textproduct: NWS Phoenix

UPDATE

06Z Aviation Discussion.

KEY MESSAGES

- Much above normal temperatures will continue through the weekend with readings approaching record levels and Extreme Heat Warnings in effect.

- Strong outflow boundary winds will move through South Central Arizona later this afternoon and this evening, with a slight chance to generate showers and storms over the lower deserts.

- 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 work week, high pressure will strengthen over the region once again late in the week, resulting in another period of unseasonably hot temperatures and widespread Major Heat Risk.

SHORT TERM /TODAY THROUGH MONDAY/

Current midlevel water vapor imagery and 500 mb analyzed heights from the RAP place the center of an expansive subtropical anticyclone approximately over the Four Corners/Colorado Rocky Mountains early this afternoon. Meanwhile, an inverted trough associated with the remnants of TC Bertha remains apparent in water vapor imagery moving west northwestward over Baja California. To its north, angling towards the center of the subtropical high, it continues to slowly drag an axis of vorticity across portions of the state. This had resulted in enough lift and a completely uncapped environment over the prominent terrain stretching from Flagstaff southward and along the Mogollon Rim such that thunderstorms began firing as early as 11 AM MST. This activity will trend southward through the afternoon, likely impacting the northern fringes of Maricopa County and Southern Gila County and sending a strong (potentially 30-50 mph gusts), somewhat organized outflow into the lower elevations later this afternoon into the evening. It is highly uncertain whether this boundary will produce enough lift to break the cap and trigger lower desert showers/storms, thus chances remain slight (10-20%). Inverted-V soundings and DCAPEs upwards of 1500- 1800 J/kg across South-Central AZ support strong, gusty winds with any thunderstorms that do develop and will also help encourage a long-lived outflow propagating through the Phoenix area and into Southwest AZ this evening. Brief heavy downpours will be a secondary hazard to contend with, especially with storms over the foothills and higher terrain to the east. Limited instability (generally under 500 J/kg MLCAPE) over the South-Central AZ lower deserts will limit the intensity of any storms that do develop, with better probabilities for severe storms focused from Pinal County south and east towards the international border, where SPC has indicated a Marginal risk today. Marginally better shear and instability further south will focus the threat for some of the most intense storms mostly towards the international border.

Across the vast majority of the forecast area, the main hazard this weekend remains dangerously hot temperatures. Despite the gradual northward migration of the subtropical high, NAEFS mean 700-500 mb temperatures and H5 heights are advertised in excess of the 90th percentile of CFSR climatology through Monday, resulting in continued above normal temperatures. After another morning with Phoenix Sky Harbor tying the all-time record warm low (97F), highs today should reach near 10F above daily normals but still likely shy of the daily records by a 1F-3F. Phoenix Sky Harbor has already reached 116F this afternoon as of writing this. With an uptick in thunderstorm activity this weekend, there will also be an increase in remnant mid and high level cloud decks, flowing downstream over the forecast area and limiting overall insolation. With outflow boundaries passing through the area, near-surface moisture will also have increased by tomorrow, further limiting the peak temperatures we may reach. Latest NBM seems to be picking up on this and reducing forecast highs by several degrees Sunday, but very warm overnight lows will maintain HeatRisk in a Major category across much of the area. Major HeatRisk coverage drops off further Monday, but is still evident across large portions of the area. As such, Extreme Heat Warnings will remain in effect across the area through Monday evening. Temperatures do not drop off considerably heading into the middle of the work week, and so extensions may be considered in future updates.

Over the next few days, the position of the high, easterly to southeasterly flow, and sufficient moisture in place will remain supportive of daily thunderstorms forming over the northern AZ high terrain and portions of Southeast AZ during the early-mid afternoon. Later in the afternoon and into the evening, chances for storms over the South-Central AZ lower deserts cannot be ruled out, with moderate instability in place and 10-11 g/kg mixing ratios. However, latest HREF has backed off considerably on storm coverage for tomorrow, perhaps picking up on subsidence from the departing weak inverted trough axis.

LONG TERM /Tuesday through Friday/

Strong, expansive ridging will remain nearly in place through next week across the southwest and central United States with the anti-cyclone center rotating back from central Colorado into New Mexico and Arizona. Early in the week, sufficient easterly flow will prevail over the region juxtaposed with marginally supportive mixing ratios of 9-11 g/kg for scattered storms. There remains considerable uncertainty regarding a more preferred time frame for higher POPS in any given location with mesoscale details likely becoming more important towards convective evolution. Overall, this appears to be a very typical monsoon pattern with alternating convectively active and inactive days. However, by the latter half of the week, there are a handful of NAEFS members indicating notable strengthening of the high pressure with 600dm H5 heights materializing and the center of the anticyclone shifting to over northern AZ/southern UT. It appears the NBM/WPC forecasts are latching onto this scenario as POPs dwindle to much lower than expected values given the ensemble mean synoptic pattern. The larger impact of this outcome would be a return to major HeatRisk and extreme heat thresholds towards the end of the week, after the very short-lived reprieve at the beginning of the week. As a result, the current Extreme Heat Warning may actually be extended as one prolonged warning event.

AVIATION

Updated at 0530Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Aviation concerns will remain minimal through the forecast period. A secondary outflow boundary will progress through the Phoenix Metro over the next few hours. This will promote a continuation of easterly to southeasterly flow at all terminals through tonight. Winds will then return out of the WSW early Sunday morning, mainly around 15Z-16Z. Westerly flow will increase through the afternoon with gusts reaching into the teens to around 20 kts. There is a much lower chance of an outflow reaching the metro TAF sites on Sunday. SCT to BKN mid to high clouds will prevail.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation concerns are anticipated through the forecast period. Aside from a gusty easterly winds at KBLH over the next hour or two, winds should follow typical diurnal trends at both terminals through the period with a predominant southerly component at KBLH and a southeasterly direction at KIPL. Speeds should remain at or below 10 kts at both terminals. FEW to SCT mid to high clouds will continue to progress over the region.

FIRE WEATHER

There will be better chances for showers and storms this weekend into early next week, than the previous few days, but with activity still mostly favoring the high terrain of northern AZ and southeastern AZ and slight chances (10-20%) in the lower deserts of south-central AZ. There is high confidence that strong gusty thunderstorm outflow winds will progress into the lower deserts late this afternoon and this evening, which may offer slightly better odds for rain over the lower deserts today than in the following few days. Extreme, well above normal, temperatures will contribute to seasonally dry afternoon conditions. Hottest temperatures will be today, with minimum afternoon RH values in the 10-20%. Temperatures then fall several degrees heading into the beginning of next week and moisture increase slightly, leading to minimum RH values improving slightly to 20-30%. Temperatures will warm back up again during the latter half of next week and minimum RHs will again fall to the 10-25% range. 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. Daily afternoon thunderstorms will continue through next week, mostly across higher terrain and 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 Monday for AZZ530>555-559.

CA...Extreme Heat Warning until 8 PM PDT Monday for CAZ560>570.