textproduct: NWS Phoenix

KEY MESSAGES

- Extreme Heat Warnings remain in effect for much of the region through this evening and portions of the Lower Colorado River valley and Imperial Valley on Monday due to widespread moderate to locally Major HeatRisk.

- Shower and thunderstorm activity will once again develop across the higher terrain of southcentral and southeast Arizona later this afternoon, which will bring strong outflows into the lower deserts and potentially result in additional storms this evening.

- Deeper moisture will provide a better opportunity for more widespread shower and thunderstorm activity during the first half of this week, along with noticeably cooler temperatures.

SHORT TERM /Today through Monday/

Latest GOES mid-lvl visible satellite imagery reveals mostly clear skies over southcentral AZ with pockets of cu beginning to percolate over the higher terrain areas including the White Mountains, Mogollon Rim, and mountains of southeast AZ. Large anti-cyclonic flow is apparent over the 4-Corner region where high pressure is currently situated. Similar to the past several days, storms are expected to develop over the high terrain this afternoon and attempt to descend into the lower deserts this evening. Atmospheric conditions for additional storm development are still not optimal as indicated by a large area of ML CIN (-80 to -100 J/kg) over Maricopa County. However, there is also much higher DCAPE present (up to 1900 J/kg noted on the 12Z KPHX RAOB). This could promote a stronger and longer lived outflow boundary this evening which is likely to sweep across Maricopa and Pinal County and could produce isolated to widely scattered convection in its wake. The main concerns with any thunderstorm related outflow will be gusty winds up to 30-40 mph along with areas of blowing dust, especially in the most dust prone areas of northern Pinal and western Maricopa County. Due to the higher DCAPE values, any storm that develops across the lower deserts could also produce marginally severe wind gusts up to 60 mph. Storm activity should wane by midnight with much calmer conditions anticipated overnight.

Environmental conditions for thunderstorm activity on Monday will drastically improve across our forecast area. The subtropical high will remain positioned near the Four Corners Region, with the addition of increasing mid-lvl flow around 30 kts. Moisture advection will also improve with PWAT values increasing to 1.5-1.8" along with low-level mixing ratios exceeding 11 g/kg. This will allow for higher instability and weaker convective inhibition across the lower deserts. Short-term models also indicate an increase in upper-level divergence as a weak easterly wave moves into northern Mexico, which will likely aid in higher thunderstorm coverage, especially across southeastern AZ. High-res guidance does show multiple outflow converging somewhere in Maricopa County by Monday evening/Monday night which would result in additional storm development. Strong downburst wind gusts, locally heavy rainfall, as well as areas of blowing dust in the typical dust prone areas will all be likely. NBM PoPS are still hovering around 40-50% across much of southcentral AZ on Monday night (mainly after 00Z) which are some of the better chances we have seen for the monsoon season thus far.

The unseasonably strong subtropical ridge over the 4-corners will continue to promote above normal temperatures through Monday, with afternoon highs peaking around 108-112 degrees this afternoon, resulting in areas of Major HeatRisk. Thus, an Extreme Heat Warning remains in effect for a majority of the forecast area through this evening. Heading into Monday, lowering heights/thickness will allow further cooling, although afternoon highs will still remain a few degrees above the daily normal. Overall, HeatRisk will fall to the Moderate category on Monday afternoon, however heat index values will still be very high (up to 115-118 degrees) across portions of southeast CA and the Lower Colorado River Valley. Therefore, a continuation of the Extreme Heat Warning was made for some areas along and west of the Colorado River including Yuma, the Imperial Valley, and Salton sea through Monday evening.

