textproduct: NWS Phoenix

KEY MESSAGES

- Persistent high pressure will help keep temperatures running above normal through the start of the workweek.

- Further strengthening of the high will result in an unprecedented heat wave for March with widespread triple digits expected by Wednesday, and widespread readings near 105 degrees by Thursday, shattering daily records by as much as 10 degrees.

- An Extreme Heat Watch is in effect for the middle portion of the week into the weekend for much of Southeast California and Southern Arizona

SHORT TERM /TODAY THROUGH TUESDAY/

Early morning analysis reveals the high that has been meandering off the Pacific Coast starting to nudge closer inland while the transient trough that passed by this weekend moving east of the Central Plains. With that trough now out of the regional picture, much of the western CONUS will experience generally quiet conditions during the upcoming workweek as that Eastern Pacific high becomes the dominant feature, deflecting any significant storm systems off to our north. It will take a bit of time for the ridge to rebuild enough to produce any significant temperatures changes over our forecast area, so highs this afternoon will be basically the same as Sunday with readings for the lower deserts in the lower to middle 90s. Tuesday is when a more notable bump in temps takes place as the high finally makes its way on shore, pushing the warmest air further eastward. Highs then will range generally in the middle to upper 90s, with a few spots, mainly around the Colorado River Valley and westward, potentially hitting triple digits.

Due to the intensity of the previously mentioned ridge and Central Plains trough, the regional pressure gradient will still be tight enough to produce some lingering breezy conditions today. The strongest gusts (25-35 mph) will once again be focused around the Lower Colorado River Valley, with locally higher gusts possible over higher ridge top areas. Other higher terrain spots, such as Joshua Tree NP and areas of South-Central Arizona, may should also anticipate some marginal breezes, 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 approach 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 this point, helping to push temperatures at the surface into the triple digits for most of the lower deserts. The forecasted high for Phoenix on Wednesday is 102 degrees, which if achieved, would become the early instance of triple digits for the station, besting the previous earliest, which is March 26th, by 8 days. March 26th, 1988 is actually the only March date on record where 100 degrees was achieved is Phoenix, but appears very likely that more days will be added to that list.

Model guidance has the ridge strengthening even further by Thursday with heights approaching 594-596dm, easily making this feature one of the strongest ever seen during this time of year. 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 heat wave also puts monthly all-time temperatures in jeopardy, as the values currently stand at 100, 102, and 101 for Phoenix, Yuma, and El Centro respectively. All of those values could be tied or eclipsed as early as Wednesday, but if not by then, they almost certainly will fall sometime between the end of the week and into the weekend.

Moderate HeatRisk will develop in response to the upcoming abnormal heat, with even some localized areas of major HeatRisk not out of the question. Therefore, an Extreme Heat Watch has been issued for the Lower Colorado River Valley and westward for Wednesday, which will expand to include much of Southern Arizona by Thursday. If this watch were to be upgraded to a Extreme Heat Warning, which almost certain at this point, it would become the earliest heat warning to go into effect, with the current earliest instance occurring between April 26th and 30th of 2020.

AVIATION

Updated at 0500Z.

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

No significant weather issues will exist through Monday night under thin, high cirrus decks. Confidence is good that wind trends will be similar to the past 24 hours across the Phoenix metro, though there may be some variability and odd directions around sunrise. Otherwise, winds should be weaker in magnitude and shorter in duration Monday afternoon/evening before reverting to an easterly component. North winds will be favored over SE California with light nocturnal westerlies at KIPL and afternoon gusts 20-30kt common at KBLH.

FIRE WEATHER

High pressure will result in temperatures 10-15 degrees above normal today before increasing to 25-30 degrees above normal by the middle portion of the week. Abnormally hot and very dry conditions will translate to MinRH near 10% the next few afternoons before readings drop closer to 5-10% by Wednesday. Overnight recoveries will follow a similar downtrend with values 20-40% through Wednesday morning, dropping to 15-35% through the remainder of the week. Breezy to locally windy conditions will once again be observed today, 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 Watch from Wednesday morning through Sunday evening for AZZ530.

Extreme Heat Watch from Thursday morning through Sunday evening for AZZ531>544-546-548>551-553>555-559.

CA...Extreme Heat Watch from Wednesday morning through Sunday evening for CAZ561>570.