495 FXUS65 KGJT 091752 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 1152 AM MDT Tue Oct 9 2018 .SHORT TERM...(Today through Wednesday) Issued at 308 AM MDT Tue Oct 9 2018 The amplified pattern continues over with ridging extended over the eastern Pacific, which is keeping the longwave trough centered over the western CONUS. Current water vapor imagery shows the midlevel trough located over the Four Corners is beginning to become negatively tilted over New Mexico. Conditions remain rather tranquil under the slow moving low pressure, with the exception of isolated showers this morning, since most of the dynamics have shifted east of the area. Instability associated with the low is expected to build later today mainly across the San Juans and central mountains where the bulk of the precipitation will develop. In those locations the snow levels will lift to 8,500 feet by this afternoon as the showers move through. The San Juans could see 2-4" of snow today mainly above 10,00 feet therefore limiting the impacts. The main question will be can breaks in the cloud cover allow for periods of sun to keep the passes wet to slushy. Convection could generate periods of moderate to heavy snow, but these on and off showers should be brief. Most of that precipitation should diminish during the early evening as the wave starts to lift northeastward over the Plains. The next trough digs across the Pacific Northwest towards the Great Basin later today. The upper-level jet on the backside begins to round the base of the trough placing northeast Utah under the left exit region. By this evening that trough continues southeastward so expect the precipitation coverage to increase as the dynamics become enhanced. By tomorrow the low will weaken as it moves over the CO/WY state line, which is where the best focus for lift will support precipitation. Some of this lift will extend southward across the central mountains with less influence over the southern mountains. This system does brings cooler 700 mb temperatures that should range from -6C in northern Colorado to near 0C in southern Colorado by the afternoon. This means snow levels could lower to valley floors in the north to around 8,500 feet in the south. The resulting snow accumulations by Wednesday evening are 4-8" in the Elkhead and Park Range as well as the Mesa into the central mountains. A few inches could be expected in the San Juans. .LONG TERM...(Wednesday night through Monday) Issued at 308 AM MDT Tue Oct 9 2018 Some showers may linger across the northern zones Wednesday evening as one shortwave trough exits the region to the north and another dives through the Great Basin on its heels. The models have consistently shown a band of precipitation forming across east- central Utah into west-central Colorado Thursday morning before shifting this band to the southeast across SE Utah and into SW Colorado by Thursday afternoon as the positively tilted shortwave trough pushes across the northern Rockies, keeping convective showers across the region through Thursday evening before drier air moves in by Friday morning. H7 temps will warm ahead of this band causing snow levels to rise above 10,000 feet MSL where the precipitation does fall. Meanwhile, models are in better agreement now that the remnants of Hurricane Sergio will move well south of the region across southern New Mexico into Texas Friday into Saturday with the upstream trough being strong enough to keep the track south and our forecast area dry. Confidence beyond Friday drops a bit as models are in disagreement with the track of the next upper level low set to move in from the northern Rockies and down the Front Range and Plains this weekend. The latest GFS has shifted the track a little further west, with the northern and central Colorado divide mountains getting clipped by the western edge of this system. The EC is the furthest west with this low bringing showers to most of the forecast area. The GFS Parallel is a bit in between the two solutions but closer to the operational GFS, keeping most of the precipitation across the northern and central Colorado mountains. Depending on how far west this system tracks, this storm has potential to bring snow levels down to valley floors with H7 temps dropping to some of the coldest air we have seen this season thus far with temps in the -8C to -12C range on Sunday. Sunday night could also end up being a season ending freeze for many lower elevations that have not seen their freeze yet. Drier air moves back in Monday through the coming week as high pressure rebuilds out west and influences our region. && .AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon) Issued at 1116 AM MDT Tue Oct 9 2018 Convective showers and snow showers will develop over the higher terrain of central and southern Colorado this afternoon. Several terminals may be affected by brief visibility reductions as well as droppings in the ceiling. This shower activity should diminish rapidly before sunset. After midnight, showers will begin to work in from the west and clouds will thicken again. A band of rain and snow will work eastward from Utah into western Colorado throughout the morning hours on Wednesday. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && $$ SHORT TERM...KJS LONG TERM...MDA AVIATION...MAC