textproduct: NWS Phoenix

UPDATE

Updated 12Z Aviation Discussion.

KEY MESSAGES

- Temperatures will hover near to slightly above normal through early next before modest cooling arrives.

- Dry conditions will prevail through early next week with typical afternoon breeziness.

- A slight chance of thunderstorms will exist over the eastern Arizona high terrain early next week, albeit with limited rainfall potential.

SHORT TERM /Today through Sunday/

Weak negative height anomalies will translate into the southern plains over the next 48 hours with shortwave ridging building into the SW Conus through the weekend. Ensemble agreement is excellent that H5 readings near 576dm will increase into a 580-582dm range forcing a modest warming trend. With narrow spread in numerical guidance, confidence is excellent temperatures will only reach 2F-4F above normal resulting in the warmest lower elevation locations just touching 100F. This is quite typical for the Memorial Day holiday weekend, as is dry and very tranquil conditions with only modest afternoon breeziness.

LONG TERM /Monday through Thursday/

The first half of next week will be characterized by height falls over the SW Conus as strong troughing entering the Pacific NW and Great Basin combines with a weak shortwave and enhanced subtropical jet propagating across southern California/northwest Mexico. A brief period of deeply meridional flow should import marginally better moisture profiles into the eastern parts of the forecast area, however models continue to indicate no more than 5 g/kg within the boundary layer. The intensity of this shortwave and associated subtropical jet are unusually strong for this time of year, and pattern recognition suggests robust jet forced ascent and increased moisture interacting with the cooling aloft and steepening lapse rates over the higher terrain of eastern Arizona Monday afternoon. While instability will likely be limited, deep mountain convection should develop over parts of southern Gila County, though thermodynamic profiles indicate a substantial amount of evaporation which would favor dry lightning strikes and gusty outflow winds versus accumulating rainfall. Assuming this evolution come to fruition, the greatest concern would be wildfire starts in remote high terrain areas.

Forecast confidence deteriorates towards the middle of next week in response to a developing blocking pattern over the western hemisphere. Recent GFS and ECMWF operational iterations have sided more towards the majority of CMC ensemble members depicting a slow moving, closed circulation hovering over the Great Basin until the end of the week. However, GEFS members are surprisingly far more dispersive than their CMC counterparts with many members showing a far less intense and less cutoff negative height anomaly. Given the preference towards a blocking pattern, would favor the slower, more closed circulation outcome in this forecast which would be more apt to yield areas of stronger winds, albeit with more impactful speeds relegated to northern Arizona. That said, it would not be unexpected to see future automated NBM/WPC mandated forecasts advertise stronger wind gusts in the Wed-Thurs time frame once these ensemble blends converge on a similar pattern.

AVIATION

Updated at 1650Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns will exist through the TAF period under mostly clear skies. Winds will generally follow their typical diurnal tendencies with speeds aob 10 kt.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation concerns are expected during the next 24 hours under mostly clear skies. Winds generally between 5-12 kts will favor the southeast at KIPL and the south at KBLH through the entire period.

FIRE WEATHER

Temperatures will remain in a slightly above normal category through early next week before slight cooling during the middle of the week. Minimum afternoon humidity levels will fall into a 10-20% range with poor to fair overnight recovery of 20-40% improving somewhat next week. A marginal increase in moisture early next week may lead to a slight chance for afternoon/evening storms and lightning strikes over high terrain of eastern districts. This would most likely occur Monday, and with little to no accumulating rainfall, the threat for new wildfire starts due to lightning strikes will be heightened. The typical afternoon spring upslope gustiness of 15-25 mph will be common through the period with the most pronounced gusts over eastern district high terrain and through the lower Colorado River valley during the middle of next week.

PSR WATCHES/WARNINGS/ADVISORIES

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