textproduct: NWS Phoenix

UPDATE

Updated 12Z Aviation Discussion.

KEY MESSAGES

- Lingering showers and thunderstorms are expected through around sunrise this morning across south-central Arizona followed by some isolated storms in the afternoon over high terrain areas.

- After a brief reprieve from the extreme heat today over south- central and eastern Arizona, highs will return to around 110 degrees by Friday, lasting into early next week.

- Extreme Heat Warnings are in effect through Sunday evening due to continued areas of high-end moderate to major HeatRisk.

- Thunderstorm chances will dip starting Friday, likely remaining confined to the Arizona high terrain through at least Saturday.

SHORT TERM /TODAY THROUGH SATURDAY/

It is amazing what can happen when the lower deserts finally gets a virtually uncapped atmosphere. Models were showing the potential for a significant outflow along with scattered storms developing into the south-central Arizona lower deserts, but there wasn't a strong consensus on its evolution. Yesterday was just one of those days where everything came together perfectly for a large MCS to develop and to produce fairly widespread strong to severe winds. Many locations within and around the Phoenix area reported wind gusts between 50-60 mph with a few reports around 65 mph. The afternoon balloon launches for KPHX both showed 1500-1800 J/kg of CAPE with similar values of DCAPE, while CIN was low enough to be easily overcome from a modest to strong outflow. The convection over the high terrain became organized enough to persist into the lower deserts with additional development occurring once the CAP was broken. The storms are even redeveloping early this morning along a mid-level convergent line from Phoenix to Show Low. Hi-res CAMs somewhat show this line of convection, but they have no idea how long it may last. Any training of storms within this southwest to northeast line could result in localized additional rainfall amounts of up to an inch through 4 AM. We anticipate this activity to gradually wind down by sunrise.

The rest of today is likely to be much more quiet as the atmosphere was effectively overturned by the widespread storm activity. Temperatures have cooled substantially with readings into the 70s over portions of the south-central Arizona lower deserts. Cloud cover is also expected to linger through a good portion of this morning, potentially keeping daytime highs from even reaching 105 degrees in the Phoenix area. Hi-res CAMs are finally catching on to the overturned atmosphere showing mostly scattered convection along the Mogollon Rim through southern Gila County this afternoon, but then dissipating before moving into the lower deserts. Have adjusted PoPs to around 10-15% in the Phoenix area to account for the more stable environment. Little has changed with the western lower deserts with temperatures remaining quite hot with elevated dew points. Forecast highs of 109-112 degrees are expected across southwest Arizona and southeast California with even higher heat indices.

The subtropical high center is favored to shift east northeastward into Colorado on Friday and then potentially wobble southeastward into eastern New Mexico and/or into western Texas by Sunday. The expected shift in the high should turn winds out of the south ushering in drier air Friday into Saturday. Guidance shows enough drying to push low level mixing ratios to 9-10 g/kg on Friday to as low as 7-8 g/kg on Saturday. PoPs for both Friday and Saturday drop to below 10% for the lower deserts to 10-20% over portions of the high terrain. There is some guidance showing more convection forming near the Mexico/Arizona border on Friday, so we can't rule out some sort of outflow emanating northward into our area Friday evening.

Temperatures will also recover by Friday with daytime highs topping out at or just above 110 degrees across the lower deserts. Forecast dew points are also shown to decrease closer to 50-55 degree range by Saturday lowering heat indices closer to 110 degrees.

LONG TERM /SUNDAY THROUGH WEDNESDAY/

Model uncertainty remains quite high for the first half of next week as the position of the high is likely to set up somewhere over eastern New Mexico and western Texas by Monday with little to no movement into the middle of next week. This position can cause higher uncertainty in moisture levels for our area with the latest guidance trying to advect some moisture into our area by Sunday or Monday, but NBM forecast PoPs are at most 10-15% for the lower deserts. At this point, we could continue to see mostly slight chances favoring the high terrain into early next week, but if moisture does eventually improve enough storm chances are likely to be higher than what the NBM is depicting. Another wildcard will be the potential influence of an upper level trough that may attempt to move into western portions of our region Sunday into Monday. If this occurs, it may be enough to improve convective potential across southeast California and western Arizona for Sunday and/or Monday.

Temperatures are expected to continue to be more influenced by the strong subtropical high as H5 heights hover between 593-596dm across the Desert Southwest through early next week. Daytime highs are not expected to change much day to day from Sunday through early next, barring any significant change with the convective chances. The Extreme Heat Warning is currently in effect through Sunday, but it seems quite likely it will need to be extended again at some point.

AVIATION

Updated at 1055Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Lingering rain and a few isolated thunderstorms continue across the Phoenix Metro early this morning. All activity is expected to exit the Metro over the next 1-2 hours. Then, dry conditions are expected through the rest of the TAF period. Winds will be light and variable this morning (but may tend to favor a SE component) prior to developing the usual westerly component this afternoon. The main aviation concern will be on if any outflows can make it into the Phoenix Metro. Current hi-res models show a NE outflow approaching the Phoenix Metro, but models differ on the exact strength of this outflow and how far it will survive into the Valley. As of now, I have added this outflow in as a FM group for KDVT and KSDL but have left it as a TEMPO at KPHX and KIWA. Then, SE winds are expected overnight and through tomorrow morning. SCT- BKN cloud decks with bases as low as 10 kft are expected through the majority of the TAF period. A period of only FEW clouds is expected this afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns are expected during the next 24 hours. At KIPL, SSE winds will continue before going W this evening. Wind speeds will generally be aob 10 kt. At KBLH, winds will be light and variable this morning before developing a S component by the mid morning hours. A brief period of SW winds is expected tomorrow evening before going back S overnight. Winds will be aob 15 kt. FEW-SCT cloud decks will be present through much of the forecast window, with the lowest bases around 15 kft.

FIRE WEATHER

Elevated moisture levels will persist today with a temporary reprieve from the extreme heat across the eastern districts. Expect mostly isolated showers and thunderstorms this afternoon and early evening over the high terrain areas. Drier air will then work its way in for Friday and Saturday with even lower chances for rain while temperatures heat back up to extreme levels. MinRHs of 20-30% today will lower to 15-25% beginning Friday. Winds should overall follow the typical diurnal tendencies with some modest afternoon upslope gustiness.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Sunday for AZZ530>544-546- 548>551-553>555-559-560.

CA...Extreme Heat Warning until 8 PM PDT Sunday for CAZ562-565>567- 569-570.