textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Temperatures around five degrees above normal will result in widespread Moderate Heat Risk and localized Major Heat Risk through Thursday.

- Extreme Heat Warnings will remain in effect for portions of the area through Thursday.

- Drying conditions and a regional increase in winds late this week will result in areas of critical fire weather, and so Fire Weather Watches have been issued for the Lower Colorado River Valley and Tonto National Forest on Friday and Saturday.

SHORT TERM /TODAY THROUGH THURSDAY/

A strong upper level high remains positioned across the region with the center situated across far southwest New Mexico. Despite the ridge being quite strong (near climatological record H5 heights) just to our southeast, plenty of mid and upper level moisture has entrained into the system over the past couple of days. Weak upper level shortwaves continue to flow around the western fringes of the ridge passing through California, supported by a persistent upper level jet situated from off the coast of California into the Great Basin. Support from the upper level jet will also continue to bring isolated 10% chances for showers (mostly virga) and maybe a few rumbles of thunder today into tonight.

The near record H5 heights of 596-598dm could support near record temperatures, but we will fall well short over the next couple of days due to marginally increased boundary layer moisture and fairly abundant mid and high clouds. NBM forecast temperatures for today and Thursday have struggled a bit over the past several runs, initially discounting the potential impacts of the cloud cover and showing potential for temperatures near 115 degrees. The increased moisture and clouds today are expected to keep daytime highs more in a 108-112 degree range, or 5-7 degrees above normal, but very warm overnight lows well into the 80s for many spots will make for little relief from the heat. Extreme Heat Warnings are in effect for today through Thursday evening, focused over urban locations. The ridge is expected to begin to weaken later today into Thursday, but highs Thursday will essentially mirror today's readings. Drier air will also begin to filter in from the west on Thursday resulting in gradually clearing skies through the first part of the day.

LONG TERM /FRIDAY THROUGH TUESDAY/

Forecast concern will turn away from the Extreme Heat and toward increasing fire danger as winds increase and much drier conditions are expected by Friday. A large Pacific trough is slated to gradually shift southeastward down the West Coast Thursday into Friday, quickly pushing the subtropical high to the south of the region. Persistent height falls from the incoming trough are forecast on Friday and Saturday while a strong gradient sets up across the Southwestern U.S. Winds will initially become breezy as early as Thursday, but pick up even more on Friday into Saturday. Guidance shows unseasonably strong gradient winds setting up on Friday and lasting through at least Saturday and potentially even into Sunday. Afternoon wind gusts of 20-30 mph should impact the entire area on Friday and Saturday with locally higher gusts of 30-40 mph. Gradual drying of the boundary layer will also lower daytime humidities to between 5-15% by Friday. The combination of the low humidities and the strong gusty winds are likely to make for dangerous fire weather conditions starting Friday. A Fire Weather Watch is currently in effect for the Lower CO River Valley and over the Arizona high terrain from Friday morning through Saturday evening.

Although H5 heights decrease to below 590dm on Friday, temperatures will be a bit slow to respond as highs will still mostly top out in a 106-110 degree range. As the trough gets closer and heights continue to fall through the weekend, temperatures are expected to drop into the normal range as early as Saturday and potentially even 4-7 degrees below normal starting Sunday. The latest NBM/WPC forecast highs shows readings only in a 99-103 degree range across southeast California to 100-105 degrees in the Phoenix area for Sunday-Tuesday.

AVIATION

Updated at 1720Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The only weather issues through the TAF period will be occasionally gusty west winds and the potential for virga showers and very low chances of VCSH/SHRA conditions at the terminals, primarily during the overnight period into Thursday morning, under FEW-SCT and at times BKN midlevel decks above 10 kft AGL. The bulk of clouds and radar echoes have remained well west of the terminals this morning. Isolated SHRA and a few strikes of lightning have been observed over western Maricopa County and will continue for the next few hours, remaining generally west of the airspace. Winds will favor westerly directions through at least this evening, with afternoon/early evening gusts to 15-20 kts. A typical nocturnal E/SE wind shift is anticipated tonight.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours under SCT-BKN mid-level cloud cover. Light southeast winds at KIPL will shift W'rly late this afternoon with gusts to 20-25 kts developing during the evening. At KBLH, S'rly winds will persist, tilting more to the SW during the afternoon and evening. A period of higher gusts around 20 kts at KBLH is possible late afternoon into the evening.

FIRE WEATHER

Locally elevated fire weather conditions due to low humidity and afternoon upslope/upvalley breeziness will continue through Thursday. Temperatures will be above normal, with afternoon highs near or slightly above 110 degrees across the lower deserts. MinRHs will be around 15% areawide today and decrease into the single digits across the western districts starting Thursday. Winds will generally remain terrain- driven, with upslope/upvalley gusts during the latter half of the afternoon into the early evening peaking around 20 mph today and 25 mph Thursday. Areas of critical fire weather conditions are likely by Friday and into this weekend as southwesterly winds increase across the region. A Fire Weather Watch has been issued for the Lower Colorado River Valley and Tonto NF/Southern Gila County zones Friday-Saturday.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Thursday for AZZ530-532-533- 537-540-542>544-546-548-550-551.

Fire Weather Watch from Friday morning through Saturday evening for AZZ131-133.

Extreme Heat Warning until 8 PM MST this evening for AZZ560.

CA...Extreme Heat Warning until 8 PM PDT Thursday for CAZ562-566-567- 569.

Fire Weather Watch from Friday morning through Saturday evening for CAZ231.