LONG TERM /Tuesday through Saturday/

Conditions will remain favorable through the middle of the week for additional monsoonal activity. Depending on how active Monday turns out, Tuesday could actually be more of a down day given a worked over atmosphere and thus even though there is still potential for storm activity, especially across the higher terrain areas, NBM PoPs for Tuesday are lower, mostly ranging between 30-50%. Another more active period appears to be in the cards for late Wednesday into the first half of Thursday. The flow by Wednesday is forecast to shift out of the south as the subtropical high shifts into Texas and a upper-level low moves off the coast of California. This southerly flow is likely to bring additional moisture northward from Mexico, with projected PWATs increasing to 1.7-2.0" and low-level mixing ratios increasing to 12-13 g/kg. In addition, there will likely more favorable upper-level forcing as both the GFS and ECMWF do indicate an upper-level wave moving into the region from Mexico, which will likely enhance the convective activity with flash flooding becoming more of a threat given the deeper moisture profiles that will be in place. The latest NBM PoPs for late Wednesday through the first part of Thursday currently range between 50-60% across much of south- central and southeast AZ.

Heading towards the end of the week and next weekend, the guidance is in good agreement in showing the subtropical high shifting towards the Gulf Coast Region while the aforementioned upper- level low remains situated off the west coast. This pattern configuration will cause the flow to shift more out of the southwest, bringing in drier air as PWATs drop under 1.2-1.3". As a result, an overall downtrend in storm activity is likely.

With the lowering heights aloft and as well as an increase in convective activity and cloud cover, temperatures throughout the upcoming work week are expected to gradually cool with afternoon highs expected to fall to near seasonal levels by Tuesday and potentially a few degrees below normal by the latter half of the week. Temperatures do eventually trend slightly back up again by next weekend, but to near seasonal levels with no extreme heat in sight.

AVIATION

Updated at 1801Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main weather concern will be strong outflow winds, blowing dust, and VCTS/TS at the terminals. There is good confidence that one or multiple outflow boundaries will move through the terminals this evening, with wind gusts up to 20-30 kts anticipated and around 50% chance for peak gusts greater than 30 kts. The outflow winds will be capable of bringing a wall of dust through the metro. There will also be slight chances (20%) for VCTS/TS. Timing of activity tonight is favored between 02-06Z, although since confidence remains low for TS possibilities a PROB30 remains in the latest TAF package. Cloud bases during thunderstorm activity are only expected to be as low at 10Kft, otherwise FEW-SCT high clouds will persist through the period.

Before this evening, winds should generally be light (AOB 10 kts), with KPHX and KIWA remaining westerly and KDVT and KSDL remaining VRB. Confidence in wind direction is then very low due to the potential outflows moving through this evening, however, southerly directions look preferred, with a tendency towards southeast late evening into the overnight hours.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns are expected through the next 24 hours under periods of FEW-SCT mid to high cloud decks and otherwise mostly clear skies. At KIPL, winds will primarily stay southeasterly. At KBLH, winds will generally fluctuate between SE and SSW. Overall, wind speeds should generally remain AOB 12 kts. A distant thunderstorm outflow reaching KBLH this evening cannot be ruled out, which would result in an abrupt wind shift and potentially some VIS restrictions due to lofted/blowing dust.

FIRE WEATHER

Above normal temperatures with afternoon highs reaching 108-112 degrees can be expected across the lower deserts today, before gradual cooling to more seasonal levels takes place during the upcoming week. Elevated moisture will maintain afternoon MinRHs above 20% each afternoon along with good to excellent overnight recoveries. Shower and thunderstorm activity will again remain isolated today and tonight, but coverage is expected to become more widespread in through the first half of the week. The chances of wetting rainfall will also increase significantly, especially across the eastern districts. Outside of gusty and erratic winds due to thunderstorms, winds will follow the familiar diurnal trends, with typical afternoon upslope breeziness around 20 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Monday for AZZ530-532.

Extreme Heat Warning until 8 PM MST this evening for AZZ531- 533>544-546-548>551-553>555-559.

CA...Extreme Heat Warning until 8 PM PDT Monday for CAZ563-566-567- 569-570.

Extreme Heat Warning until 8 PM PDT this evening for CAZ560>562- 564-565-568.