textproduct: NWS Phoenix

UPDATE

Updated 00Z Aviation Discussion.

KEY MESSAGES

- Extreme Heat Warnings continue across portions of Southeast California and Southwest Arizona through this evening ahead of slightly cooling temperatures through the weekend.

- Shower and thunderstorm chances will be focused mainly over the high terrain of Northern and Eastern Arizona over the next few days before increasing coverage and spreading into the lower deserts early next week.

- Near to slightly above normal temperatures are expected for this weekend and likely through the first part of next week.

SHORT TERM /Today through Saturday/

Current upper-air analysis reveals flat ridging over the the Desert Southwest as troughing over the Pacific Northwest displaces the sub- tropical high toward Southern Arizona and Northern Mexico. These clashing air masses have also translated to a bit of high pressure erosion which will result in some slightly "cooler" temperatures across across the region. However, the difference compared to the previous few days will not be incredibly noticeable, in fact, extreme heat conditions will continue through at least this evening for portions of Southeast California and Southwest Arizona. Afternoon highs today over these areas will range between 105-111 degrees, but it will feel even hotter thanks to abundant surface moisture. Apparent temperatures will range closer to 110-115 degrees as dew points hover in the lower to upper 60s. Elsewhere across the lower deserts, afternoon readings will run near 100-106 degrees, backing away from Major HeatRisk, but still around 5 degrees above normal for this time of year.

Although the expectation is that shower and thunderstorm activity will be focused over the high terrain areas of Northern and Eastern Arizona over the next few days, it is not out of the realm of possibility that some activity could sneak over parts of the lower deserts this evening/tonight. The two main factors that will determine if this comes to fruition are if a strong enough outflow or colliding outflows from distant convection will occur and the timing of this potential outcome. The environment will be favorable for convection this afternoon with forecast soundings indicating mixing ratios of 14 g/kg and ML CAPE values possibly near or exceeding 1500 J/kg. However, hi-res guidance is indicating that if outflows, providing the needed lift, were to approach population centers of Pinal and Maricopa Counties, it would not be until after sunset, at a time when instability would be on the decrease while the cap rebuilds. Not to mention that anything that may fire up will also have to deal with synoptic subsidence due to the sub-tropical high. The bottom line is, lower elevation convection will be heavily conditional on how activity evolves elsewhere, and therefore, rain chances are only 20-25% for areas such as the Phoenix metro tonight as a lot of uncertainty remains in the forecast. Further east over the foothill and mountainous areas, it is not surprise that slightly better chances (30-40%) for rain exist thanks to orographic influences.

As for Saturday, it appears that rain chances of backed off compared to previous forecast runs. Tomorrow might be more of down day in terms of convection across most of the state, relative to what is expected this afternoon. With the high and associated subsidence still overhead, and the atmosphere likely to be worked over, thunderstorm coverage would likely be more isolated to a few spots over The Rim and enhanced terrain features of Southeast Arizona. Even with a decent environment still in place with projected ML CAPE ~1200 J/kg and mixing rations ~13 g/kg, without a larger complex of convection sending organized outflows our way, there will be an absence of lift to spark more showers and storms downstream over the lower deserts of our forecast area. So far, the early hi-res runs favor this drier outcome. Regardless of how convection transpires, day-to-day temperatures will fall slightly, with afternoon highs ranging between 99-108 degrees. HeatRisk levels will be in the Minor to Moderate category for most of the region, warranting the conclusion of this stretch of Extreme Heat products, but a few isolated spots of Major Heat Risk will linger, mainly for the Imperial and Lower Colorado River Valleys.

LONG TERM /SUNDAY THROUGH THURSDAY/

As stated last night, forecast uncertainty remains quite high for storm potential for the first part of next week. Guidance is in good agreement with the synoptic set up for Sunday through early next week, but it's the subtle mesoscale features that differ and where the main uncertainty lies. Models are also in better agreement with increasing moisture Sunday into Monday compared to last night and this in turn is leading to higher forecast instability and even PoPs. The ridge will continue to shift farther to the east during this time as a trough moves into the Northwestern U.S. The main shortwave within the trough will track across the Northern Rockies, but the southern fringes of the trough is still likely to bring increased support for showers and thunderstorms by early next week. Models show a seasonably strong upper level jet setting up from the Northern Rockies through the Great Basin by Monday which is expected to place our region within a favorable area for ascent as early as Sunday night and potentially lasting through next Tuesday or Wednesday.

Other unknowns for early next week will be the placement of the best moisture and likely the small swatch of the best instability. It seems likely there will be considerable cloudiness somewhere over or near eastern Arizona which will limit instability, while the best instability/lower CIN sets up in a more cloud-free area to the west (South-central AZ). There is also a decent chance of having some sort of mesoscale disturbance tracking northward out of Mexico at some point early next week which will help to initiate storm activity. However, uncertainty is still too high to be able to pinpoint the timing and the potential for storms, but there is more optimism for at least one or two busy storm days early next week. NBM PoPs have increased for early next week and look fairly reasonable at anywhere from 30-70% across south- central and eastern Arizona. If moisture and instability ends up on the higher end of guidance, we are likely to have to worry about some strong storms and heavy rainfall/flooding potential. How long this increased rain potential may last is also uncertain as models really don't change the synoptic set up through at least next Thursday or Friday. It's doubtful we will have good rain chances each day next week, but it also seems likely there will not be any strong drying that will affect the area.

Temperatures next week are almost as uncertain as rain chances as fairly high model spread is seen from Sunday-Wednesday, but with at least a downtrend in temperatures. The latter half of next week shows even higher spread, but with an uptrend in temperatures with readings eventually warming back to above normal by next weekend.

AVIATION

Updated at 0005Z.

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

The only aviation weather concern will be the possibility of a distant TS outflow causing sudden gusty winds out of the E/SE and possibly some BLDU/HZ if strong enough. Occasional gusts between 15-20 kts are possible over the next couple of hours before tapering off. Chances for TSRA/SHRA directly impacting the terminals remain too low (10% or less) to include in the TAF for later this evening, but most likely time would be after 05-06Z. FEW-SCT and at times BKN mid and high cloud decks will persist through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

SE'rly winds will continue at KIPL with speeds in the low teens through this evening before lowering to aob 10 kt overnight. KBLH will maintain a southerly flow with speeds aoa 10kts and gusts between 20-25kt, gusts will relax for the overnight period, however sustained speeds will remain aoa 10kts. Gusts of 20-25 kt are expected again tomorrow afternoon. Periods of FEW-SCT mid and high cloud decks will continue.

FIRE WEATHER

Above normal temperatures will continue into the weekend, but with some gradual cooling as humidities remain somewhat elevated. Isolated chances for showers and a few thunderstorms can be expected through the weekend, with the best chances over the Arizona high terrain. Rain chances should improve during the first half of next week with temperatures eventually dipping to around average for mid September. MinRHs through early next week will generally be between 15-25% across the western districts and 25-35% across the eastern districts. Winds will somewhat follow typical diurnal patterns, but with more of a south southwesterly predominant wind across the western districts. Daily afternoon gusts of 20-25 mph will be common across the Lower CO River Valley through the weekend before becoming more widespread early next week.

PSR WATCHES/WARNINGS/ADVISORIES

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

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