Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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396 FXUS65 KPSR 291024 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 324 AM MST Sat Jun 29 2024 .SYNOPSIS... Above normal temperatures will persist well into next week, especially across the western deserts. Expect daily high temperatures around 110 degrees across the lower deserts this weekend before temperatures cool slightly across south-central Arizona starting Monday. After an overall quiet day today, shower and thunderstorm chances will ramp up later on Sunday across south-central and eastern Arizona. The increased monsoon moisture starting Sunday should last into the middle of next week giving daily chances of showers and thunderstorms to portions of southern and central Arizona. && .DISCUSSION... Dry air continues to dominate the western half of Arizona and southern California, while modest monsoon moisture stubbornly remains in place across the southeastern 1/3rd of Arizona. The weaker ridge yesterday resulted in barely any CIN and the instability driven by the lingering moisture allowed for fairly widespread showers and thunderstorms across Pinal County through southeast Arizona. For today, the sub-tropical ridge center over the Southern Plains is forecast to strengthen while also extending back through our region. This added subsidence aloft should be enough to create an overall fairly quiet day across the area. Hi- res CAMs do show some isolated convection popping up this afternoon across far southern Maricopa County and over the higher terrain in Gila County, but the coverage should be considerably less than what was seen on Friday. Over Mexico, an easterly wave can be seen within water vapor imagery early this morning and this is expected to drive a robust MCS across Sonora later today before nearing southeast Arizona tonight. This wave will help to turn our mid level flow out of the southeast tonight, allowing for moisture to surge northwestward into southern and central Arizona. We may also see some isolated elevated light shower activity late tonight into early Sunday morning associated with this moisture advection and any gravity waves emanating from the Sonoran MCS. Guidance continues to point toward a very busy monsoon day across southeast Arizona Sunday afternoon where instability is likely to be maximized and then potentially Sunday evening into south- central Arizona. The 00Z HREF and now the latest HRRR all point toward outflows and at least some showers and thunderstorms progressing northwestward out of the Tucson area by early Sunday evening. Forecast MUCAPE does decrease to around 500-750 J/kg in the Phoenix area with the axis of better instability extending toward Tucson. Forecast soundings do show some CIN in the Phoenix area, but any large robust outflow and/or continuous redevelopment of convection should be enough to overcome the expected CIN. As of right now, we are confident there will be at least some scattered showers and thunderstorms making their way into south-central Arizona Sunday evening with the best chances over Pinal and Gila Counties with PoPs of 50-60%. Lower chances of 30-40% are expected into the Phoenix area as the activity begins to enter lower instability and higher CIN. A few storms are likely to be strong to severe with damaging winds possible. Storm outflows should also have a good possibility of generating some blowing dust, especially in Pinal County. Moisture levels should also be ample enough with PWATs increasing toward 1.75" to provide for some localized heavy rainfall and potentially some minor flooding. The GFS has been consistently showing an MCV forming Sunday night over south-central Arizona with the remnant MCV slowly drifting toward Flagstaff on Monday. Guidance does overall agree with Monday showing a much quieter monsoon day, likely partially due to the subsidence behind the MCV and because the flow will begin to turn southwesterly again. This southwesterly flow should provide for a brief period of drying Monday into Tuesday, but moisture should still remain plenty adequate on Tuesday for some mainly high terrain based showers and thunderstorm activity. Forecast confidence for Tuesday and Wednesday is still somewhat low due to the lack of any good forcing as we will fall in between a developing high pressure ridge off the coast of California and the ridge center over the Lower Mississippi Valley. A couple of weak upper level shortwave troughs are also expected to pass by to our north Tuesday and Wednesday with potentially a little bit of upper level forcing over Arizona on Wednesday. This may help with some convection, but a southwesterly steering flow is generally not very favorable for convection impacting the lower deserts. Later next week, a very strong high pressure ridge with H5 heights upwards of 598dm is forecast to shift into California by Thursday and maybe into the Great Basin or over our region by the following weekend. This synoptic set up would bring northerly dry flow across our region considerably limiting monsoon activity starting Thursday. Temperatures over the next week or so are expected to stay several degrees above normal, particularly across the western deserts. Forecast highs up to around 110 degrees are likely to be seen across the south-central Arizona lower deserts to 108-113 degrees across the western lower deserts. When adding in fairly high dew points in the 50s to the expected highs around 110 degrees over much of the area, the heat will continue to be dangerous to those most vulnerable and to those not adequately prepared for the heat. An Excessive Heat Watch remains in effect for portions of southeast California and the Lower CO River Valley for the majority of next week. && .AVIATION...Updated at 0548Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: A broad swath of southerly outflow winds originating from earlier storms that were over Pinal and Pima Counties has overspread the Valley floor. Expect southerly breeziness (gusts to 20kts) to slowly weaken and become light and variable favoring southeasterly before sunrise. Expect SW/Wly winds to develop by mid afternoon. Anticipate an increase in high clouds late tonight and Saturday morning. As for storm development Saturday afternoon/evening, confidence in impacts at the TAF sites too low to reflect in the forecasts at this time. Southeast California/Southwest Arizona including KIPL and KBLH: Overall, expect familiar warm season wind patterns at the TAF sites through Saturday. At KIPL, westerly winds will shift to southeasterly by sunrise. At KBLH, SWly winds early in the TAF period will trend to southerly by 09Z or so. Southerly breeziness (gusts to 25kts) resumes around midday Saturday. Otherwise, expect clear skies. && .FIRE WEATHER... Very limited monsoon activity is expected today before fairly widespread showers and thunderstorms are expected over south- central and eastern Arizona Sunday afternoon and evening. Humidity levels today will drop slightly from yesterday with MinRHs in the upper teens across the lower deserts to 20-30% over the higher terrain with only very slight improvement through Sunday afternoon. Overnight recoveries will be fair tonight and good Sunday night as moisture increases over much of the area. Winds will follow typical diurnal trends through Sunday afternoon with occasional afternoon breeziness near 20 mph, while strong outflow south southeasterly winds look likely Sunday evening into south- central Arizona. Daily monsoon storm chances should persist through at least south-central Arizona into the middle part of next week, while humidities rebound to 20-25% across the lower deserts to upwards of 35-40% over the eastern Arizona high terrain. Temperatures next week will remain several degrees above normal each day. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Watch from Sunday morning through Friday evening for AZZ530. CA...Excessive Heat Watch from Sunday morning through Friday evening for CAZ562-566-567-569-570. && $$ DISCUSSION...Kuhlman AVIATION...AJ FIRE WEATHER...Kuhlman