textproduct: NWS Phoenix

UPDATE...Updated 00z Aviation Discussion

KEY MESSAGES

- Hotter temperatures this week will result in widespread Moderate Heat Risk 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 and 115 degrees across the lower deserts.

- Dry conditions will prevail through the next week with very limited chances for isolated showers and a few weak storms mid week for portions of Southeast California and Southwest Arizona.

SHORT TERM /TODAY THROUGH TUESDAY/

A fairly compact, W-E elongated subtropical high remains centered in the vicinity of El Paso early this afternoon, with zonal flow in place to its north across much of the Western US. As troughing in the northern stream continues eastward into the North-Central CONUS, ensembles remain in excellent agreement that the flattened subtropical high will amplify/expand, increasing heights aloft from our southeast. Guidance is in good agreement that H5 heights will peak late Tuesday into Wednesday, with ensemble mean values between 595-598 dam, around the 98th-99th percentile of climatology. This will result in increasing temperatures through the middle of the work week, with a majority of the typically hotter spots across the lower deserts expected to near or top 110F Tuesday afternoon, 3F-6F above daily normals.

As the subtropical high strengthens to our southeast, southerly flow aloft will become established and import modest midlevel moisture into the region. Current midlevel water vapor imagery does indicate a subtle plume of moisture over the Northern Baja/Gulf of California and offshore, with a collocated deformation axis and area of upper level diffluence shifting northward. Moisture may combine with this transient ascent mechanism to result in isolated shower activity over portions of the Western CWA late Tuesday, however, PoPs are mostly focused over Central Riverside County/Joshua Tree NP. The primary impact from this increase in moisture will be the introduction of midlevel cloud cover and higher humidity at the surface resulting in much warmer overnight lows heading into the middle of the week.

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 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 2310Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns under generally clear skies can be expected through Tuesday afternoon. The overall wind pattern will follow the typical diurnal tendencies with speeds aob 10 kts along with some occasional afternoon/early evening gusts in the mid to upper teens. There is a bit of uncertainty of whether or not a full easterly shift materializes overnight tonight into early Tuesday morning, especially at KPHX, with the potential for variable directions to be more prominent. Nevertheless, a full westerly shift should materialize by the mid-morning hours.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns under mostly clear skies can be expected through Tuesday afternoon. At KIPL, winds will generally be out of the southeast while at KBLH, winds will generally be out of the south. Some gusts between 20-25 kts will continue at KBLH through sunset with another round of similar gusts expected once again Tuesday afternoon.

FIRE WEATHER

Locally elevated fire weather conditions due to very low humidity and afternoon upslope/upvalley breeziness will continue through the middle of the week. Afternoon minimum humidities will improve slightly into a 10-15% range starting Tuesday, however, there will be little to no improvement in overnight recoveries as they remain mostly poor between 15-35%. Temperatures will heat up into an above normal category for much of the week with lower desert highs topping 110 degrees at least on Wednesday and Thursday, and overnight lows will offer little relief as they rise into the 80s for much of the lower deserts. Winds will remain generally terrain driven through the middle of the week with upslope/upvalley gusts between 20-25 mph during the latter half of the afternoon into the early evening. Areas of critical fire weather conditions look likely by Friday or Saturday as southwesterly winds increase across the region.

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 Warning from 11 AM Tuesday to 8 PM MST Wednesday for AZZ560.

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