textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion

KEY MESSAGES

- Temperatures hovering around 3 to 8 degrees above daily normals will result in widespread Moderate Heat Risk, with isolated pockets of Major Heat Risk at times, through the middle of next week.

- An increase in moisture today and into the weekend will lead to slight chances for showers and storms, with better chances currently focused during morning and overnight periods.

- Overnight low temperatures will be noticeably warmer as a result of the increased humidity, with many of the typically warmer lower desert locales struggling to cool below 80 degrees.

SHORT TERM /TODAY THROUGH SUNDAY/

Moisture levels have rapidly increased across the forecast area in the last 12 hours, as a push of southerly flow aloft and gulf surge yesterday evening have worked in tandem to rapidly moisten the lower and middle levels of the column. Dewpoint temperatures in the 50s are now observed across much of the lower deserts, with some readings as high as the 60s in the Imperial Valley and Yuma area. The forecast remains largely on track, with the forecast area positioned under an area of excellent upper level divergence between an upper low off the Northern Baja and a subtropical anticyclone centered somewhere close to El Paso/West Texas. An initial vorticity lobe ejected northward along the Lower CO River Valley, with 500 mb RAP analysis currently placing it near the Chocolate Mountains in Imperial County. Ahead of this feature, a shield of weak radar echoes has spread out across the area, stretching from Southeast CA to as far east as the Phoenix Metro. A couple embedded, weak convective cells were observed and produced a few strikes of lightning within this initial shield of clouds/radar echoes, and this isolated threat for lightning will remain through at least the rest of the morning. CAMs continue to provide a consensus of spotty (at best) shower activity across the area today, with almost all areas seeing no measurable rainfall.

On Saturday, the model consensus is for more favorable thermodynamics for deep, moist convection across portions of AZ, however, better synoptic ascent mechanisms become displaced to the west as the subtropical high asserts itself closer to Sonora. Near- surface mixing ratios between 9-10 g/kg are shown across Southwest and South-Central AZ Saturday, with GFS bufr soundings in Yuma and Phoenix indicating MUCAPE upwards of 250 J/kg to as much as 750 J/kg. Lingering synoptic ascent, a somewhat shallow, saturated layer around 650 mb, and gradually steepening lapse rates, would suggest that at least isolated coverage of elevated showers/storms will develop somewhere across the area Saturday morning and begin to track northeastward as flow aloft shifts out of the southwest. By the afternoon, shower and thunderstorm activity will become relegated primarily to the Northern AZ high terrain. If an organized outflow were to develop and be strong enough and push downhill into the lower elevations, it is conceivable for some convection to spark over the foothills of Northeastern Maricopa County Saturday evening, but again, rainfall potential will be fairly limited. The sustained southwesterly to westerly flow aloft will continue to dry the region Sunday, but not enough to preclude the possibility of afternoon showers and storms once again over the high terrain and portions of Southeast AZ.

As the subtropical high asserts itself closer to the region this weekend, slightly warmer temperatures are likely to materialize, though H5 heights will remain largely in a 588-591 dam range through the weekend. As such, anticipate temperatures to remain in a 3F-8F above normal range, with lower desert highs mostly between 104F-111F and overnight lows from the middle 70s to the middle 80s. This weekend, the overnight lows will be above their normal values by around the same magnitude as the highs, which would almost never be the case if conditions were seasonably dry for mid June. These temperatures will result in widespread Moderate HeatRisk; though not Major (except for a few isolated pockets), this is a level of heat that can pose a risk to anyone, especially if not taking the proper precautions.

LONG TERM /MONDAY THROUGH THURSDAY/

Although uncertainty remains in the upper level pattern next week and especially by the end of the week, ensembles continue to advertise H5 height aloft remaining fairly stable through at least the first half of the week and dry (albeit weak) westerly or northwesterly flow being maintained. Temperatures may warm a few degrees Tuesday and/or Wednesday under slightly drier conditions and a period of stronger subsidence, as suggested by deeper midlevel inversions in model soundings, resulting in slightly higher coverage of Major HeatRisk for a day or two. Extreme Heat products may need to be considered for this timeframe, but confidence is currently lower than usual on the temperature forecast, as it has been fluctuating a few degrees up or down with each run over the last few days. Regardless, temperatures 3F-8F above normal will persist through at least the middle of next week, and Moderate HeatRisk will be widespread.

AVIATION

Updated at 1100Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Winds in the near term will continue to be the main aviation question mark during the TAF period as uncertainty remains regarding the usual morning E'rly shift at KPHX. It appears that W'rly winds should remain through the morning, but a period of VRB conditions, and potentially even E'rly flow, cannot be ruled out. One other forecast uncertainty will be the presence of very light shower activity through the remainder of the morning. VCSH has been added to each terminal, but the majority of any rain is likely to fall as virga, limiting any impacts at the terminal. If a heavier shower does indeed pop, some erratic winds will be the main concern. Other than those factors, W'rly flow will be common through Friday evening, with some occasional afternoon gusts in the upper teens to around 20 kt. BKN skies will be present through much of the forecast, with the lowest bases around 12k ft.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. Winds at KIPL will follow familiar diurnal trends, while KBLH can expect generally S'rly flow with afternoon gusts near 20 kt. SCT to maybe BKN cloud decks will be present through the evening, with the lowest bases near 12k ft.

FIRE WEATHER

Temperatures 3-8 degrees above normal will prevail with lower desert highs near 110F in some of the hotter spots, especially over the weekend. Improved humidity levels will prevail through the weekend, with afternoon minimums mostly above 15% and overnight recoveries in a poor to fair category, with values as low as 25% in the far western districts and between 35-50% for most other areas. With the increasing moisture will also come slight chances for isolated shower and thunderstorm activity. Chances will become almost entirely confined to the high terrain of the eastern districts by Saturday afternoon. Rainfall potential will be limited, so locally gusty outflow winds and dry lightning potentially leading to natural fire starts are the main concerns. The overall wind pattern will remain diurnal with speeds at or below 15 mph and afternoon/early evening upslope and upvalley gusts up to 25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.