textproduct: NWS Phoenix

UPDATE...Updated 00z Aviation Discussion

KEY MESSAGES

- Hot and muggy conditions will continue through mid week, particularly across southeast California and southwest Arizona where Extreme Heat Warnings have been issued through Thursday.

- Shower and thunderstorms chances will be fairly limited this afternoon and evening, but increase again for Wednesday.

- A disturbance will help to bring more widespread shower and thunderstorm activity on Thursday and Friday with the potential for heavy rainfall and localized flooding over eastern and central Arizona.

- Temperatures will cool later this week with below normal readings across much of Arizona to near normal for southeast California by Friday.

SHORT TERM /Today through Wednesday/

Current upper-air analysis reveals an anomalously strong area of high pressure stretching from the Great Lakes to the Rockies. Although not a typical setup, this pattern has helped provide the moist southeasterly flow needed to shift our monsoon season into a higher gear. Multiple rounds of showers and thunderstorms have been seen across a good chunk of Arizona since this past weekend, and it looks like activity will continue through at least the middle portion of next week. How much convection develops will, of course, depend on what occurs the day before, so future chances are conditional, but nonetheless, PoP chances stretch into next Tuesday.

Focusing more on the near term, like what was just stated above, today looks to be more of a down day in terms of convective activity, especially across lower desert areas of south-central Arizona after what occurred yesterday evening. The atmosphere is decently turned over so it will be much more difficult to overcome any remaining cap in place by this afternoon. Best chances to see storms across any span of desert are only 10-20% and will be focused around the enhanced terrain of far southern Maricopa County and the Kofas. A rouge storm cannot be completely ruled out for the population centers of Maricopa and Pinal Counties, but any activity will be heavily dependent on strong enough outflows descending off the mountains, which appears unlikely at this time.

Better convective coverage is expected for Wednesday as atmospheric instability starts to recover. Along with better coverage, average storm intensity is expected to be greater as well creating a more favorable setup to see stronger and longer lived outflows that could descend into the lower deserts off the high terrain. Hi-res data indicates typical initiation along the Mogollon Rim and the mountains of Gila County before activity shifts down toward Maricopa and Pinal Counties. It is tough say exactly who in the lower elevations may see rain, given the spotty nature of monsoon activity, and it is likely that some will see no rainfall at all. With that being said, for the Phoenix metro and surrounding areas, PoPs are around 20-30% to reflect the uncertainty of where cells may flare up. Main impacts with this round would be heavy downpours and winds greater than 35 mph. With upper-level flow out of the northeast, widespread blowing dust appears to be less of a concern, but the potential for some patches of reduced visibility cannot be completely ignored.

For today, afternoon highs should be fairly uniform across the lower deserts with readings climbing to between 102-110 degrees. The region basically becomes split in half for Wednesday, with near normal temperatures for south-central Arizona, likely thanks to cloud and rainfall coverage, while locations in the drier, western portion of our forecast area reach toward 115 degrees. Although it may be drier in SW Arizona and SE California, since no widespread rainfall is expected, is certainly will not feel dry as dew points sit in the upper 50s to lower 60s. This will produce heat indices between 110-116 degrees, and keep overnight lows elevated in the lower to upper 80s, which would be near record high minimums. To account for these conditions, an Extreme Heat Warning is in effect for areas along the Colorado River and westward through the next few days.

LONG TERM /THURSDAY THROUGH MONDAY/

Active monsoon weather is expected to continue through the period with Thursday and Friday showing the highest potential for impactful weather. An important ingredient for more organized showers and thunderstorms is expected to be introduced by Thursday. A large easterly disturbance will continue to track across Texas into New Mexico Wednesday into Thursday before stalling out over New Mexico on Friday. This is expected to place at least the eastern half of Arizona in a favorable ascent region for Thursday and Friday, and potentially into Saturday.

Thursday has the better odds of being the best convective day as we are anticipating the highest instability to go along with the upper level support. However, there are hints of early morning rainfall moving through areas east of the Colorado River. Hi-res data shows a remnant MCV riding along the northeasterly flow, sparking this morning activity and potentially throwing the typical late day storm timeframe into doubt if lingering cloud cover limits instability. As of now, forecast soundings still show upwards of 1000 J/kg of CAPE for Phoenix, a good indication that the environment will remain favorable for robust afternoon/evening thunderstorm activity. Additional moisture transport into the region should also push PWATs to between 1.8-2.1", suggesting heavy rainfall will an increasing threat. NBM PoPs increase to 70-80% for eastern and south-central Arizona Thursday, while WPC has introduced a large Slight Risk for excessive rainfall. We will continue to monitor the latest guidance as Flood Watches are likely to be needed for some areas later this week. Shower and thunderstorm chances should also expand westward toward the Lower CO River Valley Thursday night with chances continuing areawide through at least Friday. As of now, a Slight Risk for excessive rainfall is also present for Friday, but the uncertainty is higher as instability is likely to be lower and the easterly wave should begin weakening on Friday. With all this being said, the forecast is highly conditional on what happens Thursday morning.

Abundant monsoon moisture will persist over the weekend into early next week with day-to-day storm chances continuing, but forecast uncertainty is quite high. Another easterly wave looks to be possible at some point during the first half of next week which may bring another round of higher storm/rainfall coverage.

Forecast temperatures for later this week into next week are also of low confidence due to the expected convective potential over much of the area. The latest NBM forecast highs show readings dipping below normal over the south-central Arizona lower deserts as early as Thursday, but remaining right around normal for the western deserts. Although model forecast spread for temperatures is quite high beginning Thursday, it seems likely the Phoenix area will see at least a day or two in the 90s on Friday and/or Saturday. Depending on the amount of cloud coverage and even morning/afternoon rainfall on Friday, highs may struggle to reach the low to mid 90s.

AVIATION

Updated at 2345Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns are expected through early Wednesday afternoon. The threat of any outflows/thunderstorm impacts continue to remain very low through this evening. As a result, the overall wind pattern through early Wednesday afternoon will follow the typical diurnal tendencies with overall wind speeds remaining aob 10 kts. Heading into late Wednesday afternoon/evening, there is a much better chance of a pronounced outflow moving through from the north to northeast with strong, gusty winds and potential direct thunderstorm impacts. Given the uncertainty on the overall thunderstorm coverage, a PROB30 group has been introduced for early Wednesday evening. FEW-SCT mid to high cloud decks will prevail through Wednesday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues will exist through Wednesday afternoon under periods of mid/high level cloud decks. Winds will generally fluctuate between a diurnal southeast component to nocturnal westerly direction with periods of light variability.

FIRE WEATHER

Elevated moisture levels will result in daily chances for showers and thunderstorms over the eastern districts through Wednesday before increasing in coverage and spreading westward late week. Humidities will continue to improve through the rest of the week, while temperatures eventually cool to around or even below normal by Thursday. MinRHs are expected to hover between 20-30% over the lower deserts to 30-40% over the AZ high terrain through Wednesday before improving further late week. Outside of potential thunderstorm outflows, winds will tend to be light and somewhat favor diurnal trends.

PSR WATCHES/WARNINGS/ADVISORIES

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

CA...Extreme Heat Warning until 8 PM PDT Thursday for CAZ562-563- 565>567-569-570.