textproduct: NWS Phoenix

UPDATE

Updated 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 today, but increase again starting 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 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 1030Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation weather issue during this TAF period will be the potential for a distant outflow to reach the terminals this evening and very low chances (10-20%) of TS conditions and associated impacts. Winds will generally be light, AOB 10 kts, and follow typical diurnal tendencies through the afternoon. Thunderstorms will once again form over the higher terrain NE/E/SE of the Phoenix airspace during the afternoon, however, coverage today is expected to be widely scattered. The potential for an outflow to reach the terminals late afternoon/early evening still exists but is too low probability to include in the TAF. If an outflow does reach the terminals, it will likely carry a predominant easterly component, and may do nothing more than act to switch winds out of the east earlier than they otherwise would overnight. Cloud decks will mostly be FEW-SCT and at times BKN with bases generally remaining above 10 kft AGL.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected throughout the TAF period under FEW to SCT mid to high clouds. Winds at both terminals will vary between southeast during the daytime to southwest during the evening and into the overnight hours with speeds generally AOB 10 kts sustained. Some afternoon gusts at KBLH and evening gusts at KIPL to between 15-20 kts are possible.

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 from 10 AM this morning to 8 PM MST Thursday for AZZ530-532.

CA...Extreme Heat Warning from 10 AM this morning to 8 PM PDT Thursday for CAZ562-563-565>567-569-570.