594 FXUS65 KGJT 202318 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 418 PM MST Thu Feb 20 2020 .SHORT TERM...(Tonight through Friday night) Issued at 153 PM MST Thu Feb 20 2020 A progressive ridge has begun to build over the region allowing warm air advection to become well established. There is not a cloud in the sky at this noon hour and indications are that skies should be mostly clear tonight. The main challenge will be how much the warming aloft translates to the surface. The northern valleys ended up around -25 F last night, which was much colder than expected. The 700 mb temperatures increase by 10 C over those locations so tried to follow that trend in the forecast. Tomorrow some moisture in the southwest flow might reach the San Juans in the form of clouds otherwise mostly sunny again. High temperatures will be right around normal for tomorrow. The next storm for this weekend will be a closed low pressure located just off the coast of southern California. .LONG TERM...(Saturday through Thursday) Issued at 153 PM MST Thu Feb 20 2020 On early Saturday the storm system comes ashore and the associated precipitation begins to spread northeastward through Arizona. The models are in fairly good agreement that the Abajos, La Sals and San Juans will see showers by the afternoon. Temperatures ahead of the system become maximized Saturday afternoon with many locations above normal. As a result the snow levels still appear to be relatively high when the precipitation moves in from the southwest, which are around roughly 8500 feet in the south and 5000 feet in the north. Things get interesting and complicated Saturday evening as the dynamics really increase and the midlevel low pressure tracks over northern Arizona. The exact track over our area is still uncertain with various solutions from a track of the main low south of our southern border, to two closed lows...one tracking near our southern border and the other near I-70, or the main low tracking in between I-70 and our southern border. Unfortunately, confidence has lowered again as signs of a midlevel dry slot are beginning to show up. If this materializes it would run into the southern and central areas and really decrease precipitation efficiency. Another concerning trend is a deformation band that sets up across northeastern Utah and far northwest Colorado. Temperatures Saturday night are expected to be cooler up north resulting in the potential for enhanced snowfall totals around the eastern Uinta Basin and mountains. For areas along and south of I-70 nocturnal cooling will be limited given WAA and possible cloud cover, so snow levels may drop to 6000 feet south at best and 3500 feet north. The dry slot could result in instability, which introduces the potential for graupel and more scattered coverage. This instability looks to focus over the southern half of the forecast area. On Saturday night the system tracks overhead and flow gradually shifts to the west on Sunday morning. The coldest air with this low might be -10 C at 700 mb, but most likely warmer. As a result the snow ratios remain at or below 13:1 for the entire event. With the system lacking CAA the highs on Sunday are close to normal for many locations. The most favored areas will be the southern mountains especially the southwest and west facing slopes. The central mountains are less favorable, but may still see decent snowfall in spots, while the northern mountains are the least favored. Given the amount of uncertainties and how far out we are from the start decided not to issue any highlights. The snowfall amounts still appear to be around advisory criteria for now. The potential banding and dry slot will need to be watched closely. The next shortwave trough drops southeastward over the northern Rockies on Monday but timing will be key. The models are starting to show the previous system over the plains and this one trying to interact and partially phase. This will have implications on where the northern wave tracks with the northern mountains being more favored with a southerly track. The run to run consistency is making it tough to get a handle on this system, but it could result in more mountain snow Monday into Tuesday. An expansive ridge looks to take over the western CONUS in the later half of the week. && .AVIATION...(For the 00Z TAFS through 00Z Friday evening) Issued at 418 PM MST Thu Feb 20 2020 Widespread VFR conditions will prevail across eastern Utah and western Colorado through the TAF period under high pressure aloft. Winds will generally remain light and terrain driven. High-level clouds will begin to increase after 16Z Friday as flow begins to shift to the southwest, though CIGS will remain well above ILS breakpoints. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && $$ SHORT TERM...KJS LONG TERM...KJS AVIATION...MMS