textproduct: NWS Phoenix

KEY MESSAGES

- Above normal temperatures will prevail through the rest of the week, with Extreme Heat Warnings in effect for portions of the lower deserts.

- Daily chances for scattered showers and isolated thunderstorms will continue to be focused over the Eastern Arizona high terrain. Any chances for lower deserts should be confined to late Friday and again Sunday into Monday.

- Cooling temperatures over the weekend into early next week should eventually push readings back into the normal range by around next Tuesday.

SHORT TERM /TODAY THROUGH FRIDAY/

Strong high pressure is currently planted over the Southwestern U.S., but moisture levels have improved enough to bring some isolated showers and a few embedded storms over the past several hours. The high center is currently situated over far west-central Arizona resulting in northeasterly mid-level flow, but west southwest lower level flow over much of Arizona. Moisture has increased enough in the lower levels to push mixing ratios to around 10 g/kg over much of the area to upwards of 12 g/kg over southeast Arizona, but a strong lower level subsidence inversion is also in place. Convection was quite extensive during the evening hours across southeast Arizona/southwest New Mexico with this complex sending a low level outflow and mid-level gravity wave east northeastward through the rest of Arizona. Isolated showers have popped off with the passage of these features, tapping into elevated instability. We do not anticipate anything more than very light rainfall amounts with any of these showers with a focus mostly across western Maricopa into La Paz Counties.

For the rest of today, guidance is showing much more limited convection than yesterday, in particular across southeast Arizona. The subsidence inversion is likely to too strong, keeping chances below 10% across our CWA. Guidance does show some convection developing south of the U.S./Mexico border during the evening, but it seems quite unlikely it will send anything up our way like what is currently happening. The peak strength of the ridge occurred late yesterday and this will translate to the hottest temperatures occurring today. The lower deserts of southeast California are expected to top 110 degrees with heat indices up to around 115 degrees, while the Arizona lower deserts should see highs of 107-110 degrees and heat indices closer to 110 degrees during the afternoon hours.

The upper level ridge will continue to weaken today into Friday, likely weakening enough to lower temperatures a good 2-4 degrees across south-central Arizona starting Friday and 1-3 degrees over the western deserts. The Extreme Heat Warnings will continue through Friday for much of the lower deserts of southeast California and southwest Arizona, while expiring everywhere else this evening.

Friday should also bring another influx of moisture from the south southwest, pushing area low level mixing ratios more into a 11-12 g/kg range. However, this may still not be enough to spark off more widespread convection as the the subsidence inversion will still be in place (albeit slightly weaker) and the steering flow is expected to turn out of the southwest. We are expecting better storm coverage across the eastern Arizona high terrain on Friday, but with a southwesterly steering flow it will be hard to get convection to fire over the lower deserts. The NBM does indicate 25-35% PoPs Friday evening/early Friday night from Phoenix eastward, but that seems too optimistic. If storms do actually get going over the south-central Arizona lower deserts Friday evening, they will be within a high DCAPE environment which means strong winds will be a possibility.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Forecast confidence for the weekend into next week remains quite low, especially when looking at moisture levels and rain chances. Guidance does agree the subtropical ridge will get pushed to the east of our region over the weekend as a Pacific trough moves through the northern tier states, but the GEFS is stronger and farther south with the trough compared to the EPS. The models also differ in the amount of moisture and rain chances during the period with the GEFS favoring better rain chances from Phoenix and areas east of Phoenix over the weekend, while the EPS favors better chances by Tuesday into the middle of next week. Needless to say, these differences do not inspire much confidence in the available NBM/WPC PoPs.

The latest forecast places little if any PoPs on Saturday and Saturday night, which seems fairly reasonable, but then seems to be under doing chances late Sunday. The increased PoPs of 20-40% then seem too high for Monday before gradually lowering during the middle part of next week. Lately, guidance has had a tendency to place fairly high PoPs during the 4-8 day range, just to back off as it gets into the short-term. It would seem everything would have to come together nearly perfectly with the trough passing by to our north close enough, but not too close to scour out what moisture we have in the low/mid levels. Additionally, if the subtropical ridge becomes more dominant, then we will have to worry about too strong of a subsidence inversion. Given the fairly high model uncertainty, the forecast is likely to change.

The temperature forecast is also of lower confidence with a fairly wide spread starting Monday. At the moment, NBM forecast temperatures call for readings more in the normal range by early next week, but given the higher model spread we could end up with readings slightly below or even slightly above normal. Guidance at least mostly agrees on the 'coolest' temperatures falling on next Tuesday and Wednesday before trending upward again during the latter half of next week.

AVIATION

Updated at 1000Z.

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

A distant outflow moved through overnight carrying some lofted dust, resulting in temporary reductions in VIS below 10SM. Lingering haze/lofted dust may cause slantwise visibility restrictions around sunrise but will improve through the rest of the morning. Otherwise, no major aviation weather concerns are anticipated through the TAF period under FEW-SCT mid and high cloud decks. Winds will follow fairly typical diurnal tendencies with a westerly shift likely between 17-19Z. Gusts to around 15 kts will be possible during the afternoon, relaxing after sunset. West winds are likely to increase once again late evening into the overnight hours, delaying the typical nocturnal E/SE wind switch.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns are anticipated under periods of FEW-SCT mid and high cloud decks. Winds at KIPL will favor SE through the day and veer out of the southwest this evening. Speeds will generally remain below 10 kts, but there may be a period with gusts to 15-20 kts during the evening. The possibility that a gusty outflow from distant TS to the south may reach the terminal mid to late afternoon cannot be ruled out, but is too low probability for TAQF inclusion. Winds at KBLH will continue to favor S to SW, with speeds generally AOB 12 kts.

FIRE WEATHER

Above normal temperatures will continue into the weekend with modestly elevated humidities. Rain chances will improve by Friday, but still mostly favor the Arizona high terrain. Afternoon minimum humidities of 17-25% today will improve to 25-30% for Friday and Saturday. Overnight recoveries will remain fair to good at 40-70%. Expect fairly typical diurnal winds with little if any breeziness today, but with 20-25 mph afternoon and early evening gusts on Friday and Saturday. Chances for isolated showers and thunderstorms are likely to remain possible across the eastern districts through at least early next week while temperatures eventually fall back into a normal range.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Friday for AZZ530-532-533.

Extreme Heat Warning until 8 PM MST this evening for AZZ531- 535>544-546-548>551-559.

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ562-566-567- 569-570.