textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- High pressure will center itself over Arizona and strengthen late this week resulting in record to near record temperatures and widespread major to extreme Heat Risk.

- Extreme Heat Warnings have been expanded to cover the entire forecast area starting Thursday, and continuing through the weekend.

- Drier air filtering into the region will limit any chances for showers and thunderstorms with only isolated coverage over far eastern Arizona high terrain.

SHORT TERM /TODAY THROUGH FRIDAY/

Weak troughing entering the Great Basin has displaced the monsoon high off to our east with the center of the anti-cyclonic rotation now over New Mexico. Deep southerly flow remains in place for much of Arizona, keeping moisture levels relatively elevated as PWATs continue to hover around 1.2-1.6", according to mesoanalysis, across our lower desert areas. However, it does not appear that the moisture profile will be conducive enough to spark much, if any, shower and thunderstorm activity, even for the mountainous areas of Gila County. The entire state will not be completely deprived of rainfall today though as orographically induced convection is expected for parts of the Flagstaff and Tucson CWAs. Convective activity will diminish noticeably during the back half of the week across most of Arizona, with the exception being the White Mountains, as drier air spreads over the region and subsidence increases as the sub-tropical high shifts to the west.

Due to the lack of monsoon activity, the main forecast concern during the remainder of the workweek will be the building heat and the introduction of widespread Major, to locally Extreme, HeatRisk. Temperatures this afternoon will not be too out of the ordinary with lower desert readings hovering around 108-113 degrees, but we add about two degrees each day through Friday as the previously mentioned high strengthens to around 598-600dm during this timeframe, which will be near climatological records for this time of year. With dryer air in place as well, heating will be more efficient, allowing for temperatures, to near, and even exceed, 115 degrees for many locations. The highest values the NBM shows during the short term is around 116 degrees by Friday, but given how relatively dry it will be, and how strong the high will become, it seems that those readings might be a bit underdone. It would not be surprising that during the next few subsequent forecasts the NBM values climb a few degrees, with some areas, mainly the deserts of SE CA and far SW AZ, potentially even seeing MaxT projections nearing 120 degrees.

LONG TERM /SATURDAY THROUGH TUESDAY/

Dangerous heat will impact the region over the weekend and potentially extending into early next week. The peak strength of the upper level ridge is forecast to occur on Friday and Saturday before weakening Sunday into Monday. Even though guidance is forecasting record H5 heights on Friday and Saturday, forecast highs are barely reaching daily records. This is either due to the modest low level moisture remaining in place holding down daytime temperatures a bit, or the NBM is under forecasting temperatures. We are leaning toward the NBM deterministic forecast temperatures not being hot enough, but not enough to warrant nudging the forecast upward. It would not be surprising to have actual highs closer to the 75-90th percentile of guidance, or a couple degrees higher than the current forecast. We are likely to have some areas reaching 120 degrees across the lower deserts of southeast California on Saturday and Sunday, while the Phoenix area could conceivably reach 118 or even 119 degrees. Overnight lows will also provide little relief from the extreme heat with readings mostly in the upper 80s to the lower 90s to as high as the mid 90s in the urban core of the Phoenix metro.

Monsoon activity is expected to remain very limited on Saturday with little to no shot at any outflows nearing the lower deserts, but that may change by Sunday or Monday. As the ridge begins to weaken Sunday into Monday, guidance shows evidence of increasing low level moisture and more monsoon activity across eastern Arizona and northern Mexico. However, chances still remain fairly low on Sunday and Monday with high terrain PoPs of 20-30% and 10-15% into the Phoenix area during the evening hours. As long as the ridge continues to weaken early next week, monsoon convection should also increase, especially by the middle of next week as there are hints of a potential easterly wave moving into or near Arizona. As the ridge weakens and moisture increases, temperatures are likely to drift lower, but another day or two of Major to Extreme HeatRisk looks possible for Monday and/or Tuesday, especially across southeast California and southwest Arizona.

AVIATION

Updated at 2300Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major weather issues will exist through Thursday evening under occasional mid/high cloud decks. Winds should exhibit similar behavior to the past 24 hours with substantial uncertainty whether wind complete an easterly switch overnight. Some guidance shows long traveling, weak outflow boundaries forcing an east switch by sunrise, but this is a lower probability outcome. Instead, a variable component may briefly take hold before quickly reverting to west by late morning. Any afternoon gustiness will be limited in time and space.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather concerns will exist through Thursday evening under mostly clear skies. Trends in wind speeds and directions should be very similar to the past 24 hours featuring a preference towards S/SE directions and modest afternoon/early evening gusts 15-20kt. There is uncertainty when, and how long a SW wind direction could affect KIPL later in the evening.

FIRE WEATHER

Strong high pressure will gradually build over the region through the rest of the week, while drier air also moves in from the southwest. These two factors will severely limit monsoon activity as very little activity is expected through at least Friday. Expect humidities to gradually lower over the next couple of days with MinRHs settling at 10-15% by Friday. Record to near record temperatures are also forecast for Friday and through the upcoming weekend. Winds will mostly follow diurnal patterns, but west southwesterly winds will be dominant much of the time with afternoon gusts of 15-20 mph at times.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Sunday for AZZ530-532-537- 540-542>544-546-548-550-551.

Extreme Heat Warning from 10 AM Thursday to 8 PM MST Sunday for AZZ531-533>536-538-539-541-545-547-549-552>563.

CA...Extreme Heat Warning until 8 PM PDT Sunday for CAZ567-569-570.

Extreme Heat Warning from 10 AM Thursday to 8 PM PDT Sunday for CAZ560>566-568.