Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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514 FXUS65 KPSR 210544 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1044 PM MST Thu Jun 20 2024 .UPDATE... Updated Aviation section. && .SYNOPSIS... As moisture begins to surge westward into Arizona today, thunderstorm chances will increase this afternoon with activity focused mainly east of Phoenix. Strong gusty winds and blowing dust will be the main hazards with the activity this afternoon. Very hot temperatures are expected this afternoon and again on Friday with highs topping out around 110 degrees across the lower deserts. As abundant moisture remains in place through the weekend, daily chances for thunderstorms will exist with the highest chances across the higher terrain areas. && .DISCUSSION... Latest water vapor satellite imagery as well as streamline analysis shows a broad upper-level trough over CA with an upper-level ridge centered over the eastern CONUS retrograding westward. Trapped underneath the upper-level ridge is Tropical Depression Alberto, which is moving westward across Mexico. The retrograding ridge along with influence from Alberto is enhancing southeasterly flow across most of eastern AZ with strong moisture advection ongoing. The southeasterly flow will enhance winds across southern Gila County today and into early Friday, with peak gusts ranging between 40-50 mph. As a result, a wind advisory remains in effect through early Friday. With the strong moisture advection, enough moisture in the low to mid-levels will allow for afternoon convection to develop with the latest hi-res guidance showing the activity across the eastern half of Pinal, far eastern Maricopa and through southern Gila Counties where CAPE values as high as 1000 J/Kg is expected. Forecast model soundings show DCAPE values as high as 2000 J/Kg and thus strong downbursts winds will be a major threat with these thunderstorms with HREF showing between a 10-30% chance of 50+ kt winds just east and southeast of Phoenix. Strong outflow winds, in excess of 40-50 mph, will likely surge westward into central and western Pinal County and into the Phoenix area, with the potential for dense blowing dust to materialize. As a result, a Blowing Dust Advisory remains in effect for late this afternoon into early this evening for Pinal County. Aside from the thunderstorm potential, the other main hazard for today will be the excessive heat. Building 500 mb heights from the retrograding ridge will allow temperatures to increase, particularly across the south-central AZ lower deserts, where highs are likely to top out between 110-114 degrees. The strong southeasterly flow will continue through Friday and thus additional moisture will surge further westward with ensemble mean PWATs ranging between 1.2-1.4" over much of south-central AZ. Thus, afternoon convective activity is expected once again, mainly over the higher terrain areas to the east of Phoenix. However, outflow winds emanating from the convective activity will likely impact the lower deserts. With 500 mb height fields rising above 590dm for much of the area and not much in the way of cloud cover expected through mid-afternoon, high temperatures across most of the lower desert communities will likely range between 110-115 degrees, with areas of major HeatRisk once again materializing, particularly across south-central AZ. As a result, the Excessive Heat Warning which was originally in effect through this evening, has been extended through Friday evening. Heading into the weekend, the model guidance is showing additional surges of moisture moving into central and southern AZ and as far west as the southeast California lower deserts with PWATs increasing to between 1.6-1.8". However, at the same time that the moisture is on the increase, the upper-level ridge is expected to move directly overhead, which will increase the subsidence aloft. Therefore, the overall convective coverage will likely be limited to the higher terrain areas of AZ. The one caveat is that guidance is indicating that an inverted trough moving westward through northern Mexico will reach southern AZ on Sunday, which could potentially provide greater convective coverage. This is reflected in the latest NBM PoPs, which shows the greatest areal coverage for Sunday. High temperatures over the weekend, with the increasing moisture and potential cloud cover will be slightly cooler with overnight lows expected to more elevated. In fact, overnight and early low temperatures are forecast to remain near to just above 90 degrees across central Phoenix. Thus, even though the overall HeatRisk is expected to drop into the moderate category, the necessary heat precautions should still be performed, especially that there will not be as much relief during the overnight hours. Ensembles are strongly favoring the subtropical ridge to strengthen even further heading into the first half of next week with 500 mb height fields climbing to between 594-597dm. The high will likely be centered between eastern AZ and western NM with subsidence aloft increasing even further. This will cause some moisture erosion and thus convective potential will decrease as a result starting on Tuesday. With the strong ridge positioned directly overhead, extreme heat will be a concern with temperatures heading into the middle of next week rising to near 115 degrees with the areal coverage of major HeatRisk becoming more widespread. By the latter half of the week, there are indications from the ensembles that the ridge will gradually weaken as a trough moves through the Pacific Northwest and thus a slight cooling trend would ensue. && .AVIATION...Updated at 0544Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Thunderstorms late this evening are isolated over the eastern half of AZ and will continue to wind down overnight. Easterly surface winds have overspread the Greater Phoenix area and east or southeast winds winds will remain in place through Friday morning. At most TAF sites, speeds are currently below 15kts (except KSDL). It is expected there will be an uptrend in speeds (gusts to 25kts) which could occur as early as 09Z or as late as 16Z and be southeasterly. Directions are expected to trend to southerly by late morning then southwesterly by mid-late afternoon. Anticipate improved surface and slantwise visibilities during the day Friday (compared to Thursday) though it remains to be seen just how much improvement there will be. Isolated to scattered thunderstorms are expected once again over the eastern half of AZ Friday afternoon. The main impact at the the TAF sites (if any) is expected to be a shift to southeasterly winds as early as 00Z or as late as 04Z. However, there is a 20-30% chance of thunderstorms occurring at any given TAF site which would mean substantially more significant impacts to winds (and ceilings). Southeast California/Southwest Arizona including KIPL and KBLH: Dry southwesterly flow aloft will begin weakening during the day Friday as an upper trough weakens and high pressure centered to the east begins to expand westward. Surface winds will generally follow familiar warm season patterns. A change for KIPL will be the lack of a distinct shift to westerly in the evening. Otherwise, clear skies. && .FIRE WEATHER... Moisture will continue to increase across the region into the upcoming weekend bringing daily chances for afternoon thunderstorms, with the best chances expected across the higher terrain areas of south-central AZ. MinRH values for today will range around 25% across the far eastern districts to between 10-15% across the western districts. Similar MinRH values will be seen for Friday before increasing to between 20-35% areawide for the upcoming weekend. Gusty easterly winds upwards of 35-45 mph can be expected at times across southern Gila County through early Friday morning with some breezy easterly winds extending through the south-central AZ lower deserts. High pressure is expected to strengthen over the region by early next week leading to even hotter temperatures along with slightly drier conditions. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning until 8 PM MST Friday for AZZ534-537>555- 559. Wind Advisory until 5 AM MST Friday for AZZ560-562. CA...None. && $$ DISCUSSION...Lojero AVIATION...AJ FIRE WEATHER...Lojero/Kuhlman