543 FXUS65 KPSR 232137 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 237 PM MST Wed Jun 23 2021 .SYNOPSIS... Isolated to scattered showers and storms will continue through this evening, but minimal rainfall is expected. Increasing high pressure and drier conditions later in the week is expected to push high temperatures back around 110 degrees over the lower deserts by this weekend. Unsettled conditions will be possible starting next week as increased moisture is likely to bring more shower and thunderstorm chances. && .DISCUSSION... Many Phoenicians are rejoicing today as an axis of rich moisture positioned over south-central AZ resulting in scattered light showers. PWATs climbed from less than 1" yesterday up to 1.5-1.7" this afternoon, which is above the 99th percentile for this time of year. Objective analysis this afternoon does suggest MUCAPE magnitudes are upwards of 500-1000 J/kg and sfc-6km around 40-50 kts. With this and the high moisture, you would think we would be dealing with scattered to widespread organized elevated storms. In reality, storms have been limited and rainfall amounts have generally been 0.10" or less with rain rates up to 0.25"/hr in stronger showers/storms (~45dBZ). Phoenix Sky Harbor, the official observation station for Phoenix at least has measured over 0.15" today. A couple of the limiting factors that are evident are the high CIN values (~-100 to -200 J/kg) through south- central AZ and the limited synoptic forcing. Areas of concern through the rest of the afternoon will mostly be the areas that have seen greater sunshine and heating, with chances for surface-based convection. This includes southeast CA/southwest AZ and southeast AZ down towards Tucson. Phoenix temperatures are still (2 PM MST) at daily lows for the day in the upper-70s to low-80s (quite refreshing!) while temperatures out west are in the upper-90s to low-100s. While showers and isolated storms in the Phoenix area generally move east or north of the area, strong storms will be capable of developing, that become surface based or develop over terrain features in the locations mentioned above, through the rest of the afternoon. Primary concerns with any storms will remain strong downburst winds, capable of generating blowing dust, and lightning. 12Z HREF 4-hr max wind with convection has maintained a 50% probability of speeds greater than 35 mph. Recent DCAPE calculations and analysis is around 1000 J/kg, which supports the strong downburst chances with storms. Localized gusts up to 45-55 mph cannot be ruled out. Minimal rainfall is still projected through the rest of the afternoon and evening, generally 0.10-0.20" or less with locally higher possible in any prolonged storm. .PREVIOUS DISCUSSION... Shower and thunderstorm chances (~10%) will continue across Arizona through tonight, but the activity is likely to be fairly isolated as the jet forced ascent is lost. For Thursday, models now support the closed low just off the West Coast to become mostly cut off from the main flow while beginning to drift back to the south. An increasing west to east zonal upper level jet will also start to shear out the upper level low Thursday night into Friday, essentially bringing weakening zonal flow over our region into the weekend. No matter how the upper level pattern evolves over the next few days, flow in the lower and mid levels will turn southwesterly with drying beginning by Thursday morning, persisting through Friday. PWATs are forecast to drop to around 0.75" already by Friday afternoon when we will be back to full sun and warming conditions. A warming trend will commence on Thursday as we begin to dry out, but temperature guidance is at least delaying the potential excessive heat. High temperatures are likely to reach 110 degrees by Saturday or Sunday with the western deserts potentially seeing 112-114 degrees by Sunday. While we are under the weak zonal flow into the weekend, ensembles vehemently agree on another impressive ridge of high pressure moving east southeastward over the Pacific Northwest by Saturday, likely remaining nearly stationary through early next week. Models also support the ridge expanding southward this weekend, raising our heights and providing us with the borderline excessive heat. However, this will also start to increase easterly flow across our region likely providing gradual moistening of the atmosphere starting Sunday. It is still a bit early to know the extent of the moisture return and when it will lead to more storm chances, but for now it seems the eastern AZ high terrain may see chances as early as Sunday. With the ridge likely remaining in place for several days, it seems likely we will see easterly moist flow through early next week. Shower and thunderstorm chances should then spread westward through Arizona early next week. Early next week may be our first somewhat widespread rainfall event of the monsoon season. && .AVIATION...Updated at 1822Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Current shower activity to continue over the Phoenix area to continue into the early-mid afternoon hours, then diminish/move off to the north later this afternoon as the weather disturbance that is producing the activity shifts off to the north. Main impacts will be lowered CIGS (as low as 7-9k feet) at times. An isolated TS cannot be ruled out as well, but confidence is too low to put anything more than VCTS in the TAFs at this point. Winds to remain mainly out of a westerly direction, but some wind direction variability is possible as heavier showers/TS pass near/over the terminals. A gradual improvement/lifting of CIGS can be expected during the late afternoon/evening hours, with SCT-BKN CIGS rising aoa 10k feet into tonight. There is a remote chance that new shower/TS development over SE AZ late tonight/early Wed could approach the PHX area, but confidence is far too low to include any impact from these -SHRA/TS in the TAFs at this point. Winds are expected to switch to easterly late tonight. Dry conditions and CIGS aoa 10k feet can be expected during the day on Thursday with a wind shift to westerly during the late morning hours. Southeast California/Southwest Arizona including KIPL and KBLH: A window of quieter weather is possible from around mid morning to early-mid afternoon with less shower activity. Latest high-res computer model output is showing the development of isolated to scattered thunderstorms this afternoon. These storms could produce gusty, erratic winds, area of blowing dust, and lower BKN CIGs near 7-8kft. A downward trend in clouds/showers/storms and a clearing trend is expected Wednesday evening/overnight. Winds to trend more diurnal later tonight and on Wednesday. && .FIRE WEATHER... Saturday through Wednesday: Expect drier conditions and a warming temperatures into this weekend with highs climbing to around 110 degrees. Min RH values will generally be in the single digits through this weekend with overnight recoveries only to 15-25%. RHs then improve through early next week with Min RHs around 10-15% Monday and 15-20% Tuesday and Wednesday. Overnight recoveries improve to 30-40% by Monday night. With the increased moisture, rain and storm chances will increase as well, primarily across eastern districts. Aside from any thunderstorm outflows next week, winds will follow typical diurnal patterns with afternoon gusts. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Benedict PREVIOUS DISCUSSION...Kuhlman AVIATION...Percha/Sawtelle/Iniguez FIRE WEATHER...Benedict/Kuhlman