textproduct: NWS Phoenix

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 WEDNESDAY/

Strong high pressure continues to build eastward across the Western CONUS, with the current 591-592dam anticyclone centered near the California Channel Islands. Heat is building as the anticyclone moves closer to the local area. This anticyclone will move and station directly over the local area by the end of the day tomorrow, while strengthening to record H5 height levels of 594-596dam. As has been advertised for several days, this record strength high will lead to unprecedented record-shattering temperatures across not only the Desert Southwest, but nearly all of the Southwest CONUS and even parts of Northwest and Plains.

Today, high temperatures will be around 5 degrees warmer than yesterday, with lower deserts reaching the middle to upper 90s. There will also be a chance for a few locations to reach 100 degrees today, including El Centro and Yuma. Most lower desert communities will eclipse the 100 degree mark tomorrow as the heat continues to build. 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 on Wednesday across southeast California and the rest of the area starting Thursday.

LONG TERM /THURSDAY THROUGH MONDAY/

Models all agree this ridge will become the strongest ridge the Southwestern U.S. has ever experienced during the month of March and potentially even during the entire winter/early spring time frame. H5 heights are now forecast to peak at around 595-596dm on Thursday before gradually lowering Friday and through the weekend as the ridge center barely moves. NBM forecast temperatures are still showing highs peaking on Friday and Saturday between 103-109 degrees across the lower deserts, which is between 7-10 degrees above daily records. Widespread high-end Moderate HeatRisk to low- end Major HeatRisk will impact the area from Thursday through at least Saturday before the weakening ridge finally allows temperatures to start to decrease.

The ridge will continue to weaken this weekend, especially by Sunday as a it is likely to get flattened from a passing shortwave trough well to our north. H5 heights are expected to fall to around 582- 585dm by Sunday and mostly stay in that range through the first part of next week. Even with these lower heights, they will still stay above the 90th percentile of climatology. Temperatures will be slow to drop off due to no actual front moving through the region and only relying on the gradual weakening of the ridge. Forecast temperatures still have highs topping 100 degrees Sunday and easily into the upper 90s next Monday. The weather pattern into the middle part of next week is likely to keep slight ridging in place resulting in continued dry weather while maintaining well-above normal temperatures mostly in the low to mid 90s.

AVIATION

Updated at 1710Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; No significant aviation weather issues will exist under FEW to at times SCT high cirrus. Winds will continue to follow their typical diurnal tendencies. Confidence is good that winds will switch out of the west between 20-22Z, then return to an easterly direction late in the evening (04-06z). Speeds will generally remain under 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH: Winds will vary between W and NNE with speeds generally AOB 6 kts. Periods of variability are likely. Expect FEW-SCT cirrus decks through much of the day, eventually becoming SKC this evening/overnight.

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 25-30 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 near 10% today before readings drop 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.

CLIMATE

Daily record highs through the weekend:

Date Phoenix Yuma El Centro ---- ------- ---- --------- 3/17 99 in 2007 101 in 2007 101 in 2007 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

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.