textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- A historic March heatwave through this weekend will produce record shattering lower desert high temperatures exceeding 100 degrees beginning Wednesday.

- An Extreme Heat Warning will take effect Wednesday across southeast California and Thursday across Arizona deserts, and continuing through this Sunday.

- The unprecedented afternoon heat will be dangerous, especially with any strenuous or long-duration outdoor activities without proper hydration and taking breaks in the shade, or air conditioning.

SHORT TERM /TODAY THROUGH TUESDAY/

The strong high pressure that will drive this week's historic heatwave has already developed and is centered just off the California coast. The Desert Southwest is currently on the eastern periphery of this high, with thin high cirrus crashing down from the north, after coming around the top of the high. With the high's current positioning and H5 heights around 582-584dam over the region, which is above the 90th percentile of climatology, high temperatures this afternoon will reach the low 90s in South-Central AZ to middle 90s in Southeast CA. These temperatures will be upwards of 13-17 degrees above normal. Unfortunately, today looks like the "coolest" day of at least the next 7 days. Temperatures will warm ~5 degrees heading into tomorrow, as the center of the high nudges closer to the area, becoming centered over Los Angeles by the end of the day. A few spots in Southeast CA, through the Lower CO River Valley, may reach 100F for the first time this year tomorrow, with around a 50% chance at El Centro and 25% chance at Yuma. The high temperatures the next couple of days will lead to widespread Minor HeatRisk, with areas of Moderate HeatRisk in Southeast CA and Southwest AZ. Moderate HeatRisk means everyone sensitive to heat is at risk of heat-related health impacts, especially without effective cooling and hydration.

Besides the heat, there will be some lingering breeziness today, as a result of a tightened pressure gradient following the passing short-wave yesterday. Strongest northerly breezes, gusts as high as 25-35 mph, will be along the Lower CO River Valley and parts of Southeast CA. Higher terrain spots of South-Central Arizona should also anticipate some marginal breezes today through tomorrow morning, but peak gusts for these locations should hover closer to 25 mph.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

By Wednesday, the Desert Southwest will find itself directly under the center of the high, with heights aloft reaching upwards of 592- 594dm. To put things into perspective, sounding climatology data for Las Vegas, Flagstaff, and Tucson show record H5 heights for March are around 590dm and April 591dm, 593dm, and 592dm respectively. That would put this ridge as one of the strongest ever seen in recorded history for the region, not only for March, but for April as well. With this setup, the thermal profile will also push beyond record levels, with all-time readings for March looking likely at the H7 and H5 levels. 850mb temps will be close to all-time March records by mid to late week, helping to push temperatures at the surface into the triple digits for most of the lower deserts. The forecast high for Phoenix on Wednesday is triple digits, which if achieved, would become the early instance for Phoenix, besting the previous earliest (March 26th) by 8 days. March 26th, 1988 is actually the only March date on record where 100 degrees was achieved is Phoenix, but several days will be added to this list through this heatwave.

Confidence is very high, given small model spread, that the high will strengthen even further by Thursday with heights reaching 595- 597dm, which is around max height for the middle of summer, and will cement this high pressure as the strongest on record for March. This will push temperatures into uncharted territory (at least by late winter and early spring standards), both at the surface and throughout the atmospheric column. Highs Thursday through at least Saturday will range between 103-109 degrees, with even a low end chance for a few locations to reach 110F during this window. This would put daily MaxTs 25-30 degrees above normal and daily records may be broken by 5-10 degrees. On the topic of records, this heatwave will also break monthly all-time temperatures by a few degrees, as the values currently stand at 100, 102, and 101 for Phoenix, Yuma, and El Centro respectively. The monthly records may be tied or broken as early as Wednesday, but if not by then then they will be broken on Thursday.

Widespread Moderate HeatRisk will develop across the region in response to the historic heat, with even some localized areas of major HeatRisk not out of the question. The Extreme Heat Watch has been upgraded to a Warning, beginning Wednesday across Southeast CA and Thursday for AZ deserts. The Warning will run through Sunday. This will be the earliest heat warning on record, since heat products started, with the current earliest instance occurring between April 26th and 30th of 2020. One, somewhat positive, takeaway from this heatwave is that the sun angle is lower and length of day is shorter than during the summer. For this reason, there will be relief from the daytime heat during the overnight and morning hours. Morning low temperatures will likely break daily warm low records as well, but lows in the 60s to low 70s will offer more relief than the 80s to low 90s seen during the summer. It is important to note though to not get fooled by the nice mornings and avoid getting caught off guard by the rapid heating from mid-morning through early-afternoon. Temperatures will warm very fast between 8 AM and 1 PM, with temperatures late week likely reaching the 90s before noon.

When will the heatwave end? Well, there is a good signal that this historic high pressure will weaken and flatten out late- weekend through early next week, with potentially a shortwave trough moving through the region, but coming out of a historic heatwave it will take a strong low to cool things down significantly and there is currently no sign of one in the extended outlook. So, with the change early next week, there is a good chance high temperatures will fall back below 100 (60-80% chance), but highs look to remain above 90 degrees through the end of the week, with NBM odds of >90F at 80-90% chance.

This might be a record number of uses of the word "record" in an AFD.

AVIATION

Updated at 2335Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; and Southeast California/Southwest Arizona including KIPL and KBLH:

No major weather concerns will exist through Tuesday evening under persistent thin, high cirrus decks. In the Phoenix metro, confidence is moderate that wind trends will be similar to the past 24 hours, though some variability (040v310) may occur through early evening. Otherwise, wind speeds should be weaker with an earlier than usual mid evening shift back to an easterly component. North winds will be favored over SE California with light nocturnal westerlies at KIPL and afternoon gusts common at KBLH.

FIRE WEATHER

A historic high pressure will lead to record temperatures, very dry conditions, seasonal winds, and mostly sunny skies through the next week. High temperatures will be 10-15 degrees above normal today before increasing to 25-30 degrees above normal from midweek through this weekend. Abnormally hot and very dry conditions will translate to MinRH near 10% the next few afternoons before readings drop closer to 5-10% Wednesday through this weekend. Overnight recoveries will not be great, with values around 20-40% through Wednesday morning before dropping to 15-35% through the remainder of the week. Breezy to locally windy conditions are ongoing today and will continue through tonight, mainly over the Arizona high terrain and the Lower Colorado River Valley, with the strongest gusts (25-35 mph) focused over the latter area. The breezy conditions, combined with low RH values will result in elevated to isolated areas of critical fire weather conditions. However, with the limited scope of enhanced winds, no fire weather products are needed at this time.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning from 11 AM Wednesday to 8 PM MST Sunday for AZZ530.

Extreme Heat Warning from 11 AM Thursday to 8 PM MST Sunday for AZZ531>555-559-561.

CA...Extreme Heat Warning from 11 AM Wednesday to 8 PM PDT Sunday for CAZ561>570.