Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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546 FXUS65 KPSR 111650 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 950 AM MST Thu Jul 11 2024 .UPDATE... Updated Aviation && .SYNOPSIS... Strong high pressure will continue to impact the region over the next few days allowing excessive heat to persist across the region through at least Friday. Modest moisture levels will also provide for chances for isolated thunderstorms over eastern Arizona high today and gusty winds as far west as the Phoenix area. For Friday and through the weekend, moisture levels will continue to improve as rainfall chances eventually spread over the rest of the region. Temperatures will also gradually cool away from excessive levels and closer to seasonal normals by Sunday. && .DISCUSSION... The strong high pressure system currently centered near Las Vegas continues to be the dominant feature across the region, but day by day moisture levels are improving and this will eventually curtail the excessive heat that has been a problem over the last week or so. Convective activity yesterday was fairly limited in scope and impacted areas well north and southeast of Phoenix. Increased moisture levels today are expected to bring a bit more widespread convection, initially focused along the Mogollon Rim to the White Mtns. Forecast MUCAPE of around 1000 J/kg across the majority of southern Arizona along with 20-30kt mid-level winds out of the north northeast in eastern Arizona should promote isolated strong to possible severe thunderstorms over the high terrain by mid to late afternoon. The dry lower levels will again support very high DCAPEs increasing the strong wind threat. The 00Z HREF generally shows two main areas of convection, one north northeast of Phoenix and another over southern Gila County into eastern Pinal and Graham Counties. Given the expected high CIN in place over the south-central Arizona lower deserts, any storms coming off the high terrain are expected to die out quickly, but strong outflow winds moving into the lower deserts are very likely. Guidance generally supports the strongest outflow winds across central Pinal County with another likely weaker outflow potentially moving southwestward into northern Maricopa County. Moisture levels will continue to improve into Friday, especially across the lower deserts as guidance shows a robust Gulf moisture surge occurring tonight/Friday morning. Convective potential over the higher terrain for Friday seems to be very similar to today minus less activity in far southeast Arizona due to a pocket of drier air moving out of west Texas and northern Mexico. The chance for strong to severe storms on Friday looks to be considerably lower due to lower instability and DCAPEs, but better moisture and lower CIN over the lower deserts may be enough for some storms to survive or to develop along propagating outflows into the south- central Arizona lower deserts. Excessively hot temperatures will persist through at least Friday with highs gradually lowering each day as lower level moisture increases. Forecast highs today are again between 112-116 degrees in the Phoenix area to 114-119 degrees across the western deserts before some improvement is seen on Friday across the western deserts. Surface dew points increase a good amount into Saturday with readings in the 60s across the western deserts to the mid to upper 50s in the Phoenix area. Forecast highs Saturday do take a decent step lower with readings mostly between 110-113 degrees, but given the increased humidities it really won`t feel any cooler than Friday. The one upside starting Saturday is there should be some cloud cover at times and this could help to take the edge off the heat. Forecast confidence for convection over the weekend has decreased as model guidance is now shifting the expected development of an upper level low from over New Mexico to over Texas. Guidance is now also hinting at a quiet day across at least southeast Arizona on Saturday due to the pocket of drier air, despite our flow turning solidly out of the east. An easterly flow is almost always favorable for monsoon activity, but given the models shifting toward a brief period of drying into eastern Arizona Saturday`s convection is probably going to be fairly limited. Moisture advection should improve once again Saturday night into Sunday across the entire area creating better storm chances later Sunday, but likely losing support of the upper low (now probably too far east) may hinder widespread development. However, with deeper moisture likely in place by Sunday afternoon, as long as the higher terrain convection is widespread enough, any colliding outflows over the lower deserts should result in additional development. Locally heavy rainfall may also become a concern on Sunday along with a strong wind threat from any stronger storms that may develop. Drier air is expected to gradually spread from west to east early next week likely limiting convection to south- central and eastern Arizona by Tuesday. Temperatures are also expected to continue to improve Sunday into Monday with highs falling below 110 degrees over the south-central Arizona lower deserts to around 110 degrees over the western deserts. If guidance is correct in showing the drier air by next Tuesday or Wednesday, then that should eventually push temperatures up a few degrees during the middle of next week. && .AVIATION...Updated at 1135Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: The potential for multiple strong, gusty outflow boundaries with abrupt wind shifts are the primary weather hazards this evening. Confidence is very good that light east winds will turn to a W/SW direction around noon, however forecast uncertainty grows exponentially thereafter. While actual thunderstorms should remain well removed from the Phoenix metro, outflows emanating from the east, north, and south could all impact operations. The most likely direction will initially be from the east due to afternoon storms over southern Gila/eastern Pinal counties with a 30% chance of prolonged wind gusts over 30kt. Onset timing is also somewhat uncertain with a range of 00Z-03Z among high resolution models. There is also a non-zero chance of a northerly outflow also sweeping into the metro, however odds are low enough not to include in this TAF package. Higher odds exist for a wind shift to the S/SW mid/late evening as storms in southern AZ could push an outflow north. Confidence is slightly better overnight that an easterly component will finally settle over the metro sometime after midnight. Southeast California/Southwest Arizona including KIPL and KBLH: The potential for increasing wind speeds overnight will be the main weather issue through Friday afternoon with occasional SCT mid/high level cloud decks. S/SE winds will be preferred through the entire TAF period, however a Gulf surge overnight may cause an increase in speeds and frequent gusts. Onset timing looks to be around or just prior to midnight, with the greatest uncertainty in the magnitude and duration of any stronger gusts. && .FIRE WEATHER... Unusually hot weather will persist the next few days with thunderstorm activity steadily increasing over eastern districts. Moisture will steadily increase over most districts through the weekend allowing temperatures to retreat while humidity levels and rainfall chances increase markedly. Storms will initially produce only isolated wetting rain chances in eastern districts with areal coverage likely becoming far more widespread by the weekend. Minimum humidity values will continue to fall into a 5-15% range through Friday, 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 11 118 (1958) 118 (1958) 117 (1975) July 12 116 (2020) 119 (1939) 118 (1964) && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning until 8 PM MST Friday for AZZ530>555-559. CA...Excessive Heat Warning until 8 PM PDT Friday for CAZ560>570. && $$ DISCUSSION...Kuhlman AVIATION...18 FIRE WEATHER...18/Kuhlman CLIMATE...Berislavich/Kuhlman