Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/12/19

Area Forecast Discussion
National Weather Service Phoenix AZ
551 PM MST Sat May 11 2019 .UPDATE...Updated Aviation Discussion. && .SYNOPSIS... A low pressure system will impact the region this weekend, bringing an increased threat of showers and isolated thunderstorms to the Desert Southwest. The best rain chances across south central Arizona and the Phoenix area will be during the overnight hours tonight and Sunday afternoon. Rain chances will linger across the eastern Arizona high terrain into Monday, before high pressure causes warmer and drier conditions through Thursday. A return to below normal temperatures is likely by Friday as another Pacific low pressure system affects the intermountain West. && .DISCUSSION... Scattered showers and isolated thunderstorms are currently developing along the Mogollon Rim with deeper cells suggesting the weak midlevel steering flow has become east-northeasterly. Limited instability should prevent much if any development into the lower deserts this afternoon and early evening, but weak east-northeast outflows will probably make it into the Phoenix metro after 5-6 pm. PWAT values are gradually increasing across south central Arizona and will continue to increase from 0.6-0.7 inches now to 0.8-1.0 inches overnight as the closed low currently just moving onto northern Baja California ejects east across the Gulf of California tonight. At the current moment, instability is fairly limited to primarily the higher terrain with weak midlevel stability present on aircraft soundings into KPHX. Therefore, thunderstorms are generally not expected in the Phoenix metro until after sunset, with none expected over the lower deserts of southeast California and southwest Arizona. However, the aforementioned stable layers over the lower deserts of south central Arizona are forecast to erode by early evening as moisture advecting into the region continues to increase CAPE into the 250-1000 J/kg range overnight. In addition, hi-res models show convection developing over southeast Arizona late this afternoon that should push to the east-northeast into northern Pinal and southern Maricopa Counties later tonight and possibly hold together into Yuma County. Therefore, we expect to have at least isolated to scattered showers and thunderstorms tonight into at least northern Pinal County and southern portions of the Maricopa County and the Phoenix metro, with some potential for more widespread light to moderate showers on the north side of the low tracking over the northern Gulf of California. Rainfall amounts should generally stay below a tenth of an inch with locally heavy amounts of a quarter to half inch in slow-moving convective cells. Gusty winds will be possible, but DCAPE values will be below 500 J/kg in the absence of daytime heating tonight. Model agreement remains high in the track of the closed upper level low as it slowly pushes east over northern Sonora, becoming centered south of Nogales by late tomorrow afternoon. Therefore, we should have stronger northerly or northeasterly steering flow off of the Mogollon Rim tomorrow afternoon to support movement of thunderstorms developing off of higher terrain into the Phoenix metro. However, overnight showers and thunderstorms may leave behind significant midlevel and high cloud cover that may reduce our instability if they persist into the afternoon hours. We currently expect that afternoon insolation will give way to steepening lapse rates and increasing instability enhanced by the close proximity of the closed low to allow for isolated to scattered showers and thunderstorms in the Phoenix metro. SREF paints high probabilities of widespread MUCAPEs exceeding 500 J/kg across Arizona, with 25-30 knot bulk shear values supporting organized convection with the potential for gusty winds and small hail in the strongest cells. Rainfall amounts will once again generally be below a tenth of an inch of rain, but very localized minor flooding will be possible in stronger, more organized cells that produce heavier downpours up to a half inch. The low pressure system is expected to weaken overnight Sunday into Monday, with storms dissipating in the early evening as a more subsident pattern develops. Although a ridge of high pressure will build across the region early in the week, lingering moisture will support isolated showers and thunderstorms mainly across the Mogollon Rim on Monday and Tuesday afternoon. Otherwise, the rest of the lower deserts will remain dry as increasing heights translate into a warming trend that will continue through mid-week. High temperatures will again flirt with the 100 degree mark in the Phoenix area Wednesday, with temperatures on Thursday a bit uncertain due to the timing of our next westerly upper-level disturbance. A quicker progression of this trough supported by the ECM ensemble would yield high temperatures in the upper 80s across the lower deserts, whereas a slower progression would result in highs in the upper 90s which is more in line with the NBM and our forecast. Regardless, a cooling trend is anticipated by Friday with high temperatures likely falling back below normal into the upper 80s. This next system currently looks to be relatively dry and windy under southwest flow aloft with NBM PoPs generally below 5-10 percent. && .AVIATION...Updated at 0050 UTC. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: A low pressure system currently centered just off the western coast of Baja California will continue to sponsor unsettled weather through the TAF period. Storms over the higher terrain northeast of Phoenix have produced an outflow boundary, which is currently moving to the east/southeast through the Phoenix metro. Easterly/northeasterly winds with gusts up to 20 kts will be possible as the boundary moves through. Winds may become erratic this evening as outflows from nearby showers and thunderstorms move through. Model guidance shows showers moving through the Phoenix metro early Sunday morning. Have VCSH in the TAFs starting around 14-15Z but this may change as there is still uncertainty in timing. Variable amounts of mid and high level clouds are expected with bases aoa 6 kft and cigs aoa 10 kft. More showers and thunderstorms are expected to develop tomorrow afternoon across south-central Arizona. Southeast California/Southwest Arizona including KIPL and KBLH: Winds at KIPL and KBLH will favor the S/SE this evening before KIPL takes on a westerly component tonight. Winds are expected to remain fairly light through the period with wind speeds below 10 kts. Variable amounts of mid and high level clouds are expected with bases remaining aoa 7 kft. A slight chance of a vicinity shower cannot be ruled out during the TAF period but confidence remains too low to include VCSH in the TAFs. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Tuesday through Saturday: Aside from very isolated showers and thunderstorms on Tuesday afternoon across the higher terrain north and east of Phoenix, dry conditions are expected for the rest of next week. A ridge of high pressure will quickly build across the region to usher in much drier and warmer conditions for the middle of next week as temperatures warm back up into the mid to upper 90s. Another upper-level disturbance moving in from the west will drop temperatures back below normal late in the week with wind gusts in excess of 20-25 mph across the lower deserts and stronger gusts at higher elevations. Minimum humidity levels will drop into the teens across the lower deserts and the 20-25 percent range for higher elevation locations each day with good overnight recovery. && .SPOTTER INFORMATION STATEMENT... Amateur radio Skywarn net activation is not expected at this time. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Hopper/Hirsch AVIATION...Smith FIRE WEATHER...Hopper