Area Forecast Discussion
Issued by NWS Salt Lake City, UT
Issued by NWS Salt Lake City, UT
181 FXUS65 KSLC 211018 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 418 AM MDT Fri Jun 21 2024 .SYNOPSIS...Deep moisture will spread into eastern Utah through Friday, bringing the threat of showers and thunderstorms capable of producing heavy rain, damaging winds and large hail. Drier conditions are expected over the weekend and into early next week with increasingly hot temperatures, though enough low level moisture is expected to linger across southern Utah to support isolated showers and thunderstorms through early next week. && Key Messages: * Anomalous moisture will remain in place on Friday across eastern Utah, mainly along and east of I-15. This moisture will support locally heavy rainfall and an associated flash flood risk Friday and Friday evening. Areas most as risk include recent burn scars, slot canyons, normally dry washes and slickrock. In addition, there is a slight (level 2 of 5) risk for a few severe thunderstorms with a threat of damaging wind gusts, large hail (1" to 1.5" in diameter) and even an isolated tornado. * Heat re-surges across the forecast area, allowing high and low temperatures to rise to 10-15 degrees above average by Sunday. Hot temperatures combined with warm overnight lows will bring an increasing risk of heat related illness through at least Monday, especially for the urban areas. There is also increasing heat risk across southern Utah by the middle of next week. .SHORT TERM (Through 12Z/6AM Sunday)...An active day remains in store across much of Utah today, with a continued threat for flash flooding and severe thunderstorms. Early Friday morning, a few showers with embedded thunderstorms were ongoing across eastern Utah, mainly along/near the Green River and across the Swell and Castle Country. Copious moisture has worked into areas mainly east of I-15, with impressive moisture advection. For a representative example of how quickly the low levels have moistened, we look at the Buckskin RAWS near the Arizona border over south-central Utah, where the dewpoint increased from 19F to 51F in two hours! Early morning analysis shows an axis of 1" to 1.1" PWATs in place across east-central Utah. These values will increase slightly through the day east of I-15, and are highly anomalous for the time of year, or 225-250% of normal. How might such a moist atmosphere translate into precipitation intensity? Well, on Thursday, a thunderstorm with a wet core passed over the Grand Junction, CO Airport, and produced nearly 0.50" of rain in 5 minutes, translating to an hourly instantaneous rate between 4"/hr and 5" of rain/hr. Similar precipitation rates will be observed in the strongest cores today across eastern Utah. Finally, a shortwave and associated jet streak were located just upstream over southern Nevada. The combination of highly anomalous moisture, large-scale lift courtesy of the shortwave and associated jet streak, SBCAPE values near 1000J/kg at convective initiation time and impressive deep-layer wind shear (35-55kts) will set the scene for strong, organized convection this afternoon. Latest CAMs suggests thunderstorm initiation will take place between 11AM and 12PM, as ample sunshine will warm the surface and allow parcels to reach their convective temperatures. Only cloud cover appears to be upslope-induced stratus clinging on the southeast sides of terrain features. After initiation, thunderstorms will then move off towards the northeast and east. An examination into convective mode favors discrete supercells across extreme SE Utah, with notable updraft vorticity tracks. Given forecast 0-3km helicity values in the 200-300m2/s2, these values are supportive of rotating updrafts. If an updraft can track along resident vorticity pools or other low-level convergence boundaries, we`ll need to monitor for the potential of a tornado. Storm Prediction Center has highlighted southeast Utah with a 2% tornado probability, meaning there is a 2% chance of a tornado within 25 miles of a single point. Overall, a slight (level 2 of 5) risk of severe thunderstorms has been issued in the Day 1 SPC outlook for Friday. This is only the 4th slight risk in the state of Utah in June going back to the late 80s. HREF 50kt wind probability contour of 50% now appears across southeast Utah on the 21.00Z run as well, the highest such probability this forecaster can recall across the Intermountain West and highlighting the unusually high probability of strong, damaging winds. Farther north, CAMs suggest more mixed storm modes consisting of bowing segments and merging/splitting cells suggestive of more of a damaging wind threat as the primary convective hazard, however hail and tornado risk, although lower, still remains. These more mixed convective modes are suggested generally east of I-15, and north of I-70 into the Uintas. CAMs suggest most convective activity will weaken/move east of Green River late this evening, with isolated activity left behind. While QPF has come down slightly compared to previous model runs, given the ingredients in place we don`t believe this will translate to any reduced flash flood risk compared to previous forecasts. Heavy cores with even limited residence time of less than 30 minutes across flashy areas such as slot canyons, slickrock, normally dry washes and recent burn scars carry a risk of flash flooding. Those with any outdoor plans across eastern Utah today should make alternate arrangements. West of I-15, shower and thunderstorm activity will be more isolated, and will carry more of a gusty outflow wind threat owing to dry low-levels, as juicy air will struggle to make it past the mountain spine. There is still some uncertainty regarding flash flood risk over Zion and Lower Washington County, as ensemble mean QPF isn`t particularly alarming, and there is a wider range in potential dewpoints here. Ensemble max QPF, however, would be enough to carry a flash flood risk here. While the probability of such an outcome is lower over SW Utah, it is non- zero and will need to be monitored as a more organized core or training cores could carry a flash flood risk. On Saturday, large-scale lift will have waned, but anomalous low- level moisture is forecast to remain across central and southern Utah, with PWATs forecast to be 175-200% of normal. Lack of large- scale ascent favors terrain-based initiation. Westerly mid-level flow suggests convective activity will drift eastward and then collapse, except where cold pools merge. Thus, the flash flood threat on Saturday is low, but non-zero. .LONG TERM (After 12Z/6AM Sunday)...Heat and isolated to scattered convection remain the primary story of the long term forecast through the middle of next week. Towards the latter half of the work week, longer range guidance hints at potential for increasing trough influence and slightly cooler weather. Starting with the heat, Sunday