Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/17/20

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
445 PM MST Wed Dec 16 2020 .UPDATE...Updated aviation discussion. && .SYNOPSIS... A slow warming trend will allow for high temperatures to push above the seasonal average this weekend through early next week. However, mornings will remain cold in the 30s to low 40s through the next several days with some outlying lower desert areas still flirting with freezing. A weather disturbance will bring cloudy conditions Thursday and breezy northerly winds along the Lower Colorado River Valley Friday. No precipitation is expected through the next 7 days. && .DISCUSSION... The Desert Southwest remains under dry and stable northwest flow this afternoon. Aircraft soundings in Phoenix show the low level thermal profile has warmed 5-8C over the past 24 hours. This should help this afternoon`s surface temperatures warm a few degrees from yesterday, especially across the south-central AZ deserts. Thursday morning`s lows will be slightly warmer as well, but the dry conditions and clear skies will still support strong radiational cooling and some of the typical cold valley spots will still flirt with freezing; locations like northwest Pinal, central Yuma, and central La Paz counties. Morning lows Thursday through this weekend will remain relatively unchanged day to day, with most communities, besides the aforementioned cold spots, in the upper 30s to low 40s. An upper low currently moving east just offshore of the PacNW is expected to become an open wave upon moving inland tonight and turn southeast. The wave will eventually pass through northern AZ Thursday night. As a positively tilted shortwave, it will quickly move east of the area by Friday night. Other than some light rain/snow showers in northern AZ, this will be a relatively dry system for most of AZ and southern CA. A rather thick shield of mid and high clouds will stream in ahead of the trough axis during the day Thursday. Would not be surprised to see some linear bands of virga pass over the southern AZ and CA. The clouds should clear quickly following the passage of the trough axis Thursday evening/night. With the base of the trough passing over the area, gradient winds aloft will increase, but is not expected to mix all the way down to the lower desert floors given the strongest winds pass over Thursday night. Though there may be some breezy downsloping winds into the Imperial Valley from the west Thursday evening followed by a breezy northerly wind down the Lower Colorado River Valley during the day Friday. There will be hardly any impact on temperatures with the shortwave Thursday-Friday other than briefly delaying a more distinct warming trend expected this weekend. Models suggest another subtle shortwave trough glancing by to the northeast this weekend, but overall ensemble trends support positive height anomalies across much of the western CONUS through early next week with a weak flow regime. As mentioned low temperatures will remain relatively unchanged day to day due to dry conditions and radiational cooling, but high temperatures will warm above seasonal normals with most lower deserts returning to the 70s by Sunday. Towards the middle of next week, ECMWF ensemble and GEFS mean suggest the positive height anomalies will break down and afternoon temperatures will return to seasonal levels. Still no indication of when the next chance for rain will be. && .AVIATION...Updated at 2345Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation concerns are expected over the next 24 hours across the lower deserts. In the greater Phoenix area, expect light winds tonight into Thursday; light west wind to shift to the east in the 04-05z time window. Light east wind will persist and become a bit more southeast later Thursday morning as low level flow backs ahead of an approaching weather system. Winds Thursday afternoon likely to be either light/variable or light southeast with less chance of wind shifting to a westerly component. Skies tonight generally clear with some cirrus approaching by early Thursday morning; increasing cirrus during the day becoming mostly BKN with bases lowering to around 15k feet by midday. Southeast California/Southwest Arizona including KIPL and KBLH: An approaching weather system from the west will lead to increasing mainly high clouds later tonight into the day on Thursday. Look for increasing cirrus decks with bases aoa 25kt this evening thickening by Thursday morning becoming BKN-OVC (thin at times). Bases lowering to around 15k by mid morning with clearing from the west during the afternoon. Wind will be rather light; at KIPL they favor the west with lots of light/variable and they will increase and swing to the south or southwest during the afternoon Thursday. Light north winds at KBLH tonight will become light variable after midnight and then turn to the south up to 10kt later Thursday morning as the trof approaches. Overall there are no aviation concerns at the terminals over the next 24 hours. && .FIRE WEATHER... Friday through Tuesday: Near normal temperatures will persist through the forecast period with a slow warming trend into the weekend. A few clouds are anticipated Friday morning but with no rain. High temperatures will increase from the mid 60s to the low 70s. RH maximums will be between 40-60% Friday falling to 30-50% the rest of the period. RH minimums will be 20-30% Friday falling to 10-20% the rest of the period. The lower RH values will be found to the west. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Benedict AVIATION...CB FIRE WEATHER...Hodges