture import and more widespread thunderstorm activity next week.

SHORT TERM /Today through Thursday/

Early afternoon objective analysis depicts a sprawling subtropical ridge over the SW Conus with one particular anti-cyclone center directly over central Arizona. H5 heights currently near 594dm will increase slightly to near 596dm Wednesday as the core of the high pressure system continues to retrograde towards the California coast. Model agreement on this evolution is excellent yielding very narrow spread in numerical guidance, and resulting in temperatures solidly 5F-10F above normal the next few days. Extreme Heat Warnings remain in effect despite hovering below daily records and only coincident with patchy major HeatRisk owing to the fact this episode is occurring during the peak of climatological temperatures. A gradual increase in boundary layer moisture will also restrict the amount of nocturnal cooling such that even morning lows only retreat into the 80s/lower 90s.

While the direct passage of strong subtropical ridging would typically result in widespread subsidence, this will not entirely be the case the next few days. Upper level support in the form of jet level divergence will encourage deep convective updrafts over mountainous areas of eastern Arizona as low level 7-9 g/kg mixing ratios will be sufficient for scattered thunderstorm development. Sitting under the ridge axis and given the limited moisture availability, storms may be more isolated and short-lived this afternoon/evening, however DCape values in excess of 1500 J/kg will support locally strong, gusty winds and semi-organized outflow boundaries attempting to propagate westward.

It's unlikely strong, gusty winds reach into higher population centers this evening, however HREF members indicate more aggressive storm development Wednesday with better organized outflows capable of longer duration propagation. There is also a distinct possibility of multiple organized outflow boundaries Wednesday evening, and it's conceivable boundary collisions could spark additional deep convection, though low level mixing ratios below 10 g/kg would certainly limit this possibility over lower elevations and high resolution modeling only partially alludes to this outcome. Regardless, some gusts 25-35 mph and localized blowing dust appears more likely tomorrow versus today possibly skirting the southern and eastern parts of the Phoeinx metro. Thunderstorm activity may revert back to an isolated mountain event Thursday as the northern stream jet core shifts south in the Great Basin suppressing the more favorable divergence aloft into northern Mexico/SE Arizona.

LONG TERM /Friday through Monday/

Above normal temperatures with at least high-end moderate HeatRisk is forecast for Friday, before gradual cooling and eventually a big uptick in monsoon storm activity arrives over the weekend into early next week. Forecast highs Friday are still ranging from 110-114 degrees across the western lower deserts, while lowering heights over central and eastern Arizona should allow for highs to lower to 108-112 degrees. Low level moisture is also shown to start increasing again as early as Friday with moisture streaming off the Gulf of California. However, drier air aloft will dilute some of this low level moisture increase and it's likely to take a couple days for moisture and instability to increase enough for storm potential to ramp back up. Shower and storm chances are currently expected to pick up on Saturday across at least southeast Arizona with some potential for decaying activity into the south-central Arizona lower deserts Saturday evening/night.

Starting Sunday or Monday, models are really setting up the region to become quite active as there is good agreement shifting the high center somewhere to our northeast. Guidance is favoring a very large ridge with record or near record H5 heights setting up over the Northern Plains by Sunday. The ridge is expected to be very expansive stretching as far west as the Four Corners area and as far east as the Ohio River Valley. As of right now, the large scale flow is likely to become blocked with this ridge lasting through most, if not all of next week.

The expected large expanse and strength of this ridge should be quite beneficial for increasing moisture and storm chances over much of the Desert Southwest next week. Although uncertainty remains, especially with how much moisture and with any potential disturbances that are likely to move over or near our region, we are anticipating our first decent period of monsoon activity next week. It is too early to speculate on which days will be the most active, but NBM/WPC PoPs for Sunday are already as high as 30-40% over eastern and south-central Arizona. Once the monsoon activity kicks into gear, temperatures should also begin to dip closer to seasonal normals.

AVIATION

Updated at 1740Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No significant aviation weather concerns are anticipated through the TAF period. Winds will continue to increase out of the west heading into this afternoon with a few gusts in the teens. Confidence in winds decreases this evening as guidance indicate an outflow boundary approaching the terminals from the E-SE. Confidence is low in the magnitude of this outflow and how far west it makes it into the Valley. For now, have reflected a TEMPO light easterly switch at the terminals with any stronger winds expected at KIWA. The best timing for this outflow to arrive to the terminals would be between 03Z-06Z. Rain chances during this time will be very low (<5%). Mostly clear skies with the exception of a FEW mid to high level clouds can be expected throughout the period.

Southeast California/Southwest Arizona including KIPL and KBLH: Some minor gusty winds will be the main aviation weather issue throughout the TAF period under clear skies. Winds at KIPL will favor the SE through the afternoon before switching around to the W this evening. At KBLH, winds will fluctuate between the SSE to SW. Overall sustained speeds will remain aob 12 kts, with some occasional afternoon/evening gusts near 20 kts.

FIRE WEATHER

Gusty and erratic thunderstorm winds with isolated lightning over the far eastern districts the next few afternoons will be the main fire weather concern. Outside of any potential outflows, winds should favor light and diurnal trends with typical afternoon breeziness around 20 mph. Afternoon RH values between 10-15% will be common across the region. Overnight recoveries will offer only limited to modest relief as MaxRH values range generally between 20- 45%. Temperatures are expected to run well above normal through the middle of the week, with the lowest elevations reaching between 110- 115 degrees. A decrease in monsoon activity is likely late week, before picking up again and becoming more widespread by early next week. Moisture levels and humidities should also improve considerably by early next week, alleviating much of the fire weather concerns.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Thursday for AZZ530>534- 536>551-553>555-559-560-562.

CA...Extreme Heat Warning until 8 PM PDT Thursday for CAZ562-564>570.