textproduct: NWS Phoenix

KEY MESSAGES

- A fairly typical Monsoon pattern will continue through the first half of the week, with early afternoon showers and thunderstorms forming over the Arizona high terrain daily followed by chances over the lower deserts focused later in the afternoon into the evening.

- A disturbance may approach the region from the east during the latter half of the week, bringing favorable conditions for more widespread showers and thunderstorms.

- Seasonably hot temperatures will persist through the middle of the week resulting in widespread Moderate Heat Risk and locally Major Heat Risk on Wednesday in Southeast California, followed by cooler temperatures late week.

SHORT TERM /TODAY THROUGH TUESDAY/

Upper level objective analysis depicts an anticyclonically curved polar jet stretching from the Pacific Northwest into South-Central Canada, allowing a center of strongly positive midlevel height anomalies to migrate from the Intermountain West to as far northeast as the Northern Plains early this week. At this hour, the forecast area remains under weakening southeasterly flow along the periphery of the broad midlevel anticyclone, which has aided to import abundant moisture into the region and carry storms off Eastern AZ high terrain and into the lower elevations during the past couple afternoons/evenings.

Abundant moisture will remain in place across the forecast area with ensemble mean PWATs hovering between 1.5-1.8" through the first half of the week and close to 2" at times near the Gulf of California and northward towards the Lower Colorado River Valley. These typical moisture values for the time of year are also fairly well distributed throughout the column, with 10-12 g/kg near surface mixing ratios being maintained throughout the week, supportive of deep moist convection even over Arizona lower deserts. However, the gradually weakening southeasterly flow aloft will somewhat inhibit the potential for stronger clusters of thunderstorms to develop the next few days. As such, anticipate a familiar convective pattern to take shape the next few days, with early afternoon thunderstorms firing over Arizona higher terrain areas, followed by conditional slight (10-35%) chances for thunderstorms moving over or forming in the South-Central AZ lower deserts later in the afternoon and into the evenings. The coverage of storms will vary day to day based on subtle disturbances in the flow, subtle moisture differences, and prior convective activity - either from the prior day or earlier that same day.

This afternoon/evening, instability measures seem somewhat diminished from that of Sunday, perhaps due to less favorable midlevel lapse rates (GFS bufr soundings for PHX showing 700-500 mb lapse rates around 6 C/km compares to the ~7C/km on Sunday). HREF mean MUCAPES are mostly between 500-1000 J/kg this afternoon/evening in South-Central AZ, and model soundings indicate MLCAPE values peaking around 1100 J/kg in the early evening for Phoenix. With these values, it is not out of the question to see strong storms and perhaps an isolated marginally severe storm or two, especially with inverted-V soundings resulting in DCAPEs peaking around 1000-1500 J/kg. And steering flow still elevated to around 15-20 kts will be capable of carrying clusters of storms off higher terrain and into the lower deserts downstream. However, HREF membership does not show a consensus on an organized outflow forming and triggering new convection over the lower deserts or where the greatest coverage of storms will be. Neighborhood probabilities for 35+ mph gusts near storms are lower than that of Sunday but still around 50-70%, focused along a corridor from Phoenix south/southeastward to the international border. Activity will then likely trend into Southwest AZ (La Paz County) heading into the mid-late evening, though again, this is highly conditional on upstream activity and if outflows from Central AZ high terrain storms can trigger additional cells.

Tuesday, thermodynamics may be slightly more favorable for strong storms, however, flow aloft weakens further and directions become complex as weak anticyclonic and cyclonic circulations begin to impinge on the region, creating a deformation area. The threat for lower desert storms will be even more conditional on lifting mechanisms from earlier storms or subtle lift from the deformation zones draped across the region.

In terms of temperatures, afternoon highs will increase from a 102F- 107F range today into a 105F-110F range. Tuesday Plentiful moisture (by desert standards) will make insolation less efficient at heating us up, but in turn, the increased humidity will make it feel hotter, with heat indices climbing toward 110F-117F degrees, especially on Tuesday. Moderate HeatRisk will remain widespread through the next few days, so if plans take you outside, be sure to exercise heat precautions.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

There is still excellent agreement within the EPS and GEFS members regarding the placement of the upper high over the northern Plains and Upper Midwest through midweek before retrograding over the central Rockies by the latter half of the week. The overall position of the 500 mb high will maintain deep easterly flow across the Desert Southwest, helping enhance moisture transport. Latest EPS and GEFS continue to show PWATs hovering around 1.5"-1.8" through the entire week with low-level mixing ratios remaining between 10-12 g/kg. Therefore, conditions will continue to remain conducive for daily thunderstorm activity with variations in the overall coverage. Both the mean of the EPS and GEFS show an uptick in QPF across the region by the end of this week due to an easterly wave/inverted trough approaching the area by Thursday-Friday. If this occurs, this troughing feature will likely enhance convective activity potentially introduce a risk for heavy rainfall across portions of AZ. This will have to be monitored closely in the coming days.

Temperatures are expected to peak Wednesday mostly between 105F-110F across the Arizona lower deserts, but some areas of Major HeatRisk enter the picture for portions of Southeast CA and along the Lower Colorado River Valley, where latest NBM forecast shows highs as high as 113F and lows in the middle to upper 80s thanks to the inefficient overnight radiative cooling from higher humidity and likely some mid-high cloud cover. The NBM continues to indicate temperatures cooling to well below normal by the end of the week with highs falling into the 90s to around 100 degrees across the lower deserts by Friday. This is likely due to the increased signal for thicker cloud cover and the potential for more widespread rainfall.

AVIATION

Updated at 0556Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Lingering regional showers and potential for additional isolated high-based showers and an embedded thunderstorm moving into the Phoenix area through the overnight hours has resulted in a low confidence wind forecast. An easterly shift is favored by or shortly after 07Z, followed by an early SW shift by noon Monday. Models indicate an earlier storm initiation time Monday afternoon, as early as 19-20Z, in the mountain just east of Phoenix, which may lead to an earlier outflow progression into the metro area by 23-00Z. VCTS/TSRA chances remain low, around 20-30%. Overall thunderstorm impacts are expected to be less than Sunday afternoon. Hazy skies from lofted dust may linger through Monday morning and degrade slantwise visibility. Cloud bases will mostly stay above 8-10K ft AGL.

Southeast California/Southwest Arizona including KIPL and KBLH: Winds at KIPL will favor a SE'rly component through the TAF period with winds generally between 5-10 kts. KBLH will maintain generally a southerly flow with potential for a period of gusty conditions tonight from a distant thunderstorm outflow boundary arriving from the east. Speeds otherwise will hover around 5-10 kts. There is support for an area of high-based showers with an embedded storm to progress across portions of southeast CA Monday morning. Best timing looks to be between 12-20Z. This may lead to some gusty erratic winds. SCT-BKN clouds, with bases mostly above 10K ft AGL will prevail through most of the TAF period.

FIRE WEATHER

Seasonably hot temperatures will prevail through the next several days as readings will be near to slightly above normal. Daily thunderstorm activity will be possible through this week with the potential for more widespread wetting rainfall increasing across the eastern districts later in the week. Due to increasing boundary layer moisture, afternoon humidity levels will remain in a 20-30% range which will keep any fire weather concerns at a minimum. Overnight recoveries will be fair to good, or between 40-70%. Outside of any thunderstorm driven outflow, winds should favor diurnal trends with the typical afternoon breeziness around 20-25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

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