textproduct: NWS Phoenix

UPDATE

Updated Aviation

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.

- A weather disturbance will bring 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 temperatures across much of Arizona to near normal for southeast California by Friday.

SHORT TERM /TODAY THROUGH WEDNESDAY/

The Desert Southwest monsoon is well underway and looks to remain active at least for the next couple of weeks. The subtropical ridge has moved far enough to the north to allow for very good moisture transport into the region over the past few days. Deep moisture and conditions conducive for monsoon storms are likely to be present for most days through the weekend and probably longer. There are two main forecast concerns over the next several days: First is the Extreme Heat impacting portions of the western deserts today through Thursday, and then the heavy rain potential for the eastern 2/3rds of Arizona starting Thursday.

Currently, a near record strength upper level ridge remains centered over the North-central U.S., extending through the entire central U.S. and much of the Rockies. Moist southeasterly flow continues across Arizona and even extending as far north as Montana. This fetch of moisture is likely to remain in place for a good amount of time with an active eastern Pacific tropical scene also helping to direct moisture northward through Mexico.

The convective activity on Monday was expansive enough to overturn the atmosphere throughout much of southern and central Arizona, so today's convective potential will be much more limited. Forecast temperatures today across south-central Arizona have been lowered slightly due yesterday's rainfall activity with some lower desert areas not even reaching normal readings. However, the western deserts will heat up even further today under mostly sunny skies as highs likely reach 110 degrees. Boundary layer moisture has also increased enough to push surface dew points into the mid 60s to the lower 70s. This amount of moisture will somewhat curtail temperatures, but it will make it feel much more uncomfortable as heat indices top 110 degrees across the western lower deserts to as high as 115 degrees in the El Centro area. Guidance indicates a worsening of the heat across the western deserts Wednesday into Thursday with highs peaking at 110-113 degrees, but with heat indices as high as 113-119 degrees. An Extreme Heat Warning has been issued for the lower deserts of southeast California and throughout the Lower CO River Valley starting late this morning through Thursday evening. Overnight forecast lows right around record high minimums were also a factor in the Extreme Heat Warning issuance.

Atmospheric instability is expected to recover starting Wednesday with hi-res CAMs showing good coverage of afternoon convection forming over higher terrain areas with a decent likelihood of it moving into the lower deserts during the evening hours. As of now, the coverage may end up being on par or even slightly better than what occurred last evening. The upper level flow pattern later on Wednesday should begin to become more favorable for storm development over eastern Arizona as a large upper level low moves eastward into Texas. The threat for any strong to severe storms on Wednesday looks to be very marginal as forecast instability is only around 500-750 J/kg and the winds aloft are overall weak. However, we are more likely to see at least a minor threat of heavy rainfall and localized flooding starting Wednesday as low level mixing ratios remain around 11-12 g/kg and the steering flow weakens.

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. The amount of cloud cover on Thursday is still a question and it very well may influence the amount of potential instability, but forecast soundings still show upwards of 1000 J/kg of CAPE for Phoenix. 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.

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 1700Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Winds will be the main weather issue through Wednesday afternoon under periods of mid/high cloud decks. Directions may remain variable into early this afternoon before settling on a W/SW direction. Mountain thunderstorms should be less pronounced today, and likely not descending into the terminal footprint. Even outflow winds may not impact all terminal locations, and there is large uncertainty regarding the timing of an easterly wind shift overnight. Have pushed back timing of an easterly shift, and it's not impossible that KPHX never truly turns east, and just becomes variable for a few hours Wednesday morning.

Wednesday evening outlook: Chances for stronger, gusty easterly outflow winds and direct TS impacts will increase substantially late afternoon/early Wednesday evening. Brief periods of MVFR/IFR visibility in TSRA are possible early/mid evening.

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 vary from a diurnal SE component to a nocturnal SW direction. Any gusts will be limited and directions may become variable for a period around sunrise.

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.