Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 03/11/21

Area Forecast Discussion
National Weather Service Phoenix AZ
515 PM MST Wed Mar 10 2021 .UPDATE... Updated 00Z aviation discussion. && .SYNOPSIS... A slow moving weather disturbance will move through the region the remainder of the week providing a period of unsettled and occasionally breezy weather. Temperatures will cool to nearly 10 to 15 degrees below normal the next several days before moderating over the weekend. A few light showers and even an isolated thunderstorm will be possible through Friday though many locations may not see rainfall. Precipitation amounts should not be particularly heavy with the greatest chances and accumulations over higher terrain areas of south-central Arizona. && .DISCUSSION... Drier air has moved in aloft and the atmosphere has cooled considerably. The low in Phoenix this morning, 49F, was 15 degrees cooler than yesterday morning. Full tropospheric cooling is noted in aircraft and neighboring area soundings. With the cooler air in place, and trending cooler as a cold system slowly approaches, highs this afternoon will also be notably cooler than yesterday (~5F-10F cooler). Most lower desert locals are expected to come up short of 70F today. Objective analysis and satellite also shows a strong jet streak has now positioned over the region ahead of the approaching system. Tucson, AZ sampled a 130kt wind at 250mb with this morning`s sounding. In the lower levels of the atmosphere the height packing has shifted slightly to the southeast, thus shifting the stronger geostrophic flow just to the southeast. So, while it is not expected to be as windy as it was yesterday, it will still be quite breezy through this afternoon, especially across southeast CA and southwest AZ. Gusts could reach up to 30-35 mph in this area with up to 25-30 mph further east into south-central AZ. The expansive circulation which was centered off the coast of the PacNW at this time yesterday has now slid down the CA coast to just northwest of the Bay area. Multiple embedded vorticity maxima are visible on satellite rotating around the larger circulation. One shortwave axis moved into SoCal earlier today with a weak IVT plume, spurring scattered showers and isolated convection on the windward side of SoCal. For most of the day the rain shadow effect has held on, but wave cloud coverage continues to increase across and some weak echoes are beginning to pop up across southeast CA. Through the rest of the afternoon and evening, HREF members suggest a isolated showers will be possible across the western warning area, with better chances just north of the area. Some weak showers could also work their way into central AZ tonight, but again remain mainly just north of the warning area as NBM PoPs only reach 10-20% across the northern Maricopa county foothills to Tonto Basin. A strong jet streak currently seen on the western flank of the expansive circulation will help reorient the broad low and begin turning the track more toward the Desert Southwest heading into Thursday. As this occurs, models indicate a deepening of the lower level moisture field across the western warning area through most of the day Thursday with increasing chances for showers. With better dynamics and steepening mid level lapse rates, any surface instability could also support an isolated storm Thursday afternoon. Heading into Thursday night and Friday morning, precipitation chances should increase across the eastern warning area as the low continues to drift east. Based on some short-range guidance this looks like the best timeframe for rain in south- central AZ, including the Phoenix area. 6-hour NBM PoPs are now up to 60-70% during this time. By Friday afternoon, NAEFS members are in fairly good agreement placing the cold core center somewhere over western Arizona. If anything, the forecast trend points towards a slightly slower forward progression albeit only enough to affect some subtle details. Nevertheless given the magnitude of cold air aloft entering the forecast area, some locations in Arizona will be primed for shallow topped convection. Steepening lapse rates (H7-H5 layer ~8 C/km and sfc-H7 nearly 9 C/km) will likely become juxtaposed with residual moisture and vorticity aided support. Boundary layer moisture may be the key here where low 30 dewpoints result in SBCape around 500 J/kg while more aggressive drying and dewpoints falling into the low 20s could preclude deeper convective updrafts. Kept POPs near 60% and low TS mention as conceptually this scenario historically matches ones where convective activity and coverage is much greater than models initially indicate. There are still some minor timing differences heading through the weekend which are resulting in limited forecast confidence. There a minority of ensemble membership (mostly in the CMC and EPS camps) showing the cold core and larger circulation still hovering over Arizona Saturday while the vast majority of model output indicates this center located somewhere near the New Mexico/Colorado border. At this time, will stick with the preponderance of evidence even incorporating a somewhat slower progression feeling only the eastern part of the forecast area will still be influenced by the western periphery of the cyclonic circulation and cold core. Typically, these scenarios involve a vorticty spoke rotating southeast and forcing moist ascent along the Rim. This familiar pattern with NW-SE steering flow would bring a dusting of snow across the higher terrain of Gila County, and potentially a few decaying rain showers into the northeast portions of the Phoenix metro. By Sunday and Monday, a brief period of low amplitude shortwave ridging between weather disturbances will control the SW Conus weather. This setup should bring drying and moderating conditions to the region with temperatures rebounding back close to the seasonal normal. A quick moving shortwave may rapidly descend into northern Arizona late Monday and Tuesday, however the trajectory, depth, and impacts of this next system are highly uncertain this far in advance. && .AVIATION...Updated at 0015Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Gusty westerly winds through early evening and lowering ceilings late Thursday morning and afternoon will be the main aviation impacts through the TAF period. Westerly winds currently gusting up to 20-25 knots will weaken by 03Z to around 10-15 knots before becoming even lighter overnight. Easterly winds should briefly return before sunrise at the East Valley sites, but remain westerly or southerly at the West Valley sites Thursday morning, with some uncertainty on whether they will switch at KPHX and KDVT. Winds will remain southerly to westerly at 10-15 knots by early afternoon while SCT-BKN ceilings decrease to 6-8 kft as another Pacific disturbance slowly approaches from the west. Southeast California/Southwest Arizona including KIPL and KBLH: Strong westerly winds through late evening and isolated to scattered showers over southeast California on Thursday will be the main aviation impacts through the TAF period. Westerly winds currently gusting up to 30-35 knots capable of causing patchy blowing dust will continue at KBLH and KIPL through this evening before gradually decreasing during the overnight hours. Low-level wind shear impacts may briefly occur later this evening at KIPL if winds weaken more quickly than forecast, but confidence is too low to include that at this time because surface winds should not decrease below 15 knots until late this evening. Confidence in shower activity ahead of a strong Pacific disturbance is increasing at enough at KIPL to include a PROB30 group there between 15-21Z, but still too low to include at KBLH at this time. Winds tomorrow afternoon should be a little weaker and a bit more southwesterly than today, with gusts up to 20-25 knots. && .FIRE WEATHER... Friday through Tuesday: Unusually cool temperatures at the end of the week will moderate closer to normal next week as a weather disturbance exits the area. Scattered showers will be common in eastern districts Friday with some localized areas experiencing wetting rains. There is an outside chance of an isolated thunderstorm Friday afternoon though heavy rainfall is not anticipated. More mountainous areas east of the Phoenix metro will see some light snow which could linger into Saturday. Winds will exhibit typical seasonal gustiness with speeds the strongest on Saturday and Monday. Minimum afternoon humidity levels will mostly fall into a 20-30% range this week, then falling back into the teens next week. Overnight recovery will be fair to good in a 30-70% range. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Benedict/MO AVIATION...Hopper FIRE WEATHER...MO