Area Forecast Discussion
Issued by NWS Phoenix, AZ

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234 FXUS65 KPSR 121758 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1058 AM MST Fri Jul 12 2024 .UPDATE...
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18Z Aviation Discussion.
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&& .SYNOPSIS... A strong ridge of high pressure located over the Great Basin will continue to result in excessive heat through Saturday. This ridge will finally migrate over the 4-Corners region this weekend, resulting in a slow cooling trend and a noticeable increase in moisture levels. Isolated showers and storms will be possible across south-central Arizona through Saturday with the best chances for storms reaching the lower deserts on Sunday. Drier conditions are expected to gradually spread over the area during the first half of next week. && .DISCUSSION... The strong high pressure ridge is still the most dominant feature across the region, but moisture continues to increase day by day allowing for increasing convection. The heat dome which has caused excessive heat all week will remain intact today and largely even through Saturday. Forecast highs for today are a couple degrees cooler than what we have been seeing over the past week, but they still easily reach excessive heat levels. PWATs have improved to over 1" across much of the lower deserts, but drier air working in from northern Mexico is lowering PWATs to around 0.8" across eastern Arizona. Despite the minimal moisture levels for today, the excessive heat will again create steep lapse rates with MUCAPEs mainly between 500-750 J/kg from the White Mtns through the Mogollon Rim. Higher CAPE is forecast across southwest Arizona and southeast California today, but that will be wasted due to strong CIN. The latest HREF and the 06Z HRRR show convection forming over the higher terrain east and northeast of Phoenix this afternoon with a east southeasterly steering flow mostly keeping the convection riding just north of Phoenix into this evening. The HREF does show some potential for additional convective development across Pinal County this evening, probably from colliding outflows, but this is of fairly low forecast confidence. The gusty wind threat today looks to be considerably lower than what we had yesterday due to lower instability, less mid-level flow, and marginally lower DCAPEs. Lower level moisture will continue to improve Friday night into Saturday causing surface dew points to surge to around 70 degrees in Yuma and El Centro to around 60 degrees in the Phoenix area. Lingering overnight clouds Friday night are also likely to keep temperatures very warm, likely the warmest night of this past week. Forecast highs Saturday do drop another 3 degrees or so from today`s highs, but given the increased humidities, it will still feel just as hot as Friday. Even though temperatures fall enough on Saturday to drop the HeatRisk mostly into the Moderate category, we feel it was necessary to extend the Excessive Heat Warning through Saturday due to Heat Indices essentially staying between 110-115 degrees. Convective potential for Saturday still looks to be rather limited due to the drier air temporarily working its way into far eastern Arizona, but since moisture will have worked its way all the way through southern California we can`t rule out some isolated showers and thunderstorms over the western deserts, especially within JTNP and over any locally higher terrain features. Hi-res CAMs generally agree on most of the convection staying over the higher terrain north of Phoenix with some potential for some activity across Pima County and southern Maricopa County. Deeper moist southeasterly flow is then expected to overtake the majority of Arizona late Saturday into Sunday increasing PWATs to around 1.5" over the lower deserts. Higher moisture will also spread across eastern Arizona which will provide for much more expansive higher terrain development Sunday afternoon and likely multiple outflows into the lower deserts prompting further development of convection. NBM PoPs for Sunday rise to 30-40% across the south-central Arizona lower deserts to 50-60% over higher terrain areas east of Phoenix. GFS forecast soundings for Phoenix show considerable CAPE at around 1500 J/kg Sunday afternoon, but with still some modest CIN in place. However, given the potential for high instability any deep storm outflows or colliding outflows will likely be enough for lower desert convective development. Any strong thunderstorms Sunday will likely have the potential to produce strong to possibly severe wind gusts and locally heavy rainfall. Temperatures Sunday should lower enough to fall below excessive heat levels across the board due to the overall increased moisture, convective activity, and additional cloud cover. Starting Monday, guidance is still showing some drier air working from west to east through southern California into at least western portions of Arizona. There should still be enough moisture to work with across the eastern half of Arizona for another round of shower and thunderstorm activity on Monday, but instability is likely to be lower and thus the convective potential should be noticeably lower than Sunday. The drier air should then continue to spread eastward Tuesday into Wednesday mainly limiting any convection to the eastern Arizona high terrain. Temperatures should bottom out on around Monday, but highs are still forecast to be 2-4 degrees above normals. As the drier air works through the region during the middle part of next week, temperatures should slowly rise with highs likely topping 110 degrees over much of the lower deserts by next Wednesday or Thursday. && .AVIATION...
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Updated at 1740Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: There is a lot more uncertainty regarding thunderstorm potential this afternoon and evening. Latest guidance suggests lower storm coverage across the region and therefore less confidence in outflow boundaries reaching KIWA and KPHX (10% chance). However there still around a 30% chance of a northerly or northeasterly outflow reaching KSDL and KDVT tonight, mainly after 03Z. Winds through the rest of today will be slow to veer, becoming westerly by 23Z-00Z. Winds should return out of the ESE by 09Z-10Z tonight. Cloud cover is expected to remain FEW to SCT throughout the period. Smoke from nearby fires could create haze and slantwise visibility issues through early this afternoon. Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns are expected through the next 24 hours. Winds will continue to favor an E to SE component at KIPL, while a southerly direction is expected at KBLH. Gusts will materialize this afternoon to around 20-25 kts at both sites. Otherwise, FEW-SCT mid to high clouds will continue throughout the period.
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&& .FIRE WEATHER... Very hot temperatures will be seen again today with thunderstorm chances steadily increasing over the eastern districts through the weekend as moisture improves. Temperatures will slowly retreat over the weekend while humidity levels increase. Storms will initially produce only isolated wetting rain chances in eastern districts with areal coverage likely becoming far more widespread by Sunday. Minimum humidity values will fall to around 10-15% today, however by Sunday, values should struggle to fall below 20%. Some slight drying may wash over the districts by the middle of next week when minimum values fall closer to a 10-20% range. Occasional afternoon upslope gusts near 20 mph will be common the next few days, and the increase in thunderstorm activity will produce more frequent strong outflow winds across a larger expanse. && .CLIMATE... Record Highs Phoenix Yuma El Centro --------- ------- ----------- July 12 116 (2020) 119 (1939) 118 (1964) && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning until 8 PM MST Saturday for AZZ530>555- 559. CA...Excessive Heat Warning until 8 PM PDT Saturday for CAZ560>570. && $$ DISCUSSION...Kuhlman AVIATION...Salerno FIRE WEATHER...Salerno/Kuhlman CLIMATE...Kuhlman