textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion.

KEY MESSAGES

- A low pressure system moving through the Southwest will result in scattered showers and thunderstorms today along with cooler temperatures.

- Thunderstorms will be capable of producing small hail, strong gusty winds, and brief heavy rainfall.

- Strong high pressure will build into the region by midweek leading to dry weather and temperatures rapidly warming much above normal.

SHORT TERM /Today through Wednesday/

A deep cutoff low continues to wobble west of the Baja peninsula early this morning with all indications in objective height fall data and satellite imagery suggesting the primary vorticity core slowly shifting east towards northern Mexico. Synoptic ascent mechanisms remain excellent north of the cold core where moisture advection and low level convergence becomes juxtaposed with left front jet quadrant dynamics. This is already reflected in banded, nocturnal storms along the international border, though this activity may wane after sunrise. The aforementioned cutoff will only slowly migrate into central/northern Sonora over the next 36 hours providing an extended time frame of favorable synoptic scale ascent over the forecast area resulting in scattered showers and embedded thunderstorms.

One aspect of forecast uncertainty and a primary limiting factor for more expansive and intense storms will be only modest 6-7 g/kg mixing ratios advected into the CWA combined with seasonably warm midtropospheric temperatures. As a result, CAPE measures largely only peak in a 250-500 J/kg range forming a tall, skinny unstable profile. However, 0-6km deep layer shear should increase close to 45kt providing the opportunity for organized structure and brief supercellular characteristic to the most robust and persistent updrafts. There's a general consensus among high resolution models indicating multiple bands of showers and embedded storms blossoming initially in SW Arizona during mid afternoon, then formulating eastward through the evening and overnight. Individual shower elements should propagate rapidly north (20-30kt) under the strong shear profiles limiting rainfall accumulations in any single location, though conceivably some areas over 0.25" totals would not be unexpected. Otherwise while some small hail is possible, the larger threat appears to be strong, gusty winds due to momentum transport tapping the strong jet aloft within the strongest storms.

Subsidence should gradually filter into much of the forecast area Monday night, however forecast confidence is good that the slow forward motion of the attendant cutoff will maintain moist ascent into the form of deformation banding and light showers across southern Gila County through Tuesday afternoon. Cloud cover and the proximity of the cold core will force temperatures holding near the climatological normal through Tuesday before rapid height rises ahead of building subtropical ridging envelops the SW Conus Wednesday.

LONG TERM /Thursday through Sunday/

Forecast confidence for the latter half of the week is excellent given remarkable ensemble agreement showing a low amplitude northern stream jet constricting poleward towards a Hudson Bay vortex allowing strong subtropical ridging to expand into the SW Conus. H5 heights will settle into a 585-588dm range through the majority of the period with only a marginal chance of heights/thermal fields dampening over the weekend due to shortwave energy digging into the Upper Midwest/Great Lakes. Ensemble guidance spread is extremely narrow and reflects climatologically anomalous tropospheric height/thermal measures. Surface temperatures 15F-20F above normal will become common by the end of the week resulting in widespread minor HeatRisk while also setting daily records (see Climate section). It would also not be out of the question for some the warmer, lower desert communities to achieve the first 100F of the season as the NBM 75th percentile easily covers this threshold. Just beyond this forecast period, essentially all ensemble members indicate even warmer weather as East Pacific blocking edges into the SW Conus and subsident ridging builds stronger. The ensemble guidance envelop is solidly above the 100F threshold for all lower desert communities portending a prolonged period of record setting warmth.

AVIATION

Updated at 1123Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation hazard for the forecast period will be the possibility of VCTS/TS by this evening where the main impacts will be gusty winds and small hail. Before this evening's activity no significant aviation concerns are expected with VFR conditions persisting until then. Wind directions are expected to switch from NE to NW throughout the morning hours today then beginning to shift SW'rly by the early afternoon. Wind speeds through the morning and afternoon hours will be generally aob 10kts, then by the evening (~00Z), wind speeds above 10 kts will be possible, most notably at KIWA. SCT-BKN mid-lvl clouds with bases around 8-10 kft will develop over the region by this afternoon with bases expected to decrease to no lower than 6 kft. Most of the activity is expected to the east of the terminals by late tonight but lingering VCSH may be possible after the main passage.

Southeast California/Southwest Arizona including KIPL and KBLH: The main aviation concerns at the southeast CA terminals will be scattered SHRA and a low chance (10-20%) of a thunderstorm this afternoon and early evening. Winds will favor a E-SE component at KIPL and will predominantly S-SW at KBLH through the afternoon. Wind speeds at both terminals should generally be around 8-11 kt, before becoming light and VRB by the end of the TAF period. By early tomorrow afternoon, SHRA/VCSH is expected to materialize over SE California. There is also a low chance (10-20%) of an isolated t-storm impacting either terminal, mainly between 21Z-00Z. SCT-BKN mid-high clouds with bases around 10-12 kft will persist through the evening hours.

FIRE WEATHER

A weather system moving through the region today through early Tuesday will bring cooler temperatures, increased humidity levels, and chances for wetting rainfall. Dry weather with unusually warm temperatures and low humidities will return during the latter half of the week. Minimum humidity levels will only fall into a 25-40% range this afternoon with good to excellent overnight recovery. Better than a 50% chance of wetting rain with embedded thunderstorms will exist in eastern districts through Tuesday afternoon before a drying trend enters the region. Conditions will trend markedly drier through the week with minimum afternoon humidities falling into a 5- 15% range by the end of the week while poor to fair overnight recovery of 15-30% materialize. Outside of thunderstorm wind gusts today, winds will be fairly light with only modest afternoon upslope gustiness limiting a greater fire weather danger.

PSR WATCHES/WARNINGS/ADVISORIES

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