textproduct: NWS Phoenix

UPDATE

18Z Aviation Discussion

KEY MESSAGES

- Extreme Heat Warnings continue across portions of southeast California and southwest Arizona through early evening while temperatures gradually cool into the weekend.

- Chances for scattered showers and isolated thunderstorms will continue to be focused mainly over the Eastern Arizona high terrain through the weekend before increasing in 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/

Relatively quiet weather conditions will prevail today and likely through Saturday. The subtropical ridge has weakened enough to start a cooling trend today, but above normal temperatures will still easily persist today. Extreme Heat Warnings for portions of the western lower deserts continue through this evening with highs from 106-110 degrees with even higher heat indices. The lower heights and slight cooling aloft will help to bring daytime highs across the south-central Arizona lower deserts to 103-106 degrees, or 3-4 degrees above normal. Today will also bring limited convective potential with the subsidence inversion still inhibiting convection over the lower deserts and to a decent degree over the high terrain. The best potential (10-20%) PoPs looks to be from late evening through the first part of the overnight hours from Phoenix and areas east of Phoenix.

Saturday looks quite similar to today with moisture levels remaining marginal enough to have a hard time overcoming the subsidence inversion. Although the subtropical ridge will be weaker and the center moves into New Mexico, it will still be the dominant feature across the region. PoPs Saturday afternoon and evening are still at most 10-20%, but increase Saturday night to 20-30% across the Phoenix area as there are indications of a weak mid-level disturbance moving northward into southern Arizona. This unfortunately is of high uncertainty at this point. However, if some convection does get going Saturday night, forecast soundings show 1000-1500 J/kg of elevated CAPE. We will have to watch for any signs of the CAMs picking up on this potential as it could result in some localized strong wind gusts and/or a low-end chance of heavy rainfall. Temperatures Saturday are also anticipated to drop another couple of degrees from Friday with lower desert highs mostly between 102-106 degrees.

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 1757Z.

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

The only aviation weather issue will be gusty afternoon/late evening gusts, with the possibility of a distant TS outflow causing sudden gusty winds out of the SE. Occasional afternoon gusts to between 15-20 kts are likely with gusts expected to be increase to between 20-25kts between 04-06Z. Chances for TSRA/SHRA directly impacting the terminals are too low (10% or less) to include in the TAF late afternoon/evening, however a few hours of VCSH have been introduced from 06-11Z. FEW- SCT and at times BKN mid and high cloud decks will persist through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will continue to oscillate between SE and SW at KIPL with speeds peaking between 8 to 15 kts sustained. 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. The possibility of another distant TS outflow reaching KIPL cannot be ruled out late this afternoon/early evening. Periods with FEW- SCT and occasionally BKN 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.