continues to look very warm across much of the forecast region, especially northern Utah, as a strong ridge extending from the southern Plains into the Desert Southwest yields H7 temperatures around 16C to 19C across the area. With this, afternoon highs from the Wasatch Front into the Great Salt Lake Desert look to push near to maybe slightly above 100F. While it`ll hot more or less everywhere, HeatRisk product particularly highlights the urban corridor from the Salt Lake Valley northward roughly through to Brigham City, as well as much of the Great Salt Lake Desert to Wendover. As such, opted to issue an Excessive Heat Watch for these areas with this package. Held off on including the Toole/Rush and Utah Valley zones where temperatures look just marginally cooler, but subsequent expansion to heat headlines definitely remains possible. A shortwave trough passing through the northern jet will brush by Monday, helping to flatten the ridge a bit, and nudge temperatures down a few degrees accordingly. However, following the passage of this feature, the ridge rebounds Tuesday/Wednesday, and afternoon highs and overnight lows will once again push to potentially dangerous levels. HeatRisk once again highlights this period for potential heat headlines, but given a bit more spread in guidance and still several days out, holding off on any issuance. For southern Utah, temperatures will also start off very warm Sunday with afternoon highs at lower elevations spanning generally from the mid 90s to mid 100s. Given these temperatures are more reasonable for southern Utah from a climatological perspective (around 5-10F above normal), HeatRisk isn`t as aggressive, thus no headlines planned at the moment. That said, the aforementioned flattening effect from the northern stream trough will be too weak to have much impact across southern Utah, and instead will see temperatures trend upward into midweek, likely peaking Wednesday or Thursday. Tuesday through Thursday, forecast highs for lower elevations of Glen Canyon NRA are around 100-105F, and for Lower Washington County around 105- 110F. With overnight lows near 80F offering little relief in these areas, this appears a more likely time range where heat headlines will need to be considered. In any case, it will be hot across the vast majority of the forecast region from Sunday through midweek, and naturally more heat can be expected as we push deeper into summer. Remember to keep heat related safety in mind, whether outside recreating, working, or just in general. Drink plenty of fluids, take frequent breaks (ideally in an air-conditioned area), wear lighter and looser fitting clothing, and recognize the signs of heat exhaustion or heat stroke. Never leave young children and pets in unattended vehicles. Despite the strong ridge exerting the dominant influence over the forecast region, precipitation chances remain non-zero as anomalously high moisture (PWAT values ~0.75" to 1.00") for this time of year periodically surges northward into the area. While the subsident effect of the ridge should limit widespread precipitation potential, daily isolated to scattered convection looks to fire during the afternoon. Southern Utah appears more favored to see these low end daily convective chances, though further north especially along high terrain and adjacent valleys, at least a few showers and thunderstorms can`t be ruled out, particularly by Wednesday. While the day to day coverage will be a bit isolated in nature, those planning on recreating in any rain sensitive areas (slickrock areas, typically dry washes, slot canyons, etc.) should remain weather aware for potentially quickly changing conditions. Thursday or Friday continue to offer the next opportunity for some relief from the heat, as a deepening Pacific trough moving inland may help to weaken the influence of the stout ridge. By Thursday, around 25% of ensemble membership shows some increasing trough influence, and by Friday membership increases markedly. That said, there`s still a small grouping of around 10% of ensemble members showing continued strong ridging holding on into Friday, which would yield a continuation of heat if that scenario ultimately pans out. && .AVIATION...KSLC...Winds increase out of the S by mid morning, with a shift to NW still anticipated by late morning to early afternoon Friday. That said, any afternoon convection nearby will have potential to push gusty erratic outflow toward the terminal. Higher convective coverage expected east and south of the immediate area, but high resolution models show some low potential (around 20-30% chance) of something drifting atop the terminal, which would carry additional lightning risk and briefly reduced conditions. Any lingering convection wanes moving into the evening, and winds then favored to shift back out of the S near typical diurnal time (~03z- 05z). .REST OF UTAH AND SOUTHWEST WYOMING...A bit more of an active day expected Friday, primarily at terminals east of the I-15 corridor. Increasing moisture and instability will trigger fairly widespread convection, scattered in nature, from early afternoon on into the evening. Some convective cells may be strong to severe, carrying increased risk of strong wind gusts, hail, and heavy rain with what may drift over area terminals. While VFR conditions generally prevail, stronger convection moving over a terminal would likely result in temporary categorical reductions. Convection trends downward in strength and coverage as the evening progresses, with some lingering mid/high level cloud cover remaining into the night. && .FIRE WEATHER...Unseasonably high moisture will continue to stream into portions of Utah today, especially east of I-15. An approaching trough will act on the unseasonably moist airmass to promote widespread showers and thunderstorms for areas mainly east of I-15 and south of the Uintas, with isolated activity north and west of that delineation. Any storms that develop will be capable of producing heavy rainfall, hail and gusty and erratic winds. The trough will move east of the area on Saturday, allowing high pressure to rebuild. This will bring a drying and warming trend, but a few showers and an isolated thunderstorm or two will remain possible into early next week especially over the higher terrain of southern Utah. With the ridge building over the weekend, temperatures will steadily warm, particularly over northern Utah. A modest increase in moisture is now appearing increasingly likely over southern Utah in the Wednesday to Thursday timeframe, but temperatures will remain hot. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...Excessive Heat Watch from Sunday morning through Sunday evening for UTZ101-103>105. Flood Watch from noon MDT today through this evening for UTZ113- 120-121-128>131. WY...None. && $$ ADeSmet/Warthen For more information from NOAA`s National Weather Service visit... http://weather.gov/saltlakecity