Area Forecast Discussion
Issued by NWS Salt Lake City, UT
Issued by NWS Salt Lake City, UT
175 FXUS65 KSLC 092207 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 407 PM MDT Thu May 9 2024 .SYNOPSIS...A downslope wind event for portions of northern Utah will last through much of Friday morning. Isolated to scattered daytime rain and mountain snow showers for much of Utah will continue Friday into the weekend. Fewer showers are likely for the end of the weekend. && .SHORT TERM (Through 12Z Saturday)...A closed low has established, breaking off a positively-tilted longwave trough. The low, positioned over Utah, will retrograde to near the Nevada, Arizona, and Utah border Friday prior to absorbing into the mean flow and exiting Saturday into Sunday. Cyclonic flow around the low centered from central to southern Utah has brought east synoptic flow throughout southwest Wyoming and northern Utah. Flow at 700 mb of roughly 20 knots will increase to 30-40 knots Thursday into Friday. The cold air advection from higher elevations of southwest Wyoming will strengthen a surface pressure gradient. Models have peak surface pressure differences of around 6 mb from Rock Springs, WY to Ogden, UT, which is a strong pressure gradient between those locations. That cold air advection will allow for a stable layer, or inversion near ridgetops to become established. The cold air advection, pressure gradient, and inversion near ridgetops will allow enhanced east flow to accelerate leeward, or downwind of the mountains. Hi-resolution guidance is in good agreement on wind gusts of 45 mph or more for an extended duration Thursday evening through much of Friday morning from roughly Parley`s Canyon to Brigham City and for the eastern Cache Valley. There is lower confidence on wind gusts of 55 mph or more for those locations, but nearly full agreement on wind gusts of that magnitude from Bountiful to Layton. East winds of 25-35 mph with gusts peaking around 55 mph are likely for eastern portions of the Salt Lake Valley and Cache Valley and also for Eastern Box Elder County. The typical places locally with the strongest winds in downslope wind events will be no different, with 35-45 mph winds with gusts around 65 mph from Bountiful to Layton. Locally higher gusts are likely near Farmington and Weber Canyon. As the low retrogrades southwest, downslope winds will diminish through Friday. Isolated to scattered convective showers and thunderstorms are likely during the day. Most showers will be in the mountains and will diminish once they track into valleys. .LONG TERM (After 12Z Saturday)...The long term forecast period begins with the closed low positioned over the NV/UT/AZ triple point. It is then progged to shift east through the day Saturday, with high pressure nudging into the area behind it on Sunday. Unsettled weather is expected to continue across the area through at least Saturday, as moisture continues to linger underneath the low. These should be showery in nature, focusing mostly near the spine of Utah and eastern valleys. Snow levels will continue to rise to above 8500-9000ft as H7 temperatures should remain above 0C. This should also bring surface temperatures back up closer to climatological normals. Cluster analysis of model ensembles shows good agreement regarding the egress of the low and upstream ridging. This pattern trend should bring gradually warmer and drier weather through at least Monday. Some northern Utah valleys could potentially reach 80F Monday afternoon. Thereafter, there is more uncertainty in the pattern. Cluster analysis shows a trend towards zonal flow by late Monday, then by midweek, there is quite a bit of spread regarding the degree of amplification. The latest deterministic GFS and EC are both supporting the more amplified variations, which encompass 47% of ensemble solutions. These solutions would support more wet weather across at least the northern Utah mountains midweek, with temperatures moderating a bit. && .AVIATION...KSLC...Gusty winds up to 30kts will continue at KSLC through at least 02z, though these winds will likely vary between northwesterly and east-northeasterly. Between 02-04z, winds will transition to light and westerly (70% chance)...with low-level wind shear also becoming a threat, given 40kt winds at 2000ft AGL. During the overnight hours, there is a 30% chance of occasional strong gusts out of the east due to downslope winds. .REST OF UTAH AND SOUTHWEST WYOMING...Downslope winds will be the main threat along the central and northern Wasatch Front and in the eastern Cache Valley. Strong easterly wind gusts will occur through the afternoon, especially at KOGD and KLGU. Easterly wind gusts will likely continue overnight at KOGD, however there is a low (20%) chance that winds transition to light and westerly, producing LLWS. Confidence is even lower in the winds overnight at KLGU. Strong easterly winds are more likely to persist...at least through 06z, but at times winds may become light (also with LLWS concerns). Winds are looking more likely to become light out of the west after ~06z. Thunderstorms are also a concern this afternoon through 03z across much of the area, particularly across higher terrain, with a 20% chance of thunder at many sites. Gusty and erratic outflow winds will be possible near these storms...notably at KPVU and KBCE. && .FIRE WEATHER...Low pressure will allow for east downslope winds for portions of northern Utah. Wind gusts will peak Thursday into Friday, with gusts exceeding 45 mph for locations near canyon exits, including Logan, Farmington, Weber, and Parleys Canyons. Localized wind gusts will be around 65 mph from Bountiful to Layton. With the low settling near the Nevada, Arizona, and Utah border, daily showers and isolated thunderstorms will last through the weekend, although fewer are likely Sunday. Most showers will start in higher terrain and could track into nearby valleys. Temperatures will warm through the weekend as the low exits Saturday into Sunday with more warming into the week. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...Wind Advisory until 10 AM MDT Friday for UTZ103-105-107. Wind Advisory until 6 PM MDT this evening for UTZ104. High Wind Warning until noon MDT Friday for UTZ104. WY...None. && $$ SHORT TERM...Wilson LONG TERM...Cheng AVIATION...Cunningham FIRE WEATHER...Wilson For more information from NOAA`s National Weather Service visit... http://weather.gov/saltlakecity