textproduct: NWS Phoenix

UPDATE...Updated 18z Aviation Discussion

KEY MESSAGES

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

- An Extreme Heat Warning is in effect across southeast California, will go into effect today across the Arizona deserts, and will continue through 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 FRIDAY/

Wednesday saw monthly high temperature records fall in Phoenix, Yuma, and El Centro. Unfortunately, those new records set yesterday are forecast to be superseded today and likely again on Friday. This unprecedented March heatwave is thanks to a record strong ridge of high pressure for this time of year, of which current 500 mb RAP analysis places the center over the CA-NV-AZ Tri-State area. Analyzed H5 heights have already tipped into a 595-596 dam range, with ensembles in good agreement on the strength of the ridge peaking later today around 597 dam. Though the strength of the ridge will wane Friday and especially during the weekend, strong anticyclonic subsidence should remain relatively stationary (other than a slight eastward drift) over the Desert Southwest late this week, allowing for lower level thermal profiles to rapidly warm and develop a very warm airmass across the region. This airmass may prove difficult to dislodge or even modify by a meaningful amount heading into next week.

Forecast highs across the lower deserts will range from around 100F- 106F today and 102F-108F Friday - likely the peak of this event. These forecast highs represent values around 25F above daily normals and 7F-10F above daily records (see Climate section). Widespread Moderate HeatRisk is anticipated, with even low-end Major HeatRisk for some areas on Friday. Due to the unprecedented nature of this heatwave, the timing coinciding with a weekend, and many out-of- state visitors and folks not acclimated to this level of heat, this event will be quite impactful. Extreme Heat Warnings are already in effect for Southeast CA and will go into effect today for the Arizona deserts. These warnings will remain in effect through Sunday. Remember to stay hydrated, take frequent breaks in the shade or in air conditions spaces if you are planning outdoor activities, and check on your family, friends, and neighbors! This can be a dangerous level of heat for those not taking proper precautions and for those that are sensitive to the heat.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

A few pieces of shortwave energy in the northern stream are advertised by ensembles passing overtop the ridge this weekend, acting to dampen heights aloft and dislodge the initial anticyclone east of the region. However, lower tropospheric temperatures do not have time to respond by Saturday, and NAEFS mean H8 temperatures well in excess of CFSR climatological maximum values for mid-late March should support highs similar to what is forecast today. By Sunday, forecast highs fall more into a 99F-102F range, then likely 1F-2F cooler Monday, but still flirting with the triple digits for many of the typically hotter lower desert locales. Ensembles mean H5 heights bottom out around 582-585 dam sometime Sunday into Monday, still in excess of the 97th percentile of climatology.

During the middle of next week, the trend in the last few forecast packages has been for warmer temperatures as another strong yet more transient ridge forms under an anticyclonically curved jet streak. Though the ridging should be lower amplitude compared to the current ridge, and anticyclonic subsidence will be shorter lived (likely peaking in its influence sometime Tuesday), H5 heights are still shown to rise into a 587-590 dam range. This will support a gradual warming into the middle of the upcoming workweek, with coverage of 100F+ temperatures increasing yet again. All this is to say that confidence is growing in a longer duration heatwave, with highs remaining between 15F-25F above daily normals and breaking daily records well into next week.

AVIATION

Updated at 1702Z.

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

No major aviation concerns expected throughout the TAF period. Winds will mostly follow diurnal trends with a light SSE direction before this afternoon's westerly shift. Speeds will be aob 10 kts under clear skies. Out at the western terminals prolonged period of VRB will be common.

FIRE WEATHER

A historic high pressure will lead to record temperatures, very dry conditions, seasonal winds, and mostly sunny skies into this weekend. High temperatures will be around 25 degrees above normal through Saturday before gradually dropping off Sunday and Monday. Abnormally hot and very dry conditions will translate to MinRH readings of 5-10% through the next 7 days. Overnight recoveries will be poor, with values between 15-35% through Saturday night. Humidities improve slightly early next week primarily for valley locations in the western deserts like the Yuma area, lower Colorado River Valley, Imperial Valley, and lower Gila River Valley, especially with respect to overnight recoveries.

CLIMATE

Daily record highs through early next week:

Date Phoenix Yuma El Centro ---- ------- ---- --------- 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 3/25 99 in 2025 99 in 1896 99 in 2025

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Sunday for AZZ530.

Extreme Heat Warning until 8 PM MST Sunday for AZZ531>555-559- 561.

CA...Extreme Heat Warning until 8 PM PDT Sunday for CAZ561>570.