textproduct: NWS Phoenix

KEY MESSAGES

- Extreme Heat Warnings go through early this evening before a noticeable cool down begins Sunday.

- A Flood Watch has been issued for south-central Arizona from late this afternoon through Saturday night. Localized rainfall amounts of up to around two inches will be possible.

- Showers and thunderstorms, some potentially strong to severe, are expected starting late afternoon into tonight. Additional rounds of showers and storms are also possible Sunday and Monday.

- Near to below normal temperatures are forecast starting Sunday before warming again during the latter part of next week.

SHORT TERM /TODAY THROUGH SUNDAY/

Despite the extreme heat still being present through this afternoon, it will quickly come to an end by Sunday as everything is pointing toward an active period of weather starting late this afternoon. The subtropical ridge is currently in a weakening phase while also slowly shifting eastward through New Mexico. The placement of the ridge is allowing for modest moisture advection into Arizona with 1"+ PWATs now covering all but far eastern/northeastern parts of the state to as high as 1.6" across the southwest portion. Low level moisture has so far been slow to increase, but we are expecting low level mixing ratios to increase to around 10 g/kg by late this afternoon and 11-12 g/kg by early Sunday.

In addition to the increase in moisture, the upper levels will become much more conducive for storm development by later today as a Pacific trough noses into our region from the northwest. Forecast soundings for south-central Arizona by late this afternoon show a mostly uncapped environment with MUCAPEs between 1000-1500 J/kg and very high DCAPEs of 1500-1800 J/kg. The only ingredient lacking for severe weather for today will be good shear. However, even without good wind shear we are expecting to have enough instability, high DCAPEs, and upper level support for some strong to possibility severe storms to develop. HREF guidance hints at a couple different areas supporting a strong to severe wind threat today, one over the Imperial Valley and the other centered over Pinal into Maricopa Counties. CAMs favor storm development first across Pima County late this afternoon before shifting northward with further development expected into south- central Arizona. The strong to severe wind threat should mainly fall between 5-10 pm this evening before transitioning to mostly a heavy rainfall/flood only threat from late evening into the overnight hours.

The amount of instability and upper level support available is likely to allow for a few rounds of showers and thunderstorms later today and tonight and this is where the heavy rainfall and flood threat stems from. Moisture levels are not expected to be all that much above normal, but they will be plenty adequate to support rainfall rates of 1.5-2.5"/hour with the strongest storms. Training of storms may also be an issue, especially over higher terrain areas. The heavy rainfall/flood threat is high enough for us to issue a Flood Watch from late afternoon through 5 AM Sunday morning across Maricopa, Pinal, and Gila Counties with the peak of the event likely occurring during the evening hours.

Sunday is still looking quite good for storm potential with moisture improving slightly over today, while the peak of the upper level support is expected to be in place over Arizona. Sunday morning and even a good portion of the afternoon may be a recovery period after the activity from today/tonight, but models show enough sunshine for instability to become surface based again by late afternoon with MUCAPEs again reaching 1000-1500 J/kg. The strong to severe wind threat for Sunday should again be present, but likely not to the degree of what is expected today. Assuming the atmosphere can recover enough, chances are we will face another round of localized heavy rainfall and flood potential from late Sunday afternoon through Sunday evening. This is reflected by the slight risk on WPC's Day 2 ERO.

LONG TERM /MONDAY THROUGH FRIDAY/

Model uncertainty remains quite high for much of next week, especially on Monday as at least the eastern half of Arizona should retain good moisture, while an upper level low swings through Nevada during the first part of the day. We are still unsure how much instability will be present and if the low will move to the north too quickly, but guidance is still favoring 30-60% PoPs from Phoenix and areas to the east of Phoenix for much of Monday. Monday is likely to be very conditional on any strong to severe storm threat due to the question of instability, but we are expecting better wind shear on Monday. The passing trough to our northwest should also begin to sweep drier air through a good portion of our area on Monday, but this may only impact western areas during the first part of the day leaving enough moisture from Maricopa County eastward. This drying and the eventual loss of upper level support is likely to mostly end rain chances on Tuesday before returning by next Wednesday or Thursday. Guidance suggests the tropics across the eastern Pacific will remain very active next week likely leading to more moisture getting advected into the Desert Southwest during the latter half of next week. Forecast temperatures are shown returning into or even a few degrees below the normal range early next week before rising midweek as conditions temporarily dry out. The subtropical ridge is also expected to nudge back into our region from the east helping to boost temperatures back to or a few degrees above normal at some point late next week.

AVIATION

Updated at 1009Z.

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

The main aviation concern will be thunderstorms and gusty outflows, late this evening and into the overnight period. Before then, typical diurnal tendencies are expected. Models continue to point towards an active night with a southerly to southeasterly gusty outflow with gusts between 25-30kts possible. Hi-res guidance shows storms popping up as early as 22-23Z but not over fully over the terminals until closer to 01-03Z. A PROB30 remains reflected the TAFs for in all terminals for thunderstorms however the exact timing continues to be challenging and may be subject to change as we get close to the event tonight. Any outflows from the S/SE would also bring BLDU/HZ with them, mostly expected to impact KPHX and KIWA. Winds are currently forecasted to settle back out of the east for the overnight period, once storms subside, but they could remain highly VRB until morning. Mostly SCT mid/high cloud decks will be common through the period, with the lowest CIGs forecasted at 10kft.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main aviation concern is a chance for thunderstorms this evening at KIPL. Before then winds will tend to follow their typical diurnal patterns. Oscillating between southerly and southwesterly at KBLH and southeasterly to westerly at KIPL, favoring a westerly component for a majority of the period. Wind speeds will generally be aob 10 kt, with some gusts up around 20kt expected tomorrow afternoon at both terminals. A PROB30 is reflected in the TAFs for KIPL for TS from 23-01Z tonight, but the main concern will be the increase in gust speeds during that timeframe. KIPL can expect gusts between 25-30kts with some outlier Hi-res guidance suggesting higher at times, but based on last nights evens not impossible to see gusts upwards of 35kts once more for a short period. Otherwise, FEW-SCT mid/high cloud decks will be common throughout the period.

FIRE WEATHER

A pattern shift this weekend will lead to increasing moisture and very good chances for showers and thunderstorms across the eastern districts. Gusty winds, heavy rainfall, and localized flooding will all be possible across south-central Arizona beginning late this afternoon. The western districts will generally stay dry. Expect MinRHs of 15-20% today areawide before rising to 25-35% across the eastern districts starting Sunday. Temperatures are also forecast to drop to near normal readings starting Sunday.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST this evening for AZZ530>555- 559-560.

Flood Watch from this afternoon through late tonight for AZZ534- 537-538-540>563.

CA...Extreme Heat Warning until 8 PM PDT this evening for CAZ562- 565>567-569-570.