Scientific Forecaster Discussion

000 fxus65 kslc 191558 afdslc Area forecast discussion National Weather Service Salt Lake City Utah 858 am MST Tue Feb 19 2019 Synopsis...the storm system still lingering over the area this morning will exit to the east later today. A new and potent storm system will impact mainly the southern two-thirds of the state beginning late Wednesday night and continuing into Thursday night. && Discussion... update this morning accounts for lake effect snow showers across the northern Wasatch front. These will likely fill back in southeast of the Great Salt Lake later this morning as flow becomes northwesterly once again. A tight circulation is currently over western Wyoming, which will continue to slowly migrate southeast today. It phases with the trough currently overhead and both features translate eastward this afternoon into tonight. Cold advection, will combine with destabilization today to support snow showers from the I-15 corridor east across the mountains, particularly mid day Onward. Not expecting temperatures to warm much given the cold advection in place. Adjusted hourly T/relative humidity curve based on latest observations and model guidance. Will be putting the finishing touches on the forecast for the next storm for the afternoon package. Impacts begin leading into the morning commute across the north. Even a light snow from warm advection will create travel impacts due to the cold roads. Another significant and rare storm for the valleys along the Arizona border Wednesday night into Friday morning. && Previous discussion... an elongated upper trough remains in place, extending from the northern rockies through Arizona this morning. Meanwhile, an embedded shortwave disturbance is currently noted across southeast Idaho and into northern Utah. As this piece of energy crosses the area this morning, expect temperatures at 500mb to begin warming aloft, leading to stabilization of the airmass from above. However, steep lapse rates will remain in place from the surface to above 700mb, so expect the possibility of snow showers to continue into the afternoon hours. Main concern for the next several hours is the lake-effect snow that produced locally significant accumulations in the Tooele Valley overnight. With the flow aloft backing, the band has started to transition to the Salt Lake Valley. Although the band looks less well-defined in the latest radar imagery, surface winds per latest mesowest still indicate good convergence over the gsl. This, combined with the aforementioned steep low-level lapse rates and relatively moist airmass suggest favorable conditions for the lake effect to continue through this morning, with impacts to the morning commute especially in the Salt Lake Valley. Otherwise, extensive stratus remains in place across much of western Utah through at least this morning. Temperatures will be quite cold today, with 700mb temperatures ranging from -16c across the south to -19c across the north. Afternoon maxes are forecast to range from around 15 to 25 degrees below climo today. The flow aloft will briefly become anticyclonic late tonight into early tomorrow morning as a low-amplitude ridge shifts across the area. However, warm advection will quickly develop at 700mb late tonight ahead of the next approaching trough, with snow beginning to fill in again across the north. This warm advection is expected to wane by late tomorrow afternoon. Snow accumulations across northern Utah are not expected to be particularly significant except perhaps locally near the Idaho border. The trough will end up pinching into a closed low as it dives south through western Nevada tomorrow night into Thursday. This will then shift the focus of precipitation into southern Utah. Models slowly track a 700mb circulation across southern Utah Thursday into Friday. As moisture spreads into the area ahead of said circulation, favorable southwest through southeasterly upslope flow will occur for an extended period across southern through east-central Utah. With a continued cold airmass resulting in snow levels down the all valley floors, significant snow is expected. As a result, have issued a Winter Storm Watch for this event. Northern Utah will quickly shift to an easterly flow aloft Thursday which will not Bode well for snow along the Wasatch front and adjacent mountains. In addition, the easterly mslp gradient will allow for gusty canyon winds once again along the Wasatch front, developing late tomorrow night and continuing through Thursday night. At this time, Ridgetop winds do not support a larger downslope event. The trough will fill and start shifting east at a faster pace on Friday. North to northwesterly flow aloft will spread across the entire forecast area as a result. Snow will linger especially along the mountains and adjacent I-15 corridor from Salt Lake to around Cedar City before tapering off Friday evening. By Friday night, the only precipitation continuing with the ongoing storm should be isolated to the higher terrain areas of central and eastern Utah. This should come to an end by Saturday morning, as the parent trough and surface feature continues well east of the forecast area...finally allowing for a dry period to take over across southern Utah next weekend and early next week. Switching focus to northern Utah...the next weather system is expected to move into the forecast area late Saturday into Sunday. In contrast to the previous system, this one is characterized by a low amplitude trough over the entire northwest Continental U.S.. poleward of a zonal polar front jet (pfj) will be much cooler 500 mb/700 mb temperatures. The pfj is currently forecast to align almost due east-west directly over central Utah by Saturday afternoon. This will leave the precip (all snow) confined to the northern third of Utah and southwest Wyoming Sunday as cooler temps aloft increase the instability aloft allowing for scattered snow showers to develop along higher terrain areas and other weak minor short wave troughs. The lack of significant forcing will keep any precip on light side. Heading into early next week, model consistency becomes something to be yearned for. The general trend is that the pfj will remain aligned over Utah with a jet Max moving onshore off the Pacific coast during the early part of next week. When and where this jet Max moves onshore will determine how the trough evolves. As the jet Max makes its way inland, it will increase the amplitude of the trough and create a slightly more potent system. However, due to model inconsistencies between the timing and deepness of the system attm, the current forecast package went with a blended approach of slight chance-chance pops across much of the area. && Aviation... northwest winds at the kslc terminal will persist under VFR conditions through the day, before switching to the southeast by 02z. Ceilings will remain low end VFR (4000 ft) through early afternoon before lifting above Ridgetop levels (greater than 8000 ft). Lowering cigs during the overnight hours with another round of snow is likely by Wednesday morning. && Slc watches/warnings/advisories... Utah...Winter Storm Watch from Wednesday evening through Friday afternoon for utz517-518. Winter Storm Watch from Wednesday evening through late Thursday night for utz012-013-019>021. Winter Weather Advisory until 1 PM MST this afternoon for utz002- 003. Wyoming...none. && $$ Public...10/cheng/bonnette aviation...Dewey For more information from noaa's National Weather Service visit... http://weather.Gov/saltlakecity

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