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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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165 FXUS65 KPSR 112355 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 455 PM MST Thu Jul 11 2024 UPDATE...Updated 00z Aviation Discussion. && .SYNOPSIS... A strong ridge of high pressure located over the Great Basin will continue to result in excessive heat through Friday. This ridge will finally migrate over the 4-Corners region this weekend, resulting in a cooling trend and a noticeable increase in moisture levels. Isolated showers and storms will be possible across southcentral Arizona beginning today and persisting through early next week. The best chances for storms reaching the lower deserts will be Saturday and Sunday. Drier conditions are expected to arrive by the middle of next week. && .DISCUSSION... The current upper level pattern remains unchanged from what has been in place over the past several days. However, we will finally begin to see a pattern shift this weekend which will lead to cooler temperatures and an increase in monsoonal thunderstorm activity across Arizona. 500 mb hghts this afternoon will range from 595-597 dam resulting in temperatures reaching 111F-119F across the lower deserts. Highs in the Phoenix Metro are expected to top out around 112F-116F. The forecast high of 116F is expected to fall just short of the record 118F set in 1958. A mid-lvl shortwave trough is expected to dive southward out of NM into SE AZ this afternoon which will foster enough instability and lift for a cluster of thunderstorms over the White Mountains and the Rim Country. Northeasterly steering flow will bring storms off of the higher terrain towards the lower deserts this evening. However, there is still a lot of MLCIN to overcome and it is unlikely that storms will survive into the deserts of southcentral AZ. The bigger issue will be outflow generated from decaying thunderstorms which will pose a wind risk. Currently the SPC is highlighting a marginal risk for severe winds (>50 kts) across Maricopa County and a higher (Slight) risk for Pinal County this afternoon and evening. Latest HREF shows around a 70-90% chance of winds exceeding 30 kts and 30% chance of winds exceeding 50 kts in N Pinal County this evening. Therefore, a Blowing Dust Advisory has been issued for the dust prone areas along I-10 and I-8 in Pinal County. It is possible that westward moving outflow boundaries could make it all the way to the Colorado River valley late tonight. On Friday, the 500 mb high centered over southern NV will begin to migrate slowly ewd. This will result in a slight decrease in hghts aloft and cause the upper level flow to switch from northeasterly to easterly. Thus, we should start to see an uptick in moisture beginning late Friday into early Saturday with PWAT values increasing from 1.00" to near 1.50". Despite this pattern shift, Friday will still remain very hot across the lower deserts with record breaking heat still possible in Phoenix. The current record high on Friday is 116F at Phoenix Sky Harbor and the latest NBM forecasted high is 117F. There could be another round of convection late Friday afternoon through the early evening. The main threat again will be strong outflow from any storm due to a dry subcloud environment and high DCAPE values around 1500-2000 J/kg. Heading into this weekend we will start to see a more favorable monsoon setup as the center of the subtropical high repositions over the 4-Corners area. Dewpoints will increase into the upper 50s to lower 60s on Saturday which will yield CAPE values rising above 500 J/kg. This moist and unstable environment will be prime for more widespread, robust convection across the lower deserts. Currently PoPs range from 20-25% across the central deserts to around 55% in Gila County on Saturday afternoon/evening. Temperatures will begin to cool by a few degrees on Saturday due to decreasing hghts aloft, however it will still feel very hot due to higher heat index values, especially across southeast CA and southwest AZ. Sunday still looks to include the best rain chances across the entire region as a potential stronger embedded shrtwv trough in easterly flow passes overhead. The arrival of this wave along with increasing sfc moisture and instability should result in stronger outflow potential and longer-lived storms. Rain chances Sunday afternoon and evening are around 30-40% for the Phoenix Metro and up to 60% for eastern Gila County. It will be noticeably cooler on Sunday due to the increase in moisture and cloud cover with highs remaining at or below 110F across the lower deserts. Drier air is expected to gradually filter into the region from west to east early next week, likely limiting convection to the higher terrain of southcentral and eastern Arizona by Tuesday. Temperatures are expected to remain near normal through the middle of next week as the upper lvl high remains well north of the area. .AVIATION...Updated at 2355Z. 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. While actual thunderstorms should remain well removed from the Phoenix metro, outflows emanating from the east, north, and south could all impact operations. A strong outflow out of the east southeast will reach KIWA by 00Z with gusts upwards of 30 kt possible, and that same boundary will reach KPHX likely before 01Z. Another outflow is present currently along the northern Maricopa County border with winds out of the north northwest behind it. This boundary will likely reach KDVT, but chances are lower for the boundary reaching the other terminals. Confidence is moderate for a wind shift to the south late evening as storms in southern AZ could push an outflow north. Uncertainty in wind directions decreases overnight, as an easterly component is expected to settle over the Phoenix Metro Area sometime after midnight, and light westerlies (aob 8 kt sustained) will re- establish ttomorrow afternoon between 20-22Z. 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... Very hot temperatures will persist through Friday with thunderstorm chances steadily increasing over the eastern districts. Moisture will continue to 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 fall to around 10-15% 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. Blowing Dust Advisory until 8 PM MST this evening for AZZ553-554. CA...Excessive Heat Warning until 8 PM PDT Friday for CAZ560>570. && $$ DISCUSSION...Salerno AVIATION...Whittock/18 FIRE WEATHER...Salerno/Kuhlman CLIMATE...Berislavich/Kuhlman