textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

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

- An Extreme Heat Warning will take effect today 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 THURSDAY/

The hottest temperatures the Southwestern U.S. has ever experienced in March will arrive by Thursday as a record strength high pressure center slowly shifts from southern California into Arizona on Thursday. H5 heights are forecast to continue to rise today before peaking at around 596-597dm on Thursday. Record H5 heights for the month of March for the Desert Southwest are between 591-593dm, meaning this ridge will smash the all-time March record. This high pressure system is something like we would typically get during summertime heat events. Climatological record high H5 and H7 temperatures have already spread across all of California and the bulk of the Desert Southwest with H8 temperatures surpassing records across our area by later today. Full mixing of the record heat aloft into the boundary layer will take another day or so, keeping surface temperatures from reaching their peak until Friday.

The heat over the next several days will be unprecedented for this time of year, but considering how much above climatological records this ridge will be, there are a couple of positive things. The daylength is still at least two hours shy of what we get from late May-July and the dry air will help with overnight temperatures. A wide diurnal temperature range will allow for some relief from the heat during the overnight hours with low temperatures mostly staying in the 60s, however still near 15 degrees above normal. Daytime highs today are expected to climb to between 100-104 degrees across southeast California and southwest Arizona to around 100 degrees in the Phoenix area. As the center of the ridge moves overhead on Thursday, highs will easily breach 100 degrees in the Phoenix area to as high as 103-106 degrees over the western lower deserts of SE CA and SW AZ.

As stated yesterday, reaching 100 degrees in March is highly uncommon, with the average first 100 degree days being in late- April to start of May. So, over a month early and the Phoenix climate site will set a new record earliest 100 degree, with the current earliest sitting at March 26th. It goes without saying, this will be an impactful heatwave. Extreme Heat Warnings begin today across southeast California and the rest of the area starting Thursday.

LONG TERM /FRIDAY THROUGH TUESDAY/

The hottest days this week still look to be on Friday and Saturday with highs mostly between 103-107 degrees, roughly 25 degrees above normal and 7-10 degrees above daily records. Guidance has barely deviated over the past couple of days with the ridge starting to weaken ever so slowly starting Friday and more so Saturday into Sunday. H5 heights are forecast to drop back to between 585-588dm by later Saturday before settling into a 582-585dm range starting Sunday, still well above the 90th percentile of climatology. The weaker ridge should allow for daytime highs to drop to between 99-102 degrees by Sunday.

One noticeable change with the guidance is a higher likelihood of holding onto higher heights well into or even through the majority of next week. Ensemble mean H5 heights are now shown to stay closer to the 97th percentile of climatology into at least the middle of next week and nearly certain to stay above the 90th percentile. This means the heat will not go away next week and temperatures are likely to stay at or just above daily records through at least the first half of next week. NBM forecast highs have risen by several degrees during the middle part of next week with lower desert readings staying mostly in the upper 90s, or 15-20 degrees above normal.

AVIATION

Updated at 1115Z.

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

No major aviation weather concerns will exist through the TAF period under mostly clear skies. Winds will generally follow diurnal tendencies, with speeds under 10 kts and periods of variability, particularly preceding direction shifts. Later in the afternoon, occasional westerly gusts into the teens may develop across the Phoenix area terminals.

FIRE WEATHER

A historic high pressure will lead to record temperatures, very dry conditions, seasonal winds, and mostly sunny skies through the rest of the week. High temperatures will warm to around 25 degrees above normal for Thursday-Saturday before gradually dropping off Sunday and through early next week. Abnormally hot and very dry conditions will translate to MinRH readings of 5-10% today through this weekend. Overnight recoveries will not be great, with values dropping to 15-35% through the remainder of the week.

CLIMATE

Daily record highs through early next week:

Date Phoenix Yuma El Centro ---- ------- ---- --------- 3/18 95 in 2017 96 in 2017 95 in 2007 3/19 96 in 2017 98 in 2017 96 in 2017 3/20 96 in 2017 99 in 2004 98 in 2004 3/21 97 in 2004 102 in 2004 100 in 2004 3/22 94 in 1990 98 in 2004 96 in 2004 3/23 93 in 1990 96 in 1990 93 in 1990 3/24 96 in 2025 97 in 2025 96 in 2025

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning from 11 AM this morning 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 this morning to 8 PM PDT Sunday for CAZ561>570.