textproduct: NWS Phoenix

UPDATE...Updated 00z Aviation Discussion

KEY MESSAGES

- Temperatures much above normal will persist through the weekend with readings approaching near record levels resulting in Extreme Heat Warnings.

- The high pressure system responsible for the extreme heat will also restrict thunderstorm development with chances retreating into the mountains through the end of the week.

- Temperatures should cool slightly, back to near normal to slightly above normal, early next week as increased thunderstorm coverage returns to the region.

SHORT TERM /Today through Saturday/

A broad area of high pressure is encompassing much of the southern US with the ridge axis centered over western CONUS. Additionally, Tropical Storm Bertha remains just off the coast of Louisiana and is expected to make landfall later today. With high pressure building into the region, subsidence is starting to take over along with dryer air moving into the area. PWATs are around 0.2-0.3" lower than they were yesterday (current PWATs of 1.4-1.6" across the area). Despite this, hi-res guidance does show a few isolated showers and storms popping up over the most favored higher terrain locations today and tomorrow during the afternoon and evening hours, but chances remain under 10%. The high pressure will start to migrate more over the Four Corners region this weekend allowing easterly waves into our region prompting an increase in shower and thunderstorm activity across eastern and southeastern AZ. One of the easterly waves that will move through on Saturday will be the remnants of Bertha. This wave in particular looks to remain south of our area and really only influence the activity in SE AZ. Ensemble members only show minimal moisture transport associated with this wave, however, it is expected to at least be a forcing mechanism for activity in SE AZ. While little to no activity is expected across the lower deserts of south-central AZ, models do support more robust outflows from these storms reaching the lower-deserts of south- central AZ which could spark some isolated shower and storm activity.

H5 heights are around 596 dm over AZ this afternoon. Models remain in good agreement that the high pressure system will continue to strengthen, peaking on Friday-Saturday around 597-598 dm. Temperatures early this afternoon have already reached 113 degrees at Phoenix Sky Harbor Airport. Temperatures will warm even more tomorrow and Saturday with afternoon high temperatures forecasted to reach up to 112-119 degrees across the lower deserts and 100-110 degrees across the higher terrain areas. Morning lows will also be warm and around 84-94 degrees across the lower deserts and 78-85 degrees across the lower deserts. Temperatures of this magnitude will easily result in widespread Major HeatRisk with localized areas of Extreme HeatRisk. An Extreme Heat Warning remains in effect across the lower deserts for today, Friday, and Saturday and for the higher elevation valley population centers in Gila County for Friday and Saturday. Remember to practice smart heat safety by drinking plenty of water and electrolytes and limiting your time outside, especially from the late morning through the early evening.

LONG TERM /Sunday through Wednesday/

During the first half of next week, the anti-cyclone center will propagate across the four corners into central Colorado allowing enhanced easterly flow and better chances of inverted shortwaves entering the SW Conus. However, there is considerable uncertainty regarding the timing and location of any particular feature which would enhance thunderstorm chances, not to mention mesoscale details from decayed convective complexes which further complicates the forecast. However, as early as Saturday night, long tracking, strong outflows may be sufficient to incur isolated storm development over lower elevations, while deepening moisture profiles, increasing total column PWATs, and setting the stage for enhanced convective coverage on following days. Various modeling suites highlight different days with a more preferred chance of storms sweeping into lower elevations early next week, but all models indicate at least one easterly wave influencing enhanced activity.

This easterly flow will also help erode higher midlevel heights and temperatures; and combined with the convective influences, may significantly reduce and complicate heat related impacts. A growing trend in models shows outflows and convection rapidly cooling temperatures across Arizona early in the week such that by Monday, readings return to normal levels. GFS BUFR soundings are even more aggressive towards outflows affecting temperatures suggesting extensive cloud cover much of the day Sunday, and readings falling well short of 110F. Not putting too much stock in these types of details 3-4 days in advance, but certainly the pattern supports this evolution where somewhat lower temperatures and a return to only moderate HeatRisk arrives quicker than previously forecast.

AVIATION

Updated at 2315Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns under FEW mid to high level cloud decks are expected throughout the period. The overall wind pattern will continue to exhibit light, diurnal tendencies with extended periods of variable conditions.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns under FEW mid to high level cloud decks are expected throughout the TAF period. Winds will fluctuate between the southeast to southwest with overall speeds aob 10 kts. Some gusts between 20-25 kts are likely to materialize at KBLH Friday afternoon.

FIRE WEATHER

Thunderstorm chances will remain quite low the rest of the week with only slight chances remaining over high terrain areas of far eastern districts. Better chances for more expansive storm coverage and wetting rainfall will arrive next week, but minimal activity is still expected across the lower deserts. Well above normal temperatures resulting in Extreme Heat Warnings for lower elevations and foothill locations are expected through at least the weekend. Despite the warmer and drier conditions, humidity levels will not fall substantially with minimum afternoon readings in a 10-25% range improving closer to 15-30% next week. Correspondingly, overnight recovery will mostly be in a fair to good range of 30-60% with somewhat higher values next week. Winds should be fairly light through the period with only local afternoon gusts 20-25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Monday for AZZ530>555-559.

Extreme Heat Warning from 10 AM Friday to 8 PM MST Saturday for AZZ556-560>562.

CA...Extreme Heat Warning until 8 PM PDT Monday for CAZ560>570.