textproduct: NWS Phoenix

UPDATE...Updated 06z Aviation Discussion

KEY MESSAGES

- A historic March heatwave will produce record shattering lower desert high temperatures exceeding 100 degrees through at least the weekend.

- An Extreme Heat Warning is in effect across southeast California and will go into effect Thursday across the Arizona deserts, and will continue 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 currently around 592dm and will rise to around 596-597dm tomorrow. 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, H7, and H8 temperatures have already spread across all of California and the bulk of the Desert Southwest. 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 to low 70s, however still near 15 degrees above normal.

Temperatures early this afternoon have already climbed into the upper 90s to low 100s across the lower deserts and all three of our climate sites (Phoenix, Yuma, and El Centro) have already set new daily records, surpassing the previous records by 4-6 degrees (see CLIMATE section below for the previous records). With some more daytime heating still left for today those temperatures are expected to climb another couple degrees. If Phoenix reaches 100 degrees this afternoon it will be the earliest 100 degree day in Phoenix, which the current earliest 100 degree day is March 26, 1988. Additionally, if Phoenix reaches 101 degrees it will break the all time record high temperature for the month of March, which stands at 100 degrees on March 26, 1988. Additionally, our other two climate sites are expected to near or surpass their monthly March record high temperatures. Yuma's current March record is 102 degrees, set on March 21, 2004, and El Centro's current March record is 101 degrees, set on March 17, 2007 and March 30, 1934. By tomorrow afternoon high temperatures are forecasted to be in the 102-106 degree range across the lower deserts and in the mid to upper 90s across the higher terrain. If any of the three climate sites don't break their monthly March record today they should tomorrow.

It goes without saying, this will be an impactful heatwave. Extreme Heat Warnings are already in effect across southeast California and will go into effect across the rest of the area starting tomorrow morning.

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 0455Z.

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

No aviation weather concerns under mostly clear skies can be expected throughout the TAF period. Winds will continue to exhibit the typical diurnal tendencies with speeds aob 10 kts. Periods of variable to even calm conditions can be expected, especially before the diurnal switchover.

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. However temperatures next week will still be around 20 degrees above normal. Abnormally hot and very dry conditions will translate to MinRH readings of 5-10% today continuing into next week. Overnight recoveries will not be great, with values dropping to 15-35% through the weekend.

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 until 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 until 8 PM PDT Sunday for CAZ561>570.