Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/24/19

Area Forecast Discussion
National Weather Service Phoenix AZ
518 PM MST Mon Sep 23 2019 .UPDATE...Updated Aviation Discussion. && .SYNOPSIS... Multiple periods of scattered to widespread showers and thunderstorms are expected through Tuesday morning across south central and southwest Arizona. Locally heavy rainfall capable of producing flash flooding along with isolated severe thunderstorms capable of producing large hail and damaging winds are expected. Although rain chances will begin to decrease on Tuesday, elevated rain chances will linger through Thursday as the upper level disturbance remains near the region with the best rain chances Wednesday night into Thursday. This disturbance will move east of the region Friday, with drier conditions expected this weekend. && .DISCUSSION... Very unusual setup has materialized across the Desert Southwest today. An anomalous trough continues to dig southward towards Las Vegas with abundant tropical moisture streaming northeastward ahead of the vort max across Arizona. Heavy rain with rates of up to 2 inches per hour developed early this morning along essentially what could be classified as a warm front. Training cells within this area produced as much as 3 to 7 inches of rain along portions of the Salt River, near Horse Mesa Dam and Roosevelt Lake. Latest mesoanalysis depicts a widespread area of 2000+ J/kg of CAPE with little CIN, particularly across west-central Arizona. However, lapse rates have been poor due to the lack of insolation, which has inhibited the coverage of storms. Nevertheless, conditions remain favorable for additional strong to severe storms into this evening and a Severe Thunderstorm Watch remains in effect. Models indicate the upper low to our northwest will close off, with cooler air in the mid-levels overspreading the region. Synoptic-scale jet-forced ascent will also be present in the entrance region of a jet streak near the Four Corners. CAMs are not completely in agreement though the general consensus suggests that multicellular activity will gradually migrate west to east ahead of the closed low and persist late this afternoon into this evening. Although low-level temperatures have cooled considerably behind an outflow boundary across central Arizona, larger-scale forcing for ascent will predominate, tapping into the elevated instability indicated in the latest ACARS soundings. Main threats remain strong winds, hail and heavy rain. Meanwhile, high probabilities of significant updraft helicities suggest that storm-scale rotation and isolated tornadoes will also remain a threat into late afternoon, but then diminish this evening. Later tonight, latest HREF suggests another batch of convection will move through central Arizona and PoPs of 60-80 near the National Blend were retained. && .PREVIOUS DISCUSSION... Activity should wane in our area by late Tuesday morning although enough moisture will be present that lingering showers will be possible. However, given the close proximity of the upper-level disturbance that should eventually stall over the northern Gulf of California and uncertainties in when third more linear period of storms will occur, we elected to keep the Flash Flood Watch in place through 11 AM MST Tuesday. Some hi-res guidance shows isolated showers and storms over Gila and eastern Maricopa Counties Tuesday afternoon as a cutoff low takes shape, but there is still considerable uncertainty there. Residual moisture and a second Pacific trough later in the week justify elevated PoPs in the forecast, primarily for Maricopa County eastward, late Wednesday and Thursday as the trough finally opens up and ejects to the east as it is kicked out by a much stronger longwave trough upstream. As for temperatures, the persistent troughing and shortwaves will keep a cooler airmass in place resulting in daytime temperatures below seasonal normal values through the forecast period. && .AVIATION...Updated at 0016Z South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Latest radar shows scattered showers along with a few thunderstorms across the Phoenix metro this afternoon. Vicinity showers and thunderstorms will be possible over the next couple of hours though can`t rule out a few of these directly impacting the terminals at some point during that time. CIGs look to remain aoa 6 kft for the most part but can`t rule CIGs briefly dropping below that. Thunderstorm activity should mostly diminish by 02Z with lingering showers possible into late this evening. HiRes guidance suggests redevelopment early Tuesday morning so have added VCTS to the TAFs. Thunderstorm activity early Tuesday morning should wrap up by 12Z with some lingering showers possible. Winds should favor an east to northeast component through Tuesday morning but showers and thunderstorms may result in temporary wind shifts. Confidence in the directions of those temporary shifts is low. Winds may gust upwards of 25 kts at times at the terminals. Southeast California/Southwest Arizona including KIPL and KBLH: Mostly clear skies continue across much of southeast California this afternoon. Expect bases to remain aoa 7 kft at the terminals. Not expecting any shower or thunderstorm activity at the terminals so have made no mention in the TAFs. Varying amounts of mid and high level cloud cover with bases aoa 7 kft will be possible through much of the TAF period. A few gusts upwards of 20-25 kts will be possible but should taper off over the next couple of hours. Winds may become light and variable at times during the early morning hours of Tuesday. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Thursday through Monday: The threat of scattered precipitation will continue into Thursday as a low pressure system remains situated across the Desert Southwest. A slight warming and drying trend is then expected Friday and Saturday as high pressure builds across the region. Cooler conditions are likely by Sunday as a broad area of low pressure settles across the intermountain West. Minimum relative humidities are generally expected to remain above 15 percent through next week, even across southeastern California. Somewhat stronger winds are possible Sunday associated with the low pressure system. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Flash Flood Watch through Tuesday morning for AZZ531-533-534-537- 538-540>558-560>563. CA...None. && $$ DISCUSSION...Hirsch PREVIOUS DISCUSSION...Hopper/Deems AVIATION...Smith FIRE WEATHER...Hirsch