textproduct: NWS Phoenix

UPDATE

18Z Aviation Discussion

KEY MESSAGES

- Extreme Heat Warnings remain in effect through Saturday with areas of Major to Extreme Heat Risk, despite temperatures slowly decreasing.

- Today will bring low end chances for showers and isolated thunderstorms with increasing chances beginning Saturday.

- An approaching low pressure system will bring much better chances for rain, including a chance of strong to severe storms across Southern Arizona mainly on Sunday and Monday.

SHORT TERM /TODAY THROUGH SATURDAY/

Current objective analysis shows the subtropical high center along the Arizona/New Mexico state line while a slug of decent mid and upper level moisture streams northeastward through California into the Great Basin. Some of this moisture did make its way into Arizona earlier on Thursday, but a pocket of drier air is already moving into the state from the south around the high center. The ridge is also in the first stages of weakening while slowly starting to shift farther to the east. The drier air should limit convective potential today, but CAMs are at least showing some potential focused over the eastern Arizona high terrain and over southeast California. Overall, we are not anticipating much with this activity, but given the dry boundary layer we can't rule out some strong gusty winds with any modest thunderstorms that may develop. This wind threat will mostly cover Imperial County where the highest instability is expected. Temperatures today are forecast to be very similar to yesterday's highs of 110-114 degrees, but clouds may hold temperatures a couple degrees cooler in some areas.

As the ridge shifts more into New Mexico and Texas tonight into Saturday, it will be the start of a significant pattern shift. A large Pacific trough off the Northwest U.S. coast will get reinforced and slide farther to the south and eventually to the east. During this transition this weekend, our region will see modest to strong moisture advection with low level mixing ratios gradually improving from the current 7-8 g/kg to 10-11 g/kg late Saturday before peaking on Sunday and Monday at a respectable 11-12 g/kg. Guidance suggests some potential for elevated showers and weak thunderstorms Friday night with the first big push of moisture. As of now, the best chance for this rain should be from Phoenix to just to the east of Phoenix.

What occurs Saturday afternoon/evening is still quite uncertain as it will be more of a conditional type environment for storm development. Moisture is expected to have increased enough to bring MUCAPEs up to around 1000 J/kg into south-central Arizona, but not enough to erase all of the CIN. The question is where exactly will showers and storms develop Saturday afternoon and if a large enough cluster can develop to force a strong or multiple outflows into the lower deserts. It is looking more encouraging for storm potential as the amount of CIN does not look too high and any decent outflow should overcome it. We will also start to see some upper level support from the incoming trough. The latest 06Z HRRR hints at a cluster of storms firing across Pima County late Saturday afternoon and then progressing northward or at least additional storms developing along a northward progressing outflow during the evening. With a high DCAPE environment expected, strong gusty winds are very possible across Maricopa and Pinal County Saturday evening. Saturday should also be the last of the extreme heat with forecast highs dropping to 107-111 degrees before a much more noticeable cool down begins Sunday.

LONG TERM /SUNDAY THROUGH THURSDAY/

Sunday and/or Monday are still looking quite interesting for thunderstorm potential with the environment potentially supporting some severe storms. Some ingredients for strong to severe storms are expected to be present, like upper level support and sufficient instability, but wind shear will be somewhat lacking. Moisture is forecast to improve enough to support a nearly uncapped environment across south-central Arizona Sunday and potentially again Monday afternoon with MUCAPEs as high as 1500-1800 J/kg. Both days have the potential to bring scattered to numerous showers and thunderstorms across south-central and eastern Arizona. Strong to localized severe wind gusts, blowing dust, heavy rainfall, and flooding all look possible, but as stated above the threat may be localized due to the fairly weak shear which is oftentimes needed to sustain strong storms or clusters of storms. We should have a better idea on how this strong to severe storm threat may evolve by Saturday/Saturday night. Guidance then suggests some drying will occur by Tuesday which should decrease rain chances substantially through the middle of next week.

Temperatures will drop into or even just below normal early next week, but highs are still forecast to top 100 degrees each day across the lower deserts. Once the drier air moves into the region by the middle of next week, we expect temperatures to quickly warm to a few degrees above normal but with little chance of reaching 110 degrees again.

AVIATION

Updated at 1735Z.

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

A gusty southerly outflow with some lofted haze this evening will be the primary aviation weather issue during the next 24 hours. In the meantime, winds will gradually shift out of the west by the early to mid-afternoon hours with a period of southerly winds before the eventual westerly shift. Overall speeds through this afternoon are forecast to remain aob 10 kts. Thereafter, guidance continues to show a southerly outflow from convective activity in southeast AZ propagating into the area, sometime between 03-06z, with some gusty winds potentially upwards of 25-30 kts as well as some reduced VIS due to lofted HZ/BLDU. After the outflow passage later this evening, VCSH is possible during the overnight period, mainly between 06-12z, with minimal operational impacts expected. SCT to at times BKN mid/high cloud decks will be common through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

Isolated TS activity, especially at KIPL, this afternoon will be the primary aviation weather issue during the next 24 hours. Winds will fluctuate between the southeast to west at KIPL and south to southwest at KBLH. Heading into this afternoon, TS activity over the Baja Spine may drift into the Imperial Valley, affecting KIPL, however, there continues to be uncertainty in the overall coverage and intensity of the TS activity and thus a PROB30 remains in the latest TAF package. If a TS would to materialize, gusty winds and reduced VIS due to HZ/BLDU will be the main impacts. KBLH may see an outflow from the activity to the southwest, but confidence remains too low for inclusion in the latest TAF package. FEW-SCT mid/high cloud decks will be common throughout the period, with occasional BKN decks with any convective activity.

FIRE WEATHER

Hot and relatively dry conditions will continue into this weekend, but with slight chances for isolated showers and thunderstorms. Chances then increase considerably by Sunday, especially across Southern and Central Arizona. Afternoon minimum humidities will improve slightly to 15-20% today and then 17-25% Saturday. Winds will be relatively light and follow diurnal tendencies today before south to southwesterly winds increase Saturday, particularly across the western deserts, where afternoon gusts increase up to 20-30 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Saturday for AZZ530>555-559- 560.

Extreme Heat Warning until 8 PM MST this evening for AZZ556-561- 562.

CA...Extreme Heat Warning until 8 PM PDT Saturday for CAZ562-565>567- 569-570.

Extreme Heat Warning until 8 PM PDT this evening for CAZ561-563- 564-568.