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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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703 FXUS65 KPSR 112035 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 135 PM MST Sun Aug 11 2024 .SYNOPSIS... Ample monsoon moisture over much of the area will continue into Monday, but shower and storm chances will generally favor higher terrain areas in Arizona. A significant downtrend in monsoon activity is likely by the middle of the week across the entire region as drier air filters into the region. Temperatures through the upcoming week will average near to slightly above normal. && .DISCUSSION... Little has changed with the overall synoptic pattern over the past couple of days with the subtropical high still centered over Texas stretching into eastern Arizona and the weak upper level cyclonic circulation positioned over the Baja peninsula. Additionally, a leftover MCV is currently slowly progressing to the west northwest across far southern Arizona. This feature has brought considerable cloudiness for much of the morning and as a result kept instability lower than forecast while not allowing the CAP to weaken very quickly. Moisture levels early this afternoon are still quite high, especially from south-central Arizona through the Lower CO River Valley with PWATs from 1.5-1.8" on average and low level mixing ratios around 12 g/kg. The main forecast question for the rest of today is whether or not the clearing skies early this afternoon will be enough to spark off much convection. Current objective analysis still only shows temperatures mostly in the mid to upper 90s as of 1 PM and it seems likely we will fall just short of our forecast highs of 105-109 degrees across the lower deserts. The last several runs of the HRRR show very limited convective development through the rest of the afternoon, and this may very well be what happens as the CIN may be too much to recover from due to the morning cloud cover. The most likely areas of shower and storm development are likely to be across the higher terrain well east of Phoenix and across western Maricopa County. Given the lack of strong instability and relatively low DCAPEs today, the threat for any stronger storms should be limited. Heading into Monday, the overall pattern will remain nearly unchanged as PWATs will continue to remain elevated at 1.5"+ and additional lobes of vorticity emanating from the Baja low will remain possible. Thunderstorms will once again materialize during the afternoon and evening, with the best chances confined across the higher terrain areas of eastern and southeastern AZ, including southern Gila County as well as the foothills east of Phoenix. For the lower deserts of south-central AZ, including Phoenix, any convective potential will be conditional as to what happens later today. If later today does not turn out to be convectively active such that the atmosphere is not overworked, then the chances for convection across the lower deserts of south-central AZ Monday evening would increase given that there will be plenty of instability available as MLCAPE values will likely exceed 1000-1500 J/KG based on the latest forecast soundings. A significant pattern change is still anticipated by the middle to latter half of the week as a polar trough traverses from the Pacific Northwest through the Intermountain West Region. As this pattern evolves, the flow aloft will be shifting out of the southwest, which will usher in significantly drier air into the region. This would likely shut off thunderstorm activity across the region for a couple of days, mainly Wednesday and Thursday when ensembles indicate PWATs dropping below 1.2" across the lower deserts and below one inch across the higher terrain areas. By Friday into next weekend, as the polar trough moves into the northeastern CONUS, allowing the subtropical ridge to build over the southern Plains, a return to south to southeasterly flow is likely, which will once again cause moisture levels to increase with storm chances returning. Temperatures heading into the first of the week will be slightly cooling down to near normal levels as highs across most of the lower deserts retreat close to 105 degrees. Then, by mid to late week as upper-level height fields rise, temperatures will slightly warm but most likely remain below 110 degrees. Overall, not too bad of a temperature forecast expected this upcoming week with the overall HeatRisk expected to be mostly in the moderate category across most of the region. && .AVIATION...Updated at 1837Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: There is moderate confidence in the thunderstorm outlook for this afternoon and evening with closest storm activity favored just to the southwest of the Phoenix area with around a 20% chance at the terminals. At this time, there is no mention of VCTS or VCSH in the TAFs due to the uncertainty. Confidence on wind directions through the TAF period is lower. Current light variability, with a south to southeast component, may not come around to the west until as late as 21-22Z. Initial outflow winds are favored from the southwest late this afternoon and evening, with potential for gusts in excess of 20 kts. A later outflow from the southeast is possible, but less certain. There is then potential for winds to remain westerly most of the night with low confidence on a diurnal east shift. FEW cumulus bases will be around 7-8K ft AGL this afternoon, while SCT to BKN mid and high clouds this evening and tonight will mostly be aoa 15K ft. Southeast California/Southwest Arizona including KIPL and KBLH: Winds at KIPL will primarily be southeasterly with speeds generally aob 10 kt through the TAF period, with the exception of some gustiness the next couple hours. Some isolated storm activity is possible this afternoon and evening in the mountains to the west of KIPL, which could push outflow winds into the Imperial Valley and cause a brief westerly shift. At KBLH, southerly winds will prevail through the TAF period. Winds speeds will generally be in the teens through this afternoon with gusts reaching into the middle 20 kts. FEW-SCT mid and high level clouds will continue through the period. && .FIRE WEATHER... Shower and thunderstorm chances will persist through the next couple of days, with the highest chances expected across south- central AZ. With any thunderstorm activity, strong winds with potential for outflows to travel long distances and cause sudden wind shifts as well locally heavy rainfall and frequent lightning can be expected. Storm activity is still expected to trend downward significantly by the middle of the week as drier air filters into the region. Aside from the thunderstorm winds, the overall general winds will continue to follow the typical diurnal tendencies with some afternoon upslope breeziness. MinRH values through Monday will remain elevated at between 20-35%, but then trend slightly downward heading towards midweek with values ranging between 15-25%. Temperatures during the next several days will average near to slightly above normal. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Kuhlman/Lojero AVIATION...Benedict/Berislavich FIRE WEATHER...Lojero/Kuhlman