textproduct: NWS Phoenix

KEY MESSAGES

- A weak disturbance will spark scattered showers and isolated thunderstorms this afternoon, mainly across the Arizona higher terrain east of Phoenix.

- Temperatures Tuesday will warm near to slightly above normal before abrupt cooling takes place by the latter half of the week.

- A Pacific low pressure system moving into the region during the middle portion of the week will bring increasing rain chances, mainly across south-central and eastern Arizona, with the potential for multiple rounds of moderate to heavy rainfall as well as some strong thunderstorms.

SHORT TERM /Today through Tuesday/

Early afternoon satellite WV imagery shows a weak shortwave trough progressing across Arizona, aiding in shower and isolated thunderstorm development across the higher terrain. Sufficient moisture and instability combined with orographic and dynamic ascent will allow for continued shower and thunderstorm development this afternoon. The greatest focus of this activity will reside across southeastern portions of the state where moisture and instability parameters are best. However, HREF guidance supports some of this convective activity making its way into southern Gila County this afternoon with gusty outflow winds and locally heavy rainfall being the primary concerns. Across the south-central Arizona lower deserts, conditions won't be as favorable, but a few isolated showers and storms may develop, particularly around the Table Top area. Quieter weather is expected Tuesday as the weak shortwave exits to the northeast and the greatest rain chances stay confined to the White Mountains Tuesday afternoon.

Heights rebound across the region Tuesday in the wake of the exiting shortwave and ahead of a potent approaching weather system digging southward along the West Coast. Forecast highs warm several degrees Tuesday afternoon compared to today as temperatures climb into the upper 80s to lower 90s.

LONG TERM /Wednesday through Sunday/

A much more active weather pattern is shaping up to take place by the middle portion of the week as a deep upper-level low amplifies off the CA coast and then moves inland over the Desert Southwest through the remainder of the work week. Latest ensemble and deterministic model suite have come into a better agreement in showing a deep upper-level low with strong negative height anomalies becoming established off the southern CA coast on Wednesday and potentially obtaining a slight negative tilt. This will cause the flow aloft to become established more out of the south, advecting deeper moisture northward into the region starting Tuesday night. In fact, both the EPS and GEFS show PWAT values peaking above 250% of normal with the North American Ensemble Forecasting System depicting moisture levels approaching near record values for mid-October. In addition, the combination of the deeper moisture as well as steeper mid-level lapse rates will likely lead to significant instability with both the ECMWF and GFS indicating MUCAPE values approaching 1000 J/KG. Therefore, the combination of the strong synoptic ascent from the trough and favorable thermodynamic parameters will likely lead to multiple rounds of showers and thunderstorms to break out across much of south-central AZ Wednesday through Thursday afternoon, period when rainfall chances will be highest, while much of the western deserts should remain mostly dry given that the greatest moisture advection will occur mostly over central and eastern AZ.

With the moderate levels of instability likely to be in place, any shower and thunderstorm activity will be capable of producing rainfall rates in excess of 1-2"/hr with training of activity likely to be an issue. Thus, there will be a heightened potential for flash flooding, especially across the foothills and higher terrain areas to the north and east of Phoenix where upslope flow will likely enhance the activity. Given this heightened potential for flash flooding, the Weather Prediction Center has placed areas from Phoenix and points north and east in a slight risk for excessive rainfall leading to flash flooding for both Wednesday and Thursday. In terms of rainfall amounts, the latest forecast from the Weather Prediction Center has much of the foothills and higher terrain areas north and east of Phoenix potentially receiving upwards of 1.00- 2.50" while across the lower deserts, including the Phoenix metro, amounts are likely to be lower ranging between 0.25-1.00", although locally higher amounts will be likely in areas that see training of activity. In addition to the heavy rainfall threat, given that the 0-6 km shear will be in excess of 50 kts combined with the moderate levels of instability, there will also be the threat of a few strong to severe storms capable of producing strong wind gusts and hail.

As the upper-level low moves inland into the southern Great Basin late Thursday, strong dry air advection will gradually push eastward ending rain chances by Thursday afternoon across the south-central AZ lower deserts with rain chances lingering into early Friday over the higher terrain. There are indications from guidance of a follow-on shortwave moving through area late Friday through Saturday, however, it should be inconsequential given that there will be an abundance of dry air in place limiting any potential for additional rainfall.

Under the influence of the general troughing pattern as well as the heightened rain chances, temperatures by the latter half of the week will rapidly cool off towards readings that will be several degrees below normal. The latest NBM shows much of the lower deserts struggling to reach 80 degrees for afternoon highs both Thursday and Friday, with Friday likely to be coolest day. Temperatures will gradually warm up next weekend but still remain below normal.

AVIATION

Updated at 2330Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation concerns are expected during the TAF period. Light and variable conditions look likely for this evening ahead of the typical overnight/morning easterly flow. It odes not appear that the familiar west shift will occur later in the period either as signs point toward a more southerly component taking hold. VCSHs cannot be ruled out completely in the near-term, but chances are 10% or less so no mention of rain has been included in the TAFs. FEW-SCT cloud decks will be common through the forecast window with the lowest bases around 6-8 kft.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation concerns are expected over the next 24 hours. Light and VRB conditions will be common through Tuesday morning before a S/SE'rly component takes hold at both terminals. Clear skies will give way to increasing low and mid-level cloud decks with the lowest bases around 7-9 kft.

FIRE WEATHER

Wind speeds through Tuesday will generally be light with limited afternoon gustiness before increasing towards the middle of the week ahead of an approaching weather system. Scattered showers and isolated thunderstorms are expected across the higher terrain areas of the eastern districts this afternoon. Rainfall chances temporarily decrease on Tuesday before increasing substantially across much of south-central and eastern AZ by the middle of the week with rainfall amounts likely to be greater than 0.50-1.00", especially east of Phoenix. Minimum humidity levels in a 30-50% range will be common throughout the week with even higher readings likely across eastern districts with good to excellent overnight recoveries. Temperatures will be in a near normal range during the next couple of days before falling into a below normal category by the latter half of the week.

PSR WATCHES/WARNINGS/ADVISORIES

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