textproduct: NWS Phoenix

KEY MESSAGES

- Very hot temperatures are expected through this week, resulting in widespread Moderate Heat Risk and areas of Major Heat Risk through at least Thursday.

- Due to the expected Major Heat Risk, an Extreme Heat Warning has been posted for the lower deserts of south-central and southwest Arizona, and southeast California, through Thursday.

- Outside of an isolated chance of a shower or thunderstorm over higher terrain areas of eastern Arizona, dry conditions will prevail for the majority of our forecast area through Friday.

SHORT TERM /Today through Thursday/

The main forecast concern over the next few days will be the building heat creating areas of Major HeatRisk. Extreme Heat Warnings go into effect later this morning for the bulk of the area and last through Thursday. A fairly strong subtropical ridge is currently shifting westward through the region with H5 heights of 592-594dm. The ridge is forecast to strengthen into tonight reaching H5 heights of 594-596dm, before the ridge center shifts west of the area on Wednesday. The building heights and warming aloft will allow for daytime highs reaching between 110-114 degrees today before peaking Wednesday and/or Thursday between 111-116 degrees. Low level moisture has also increased enough to curtail efficient nocturnal cooling resulting in very warm overnight temperatures over the next few days. Expect lows mostly in the 80s for rural lower desert locations to as warm as the low 90s in central Phoenix.

Although low level moisture has increased with surface dew points anywhere from the mid 40s to the mid 50s, moisture within the rest of the boundary layer remains on the lower side. Subsidence under the ridge will be a limiting factor for potential convection over the next few days, but it won't stop it from developing over the eastern Arizona high terrain. Hi-res CAMs show scattered showers and storms developing both this afternoon and Wednesday afternoon, but limited CAPE and very weak steering flow should keep any storms on the weaker side and short-lived. However, strong gusty winds of 30-40 mph should be possible under any thunderstorms that develop. Guidance also is now suggesting a westward propagating outflow may reach eastern portions of the Phoenix area this evening and maybe again Wednesday evening. This outflow is not expected to be enough to spark off any convection within the lower deserts and wind gusts should mostly stay under 30 mph. Guidance is still favoring a decrease in convective potential by Thursday due to the ridge shifting to our west and slight drying occurring over the region. Subsidence is also expected to increase late week which should further limit any high terrain convective potential.

LONG TERM /Friday through Monday/

Above normal temperatures with at least high-end Moderate HeatRisk is forecast for Friday, before some gradual cooling and then eventually a big uptick on monsoon storm activity is likely 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 for 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 next 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 0900Z.

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

An easterly outflow producing some minor gusty winds early this evening will be the main aviation weather issue throughout the TAF period. In the meantime, easterly winds early this evening will transition out of the west by the mid/later morning with some occasional afternoon gusts into the mid to upper teens. Isolated to widely scattered storms over the higher terrain east of Phoenix is likely to send an easterly outflow into the area terminals early this evening, with peak gusts upwards of 20 kts. 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, especially at KBLH, will be the main aviation weather issue throughout the TAF period under clear skies. At KIPL, southeast winds will prevail through this afternoon before shifting out of the west for a period early this evening. At KBLH, winds will generally fluctuate between the south-southeast to southwest. Overall sustained speeds will remain aob 12 kts, with some occasional afternoon gusts near 20 kts at KBLH.

FIRE WEATHER

Gusty and erratic thunderstorm winds and isolated dry lightning over the far eastern districts the next few afternoons will be the main fire weather concerns. 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 from 10 AM this morning to 8 PM MST Thursday for AZZ530>534-536>551-553>555-559-560-562.

CA...Extreme Heat Warning from 10 AM this morning to 8 PM PDT Thursday for CAZ562-564>570.