textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- Bands of light to moderate rain will be present over the region for the remainder of today with isolated heavier shower and thunderstorms possible over the western deserts during the early evening.

- A Flood Watch has been issued for portions of the region with a focus through much of Tuesday for the majority of south-central Arizona.

- Below normal temperatures are expected through the week before warming to slightly above normal by the weekend as much drier conditions eventually spread over the region.

SHORT TERM /Today through Tuesday/

The combination of tropical moisture from Hurricane Polo and an approaching Pacific low pressure system will continue to promote rounds of shower and thunderstorms through the next 24-30 hours or so. Through the remainder of today, bands of light to sometimes moderate rainfall will be present across portions of the region as enhanced southwesterly flow diverges from its typical dry status to provide the region with PWATs upwards of 175-250% of normal. However, the best mid and upper-level forcing and instability will be focused more over the westernmost portion of our forecast area, so most locations will see primarily a nice periodic light to moderate rain through at least this evening.

The one exception to the lighter rainfall will be those aforementioned western areas (mainly locations around the Colorado River Valley), where heavier convection may be realized. There are signs that enough clearing will take place to allow some decent destabilization to take place, pushing MLCAPE values toward 500-750 J/kg. With these areas also located under the best ascent due to their closer proximity to the deepening low, the environment will be favorable for the development of at least some isolated thunderstorms. Add in 0-6km shear of 50-60 kt and the potential for strong to even severe storms capable of damaging winds and even small hail will exist as well, though these should be very isolated at best. This heavier convection will also present the concern for flash flooding, as rain rates could reach upwards of over an inch an hour. Guidance points towards the best coverage of heavy rain being focused over La Paz County, and because of that, this area was added to the existing Flood Watch (in effect from this evening into the early overnight hours) last night.

As the Pacific low continues its track southeastward into Tuesday, the more favorable environment to see further rainfall activity will begin to shift toward South-Central Arizona. As this progression occurs, shower and thunderstorms will begin to blossom over Maricopa and Pinal Counties soon after midnight. Activity will last through the remainder of the morning and into the afternoon, with the easternmost portions of our coverage area even seeing the rainfall window stretch into the evening. 0-6 km shear will continue to be quite impressive hovering around 50-60 kt, but even with a bit of low-level warm air and moisture advection leading to a slight increase in instability, the severe risk should be fairly low as any storms that initiate and can be maintained should only be isolated. Perhaps a low potential severe threat reaches into the afternoon if daytime heating can aid in further destabilization, but the areas that will see the best insolation during the warmest point in the day will likely be behind the best atmospheric support and moisture. By the this timeframe, any severe storm chances would likely be confined from around central Phoenix and eastward.

The main concern for Tuesday will be the potential for heavy rain and flash flooding. The greatest flooding threat will be for foothill and high terrain areas to the north and east of the Phoenix metro, where rain totals may reach upward of 1-2.5". The Flood Watch that was issued Saturday will go into effect late tonight, lasting through the evening timeframe. As for the lower deserts of Maricopa and Pinal Counties, the heavy rain signal has increased. HREF members have been consistent since yesterday indicating at least a 30-50% chance of seeing greater than 1" within 25 miles of any point for these counties, while the ECMWF ensemble now has around 25% of its members showing upwards of 2" or greater at Sky Harbor Airport. Not to mention that EFI (Extreme Forecast Index), along with model shift-of-tails, a value indicating that anomalously high QPF values are showing up in numerous ensembles members, have been pulled back toward lower elevation areas. The preponderance of guidance led to an expansion of of the Flood Watch to include all of Maricopa and Pinal Counties.

There are some signs of lingering convective showers and thunderstorms behind the center of the low as warp-around moisture and insolation will help keep instability in place. Due to wake subsidence, anything that does pop will be reliant on enhanced terrain features providing orographic ascent. Hi-res data has been inconsistent on where it places this convection, but isolated cells may be seen as far west as La Paz County. By the time all this rain is said an done, most lower desert east of the Colorado River will anywhere from 0.5-1.50", while high terrain areas can expected closer to 1.50"-2.50". Locally higher amounts will be possible under heavier showers and storms. West of the Colorado River, rain totals will vary widely from a few hundredths in the driest spots to potentially greater than 1" if the previously mentioned strong convection is realized.

The temperature forecast has taken a back seat to all this talk of rain, but for the fans of non-triple digit readings, you are in for a treat. Below-normal readings will be common across the region through today and tomorrow, with highs this afternoon ranging in the upper 70s to lower 90s, a good 10+ degrees below what is typical for late September, with much of the same expected for Tuesday.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

The low is forecast to move into New Mexico by Wednesday morning and well into the Plains by Thursday with dry air advection and subsidence increasing across our area. Generally clear skies are expected for the lower deserts on Wednesday and eventually over 0808eastern Arizona by later the afternoon. Fairly strong ridging is then forecast to move into the region on Thursday with H5 heights quickly rising to between 586-590dm. Despite the quick boost in heights aloft, temperatures will be somewhat slow to recover, especially over the eastern half of Arizona. Highs should stay below 90 degrees in the Phoenix area through Thursday with the lower deserts warming back into the mid to upper 90s. As the boundary layer gradually dries out going into the weekend, we are likely to return to our typically lower humidity levels while temperatures warm to slightly above normal.

AVIATION

Updated at 2320Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Weather issues through Tuesday will include periods of SHRA/TSRA, lower CIGs, and erratic wind directions. Confidence is good that banded -SHRA will gradually clear the PHX metro by mid evening with light E/NE winds becoming more variable and cloud decks flirting with 060 cigs. A break in -SHRA and lower cloud decks is likely late evening with moderate confidence of additional SHRA and lowering flight categories moving into the region overnight. Periods of MVFR vsby are almost certain in moderate SHRA with cigs falling below 6K ft AGL and winds becoming N/NW. Brief instances of IFR vsby/MVFR cigs is possible around sunrise, however confidence is too low to include in this TAF package. Winds should turn E/NE as SHRA moves east of terminals mid morning, however guidance suggests directions quickly shifting to W/SW early Tuesday afternoon as 040-050 BKN decks develop with afternoon heating.

Southeast California/Southwest Arizona including KIPL and KBLH: The primary weather concern will be the potential for SHRA/TSRA at KBLH tonight, then gusty NW winds Tuesday. Confidence is good that S/SE winds will prevail this evening before veering to W/NW by late evening. While confidence is rather low, scattered SHRA/TSRA may develop near KBLH around midnight before moving eastward overnight. Confidence is excellent that winds will become N/NW Tuesday with gusts 20-25kt developing in the afternoon.

FIRE WEATHER

Elevated moisture levels and periods of showers and thunderstorms are expected through Tuesday. The peak time frame for rainfall should be today over the western districts and tonight through Tuesday afternoon for the eastern districts. Chances for wetting rain between 40-80% should greatly limit any threat of dry lightning starts. Outside of enhanced gusts associated with stronger storms and some marginal northwesterly breeziness for the western districts Tuesday, winds will follow light and diurnal trends otherwise. Drier conditions and near to slightly above normal temperatures are forecast for later this week and through next weekend.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch from midnight MST tonight through Tuesday evening for AZZ534-537>563.

Flood Watch from 6 PM MST this evening through late tonight for AZZ533.

CA...None.