textproduct: NWS Phoenix

UPDATE

Updated 00Z Aviation Discussion.

KEY MESSAGES

- Seasonably hot temperatures will be common through this weekend and into next week, with today being the hottest as areas of Major Heat Risk linger across portions of the region, keeping Extreme Heat Warnings in effect.

- Chances of showers and thunderstorms will remain confined to southeastern Arizona through Saturday, though distant outflows may bring gusty winds and areas of blowing dust to eastern portion of the lower deserts Saturday evening.

- Deeper and more widespread moisture will increase shower and thunderstorm chances for the lower elevations of south-central and southwestern Arizona by Sunday.

SHORT TERM /Today through Saturday/

500mb analysis revealed through the morning that the sub-tropical high has begun its northeastward migration, setting up what looks to be an active convective period by the end of the weekend and into next week. Before we get there though, most our forecast area will continue to remain dry through Saturday as relatively dry northerly flow and subsidence aloft combine to subdue shower and thunderstorm activity for the most part. With plenty of insolation, and heights associated with the high hovering around 594dm, widespread readings between 110-115 are expected for lower desert areas this afternoon, resulting in significant coverage of high-end Moderate HeatRisk, with patches of Major HeatRisk for the Phoenix metro, Lower Colorado River, and Imperial Valleys. As a result, Extreme Heat Warnings for these areas under the Major category will linger for one more day.

The entirety of Arizona will not be void of rainfall though through this timeframe, as sufficient enough moisture remains in place across the southeastern quadrant of Arizona. The main focus of convection will be across the Tucson CWA, but areas along the Mogollon Rim will start to see some activity on Saturday as moisture starts to work its way further north. For our coverage area, the best chances (20-30%) of showers and storms will be focused over far Gila County Saturday afternoon. With that being said, thunderstorm impacts could have a much farther reach as there is a good signal that a strong outflow will work its way through portions of Maricopa and Pinal Counties by Saturday evening. Exact timing and strength of a potential boundary still needs to be worked out by hi-res data, but the main impacts would be gusts near or exceeding 35 mph and areas of blowing dust. A rogue shower or thunderstorm initiated by this potential boundary over Pinal County cannot be ruled out, but chances are only around 10% at most as model soundings do not indicate an overly impressive thermodynamic profile. However, the overall setup by Sunday looks much more conducive for lower desert rainfall.

One other more subtle aspect of the forecast will be the potential for breezy to windy conditions for parts of the Lower Colorado River Valley the next few mornings. Thanks to an outflow from storms over Northern Sonora last night, locations such as Yuma and Blythe, CA observed gusts ranging between 30-40 mph prior to sunrise, and accompanying dust kept visibilities reduced well into the morning hours. Hi-res guidance paints a very similar picture for tonight into Saturday and once again Saturday night into early Sunday with gusts potentially reaching 40 mph which could generate areas of blowing dust. It is not a forgone conclusion that these enhanced winds will occur, as they will be heavily conditional on how distant convection evolves, but given the impacts observed from the fist round seen last night, near- term instances should be monitored at least.

LONG TERM /Sunday through Thursday/

Excellent agreement remains in place amongst the guidance of the upper high quickly migrating northeastward into the Plains and Upper Midwest regions through next week. This overall pattern setup will maintain easterly flow in place across the region, helping to transport moisture and transition the Desert Southwest into a more active monsoonal pattern. The latest EPS and GEFS indicate PWATs climbing and remaining above 1.5" through most of next week along with low-level mixing ratios remaining above 10 g/kg.

As mentioned previously, Sunday will offer the first real shot this monsoon season for more widespread convective activity to affect the south-central AZ lower deserts, given the more favorable thermodynamic environment that will be in place and this is reflected in the latest NBM with PoPs solidly in the 40-60% range by the early evening hours. The chances for additional convective activity will continue each day through next week and will expand further westward to include the western deserts with daily variations in the overall coverage. There are indications from guidance that a rather organized easterly wave/inverted trough may move in from the east and affect the region sometime towards the latter half of the week. However, given that the synoptic models are notoriously poor in the overall handling of these easterly waves/inverted troughs, high uncertainty exists in the overall forecast. If the easterly wave/inverted trough scenario pans out, then there is a good chance for enhanced convective activity to materialize across much of the region. This is something that will have to be monitored during the next several days as it could be highly impactful.

With the elevated moisture levels and more cloud cover that will be in place, temperatures through the much of next week will remain near to slightly above normal with readings across much of the lower deserts in the upper 100s to around 110 degrees for afternoon highs and overnight lows in the 80s. This will maintain the overall HeatRisk levels solidly in the moderate category.

AVIATION

Updated at 0045Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Lofted dust/haze impacting slantwise visibility and uncertainty in wind shifts and gusts are the main weather issues under a gradual increase in mid/high cloud decks. Westerly winds with some occasional gusts 15-20kt will affect some terminals late through early evening. Gusts may be more evident and persist through the evening into the overnight with a low chance of additional lofted dust. Very low confidence exists regarding a wind shift back to E/SE overnight and timing. Distant outflows may force a wind shift around 08-09Z (similar to this morning), or a lack of outflow may create just a few hours of variable winds around sunrise before reverting back to W/SW.

Saturday outlook: Confidence is increasing that storms in eastern and southern Arizona will send multiple strong outflow boundaries towards Phoenix Saturday evening. While thunderstorms are not anticipated to directly affect the terminals, abrupt wind shifts with blowing dust appear likely mid/late evening which may incur a period of MVFR or IFR visibilities. Exact wind directions are uncertain and multiple outflows could affect the terminals (from different directions). The more definitive outflow currently looks to be from the SE late tomorrow evening, and this has been introduced into the KPHX TAF with this package, however, have left reduced VIS and any HZ/BLDU out of the TAF at this time.

Southeast California/Southwest Arizona including KIPL and KBLH: A similar set up to last night is expected again tonight with an outflow boundary moving in from the SE. Uncertainty still remains on the exact strength of this outflow. But it looks to arrive at KBLH around 09Z. A TEMPO has been introduced into the TAF with this package with gusts up around 20-25kt, reduced VIS (4SM), and HZ. This outflow is not expected to make it to KIPL, like last night. So, at KIPL current southerly winds are expected to go SE'rly late tonight, with wind speeds generally aob 10 kt.

FIRE WEATHER

Seasonably hot temperatures will prevail through the next several days as readings will be near to slightly above normal. Dry conditions will prevail today before thunderstorm chances increase Saturday across the higher terrain areas of south-central AZ. More widespread monsoonal activity is likely starting Sunday and continuing through next week. Afternoon humidity levels today will range between 10-20% across the western districts and 15-25% across the eastern districts. Afternoon humidity levels Saturday increase to 15-25% areawide. Starting on Sunday and persisting through much of next week, afternoon humidity levels will increase even further and bottom out above 20% as moisture levels increase, and thus alleviating much of the fire weather concerns. Outside of any potential outflows/thunderstorm winds, winds should favor diurnal trends with the typical afternoon breeziness around 20 mph. Some stronger afternoon gusts between 20-30 mph will be common along the Lower Colorado River Valley today.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST this evening for AZZ530-537- 540-542>544-546-548-550-551.

CA...Extreme Heat Warning until 8 PM PDT this evening for CAZ562-566- 567-569-570.