textproduct: NWS Phoenix

UPDATE

Updated 18Z Aviation Discussion.

KEY MESSAGES

- Scattered showers and thunderstorms will develop over the high terrain of southcentral AZ this afternoon with much higher chances for storms surviving into the lower deserts. There is also a slight risk for severe thunderstorms in Maricopa and Pinal Counties today.

- With high pressure shifting into the Great Plains, daily shower and thunderstorm chances will continue over the Desert Southwest through much of this week.

- Seasonably hot temperatures will persist through the next several days resulting in a widespread Moderate Heat Risk.

SHORT TERM /Today through Monday/

Latest IR satellite imagery shows a decaying complex of thunderstorms progressing through Sonora MX. Gusty outflow winds from this complex may reach the Lower Colorado Valley early this morning causing winds to increase and generating patchy blowing dust in Yuma and Imperial Counties. It is possible a few showers and even some embedded lightning strikes could occur early this morning over southcentral AZ where a pocket of higher mid-lvl instability is present. As we progress into this afternoon, high pressure currently located over the Rockies will continue migrate into the Northern Plains. This pattern shift will allow 500 mb steering flow over AZ to transition from E to SE allowing for much better moisture to advect into central portions of the state.

Most of the CAMs indicate storms initiating along the Mogollon Rim and S Gila County this afternoon and generating a strong SW moving outflow boundary which will encroach on the lower deserts late this afternoon into early this evening which will likely generate additional showers/storms and areas of blowing dust. The environment across the lower deserts will consist of modest instability with ML CAPE values ranging between 500-800 J/kg and a narrow axis of higher values up to 1000-1200 J/kg mainly over the foothills E of Phoenix. Forecast soundings also indicate ML CIN values around 50-70 J/kg which may inhibit greater storm coverage across the lower deserts. Overall this setup poses a much more conditional severe threat depending on the strength of the initial outflow. Nevertheless, any storm that does develop could produce strong wind gusts up to 50-60 mph due to high DCAPE up to 1200-1500 J/kg. The Storm Prediction Center has still maintained a slight chance (15%) for severe level wind gusts (>58 mph) which includes the Phoenix Metro and the I-10 corridor south to Tucson this afternoon/evening. By late this evening into the overnight, colliding outflows from storms in Pima county and western Maricopa County will likely generate additional showers/storms across the lower deserts east of the Colorado River, where NBM PoPs are 30-40% after 00Z.

The upper-lvl pattern is not expected to change heading into Monday which will result in a very similar evolution as storms first develop over the high terrain E and SE of Phoenix and attempt to survive into the lower deserts by the later afternoon/early evening. The environment will be similar to today with maybe slightly less overall wind shear. Thus, a few strong to marginally severe storms will be possible Monday afternoon. NBM PoPs are indicating 30-40% coverage across southcentral AZ and up to 20% for the lower deserts E of the Colorado River.

Due to the increase of boundary layer moisture and 500 mb hghts ranging from 593-595 dam, high temperatures will run near to slightly above normal with lower desert locations hovering between 102-109 degrees each afternoon. The added humidity will unfortunately make it feel worse with apparent temperatures (heat index) reaching 110-115 degrees across the lower deserts. Moderate HeatRisk will remain widespread across the region, so heat precautions should continue to be exercised if plans take you outside.

LONG TERM /Monday through Friday/

There is still excellent agreement within the EPS and GEFS members regarding the placement of the upper high over the northern Plains and Upper Midwest through midweek before retrograding over the central Rockies by the latter half of the week. The overall position of the 500 mb high will maintain deep easterly flow across the Desert Southwest, helping enhance moisture transport. Latest EPS and GEFS continue to show PWATs hovering around 1.5"-1.8" through the entire week with low-level mixing ratios remaining between 10-12 g/kg. Therefore, conditions will continue to remain conducive for daily thunderstorm activity with variations in the overall coverage. Both the mean of the EPS and GEFS show an uptick in QPF across the region by the end of this week due to an easterly wave/inverted trough approaching the area by Thursday-Friday. If this occurs, this troughing feature will likely enhance convective activity potentially introduce a risk for heavy rainfall across portions of AZ. This will have to be monitored closely in the coming days.

Temperatures through the first half of the workweek will generally remain steady state at near to slightly above normal with highs generally between 102-110 degrees, resulting in the continuation of widespread Moderate HeatRisk. The NBM continues to indicate temperatures cooling to well below normal by the end of the week with highs falling into the 90s to around 100 degrees across the lower deserts by Friday. This is likely due to the increased signal for thicker cloud cover and the potential for more widespread rainfall.

AVIATION

Updated at 1818Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary concern during the TAF period will be the potential for strong gusty outflow winds and TS impacts at the terminals late this afternoon into the evening. Westerly winds will continue through the afternoon hours with speeds hovering around 10kts with periods of gusts in the mid teens this afternoon. Confidence remains high that showers and storms will develop over the eastern higher terrain and to the southeast of the terminals where an outflow boundary from the NE is expected to push into the metro as early as 22-00Z but most likely around 00-03Z as is reflected in the TAF. Wind gusts during this time remain in high agreement (70-90%) that speeds in excess of 30 kts will be possible in the Phoenix Metro. Unfortunately confidence remains low on locations where new storms might develop and the coverage of additional storms along and behind the outflows. As such, continued the PROB30 group at each of the terminals. A strong outflow out of the northeast is not favorable for producing blowing dust and thus causing reductions in visibility, but the possibility for some areas of blowing dust, VIS restrictions, and hazy conditions cannot be ruled out, especially at KIWA. Skies will be mostly SCT- BKN mid-high level clouds with the exception of any storm development where FEW low level clouds (050-060 kft) will be possible.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected through the TAF period. Winds at KIPL will remain SE'rly with winds generally between 8-14 kts, with a period of possible gusts in the upper teens near 10-13Z tomorrow morning. KBLH will maintain generally a southerly flow with gusty conditions over the overnight hours out of the ESE, otherwise continuing teetering between SSW and SSE with speeds hovering around 10 kts. Skies will remain SCT-BKN, with bases mostly above 15 kft AGL.

FIRE WEATHER

Seasonably hot temperatures will prevail through the next several days as readings will be near to slightly above normal. Scattered showers and thunderstorms will be possible this afternoon and evening bringing a threat of strong winds. Daily thunderstorm activity will be possible through this week with the potential for wetting rainfall increasing across the high terrain later in the week. Due to increasing boundary layer moisture, afternoon humidity levels will remain in a 20-30% range which will keep any fire weather concerns at a minimum. Overnight recoveries will be fair to good, or between 40-70%. Outside of any thunderstorm driven outflow, winds should favor diurnal trends with the typical afternoon breeziness around 20-25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Blowing Dust Advisory from 4 PM this afternoon to 8 PM MST this evening for AZZ539-553-554-559.

CA...None.