textproduct: NWS Phoenix

KEY MESSAGES

- Extreme Heat Warnings remain in effect through early this evening as afternoon high temperatures approach the 110 degree mark across the lower deserts.

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

- A Flood Watch remains in effect for south-central Arizona from late this afternoon through tonight as heavy rainfall, producing localized amounts in excess of 1-2 inches leading to flash flooding, will be possible.

- Near to slightly below normal temperatures are expected starting Sunday and lasting into the first half of next week.

SHORT TERM /TODAY THROUGH SUNDAY/

Latest water vapor satellite imagery as well as objective analysis depict the subtropical high displaced east of the region, centered across eastern New Mexico into western Texas, as a broad upper trough is encompassing much of the west coast. This overall pattern configuration is resulting in a southerly wind flow, which is aiding in moisture advection across much of the region. The latest SPC Mesoanalysis shows PWATs in excess of 1"+ across much of Arizona with values in excess of 1.5" covering much of the lower deserts of south-central and southwest Arizona, allowing low-level mixing ratios to reach 10+ g/kg.

The increased moisture levels and MUCAPE values of around 1000 J/kg combined with some increased upper-level forcing for ascent from the aforementioned west coast trough will support a favorable environment for widespread storm activity heading into this afternoon and tonight across much of south-central AZ with much more isolated activity (10-20% chance) near the Imperial Valley and portions of La Paz County. Latest HREF guidance is in decent agreement that activity this afternoon will develop across the AZ High County as well as across southeast AZ, with the potential for multiple outflows to materialize. Latest forecast soundings for late afternoon/early evening hours show a nearly uncapped atmosphere and thus it will not take much low-level forcing to spark storms across the lower deserts with a singular outflow likely sufficient to cause convective initiation. With DCAPE values expected to peak at around 1500-1800 J/KG, any thunderstorms will be capable of producing strong to severe downburst wind gusts and thus, generate strong outflows. Latest HREF neighboring probabilities show a 50-70% chance of winds exceeding 35 mph with around a 10% chance of winds reaching severe levels (i.e. greater than 58 mph) across much of south-central AZ. With the potential for a strong outflow to materialize from a storm complex that is likely to develop across southeast AZ, significant blowing dust will be a possibility as well across Pinal County into at least southern portions of the Phoenix Metro Area. Given this potential, in coordination with WFO Tucson, a Blowing Dust Advisory has been issued between 4 PM and 9 PM MST for these areas. In addition to the strong winds and blowing dust potential, heavy rainfall leading to flash flooding will also be a concern as there is a potential for training of storms, especially across the higher terrain areas. Given this potential for flash flooding, a Flood Watch is in effect from late this afternoon through tonight across much of south-central AZ.

Heading into Sunday, conditions will remain favorable for additional storms affecting much of south-central AZ once again as an upper- level jet streak moving through the Great Basin will increase the upper-level divergence and thus the upper-level forcing for ascent. The main question and uncertainty for how active Sunday turns out to be is the degree of recovery from the convective activity expected later today. However, if the atmosphere is able to recover enough, then another very active day is likely as MUCAPE values are forecast to exceed 1000 J/KG with scattered to numerous thunderstorms capable of producing strong wind gusts, areas of blowing dust, and heavy rainfall leading to flash flooding all on the table once again.

With 500 mb height fields still hovering around 594-596dam today, another day with highs near to just above 110 degrees can be expected across much of the lower desert communities, resulting in the continuation of Major HeatRisk. Therefore, the Extreme Heat Warning remains in effect through 8 PM MST. The further lowering of the upper-level height fields as the ridge shifts further east into the southern Plains on Sunday will cause afternoon high temperatures to cool 3-6 degrees, with readings ranging in the low to mid 100s across the south-central AZ lower deserts and mid to upper 100s across the western deserts. This will be enough to cause HeatRisk levels to drop into the moderate category and thus put an end to the prolonged extreme heat episode that has been in place for more than a week.

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 1807Z.

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

The main aviation concern will be thunderstorms, gusty outflows,and blowing dust this evening and into the overnight/early morning period. Model ensembles continue to point towards an active night with two potential outflow events. The first, a northerly flow from thunderstorm development to the north of the terminal area. Then, a southerly to southeasterly gusty outflow with gusts between 25-30kts possible from thunderstorm development to south. Look for the northerly event to occur around the 00-02Z timeframe while the southerly outflow during the 02-05Z timeframe. TEMPO lines are now in the TAFs for all terminals and may need to be adjusted as the event progresses. Any outflows from the S/SE would also bring BLDU/HZ with them, mostly expected to impact KPHX and KIWA, potentially reaching the northern terminals. 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 during thunderstorm activity. Potential thunderstorm activity for Sunday is forecast, but highly dependent on how storms forecast for this evening into tomorrow morning unfold.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main aviation concern is a low probability(<30%)chance for thunderstorms this evening at both terminals. Before then winds will tend to follow their typical diurnal patterns. Shifting 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 this afternoon at both terminals. A PROB30 is reflected in the TAFs for KIPL for TS from 23-01Z tonight and 00-02Z at KBLH, but the main concern will be the increase in gust speeds during that timeframe. Expect gusts between 25-30kts. Otherwise, FEW-SCT mid/high cloud decks will be common throughout the period with periods of mostly clear skies.

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.

Blowing Dust Advisory from 4 PM this afternoon to 9 PM MST this evening for AZZ539-550-551-553-554-559.

Flood Watch from 5 PM MST 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.