Area Forecast Discussion
Issued by NWS Salt Lake City, UT
Issued by NWS Salt Lake City, UT
209 FXUS65 KSLC 201016 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 416 AM MDT Thu Jun 20 2024 .SYNOPSIS...Temperatures will trend warmer across the area through the weekend west of I-15 before anomalous moisture spreads into eastern Utah late Thursday through Friday, bringing showers and thunderstorms with locally heavy rainfall and an associated flash flood risk along with cooler temperatures. Drier conditions are expected over the weekend and into early next week with hot temperatures, although enough low level moisture is expected to linger across central and southern Utah to support a daily shower and thunderstorm threat through early next week. && Key Messages: * Anomalous moisture will work into eastern Utah, mainly along and east of I-15 by Thursday night and Friday. This moisture will support locally heavy rainfall and an associated flash flood risk Friday and Friday evening. In addition, there is a risk for a few severe thunderstorms with a microburst threat Thursday, along with a microburst and large hail threat on Friday. Areas most as risk include recent burn scars, slot canyons, normally dry washes and slickrock. * 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 Saturday)...Main focus in the short term period of the forecast is the heavy rain and severe weather potential, particularly on Friday. Anomalous moisture for mid June standards is already in place across adjacent states in Colorado, New Mexico and northeast Arizona, with surface dew points having risen into the 40s and 50s across these areas. Model guidance continues to be in good agreement in showing this moisture gradually advecting north and west into eastern Utah over the next 24 hours. On the leading edge of the axis of higher moisture content air, isolated to scattered showers and thunderstorms are expected to develop this afternoon across southeast Utah, with the primary hazards including isolated dry lightning and gusty and erratic microburst and outflow winds. Locations with the greatest risk of experiencing these winds late this afternoon and evening include locations generally east of I-15 and south of I-70, with probabilities steadily increasing as one travels south and east. On Friday, near-record values of total column moisture are forecast to be in place across eastern Utah, when PWATs are forecast to surge to 225-250% of normal, or up to 1.2-1.3". In addition, a broad trough will have moved into the Great Basin, sending a jet streak across central and northern Utah during the day. This jet streak will place favorable entrance region dynamics across southeast Utah. In addition, the jet will support increasing shear, with 0-6km bulk shear values increasing into the 40-60kt range. Finally, owing to the anomalous moisture in place, SBCAPE values are forecast to increase into the 500-1500J/kg range, (highest values south and east earlier in the day). Therefore, more than sufficient ingredients are in place (moisture, lift, wind shear and instability) to support organized, strong convection. An analysis of available CAMs suggest convection on Friday will initiate over terrain features of central and southeast Utah and propagate off to the north and east through the afternoon and evening. Ensemble QPF tracks are in the 1-2" range along the heaviest convection, with up to a 60% chance of exceeding 10-yr ARI and a 20% chance of exceeding 100-yr ARI. Put in simpler terms, the QPF forecast on Friday afternoon and evening has a 20% chance of being a 1 in 100-year event, with the highest signal across the Swell, where several slot canyons reside. EFI QPF values approach the 90th percentile over SE Utah, with SoT values of 8, suggesting both a high risk for an anomalous event while suggesting a portion of the distribution is well above the QPF mean. Given the risk of heavy rainfall, flash flooding is probable in/near recent burn scars, slot canyons, normally dry washes and slickrock. We have issued a Flood Watch to highlight this risk, along with our neighbors in Grand Junction. Coordination with WPC has placed a slight (level 2 of 4 risk) of excessive rainfall outlook over these areas as well. In terms of the severe weather potential, CAMs all show a signal for a few long-track supercells across SE Utah on Friday afternoon. EFI values for shear and CAPE are remarkable, with extreme forecast index values above the 90th percentile generally east of I-15 and south of I-70, with a pocket outside of climatology over SE Utah. CAPE profiles on forecast soundings are impressive. HREF 30% 50kt wind contours are filled in across SE Utah once again on Friday, highlighting the risk for damaging thunderstorm wind gusts. Given the CAPE and shear profiles, the risk of large hail exists as well. Finally, EHI and helicity parameters suggest a healthy potential for rotating updrafts, especially near the SE corner of Utah. CAMs suggest activity will quickly wane late in the evening. After coordination with SPC, eastern portions of Utah were included in a marginal risk for severe weather both Thursday and Friday. Despite the quality of the ingredients in place to support both a flash flood risk and severe thunderstorm risk, we need to discuss some of the uncertainties with this event. One uncertainty is how far west thunderstorm initiation will occur. The ensemble mean products suggest the bulk of the convection will remain east of I-15 and south of I-70, with some heavier convection also extending as far north as the Uintas. If we look at the spectrum of the distribution, the high end of the distribution does suggest a potential for heavy rainfall and strong thunderstorms as far west as Zion NP, Lower Washington County and perhaps even the Wasatch Front. Additionally, there is some uncertainty as to how much cloud cover will be in place Friday morning, as higher than forecast amounts of cloud cover will limit instability/buoyancy generation. Finally, with deep-layer shear values forecast to be so high, it`s possible that shear/updraft/cold pool balance could be difficult to achieve, resulting in thunderstorms that struggle to sustain