textproduct: NWS Phoenix

UPDATE

Updated 18Z Aviation Discussion.

KEY MESSAGES

- Above normal temperatures will continue through the start of the workweek with Extreme Heat Warnings remaining in effect.

- Typical monsoon conditions are expected into next week with daily thunderstorm chances favoring Northern Arizona high terrain and Southeast Arizona and low chances in the South Central Arizona lower deserts.

- After a slight drop in temperatures during the first half of the coming workweek, high pressure will strengthen over the region once again late in the week, resulting in another period of unseasonably hot temperatures and a return of widespread Major Heat Risk.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Satellite observations early this morning shows the large monsoon high encompassing the Desert Southwest and the Plains, with the central anticyclone near the CO-NM border. There were also two features evident in IR, remnant weak vorticity over northern AZ and the remnants of TC Berth still moving northwest through northern Baja and SoCal. The interaction of Bertha remnants with the far-traveling outflow boundary from Saturday afternoon AZ storms helped spark spotty convection in Southeast CA. The synoptic pattern Today through Tuesday will see minimal changes, with overall low-grade monsoon conditions favored. Guidance keeps the monsoon high positioned over northern NM through the first half of the week, with a gradual weakening of the high pressure as well - H5 heights over AZ dropping from 597-598dam today to 594-594dam Tuesday. Boundary level moisture will remain decent across the AZ deserts - mixing ratios around 9-11 g/kg and average surface dew points in the upper 50s to lower 60s - to support daily afternoon thunderstorms, but storms will mostly remain focused over the northern AZ high terrain and southeast AZ. Meanwhile the environment over the lower deserts of south-central and southwest AZ will mostly be sub- optimal for any robust convective development, with MUCAPE values forecast to mostly stay below 1000 J/kg and a modest cap with CIN staying mostly <-25 J/kg. Still, a few storms may develop over terrain features of La Paz County and the Table Top area in southern Maricopa County.

Through the short term, and long term, the primary weather hazard will continue to be dangerously hot temperatures. An Extreme Heat Warning remains in effect through Monday for the majority of the deserts. High temperatures will not be quite as high the next few days as they were the previous few days, with the slight weakening of the high pressure, and there will be a downtrend in the HeatRisk level with a reduction in the coverage of Major HeatRisk. Lower desert highs the next few days will be in the 107-114F range, which is around 3-6F above normal. Although it is not too unusual to see highs of this magnitude this time of the year, the low temperatures will continue to be very warm each morning and will be the primary driver of the lingering areas of Major HeatRisk. Most lower deserts will only cool off into the upper 80s to lower 90s. This does not offer much relief from the heat and allows temperatures to soar quickly into the 100s by 10-11 AM each morning. The Extreme Heat Warning is still being considered for extension beyond Monday to carry right into the next extreme heat event later in the week as the monsoon high restrengthens and repositions directly over AZ. This will leave only a brief reprieve from widespread Major HeatRisk. Tuesday currently looks like the "coolest" day, with only isolated coverage of Major HeatRisk.

LONG TERM /THURSDAY THROUGH SUNDAY/

By the latter half of the week, most global ensembles support notable strengthening of the high pressure, with 600dam H5 heights materializing and the center of the anticyclone shifting to over northern AZ/southern UT. The primary impact of this outcome will be another extreme heat event with a return of widespread Major HeatRisk and area of Extreme HeatRisk, beginning as early as Thursday and the heat likely peaking on Friday and Saturday. There is already high enough confidence to warrant extreme heat headlines and the issuance of an Extreme Heat Warning, but it is still being considered whether to just extend the current Extreme Heat Warning through the brief reprieve Tuesday-Wednesday or separate the events. This forecast outcome will also likely add more subsidence over the area and subdue afternoon convection. Global ensembles also show a slow downtrend in PWATs, falling under 1.5" by Thursday-Friday. So, this is not great for the prospects of seeing storms push into the lower deserts. Latest NBM keeps slight PoPs focused in the AZ high terrain. The one potential outcome that could increase chances for storms into the lower deserts however is if the anticyclone can reposition closer to southern NV, in a Maddox Type II position, to provide stronger E-NE winds over the Rim.

AVIATION

Updated at 1905Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Westerly winds will continue through the overnight hours. Wind speeds will generally be around 7-13 kt through the afternoon, but some occasional gusts into the mid to upper teens is possible. A Gulf surge looks to move into the area this evening (00-04Z), keeping winds westerly, but some gusts up around 20 kt is possible at KPHX. The Gulf surge looks like it will wash out or at least not have gusts with it by the time it reaches the other three terminals. A complex of thunderstorms is expected to develop in Sonora Mexico this evening. This complex may send an outflow into the Valley around sunrise tomorrow morning. No gusts are expected, it would just be a shift to S/SE around 11-12Z at KPHX and KIWA. This outflow is not expected to reach KSDL and KDVT and their winds are just expected to go light and variable tomorrow morning. If the outflow washes out prior to reaching KPHX and KIWA, winds may stay westerly tomorrow morning or go light and variable. Winds are then expected to return to a more diurnal pattern late tomorrow morning with westerly winds developing and some occasional gusts into the upper teens tomorrow afternoon. FEW to SCT mid and high level clouds will continue.

Southeast California/Southwest Arizona including KIPL and KBLH: Some light showers have moved into SW Imperial County and the Imperial Valley. These showers are expected to dissipate around 21Z. Southerly winds are anticipated at KIPL through the evening with speeds in the low to mid teens, with some occasional gusts up around 20 kt possible. Winds will then go back SE'rly during the overnight hours, with speeds going back below 10 kt. At KBLH, southerly winds are anticipated through the whole period with speeds around 10-15kt, but some occasional gusts around 20 kt are possible this afternoon. A Gulf surge is then expected late tonight. This will keep winds southerly, but gusts up around 20 kt will again be possible between 05-08Z. Current SCT-BKN mid and high clouds will become FEW-SCT during the overnight hours.

FIRE WEATHER

Typical monsoon conditions are expected through much of the forecast, with daily thunderstorm activity mainly favoring the high terrain of northern AZ and southeastern AZ and low chances (10%) in the lower deserts of south-central AZ. Above normal, temperatures will contribute to seasonally dry afternoon conditions each day through the first half of the week, before temperatures once again heat up during the second half of the week. Minimum RH values of 20-30% will be common through the first half of the week before dropping into the 10-25% range late in the week as temperatures heat up again. Overnight recoveries will be fair most nights with RH values mostly in the 35-50% across lower deserts to 45-65% across southern Gila County. Outside of thunderstorm outflows, wind speeds should be fairly light through the period with only local afternoon gusts 20-25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Wednesday for AZZ530-532-537- 540-542>544-546-548-550-551.

Extreme Heat Warning until 8 PM MST Monday for AZZ531-533>536- 538-539-541-545-547-549-552>555-559.

CA...Extreme Heat Warning until 8 PM PDT Wednesday for CAZ567-569-570.

Extreme Heat Warning until 8 PM PDT Monday for CAZ560>566-568.