Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/31/19

Area Forecast Discussion
National Weather Service Phoenix AZ
855 PM MST Wed Jan 30 2019 .UPDATE...Updated aviation discussion. && .SYNOPSIS... Ample high cloudiness with above normal temperatures and humidity will continue through Saturday as a pair of weather systems move through the region. The first disturbance will bring a quick chance of showers and possibly a few thunderstorms overnight Thursday into Friday. The second and more significant system will impact southern California and Arizona this weekend with the best rain chances beginning Saturday morning in southeast California and Saturday afternoon or evening in Arizona. Rain chances should decrease on Sunday with below normal temperatures and slight rain chances expected to continue through the first half of next week. && .DISCUSSION... Above normal temperatures with most lower desert locations in the low to mid 70s have continued today under midlevel broken cloud decks with ceilings as low as 10-12 kft. Aircraft soundings display a fairly thick 3-4 kft layer of saturation at midlevels, but they also show fairly dry conditions at lower and upper levels. This midlevel cloudiness with relatively dry near-surface conditions is expected to continue overnight and through most of the day tomorrow, allowing low temperatures tonight to stay up in the upper 40s to mid 50s and high temperatures tomorrow to stay in the lower to mid 70s. Moisture will begin to increase at lower and upper levels during the day tomorrow ahead of a compact closed upper level low approaching from the W-NW. Precipitable water values increasing to 0.6-0.8 inches with S-SW to N-NE oriented isentropic ascent and PVA at midlevels ahead of this system will cause scattered to widespread showers over southeast California late tomorrow afternoon and evening. Rainfall amounts out there will be around a tenth to quarter of an inch for many locations, with a few spots seeing up to a half inch particularly near Joshua Tree NP. There is more uncertainty as to how much this will hold together into Arizona, but the increase in moisture with increasing isentropic ascent should support at least isolated to scattered showers overnight Thursday into Friday. Rainfall amounts should generally remain below a tenth of an inch, but the presence of modest cold air and weak elevated instability aloft may allow for a few thunderstorms to occur that would cause local amounts of a quarter to half an inch. However, it is more likely that locations will receive only a few hundredths of an inch to nothing at all than see these higher totals, especially over southwest Arizona. This fast-moving shortwave disturbance will exit the region by late Friday morning to perhaps allow some clearing on Friday afternoon as drier air briefly fills in aloft. Thus, Friday generally looks to be a pleasant day with highs in the lower 70s. Moisture begins to increase again on Saturday as a larger and more significant shortwave disturbance approaches from the west. Models continue to waver a bit on the timing and amount of rain expected from this system, but the latest model cycle suggests the timing of rainfall may be pushed back a bit to Saturday morning for southeast California and Saturday afternoon or evening for Arizona. This system will have better upper level support with the northern jet stream phasing with a southern branch as it approaches the California coast and the midlevel shortwave itself maintains reasonable strength as it ejects to the northeast as it traverses the Desert Southwest. The heaviest rain showers will be expected just ahead of an along a surface front that should push through southeast California Saturday afternoon and south central Arizona during the overnight hours from Saturday into Sunday. Deeper moisture and better dynamics with this system should result in widespread rainfall amounts of around a quarter inch over southeast California and southwest Arizona with around a half inch over south-central Arizona. However, locally heavy rainfall up to an inch will be possible for most areas with higher terrain locations north and east of Phoenix possibly receiving up to an inch and a half. Localized minor flooding issues in flood prone areas will be possible with this rainfall, especially if any convective shower or thunderstorm activity is enhanced by terrain. There should be a brief break later on Sunday through the first part of Monday, but another weaker system is shown approaching from the west sometime early next week. This system should have much less moisture to work with as models show the system moving through Monday night into early Tuesday. A slight chance of rainfall has been maintained on Monday and Tuesday, but it seems unlikely that this system will provide widespread rains. Snow levels do look to 5000-6000 feet by Monday and Tuesday, so snow showers at higher elevations will not be out of the question. Below normal temperatures are also expected for the first half of next week as broad trough remains situated across the western U.S., but there is significant uncertainty as to how the pattern will evolve behind the third system expected early next week. && .AVIATION... South-Central Arizona including KPHX, KIWA, and KSDL: Expect moist northwest mid-level flow to continue across the terminals tonight and through the day Thursday ahead of a wet Pacific low pressure system. SCT-BKN mid level decks 10-14k feet will persist until Thursday evening when cloud bases will lower and scattered showers will develop over the Phoenix area. Most of the lower CIGs will develop after 11 pm Thursday but bases will commonly be 5-6k feet into the morning hours on Friday. Do not expect much in terms of wind Thursday; initially winds should be east to southeast and generally less than 10kt; by mid to late afternoon they will swing towards the south/southwest but cloud cover should limit mixing and as such afternoon speeds should stay below 12kt for the most part. Southeast California/Southwest Arizona including KIPL and KBLH: Do not expect any significant aviation impacts rest of tonight and through the morning on Thursday as moist northwest mid level flow continues across the western deserts. Look for SCT-BKN decks 10-14k feet into Thursday afternoon before bases lower as a wet Pacific low starts to move east and into far SE California. Scattered showers should start after 21z Friday with CIGs lowering to 8k feet or so; by evening some CIGs could drop to around 6k feet depending on how much rain develops. No visibility restrictions are expected. Winds will be relatively light, favoring the southeast at KIPL and KBLH mainly during afternoon hours. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Friday through Tuesday: There will be no fire weather concerns. A series of weather systems will move over the region through the early part of next week bringing an increase in moisture. Minimum humidities can be expected to rise from 40-50 percent today to 45-60 percent Saturday. Moisture will decrease slightly Sunday through Tuesday as the main system pushes off to the east. The wettest system will move through the region on Saturday with the breeziest conditions expected during the day with gusts to 15 to 25 mph. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Hopper/Kuhlman AVIATION...CB FIRE WEATHER...Hernandez