textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion.

KEY MESSAGES

- Extreme Heat Warnings are in effect through this evening for Western Imperial County as well as along the Lower Colorado River Valley as heat indices are forecast to peak between 110 and 120 degrees.

- Deeper moisture will provide favorable conditions for daily shower and thunderstorm activity to materialize across the area through the middle of the week, along with noticeably cooler temperatures.

- Thunderstorm chances will decrease noticeably by the end of the week into next weekend as drier air filters in.

SHORT TERM /Today through Tuesday/

A very active evening was observed across much of south-central AZ as strong to severe thunderstorms developed and impacted near I-8 between Casa Grande and Gila Bend as well as across portions of the Phoenix area, especially from Central Phoenix westward through the Phoenix West Valley. Peak wind gusts with these storms topped anywhere between 60-80 mph. Activity gradually drifted westward into western Maricopa through La Paz Counties while gradually weakening in the process. Given the very active evening that was observed and with the atmosphere over-worked somewhat, the forecast for later today is highly uncertain in terms of overall storm coverage as it will be highly conditional on how fast the atmosphere can recover. However, if the atmosphere can recover fast enough, then there is the potential that late this afternoon through this evening conditions could turn active once again, especially since there are indications from the guidance of a mid- level shortwave trough moving through the region and could enhance the convective activity. East to southeast mid-level flow along the southern periphery of the subtropical high located near the Four-Corners Region will continue to transport higher moisture content as PWATs are projected to increase to around 1.5" with MLCAPE values peaking at around 1000 J/KG. Most of the hi-res guidance for later this afternoon through this evening show storms developing across the Mogollon Rim and White Mountains as well as across southeast AZ and gradually drifting westward into the lower deserts of south-central and southern AZ. With DCAPE values peaking at around 1500 J/KG, strong outflows will once again be a threat with the potential, similar to last night, that these outflows could spark new storm development in the lower deserts of south-central AZ, including for portions of the Phoenix area, with strong downburst wind gusts and locally heavy rainfall being the main hazards. There will also be a potential for areas of blowing dust to materialize as well in the dust prone areas. The environmental conditions will not change that much for Tuesday so the threat for additional storms will continue, especially across the higher terrain areas. However, if this evening turns out to be active across the lower deserts, then Tuesday could turn out to be more of a down day for the lower deserts.

Afternoon high temperatures today will range between 105-110 degrees across the lower deserts, enough to cause the HeatRisk levels to drop back into the moderate category. Across the western half of Imperial County and along the Lower Colorado River Valley, with dewpoints expected to range from the mid 60s to low 70s, with the highest dewpoints along the Imperial Valley, heat indices are expected to peak between 110-120 degrees later this morning into this afternoon. Therefore, given these high heat indices, an Extreme Heat Warning is in effect for this region through this evening. Temperatures on Tuesday will cool by an additional 2-4 degrees with afternoon highs near seasonal levels.

LONG TERM /Wednesday through Sunday/

Model guidance continues to be consistent in indicating another more active period materializing Wednesday into the first half of Thursday. The flow by Wednesday is forecast to shift out of the south as the subtropical high shifts into Texas and a upper-level low meanders off the California coast. This southerly flow is likely to bring additional moisture northward from Mexico, with projected PWATs increasing to 1.7-2.0" and low-level mixing ratios increasing to 12-13 g/kg. In addition, there will likely more favorable upper- level forcing from the aforementioned upper-level low in the form of divergence aloft as well as the potential of an easterly wave moving into the region from Mexico as indicated by both the GFS and ECMWF. The more favorable upper-level forcing is likely to enhance the convective activity, with flash flooding becoming more of a threat given the deeper moisture profiles that will be in place. The latest NBM PoPs for Wednesday through the first half of Thursday remain elevated, ranging between 40-60% across much of south-central and southeast AZ.

Heading towards the end of the week and next weekend, the guidance is in good agreement in showing the subtropical high relocating towards the Gulf Coast Region while the aforementioned upper- level low remains situated off the west coast. This pattern configuration will cause the flow to shift more out of the southwest, bringing in a drier airmass as PWATs drop under 1.2-1.3". As a result, an overall downtrend in thunderstorm activity is expected, with afternoon activity limited to the eastern third of AZ.

Temperatures during the middle to latter half of the week will run slightly below normal as afternoon highs will top out in the low 100s across the south-central AZ lower deserts to the mid 100s across the western deserts. In fact, with the potential for abundant cloud cover as well as lingering rainfall activity, afternoon highs on Thursday may struggle to reach the triple digits across the south- central AZ lower deserts. Temperatures will gradually rebound Friday and into next weekend but remain near seasonal levels with no extreme heat in sight for at least the next seven days.

AVIATION

Updated at 1800Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary aviation concerns this evening will once again be the potential for gusty outflow winds, blowing dust, and reduced vsby due to TS at the metro terminals. Over the next several hours, westerly winds around 10-11 kts are expected with gusts into the teens to around 20 kts developing during the afternoon. Confidence in the wind forecast decreases considerably this evening with many discrepancies still existing from the arrival of potential outflow boundaries. Thunderstorms are expected to again form over the high terrain to the north and east of Phoenix and in southeast AZ. Both complexes of storms will be capable of producing long-lived outflows which will push towards the terminals by this evening. The northeasterly outflow may briefly cause an easterly shift, but the southerly outflow is likely to be the stronger of the two. These outflows could carry some blowing dust, especially the southerly outflow and then may be followed by TS development in the Phoenix area mainly between 04Z-06Z tonight. Amendments will likely be needed to the current TAF as conditions evolve.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns are expected through the next 24 hours under periods of FEW-SCT mid to high cloud decks and otherwise mostly clear skies. At KIPL, winds will primarily stay southeasterly with a switch to westerly by this evening. There is a low chance (10%) for an isolated strong TS within the vicinity of KIPL this afternoon, which could cause an abrupt wind shift (likely out of the southwest) and strong gusts. At KBLH, winds will generally fluctuate between SSE and SSW. Overall, wind speeds should generally remain AOB 12 kts.

FIRE WEATHER

Gradual cooling temperatures to near to slightly below normal levels can be expected throughout the week. Elevated moisture will maintain afternoon MinRHs above 20% each afternoon along with good to excellent overnight recoveries. The chances for shower and thunderstorm activity as well as wetting rainfall will remain elevated through the middle of the week, especially across the eastern districts. The chances for showers and thunderstorms will decrease noticeably by the end of the week through next weekend as drier air filters in. Outside of gusty and erratic winds due to thunderstorms, winds will follow the familiar diurnal trends, with typical afternoon upslope breeziness around 20 mph.

PSR WATCHES/WARNINGS/ADVISORIES

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

CA...Extreme Heat Warning until 8 PM PDT this evening for CAZ563-566- 567-569-570.