textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion.

KEY MESSAGES

- Chances for scattered showers and isolated thunderstorms are expected today and Thursday with the most activity focused over foothill and high terrain areas.

- Along with the chances for rain, gusty storm driven outflow winds will be possible today and Thursday, potentially leading to patchy blowing dust.

- Near to slightly above normal temperatures will persist through the coming weekend before additional cooling is expected early next week.

SHORT TERM /Today through Thursday/

Broad upper level troughing remains positioned over the Southwestern U.S. with several smaller scale disturbances within the larger scale trough and abundant moisture from eastern Arizona through New Mexico. The main forecast concerns for today and Thursday will be the potential for isolated to scattered thunderstorms which will be capable of producing strong downdraft and outflow winds, along with very localized heavy rainfall. Current objective analysis shows lower level easterly flow increasing across southeast into south-central Arizona spreading moisture westward while water vapor imagery shows multiple disturbances south and east of our area.

For the rest of this morning and into the afternoon, the easterly flow is expected to increase moisture levels with low level mixing ratios of 8-10 g/kg shifting into south-central Arizona. The forecast for this afternoon through early tonight remains somewhat uncertain as the main convective potential should be focused well to our east in New Mexico associated with disturbance moving north northeastward through the state. For our area, we will fall under larger scale ascent associated with the trough and a jet max to our north leaving the higher terrain as the best area of focus for shower and thunderstorm development this afternoon. Given the available moisture and modest heating today under partly sunny skies, forecast MUCAPE should reach 1000 J/kg over the south- central Arizona lower deserts to 500-750 J/kg to our north and east. Hi-res CAMs have been inconsistent with the amount of convection for today into tonight, but they have mostly kept with initiation over the higher terrain later this afternoon. Given a somewhat dry environment in the lower deserts, DCAPEs of 1200-1500 J/kg are expected which should lead to high chances of one more more outflows being generated. Even though the CAMs have been fairly inconsistent, storm outflows are expected which in turn may be enough to initiate further storm development over the lower deserts this evening. These chances (20-30%) will be contingent upon at least one fairly strong outflow, or even colliding weaker/smaller outflows. Some iterations of the CAMs have even suggested a large strong outflow progressing as far west as La Paz and Yuma Counties, sparking brief showers and storms late in the evening to as late as 2 AM tonight.

Thursday's forecast looks even murkier as moisture is expected to continue to spread westward through southwest Arizona into southeast California, while the upper level ascent is likely to remain positioned over the eastern half of Arizona. HREF shows even higher MUCAPEs for Thursday with the bulk of area reaching 1000-1250 J/kg during the afternoon hours while an upper level shortwave is forecast to move through southeast Arizona. However, there seems to be even more uncertainty within the models as the CAMs show little to no convection developing across southern Arizona with the majority staying to our north. Given the higher instability and at least some upper level ascent, it would seem we'd have at least similar chances as today, but NBM PoPs are mostly below 10% for the lower deserts. Forecast soundings do still show weak to possibly modest convective inhibition over the lower deserts, so like today we'd mostly have to rely on some sort of strong outflow or outflow collisions for storms to develop over the lower deserts. Needless to say, uncertainty remains quite high for Thursday's rain chances. If storms actually do get going on Thursday, gusty winds and localized heavy rainfall will again be possible given the expected decent instability and modest DCAPE.

LONG TERM /Friday through Tuesday/

By Friday, the main upper level trough is forecast to shift northeastward out of a our region leaving dry southwesterly flow in place. Guidance shows low level mixing ratios quickly dropping to 5-7 g/kg across the lower deserts to only 6-8 g/kg over the Arizona high terrain. As a result, Friday will begin another dry period with only lingering chances for some isolated showers and maybe a few thunderstorms over the eastern Arizona high terrain. Temperatures are forecast to remain quite stable over the next few days with highs right around to slightly above normal and lows a few degrees above normal.

Westerly dry flow should persist through at least Saturday with large scale troughing remaining to our northwest while the subtropical ridge slowly shifts southwestward from east Texas into the interior of Mexico. This shift in the ridge is likely to bring higher heights temporarily for our region later Friday through Saturday, so daytime highs Saturday are likely to top out between 100-104 degrees (4-6 degrees above normal) under mostly sunny skies.

Forecast uncertainty increases dramatically going into early next week as it will likely depend upon what happens with Hurricane Polo, the positioning of the subtropical ridge, and an upper level trough that may swing through our region on or around next Monday. The latest NHC track for Polo has it slowly making its way westward over the next few days before turning more to the north this weekend, but staying west of southern Baja. This track may be enough to help advect tropical moisture northward as far as our area, but uncertainty remains quite high. There is a low end potential of tropical moisture making its way northward into our area just before the aforementioned trough swings through our region and this could lead to chances for strong to severe storms along with heavy rain potential on or around Monday. The more likely scenario would involve modest moisture making its way into our area, but still with the support of the trough. The trough may still be strong enough to support some strong to severe storms due to much higher than normal shear, but with less moisture it would lower the severe threat as well as well as the threat for heavy rainfall. At this moment, we are unsure which solution will win out, but we expect to have a better idea by the weekend.

AVIATION

Updated at 1130Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Potential for SHRA/TSRA and gusty outflow winds will be the main aviation concerns through this evening. Light E flow will continue before a more pronounced E/SE wind develops after sunrise with speeds up to 8-12 kts and occasional gusts into the mid-teens to around 20 kts. Storms will likely develop over the high terrain and foothills within 50 miles of the Phoenix airspace this afternoon. These storms will likely send a NE outflow into the valley by early this evening. Timing currently looks to be around 01Z-02Z for the arrival of this outflow with gusts reaching 30-35 mph for a brief period of time. There is still around a 20-30% chance of TS impacting any of the terminals this evening so have maintained a PROB30 group from 02Z-05Z. After any lingering showers clear east of the region, winds should remain E-SE tonight. FEW to SCT clouds bases down to 8-10K ft AGL will be common through the TAF period with SCT to BKN mid and high clouds.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather impacts are anticipated through the next 24 hours with mostly clear skies. Winds will favor W/NW at both KIPL and KBLH through this morning and then a shift toward E/SE by the afternoon, with wind speeds mostly staying below 10 kts along with extended periods of light variability.

FIRE WEATHER

Today into tonight will bring modest chances 30-50% for showers and thunderstorms, focused over the eastern districts, while the western districts should mostly remain dry. Gusty outflow winds with abrupt wind shifts are expected during the first part of the evening. MinRHs will fall into a 15-20% range today for the western districts and 20-25% Thursday, while the eastern districts will fall to 25-40% today and Thursday. Winds across eastern districts will generally maintain a east southeast direction, frequently becoming gusty while lighter, diurnally driven winds will persist in western districts.

PSR WATCHES/WARNINGS/ADVISORIES

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