themselves. Either more cloud cover than currently forecast and/or too much shear would act to limit the extent and severity of both the heavy rainfall and severe thunderstorm risk. Either way, those with plans to recreate on Friday, especially along/east of I-15 should consider alternate plans. .LONG TERM (After 12Z/6AM Saturday)...Saturday looks to have at least some potential to be another interesting day across portions of the area. Similar to Friday`s setup, forecast region will be positioned on the NW periphery of a strong ridge extending throughout the southern Plains, though the northern stream trough influence will be lessened as it exits. Still, southern and eastern Utah in particular will see continued favored moisture content with elevated PWAT values (around 125-200% of normal) in addition to enhanced upper level jet flow overhead. While less than that of Friday, this jet flow should still yield marginal deep layer shear around 20-30 kts or so. Provided any lingering cloud debris from Friday has cleared, modest destabilization (MUCAPE ~250-750 J/kg) should also occur. Given the departure of the northern stream trough, a little less trigger mechanism is noted, but combination of terrain and weak PVA should yield some scattered convective development by the afternoon, albeit likely with lesser of an areal extent. Given the aforementioned ingredients, the ECMWF Extreme Forecast Index (EFI) continues to also highlight southern to eastern Utah in shading depicting anomalous CAPE-Shear values, further supporting at least some potential for more mature and strong convection. Additionally, given the moisture profile, any mature convection should be able to efficiently produce rainfall, so those recreating in rain sensitive areas (slickrock areas, slot canyons, dry washes, etc.) should remain weather aware. Elsewhere, dry conditions are expected, with increasingly hot weather as H7 temps continue upwards. Sunday onward into midweek, anomalous moisture lingers especially across southern and eastern Utah leading to continued daily chances of isolated to scattered convection, and the nearby strong ridge continues to yield very warm temperatures. By Sunday, northern Utah warms sufficiently that the urban corridor in particular will see heat approaching potentially dangerous levels (upper 90s to low 100s across much of Wasatch Front). Opting to hold off at least one more set of guidance given it is still a few days out, but it appears increasingly likely some sort of heat related headlines may be needed for a portion of the region. The following several days show similar levels of heat to maybe ever so slightly cooler, though spread in guidance also increases a bit. For the southern half of Utah, conditions will gradually warm day to day moving into the middle of the week, with increasing potential for heat related risk accordingly. By Wednesday, current suite of guidance shows widespread upper 90s on into the 100s, with current NBM giving ~25% chance to hit or exceed 110 at KSGU. Given the prior mentioned lingering moisture and limited daily convective potential, these high marks will have some potential to be modulated by whatever does manage to develop. In any case, suffice to say the heat is very much back as we move into astronomical summer. Looking Thursday onward a little beyond the coverage of this forecast package, guidance shows increasing consensus on a trough deepening southward from the PacNW. Less consensus is noted on the eventual evolution including how quickly it advances inland and how deep it remains when it does. While some guidance shows any trough influence could come as early as Thursday, ensemble membership increases by Friday, with only a limited number of members showing little to no influence. Still, this looks to be the next window to see the heat abated a bit, at least across the northern half of the forecast region or so. && .AVIATION...KSLC...Fairly benign weather expected in near term. Some mid/high level moisture will result in SCT to BKN cloud cover accordingly. Weak southerly drainage flow likely becomes more light and variable prior to flipping back northwest mid to late Thursday morning. Typical diurnal shift back to southernly drainage flow then expected late Thursday evening. .REST OF UTAH AND SOUTHWEST WYOMING...Advection of mid/high level moisture will result in SCT to BKN mid/high level clouds across northern terminals, while southern terminals more favored to remain clear. Otherwise, anticipate winds to follow a fairly typical diurnal directional pattern at area terminals. && .FIRE WEATHER...Warm and dry conditions will give way to increasing moisture spreading into southern/eastern Utah by later in the day Thursday through Friday. Initially, on the fringes of the moisture surge, midlevel moisture will bring the threat of isolated dry lightning Thursday afternoon across south-central through southeast Utah, along with isolated strong microbursts potentially in excess of 50 mph. Additionally, minimum relative humidities Thursday will remain on the low side at least through the early part of the afternoon. This, combined with general southerly winds gusting to around 30 mph will bring isolated or borderline critical fire weather conditions to portions of south-central Utah where fuels have cured. As the moisture continues to increase Thursday night into Friday, showers and thunderstorms will increase, with the threat gradually transitioning from dry lightning and microbursts to locally heavy rain, still primarily focusing on southern/eastern Utah, once again mainly along and east of I-15. West of I-15, moisture will increase just slightly. Thereafter, the airmass will trend a bit drier, but lingering low-level moisture may maintain at least isolated thunderstorms through early next week across central and southern Utah. Temperatures will trend cooler across eastern Utah for Friday in conjunction with the deeper moisture, then rebound for the weekend and becoming very hot again Sunday into the early part of next week. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...Flood Watch from Friday afternoon through Friday 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