609 FXUS65 KBOU 100343 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 943 PM MDT Tue Oct 9 2018 .UPDATE... Issued at 858 PM MDT Tue Oct 9 2018 Temperatures are barely moving again tonight across the urban corridor and eastern plains with the thick cloud cover persisting. Have increased low temperatures tonight a degree or two over this area. However, dew points are running a couple of degrees cooler which should allow temperatures to reach freezing tonight. The other update to the forecast was to increase the POPs and QPF across the urban corridor Wednesday afternoon. A strong shortwave trough, which is currently evident on water vapor imagery over the Great Basin region, will move eastward and provide the forcing for snow showers to develop over the mountains in the morning and these will move off the mountains and onto the urban corridor by the afternoon. Soundings indicate snow will be the predominant precip type despite surface temperatures in the mid 30s. With that being said, the temps in the mid 30s will keep any accumulations below an inch tomorrow. && .SHORT TERM...(This evening through Wednesday) Issued at 327 PM MDT Tue Oct 9 2018 Main concerns are the existing snow on the plains, and the next system coming into the mountains tonight. The challenges in this active weather pattern continue. On the plains, there has been moderate snowfall observed in the Limon to Akron corridor, with lighter snowfall around that and a mix of rain/snow on the northwestern fringe and in lower valley locations with no accumulation. The main band has probably produced an inch or two but overall accumulations have been limited by the marginal surface temps and daytime insolation. The eastern plains are in a weak TROWAL (trough of warm air aloft) and that coupled with a deformation zone are aiding the band of heavier snowfall. That will continue into early evening and with sunset there will be more propensity for accumulating snow. We think this would be the best opportunity to get more widespread 1-4 inch total snowfall amounts out on the eastern plains - favoring ridges east of a line from Sterling to Akron and Limon. The impacts should be minimized by warm road temps, so will keep these lower end and sub Advisory amounts in place for now. That said, there may still be some slush/snow accumulation on bridges and overpasses should heavier snow bands continue longer into the evening. However, the disturbance is already beginning to shift east as evidenced by the erosion from the northwest across Denver, so fairly confident in this forecast at this point. Next concern is in the mountains as the next upper level disturbance moves in later tonight. For the evening hours, only scattered convective showers, but then there's a slug of moisture and lift arriving later tonight. This is already evidenced in the upstream satellite and radar imagery over Idaho, northern Nevada, and northern Utah. This feature will move across our northern mountains on Wednesday and may bring periods of heavy snow. The forecast of 4-8 inches for this event through tomorrow looks good, and enough road impacts given colder temperatures to issue a Winter Weather Advisory for the mountains above 9000 feet. With regard to Freeze Warning, more places will freeze tonight with additional cooling and slight drying through the night. Did bump up forecasts a degree or two due to clouds, but most areas should dip to or a few degrees below the freezing mark. .LONG TERM...(Wednesday night through Tuesday) Issued at 327 PM MDT Tue Oct 9 2018 Mountain areas will continue seeing some snow through the evening as moist westerly flow aloft continues. Drier conditions are expected on the plains due to downsloping effects in the lee of the mountains. Cool temperatures will continue. The main upper trough axis is forecast to hang back over Nevada and the northern Rockies, with westerly flow continuing across northern Colorado. Through Thursday, moisture is forecast to be drawn up into the westerly flow from out of the desert southwest. This will bring snow more snow into the mountains Thursday afternoon and overnight due to the continuing orographic flow pattern. Another few inches of snow accumulation can be expected from the continuing moist pattern. From Friday through Saturday, the upper trough over Nevada is forecast to cut-off from the northern storm track and dive southwestward of the coast of southern California. This will allow a weak upper level ridge to develop over Colorado. This will cut off the flow of moisture out of the desert southwest and allow the airmass over Colorado to begin drying out. Friday and Saturday look like they will have much less cloud cover and temperatures on the plains returning to the 50s by Saturday. Mountain areas will also be dry, but the presence of fresh snow on the ground will help keep the temperatures from warming too much. The return of dry weather looks to be short-lived as all of the medium range models are in agreement in bringing a strong short wave out of southwest Canada that sweeps over Colorado Saturday night and Sunday. A strong cold front and significant shot of cold air aloft will accompany the diving trough. Snow is forecast to spread from the mountains out onto the plains Saturday night and Sunday morning. Much colder temperatures will follow on Sunday with highs on the plains possibly only in the lower 30s. Significant snow accumulations will be possible. We will wait to see if subsequent model runs continue to forecast this pattern. No weather highlights for this time period yet, but that may change. The upper trough is presently forecast to move through quickly with improving conditions possible by Sunday evening. After that, strong upper ridging is expected across the western U.S. which should bring a drier and more tranquil weather pattern back to the state. && .AVIATION...(For the 00Z TAFS through 00Z Wednesday evening) Issued at 858 PM MDT Tue Oct 9 2018 Dew point depressions are trending closer to zero at the airports which would indicate lowering ceilings as we head into the overnight hours. Visibilities may begin to slowly decrease as a few showers are evident on radar moving off the mountains. The most likely time that the visibilities will drop into the IFR categories will be between 12-16Z tomorrow morning although the confidence in the longevity of the lower visibilities is not high enough to have a from group during that time. A strong disturbance will move eastward out of Utah tomorrow and will produce snow showers over the airports during the afternoon. There is some uncertainty is how much these showers will bring down the visibility but they may be brought down lower than 4 miles. && .BOU WATCHES/WARNINGS/ADVISORIES... Freeze Warning until 10 AM MDT Wednesday for COZ038>051. Winter Weather Advisory from midnight tonight to 9 PM MDT Wednesday for COZ031-033-034. && $$ UPDATE...Danielson SHORT TERM...Barjenbruch LONG TERM...Dankers AVIATION...Danielson