textproduct: NWS Phoenix

UPDATE

00Z Aviation Discussion.

KEY MESSAGES

- Above normal temperatures will prevail through the rest of the week, resulting in widespread moderate Heat Risk and areas of Major Heat Risk.

- Extreme Heat Warnings will remain in effect for portions of the lower deserts through the end of the week.

- Thunderstorms chances will continue to focus over Southeastern Arizona and the Eastern Arizona high terrain, with low chances for lower desert storms beginning Thursday and slightly better Friday.

- A pattern shift by the weekend will result in decreasing temperatures and eventually drying conditions from west to east across the forecast area.

SHORT TERM /TODAY THROUGH THURSDAY/

Southeasterly flow aloft continues over the region as the subtropical high is now centered over Arizona. The high will peak today with heights ranging between 594-597 dam, continuing to be near to at the max of NAEFS climatological percentile. H5 heights will continue to be unseasonably high, however the positioning of the high will continue to retrograde further to the southwest, and weaken/flatten slightly by the end of the week. Despite the positioning of the high, afternoon temperatures will continue to be 3-7 degrees above normal for mid September. Morning low temperatures are also being affected and are expected to remain 8-10 degrees above normal through at least the end of the week, providing very little relief from the heat during the day. A gulf surge last night brought an increase in moisture into the lower deserts. This can especially be seen on this morning's 12Z sounding where compared to 24 hours ago PWATs increased from 1.47" to 1.79" and mean mixing ratios increased from 8.5 g/kg to 13.6 g/kg. This increase is also notable in the morning dew points, where SC AZ had dew points in the mid to upper 60 and the western lower deserts were even higher in the mid to upper 70s. This moisture and thick overnight cloud coverage, kept lows well above normal, even setting new record warm lows by 3F-6F at all three of the climate cites (Phoenix, Yuma, and El Centro). With the above normal high and low temperatures, in conjunction with more humid conditions, widespread Moderate to Major HeatRisk remain over the region, especially over the lower desert areas. Due to this the Extreme Heat Warning remains in place for portions of the lower deserts through the end of the week. Since the region has more moisture in place/ higher dew points the heat will be less manageable and individuals should take extra care when spending time outdoors, as apparent temperatures will be closer to 108F-117F, with the high range most common across SW AZ and SE CA.

While heat is currently a dominating talking point, thunderstorm chances are also worth mentioning. As mentioned above, we have plenty of moisture in place over the region, and GFS forecast soundings show MUCAPE values between 500-750 J/kg late tonight into the overnight period. During that window though CIN will also be quite high (between -100 to -150 J/kg) so development in the lower deserts remains unfavorable. The latest CAMs are still showing good signal for development in southeastern AZ, and are trying to show isolated showers developing in Pinal County late tonight, although like the past few nights this will remain conditional. In addition, the HREF shows between a 10-30% chance of thunderstorms mostly confined to the far eastern high terrain, while the best chances remain in southeastern AZ outside of our CWA, both tonight and again Thursday night.

LONG TERM /FRIDAY THROUGH TUESDAY/

The upper level pattern will begin to shift Friday, as troughing enters the Northwest US and displaces the subtropical high east of the region this weekend. Flow aloft will transition back out of the southwest, ending the brief, more typical monsoon flow pattern experienced during the middle of the work week to something more akin to a transition season setup. However, before more pronounced southwesterly drying begins in earnest this weekend, weaker flow aloft and perhaps the best moisture profiles of the week over South- Central AZ (near-surface mixing ratios 11-12 g/kg) will present an opportunity for at least one active monsoon thunderstorm day across the lower deserts (if the highly conditional threat is not realized on Thursday). KPHX GFS bufr soundings, which admittedly have been very sensitive to subtle changes in low level moisture, indicate a balance of CAPE/DCAPE around 1500 J/kg late Friday afternoon into the early evening, with weak (greater than -50 J/kg) CIN for several hours. On Saturday, thermodynamics also look similarly favorable, with the cap almost completely disappearing. Both days, the threat is once again highly conditional, as there is no obvious upper level ascent mechanism.

Ensembles continue to disagree on how far south the jet will dip with the incoming Northwest US troughing this weekend, however, it will likely remain far enough removed that jet dynamics do not come into play to influence thunderstorm chances locally. Regardless, moisture will not be entirely scoured out of Eastern/Southeastern AZ, and daily monsoon thunderstorm activity will be maintained over prominent terrain features of Eastern AZ, Southeastern AZ, and Sonora into next week. Eventually, if the subtropical high can reassert itself east of the region and/or troughing weakens and retreats north, daily convection to our south could push better moisture into South-Central AZ and once again give our area a shot at more widespread thunderstorm chances by the middle of next week. Anticipate temperatures to decrease back into a near normal range over the weekend as a result of the pattern shift, and possibly slightly below normal next week.

AVIATION

Updated at 2330Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns will exist through the forecast period under occasional passing mid to high lvl clouds. Westerly winds will continue through sunset with intermittent gusts into the upper teens possible. There is a moderate potential for a weak outflow boundary to reach KIWA and KPHX tonight, causing an earlier than usual easterly shift between 05Z-07Z. Any lingering shower activity should remain well south of the Phoenix Metro this evening and overnight.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather concerns will exist through the TAF period under periods of mid/high cigs. Winds should follow typical diurnal patterns at both terminals with speeds remaining aob 8kts along with extended periods of nearly calm conditions during the evening and overnight hours.

FIRE WEATHER

Above normal temperatures will prevail through the end of the work week, then temperatures are likely to decrease closer to normal over the weekend. Afternoon minimum humidities will generally range between 18-30% areawide through much of the week following fair to good overnight recoveries between 35-70%. Fairly typical diurnal winds with occasional afternoon upslope breeziness can be expected through Thursday, followed by an increase in southerly to southwesterly winds Friday into this weekend. Rain and thunderstorm chances will remain low and mostly limited to the eastern Arizona high terrain through midweek before slight chances become introduced to the lower deserts of at least the eastern districts late week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Friday for AZZ530-532-533.

Extreme Heat Warning until 8 PM MST Thursday for AZZ531-535>544- 546-548>551-559.

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ562-566-567- 569-570.