textproduct: NWS Phoenix

UPDATE

Updated 18Z Aviation Discussion.

KEY MESSAGES

- Hotter temperatures this week will lead to widespread Moderate Heat Risk much of the week and localized Major Heat Risk on Wednesday and Thursday.

- An Extreme Heat Watch has been issued for portions of the area on Wednesday and Thursday with forecast highs between 110 to 115 degrees across the lower deserts. - Overall dry conditions will prevail all week with only very limited chances for isolated showers and a few weak storms mid week across the western deserts.

SHORT TERM /TODAY THROUGH TUESDAY/

The subtropical high is gradually taking over across the region with the high center now positioned just south of southwest New Mexico. H5 heights have increased to 589-591dm across our area with heights forecast to continue to rise into mid-week. Temperatures will also heat up further over the next few days with the rising heights and continued dry conditions. The latest guidance has shifted back to the higher end of potential H5 heights (595-598dm) centered on Wednesday before lowering late week. Forecast highs have ticked upward 1-2 degrees from yesterday's guidance with the bulk of the lower deserts reaching 110 degrees starting Tuesday.

Despite the continued overall dry conditions, the repositioning of the high just to our southeast will shift our flow out of the south allowing for some modest moisture return beginning later today and continuing through Wednesday. Even with the southerly flow, there is not a plethora of moisture available to our south. Guidance shows surface dew points increasing to into the 40s to the lower 50s starting Tuesday, but the average moisture in the low to mid levels is not expected to increase enough to bring much in the way of convective potential. Any shower and isolated thunderstorm chances should be quite limited with a 10-20% chance focused across central Riverside County late Tuesday and then maybe extending into southwest Arizona for Wednesday.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

The increasing influence from the subtropical high pushing temperatures even hotter Wednesday into Thursday will be the main forecast concern later this week. The peak strength of the ridge is expected to occur on Wednesday with H5 heights ranging from 593-595dm across southeast California to as high as 596-598dm across southeast Arizona. This will put all of southern and central Arizona above the 97th percentile of climatology for H5 heights and near record strength across southeast Arizona. The latest NBM forecast temperatures have bounced back from the slight dip with yesterday's guidance with the bulk of the lower desert showing highs between 111-114 degrees for both Wednesday and Thursday. The increase in low level moisture will also lead to much warmer overnight lows with readings only dipping into the low to mid 80s for much of the area to as warm as the upper 80s to around 90 degrees for much of the Phoenix metro. Due to the building heat, we have issued an Extreme Heat Watch for portions of the area, including Phoenix, Yuma, and El Centro for Wednesday and Thursday.

An upper level trough will slide southeastward into the Northwestern U.S. starting Wednesday and eventually closer to our region by Friday. This feature will help to lower heights across our region starting Thursday, but there will be a noticeable lag to any decent cooling. Forecast highs Friday are still between 110-113 degrees, but overnight temperatures should cool off more quickly due to the expected drier air that will work into the area from the west starting on Friday. The trough to our north should really start to influence our region by the coming weekend, knocking down H5 heights more into a 585-588dm range Saturday to as low as 582-585dm on Sunday into next Monday. NBM forecast highs are shown to dip into or even a few degrees below normal by Sunday. The main forecast concern will turn away from Extreme Heat Friday into the coming weekend and more toward breezy to locally windy conditions causing critical fire weather concerns.

AVIATION

Updated at 1725Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns are expected during the TAF period. Winds will will follow light and diurnal trends, with perhaps a few afternoon gusts in the mid to upper teens. There is some uncertainty regarding if we see the usual E'rly shift at KPHX, and potentially even KIWA Tuesday morning as guidance does not appear too keen on this occurring. It does appear that there will be a break from W'rly flow overnight, but whether or not these locations see more VRB conditions or a full E'rly shift remains up in the air at this time. Regardless, it does appear that the shift back to the west will take place early than normal. Mostly clear skies will be common through Tuesday morning.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns under mostly clear skies can be expected during the next 24 hours. At KIPL, outside of VRB conditions during the nighttime hours, winds should favor an E'rly component. At BLH, S'rly winds will be common through most of the period, with gusts near 20 kt this afternoon.

FIRE WEATHER

Locally elevated fire weather conditions due to very low humidity and afternoon upslope/upvalley breeziness will continue early this week. Afternoon minimum humidities in the single digits today will improve into a 10-15% starting Tuesday, following generally poor overnight recoveries between 15-35%. Winds will generally remain terrain-driven, with upslope/upvalley gusts during the latter half of the afternoon into the early evening peaking around 15-20 mph today and increasing slightly up to as much as 20-25 mph Tuesday. Temperatures will warm to above normal for much of the week with lower desert highs topping 110 degrees at least on Wednesday and Thursday. Elevated fire weather conditions are expected to continue through the rest of the week with afternoon minimum humidities staying between 10-15% while winds increase further by Thursday. Areas of critical fire weather conditions look likely by Friday or Saturday.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Watch from Wednesday morning through Thursday evening for AZZ530-532-533-537-540-542>544-546-548-550-551.

Extreme Heat Watch from Tuesday morning through Wednesday evening for AZZ560.

CA...Extreme Heat Watch from Wednesday morning through Thursday evening for CAZ562-566-567-569.