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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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238 FXUS65 KPSR 142041 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 141 PM MST Sun Jul 14 2024 .SYNOPSIS... Increased thunderstorm activity is expected through this afternoon and evening, with a slight chance for activity to survive into the Valley. Strong winds and localized dense blowing dust will be the primary concerns with these thunderstorms, along with localized flash flooding. Daily thunderstorm activity will continue through the early work week, but mostly be confined to the higher terrain before chances increase again across the lower deserts for the second half and into next weekend. Temperatures through the week will remain above normal with lower desert highs in the 110 to 114 degree range. && .DISCUSSION... A gulf surge last night has created pretty muggy conditions as morning dew points rose into the middle to upper 60s across southern AZ to even lower to middle 70s in Yuma to the Imperial Valley. Some of this lower level moisture will scour out with daytime mixing through this afternoon, but enough PBL moisture will remain to support regionally high MLCAPE values (1000-1500 J/kg) this afternoon across the southern deserts. DCAPE values will also be quite high (up to 2000 J/kg) today as afternoon mixing steepens the low level lapse rates and the PBL depth increases up to around 10K ft. Considering these parameters, plus the overall better moisture, with PWATs up to 1.5-1.7" and mean mixing ratios around 11-13 g/kg, there is potential for all manner of monsoon thunderstorm impacts today (i.e. strong-severe wind gusts, blowing dust, localized flash flooding, and frequent lightning). There is potential for high impacts from thunderstorms later today, given the environment, but it will be a matter of realizing the instability. For the lower deserts there is a lot of CIN (i.e. a strong cap) that will have to be overcome. So, it will likely take strong, deep, outflow boundaries and/or colliding outflow boundaries from the storms that develop over the terrain features this afternoon and evening. Given the high DCAPE, this is certainly possible. For AZ, objective analysis and the latest HREF paint the greatest arc of instability from eastern Pinal, through Gila, and into central Yavapai County. Based on the latest HREF, there are better chances of outflow winds and storms descending from the north and northeast of Phoenix, with a little less certainty from the south due to uncertainty in storm coverage in Pinal County. Timing on activity into the South-Central AZ lower deserts looks to be late- evening/early tonight, after 6-7 PM MST. In Southwest AZ, the main area of storm activity will likely be the Kofas region, in northern Yuma and central La Paz counties. Further west, there is a risk for high impacts from storms in Southeast CA as well, with potential for strong storms to drift off the Peninsular Mtns of northern Baja and SoCal into parts of Imperial County. Greatest risk in Imperial County will be strong outflow winds that could kick up dust/sand. In Riverside County, upslope flow could lead to localized flash flooding concerns in or around Joshua Tree NP. There will be a localized flash flooding risk elsewhere this afternoon and evening, as storms will be capable of producing rain rates in excess of 1"/hr. On Monday, the overall synoptic setup will be similar to today, with the monsoon high near the Four Corners region, inducing deep southeasterly flow across the Sonoran Desert. There even looks to be more of the morning-midday ACCAS convection across Southwest AZ and Southeast CA. The main difference noted in modeling is a slight decrease in moisture, a degree or two cooler on the temperatures, and thus slightly lower instability. So, it is not out of the question that storms could propagate on outflows into South-Central AZ, but if the environment is overturned this evening-tonight, then storms Monday will be even less likely. After Monday, afternoon and evening thunderstorms are expected to develop daily as seasonal to above normal moisture lingers in the region. Most activity will remain confined to higher terrain areas of AZ, with outflow winds still capable of descending through the lower deserts. Toward the second half of this week and heading into next weekend global models indicate a slight increase in moisture again and a repositioning of the monsoon high back toward the Vegas area. This setup, a Maddox et al. Type II setup could lead to a much more active period with a more favorable Rim to Valley storm trajectory. It will remain to be seen, but the second half of July through mid to late August is typically when the monsoon is most active in terms of severe and flood warning counts. Besides the storms, the Excessive Heat Warning which was in effect for over a week came to an end for the AZ deserts yesterday, but was continued through this evening for Southeast CA mainly due to heat indices being forecast to reach 114-117F this afternoon. Beyond today, temperatures will remain hot, just not quite excessive, with widespread Moderate HeatRisk. Lower desert highs are forecast to reach 110-114F, ~3-7 degrees above normal, through this week with overnight lows remaining quite warm in the middle 80s to lower 90s. Folks should continue to exercise the proper heat safety precautions this week, by staying cool, limiting time outdoors, and staying hydrated. && .AVIATION...Updated at 1710Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Thunderstorms and at least one outflow boundary causing gusty winds with abrupt wind shifts will be the main weather impacts through tonight. Confidence is excellent that west winds with occasional modest gusts near 15kt will prevail into the early evening hours with only mid/high clouds decks. Thunderstorms are likely to develop across higher terrain areas well northeast of the terminals by late afternoon, however forecast confidence is relatively low with respect to how far into the metro storms can survive/re-form during the evening hours. At this time, there is enough evidence suggesting storms skirting the northern edge of the Phoenix airspace to include VCTS mention for KSDL/KDVT. Outflows with abrupt wind shifts appear far more likely across the entire area with better than a 50% chance of some northerly component incorporating gusts around 30kt during a 03-05Z time frame. Wind directions thereafter become tied to locations of storms and additional wind shifts are likely, though timing and magnitude remain very uncertain. Should several more intense outflows happen to collide within area, there could be more direct thunderstorms impacts for all terminal locations, however current odds are less than 20% and preclude stronger mention in this TAF package. Southeast California/Southwest Arizona including KIPL and KBLH: Wind gusts with shifting directions will be the greatest weather concern through Monday morning under occasional midlevel cigs. While southeast winds will be favored at KIPL and south winds at KBLH, there may be wind shifts due to outflow boundaries from nearby thunderstorms. Thunderstorms are possible this afternoon over the higher terrain areas west of KIPL which could result in gusty W/SW outflow winds surging into lower elevations. Gusts over 25kt would be most likely at KIPL late afternoon/early evening with lesser probabilities at KBLH. There is around a 15% chance of TSRA directly impacting KIPL this afternoon, and radar trends will need to be monitored. && .FIRE WEATHER... Chances for thunderstorms and wetting rains will increase this afternoon and evening, particularly over the higher terrain north and east of Phoenix, as well as in Riverside County, with lesser chances in the lower elevations. Thunderstorms will be capable of producing strong erratic outflow winds with up to a 50-70% chance of outflow winds in excess of 35 mph across South-Central AZ and parts of Southern CA this afternoon/evening. Through the first half of the week, thunderstorms and wetting rainfall chances will be primarily concentrated over the higher terrain of eastern districts before chances across the lower deserts begin to increase during the latter half of the week. Gusty outflow winds will be possible on most days this week exacerbating issues on any ongoing wildfires and new starts. Otherwise, afternoon upslope gustiness of 20-25 mph will be periodically common. Min RHs will be in the 20-30% range today and fall slightly to the 15-20% range by the middle part of the week. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ530. CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ560>570. && $$ DISCUSSION...Benedict AVIATION...18 FIRE WEATHER...Benedict/Smith