Area Forecast Discussion
Issued by NWS Salt Lake City, UT
Issued by NWS Salt Lake City, UT
031 FXUS65 KSLC 062127 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 327 PM MDT Mon May 6 2024 .SYNOPSIS...A cold, unsettled pattern will continue through midweek across much of the region. A gradual warming trend can be expected late in the week into the weekend with the potential for lingering mountain showers and thunderstorms. && .SHORT TERM (Through 12Z Wednesday)...A gradual decline in coverage of precipitation is noted in the last hour as shortwave ridging builds into the area in the wake of a cold upper level low. This isn`t the end of the precipitation however, as the longwave trough will remain in place across the region through midweek. Afternoon upper air and satellite analysis indicates that an upper level low is advancing into the northern High Plains. A shortwave ridging is crossing the Great Basin. An active, relatively strong polar jet is noted across the Pacific Northwest into northern California. The longwave trough will be slow to depart the West, with the next shortwave trough pushing a reinforcing cold front through northern Utah late Wednesday night into Tuesday morning. Lingering showers will again shift to a period of moderate to occasionally heavy precipitation across portions of northern Utah Tuesday morning. With loss of synoptic support and daytime heating, expect showers to gradually decrease in coverage Tuesday evening into Tuesday night. Given 700mb temperatures near -10C or so Tuesday morning into Tuesday afternoon, rain may mix with or change to snow across the Wasatch Front. Any accumulations are expected to be limited to grassy and elevated surface but it is something to monitor. Given the longer duration of winter storm conditions for the northern mountains, extended the winter storm warning through 06Z Wednesday. A wind advisory was also added with the morning package for the western Uinta Basin, as wind gusts in the 45-50 mph are fairly widespread across the Basin. .LONG TERM (After 12Z Wednesday)...Long term forecast features a lobe of a positive tilted trough breaking off Wednesday into Thursday, and then retrograding southwestward through Utah before finally getting picked back up into southern stream jet during the weekend. Pattern evolution then becomes a bit uncertain late weekend into early next week, though generally models favor ridging with somewhat quieter and more mild conditions. Starting with temperatures, system carries some winter-like characteristics with H7 temperatures around -7C to -11C overhead with the initial cold air surge early Wednesday. At lower levels, this will translate to likely the coldest temperatures of the forecast period. To that end, a number of lower elevation locations will see temperatures push near to even below freezing for a period of time Wednesday morning. Of those zones where more of the agricultural growing season has started, higher confidence is noted in Eastern Box Elder County (especially the northern half), Toole/Rush Valleys (especially the southern half), Utah Valley (particularly western half), and much of the San Rafael Swell. A Freeze Watch has been issued for these locations accordingly, and if guidance trends colder, further expansions to the watch area may be necessitated. Currently for the Salt Lake Valley north to around Ogden, for example, NBM probabilities show around 10-20% chance of subfreezing temperatures at lower elevations, and around a 30% chance at bench locations. Temperatures Wednesday afternoon will also be quite cold for this time of year, with highs areawide running about 10 to 20 degrees below climatological normal. The good news (if you aren`t a fan of this colder weather) is that as the low becomes cutoff, H7 temps will quickly moderate through the work week, resulting in a fairly quick surface warmup in turn. By the weekend, afternoon highs likely return near to slightly above seasonal normal. Next aspect of the system to monitor is in regards to downslope wind potential in prone areas of northern Utah as the cutoff low slides further southward. General consensus is that best juxtaposition of ingredients (such as easterly low level flow and cold air advection over the terrain) would likely be Friday morning, and potentially also Thursday morning. As it stands, ensemble clusters show ~55% of members with at least some combination of ingredients (albeit weaker signal) coming together Thursday morning. Friday morning almost all members show a favorable combination of ingredients, with ~40% carrying a bit stronger signal. In any case, given the nature of downslope events and how things need to align just right (including in time), it currently remains something to keep an eye on in regards to how things ultimately trend, especially once we get within the high resolution model window. If it does occur, would anticipate a bit more of a diurnal trend as stronger May sun angle helps lessen daytime cold advection, but downslope events tend to be a bit tricky in nature. As the low moves overhead, at least something of a mildly unsettled pattern persists in regards to precipitation chances. First, with the anomalously cold airmass and strong post frontal deep north- northwesterly flow Wednesday morning, GFS derived lake effect probabilities maintain ~20-30% chance of showers/enhancement downstream of the Great Salt Lake. Given the cold airmass, snow levels are forecast to be near valley floors (~4500 ft MSL), and higher precipitation rates could result in further lowering. Otherwise, most activity looks to be somewhat diurnally triggered given the less stable environment, and generally over the higher terrain day to day through the remainder of the week. Additionally, as the airmass moderates through the week, will see snow levels gradually increase each day. Eventually by the weekend guidance favors the cutoff low catching the stronger southern stream westerlies and shifting out of the region. That said, a small amount (~25%) of mostly ECMWF ensemble members (and the ECMWF deterministic run) do show some potential for the low to hang around, which would result in cooler and more unsettled conditions than what this forecast includes. Given ridging is favored however, current forecast indicates a continued warmup and limited (and mostly confined to high terrain) diurnal precipitation chances. && .AVIATION...KSLC...Broken to overcast ceilings in VFR range will prevail through the evening. West winds will prevail, but intermittent southeast winds could occur through roughly 01Z when winds transition to southeast. Scattered rain showers will push in by around 06Z, dropping ceilings into MVFR range. Winds will transition to northwest around then, but directions could change from 06-16Z, largely depending on location of showers. Drier conditions with gusty northwest winds are likely around 16Z. Gusty winds will continue through the day, with scattered showers from 19Z into the evening. .REST OF UTAH AND SOUTHWEST WYOMING...Isolated valley rain showers snow showers in southwest Wyoming will last through the evening. VFR conditions will prevail, but ceilings and visibility in MVFR range or lower are likely with snow showers. Scattered showers will push in by around 06Z and last through much of the morning. MVFR ceilings are likely for most locations, with MVFR or lower ceilings and visibility in southwest Wyoming with snow showers. Drier conditions are likely later in the morning with gusty west to northwest winds. Winds will stay gusty through the day with scattered showers by around 19Z. Dry conditions will last through the day in southern Utah with gusty west to northwest winds. && .FIRE WEATHER...The cool and unsettled pattern continues across the region, with a gradual transition to a more showery mode across the northern Utah mountains. Another cold front will cross the area Tuesday, reinforcing the cooler than normal temperatures. By Wednesday morning, the majority of the guidance begins a gradual warming and drying trend...with a subset keeping a weakening low across southern Utah into the weekend. The main impact if this slower transition occurs will be to keep the threat of afternoon and evening thunderstorms across the higher terrain of southern Utah and adjacent valleys. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...Freeze Watch from late Tuesday night through Wednesday morning for UTZ102-103-106-121. Winter Storm Warning until midnight MDT Tuesday night for UTZ110>112. Wind Advisory until 8 PM MDT this evening for UTZ114. WY...None. && $$ Kruse/Warthen/Wilson For more information from NOAA`s National Weather Service visit... http://weather.gov/saltlakecity