![National Oceanic and Atmospheric Administration](/build/images/header/noaa.d87e0251.png)
![National Weather Service](/build/images/header/nws.4e6585d8.png)
![United States Department of Commerce](/build/images/header/doc.b38ba91a.png)
Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
848 FXUS65 KPSR 102355 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 455 PM MST Wed Jul 10 2024 .UPDATE...Updated Aviation Discussion. && .SYNOPSIS... Strong high pressure will allow extreme heat to persist across the region through Friday as many high temperature records will be threatened. Moisture will also slowly move back into the area providing modest chances for isolated thunderstorms over eastern Arizona high terrain. Over the weekend, far better moisture should be imported into lower elevations supporting improved rainfall chances, as well as pulling temperatures away from excessive levels and closer to seasonal normals. && .DISCUSSION... The pronounced anti-cyclone responsible for the current extended period of excessive heat has only shifted from SE California yesterday into NW Arizona this afternoon. As a result, with central H5 heights continuing to hover near 597dm, high HeatRisk with temperatures around 10F above normal and near record highs will persist the next couple days (see Climate section). Guidance spreads remain quite narrow resulting in seemingly excellent forecast confidence, however consequences from thunderstorms in the form of outflows, moisture surges, and debris clouds provide more uncertainty than usual given the gradual increase in convective activity the remainder of the week. By the weekend, high pressure will have finally shifted towards SW Colorado leading to greater easterly flow with more prevalent outflows and moisture intrusions, as well as lowered heights/thermal profiles aloft precluding any additional short term excessive heat. Early afternoon WV imagery shows a pocket of drier air actually has rotated around the periphery of the anti-cyclone from central California into eastern Arizona over the past 24 hours with objective analysis indicating low level mixing ratios falling below 8 g/kg. With this limited instability and primarily convergent flow aloft not particularly conducive towards thunderstorm sustenance, convective activity this afternoon should be less vigorous across higher terrain locations. However, HREF membership still suggests modest convective development in central/southern Yavapai County likely sending gusty outflow winds towards the Phoenix area, albeit weakening on their travels south. With models suggesting some form of a Gulf surge overnight in response to storm complexes over Mexico, moisture should increase again across southern Arizona Thursday resulting in better thunderstorm chances with upslope winds driving the shallow moisture into higher terrain. Forecast BUFR soundings Thursday afternoon show modest low level instability (MLCape up to 1000 J/kg) feeding convective activity, although CinH in excess of 100 J/kg should preclude storms from propagating much past foothills surrounding the city. On the other hand, DCape values will peak in excess of 2000 J/kg across the region fostering long-lived, and likely multiple outflow boundaries. While the preponderance of HREF output suggests convection over eastern Arizona targeting the Tucson area with the strongest winds and impacts, hi-res models are forecasting at least a 50% chance of 35 mph winds sweeping into the forecast area. Based on historical outcomes, feel more widespread 35 mph winds are likely with some blowing dust in the more preferred area of recent disturbed land use south of the metro. The high probability of multiple deep outflows and renewed Gulf surges Thursday evening acting to deepen better quality moisture will set the stage for more widespread direct thunderstorm impacts through the weekend. Activity Friday may mirror that of Thursday though more uncertainty exists considering the influence of previous days evolution on the flow pattern and ascent structure. Nevertheless, the improved moisture profiles Friday may allow for some high terrain convection to survive closer into lower elevation population centers of south-central Arizona with higher NBM POPs displaying more constriction around the Phoenix metro. Gusty outflow winds should still be the primary threat for the majority of the area, though with mixing ratios increasing around 9-10 g/kg and total column PWATs approaching 1.50", heavy rainfall and localized flooding will start to become a greater threat with time. The best potential for more widespread and potentially strong thunderstorm activity arrives Saturday and Sunday when models continue to show excellent upper level support in the form of enhanced divergence from the combination of an upper level low over central New Mexico and the high pressure system in SW Colorado. Forecast BUFR soundings show a favorable combination of 1000-1500 J/kg MLCape and DCape capable of support both strong, damaging winds and excessive rainfall. Conceptually, this pattern matches some very active thunderstorm days, though NBM POPs lag over most lower desert locations, and have pushed values closer to a 30-40% chance across much of the Phoenix metro (and this may even be conservative). At this time, forecasts don`t differentiate much between the weekend days though historically, if Saturday happens to be very active, then Sunday would expect to be far more quiet due to convective overturning of the atmosphere. This enhanced thunderstorm potential could easily carry over into early next week as the upper low is forecast to remain over New Mexico providing continued favorable divergence. By this time, mesoscale influences may be the greater determining factor in convective potential as adequate moisture should remain in place with daily modest instability ready to be tapped. Mandated NBM POPs appear too low given the overall flow pattern and setup, however have made no changes given the heightened uncertainty. By midweek, an upper level trough should reach the East Pacific while mean longwave troughing becomes re-established over the Great Lakes causing a re-alignment of the subtropical high. Unfortunately, there is notable spread among ensemble members on where the central anti- cyclone core shifts with a split between a shift back into California and intrusion of drier air cutting off storm activity (heavily EPS membership), or remaining near the four corners and supporting continued deep convection (mostly GEFS members). Due to this uncertainty, POPs towards the end of the forecast period naturally trend towards climatology. && .AVIATION...Updated at 2355Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary aviation weather concern through the period will be the potential for gusty outflows today and tomorrow during the late afternoon into the evening. Currently, breezy westerly winds continue this afternoon with gusts to around 20 kts at times. However, strong thunderstorms north of the Valley are now sending an outflow traveling southward toward the metro area. There is still some uncertainty related to how strong the outflow will be as it pushes into the Phoenix area and when/where it will dissipate. KDVT and KSDL will stand the best chance of seeing a gusty NNW outflow with gusts to around 20-30 kts. There is about a 10-30% chance of seeing outflow gusts in excess of 30 kts. The outflow is expected to weaken as it travels further south toward KPHX with uncertainty related to how strong it will be by the time it reaches the TAF site. This outflow will travel through the Valley between 00-03Z this evening before a more typical diurnal pattern resumes. Greater forecast uncertainty exists for tomorrow evening with the potential for multiple outflows coming in from different directions. Otherwise, mostly clear skies will continue with FEW-SCT mid and high clouds at times. Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns are expected through the next 24 hours. Winds at KIPL will continue to favor an E to SE component, while winds maintain a S component at KBLH. Winds will remain elevated at KBLH with gusts to around 20 kts through around 03Z and sustained speeds up to around 10-12 kts thereafter. Otherwise, mostly clear skies will persist with a FEW mid and high clouds at times. && .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 10 115 (1958) 118 (1959) 116 (2021) 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...18/Kuhlman AVIATION...Smith FIRE WEATHER...18/Kuhlman CLIMATE...Berislavich/Kuhlman