382 FXUS65 KSLC 061001 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 401 AM MDT Mon Sep 6 2021 .SYNOPSIS...A ridge of high pressure will build over the area through middle of the week resulting in very warm to hot and dry conditions. The ridge will be nudged east due to an incoming but weakening trough moving to the West Coast by late Thursday or Friday. && .SHORT TERM (Through 12Z/6AM Thursday)... A ridge of high pressure remains on track to build into the Great Basin today and park over Utah Tuesday and Wednesday. Deterministic and ensemble guidance continues to strengthen the ridge with successive model runs. What was initially looking like a seasonably strong ridge a few days ago with a 0% chance of 500mb heights reaching 2 standard anomalies has now evolved into a 100% chance of this threshold being reached Tuesday night into early Wednesday. EFI values approach 0.8-0.9 across far SW Utah for Wednesday, and a closed SoT contour is now indicated across far SW Utah. What this means is that there is an increasing potential for record heat for Wednesday across this area. Indeed, the forecast high for St. George on Wednesday is now around 106, and the previous record for the date is 104. Similar high temperatures are expected Tuesday as well, when the record high is 111. Additionally, forecast low temperatures Tuesday through Thursday morning are forecast in the low to mid 70s for St. George. The low temperature forecast has been steadily rising with successive runs. Looking slightly northeast towards Zion National Park, similar high temperatures are expected in the lower elevations Tuesday and Wednesday, with slightly cooler lows in the upper 60s. Given recent forecast trends and giving consideration towards increasingly warming nighttime lows, have issued an Excessive Heat Warning for lower elevations of Washington County and Zion NP from 12PM Tuesday through 9PM Wednesday evening to account for the anomalous heat. Heat Risk product supports this idea for lower Washington County. Have expanded the headline into Zion NP to raise awareness of the potential for record heat here. Elsewhere across Utah and SW Wyoming, temperatures are expected to remain anomalously hot, topping out generally 5 to 10 degrees above normal, with a lower chance of record highs being reached. Record highs at Salt Lake City International Airport Tuesday-Thursday are 99, 100 and 95 degrees, respectively. Records for Tuesday and Wednesday appear safe. Thursday's record high may be challenged, however. No precipitation is expected through the period. The HRRR smoke continues to advertise an increase in smoke, especially across northern Utah and SW Wyoming on Tuesday, owing to the development of W/WSW flow around the periphery of the ridge, which center is forecast to drift from east-central Nevada Monday night to central Utah from Tuesday into Wednesday. .LONG TERM (After 12Z/6AM MDT Thursday)... Ensemble members are in agreement with shifting the strong high pressure, that brought very warm temperatures to the area, eastward into the Four Corners region by late Thursday night. With a weaker southerly flow not progged to develop and the bulk of the moisture displaced well south of Utah, PoP chances were removed for Thursday, although there could be a storm or two that may develop across the higher terrain of southern Utah. By Friday, there is some disagreement amongst the ensemble members with a trough entering the Pacific Northwest. The biggest influence with these differences will be the strength of this trough and how it interacts with the ridge. The overall trends have been signaling an overall weaker trough in all the ensemble member outputs compared to prior runs. Taking a consensus of all the outputs, the ridge is expected to begin to flatten and elongate, west to east as it begins to move east into the Rockies. Some moisture looks to advect into Utah Friday afternoon and PoP chances of been introduced Friday afternoon. Confidence heading into next week is low with large uncertainty regarding the amplitude of the next trough approaching northern Utah. With most ensemble members continuing to show a high pressure system to the east, moisture aided by weak flow should be enough to initiate some thunderstorms chances during over the weekend. The highest confidence in this forecast will be for temperatures returning to normal across much of the area. && .AVIATION... KSLC...VFR conditions are expected across the terminal through the valid TAF period under clear skies. Light southerly flow will transition to a northerly flow around 19z, with wind speeds remaining below 10 knots. Drainage winds will develop and shift the flow back to the south around 03z. REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions are expected to prevail across the airspace through the valid TAF period under mostly clear skies. Smoke will remain aloft and not cause any aviation impacts across the terminals. && .FIRE WEATHER... A ridge of high pressure remains on track to slide directly over Utah on Tuesday and remain in place through Wednesday before gradually sliding off into the four-corners region by late in the week. As a result, above normal temperatures are expected to remain in place through Thursday, with near-record high temperatures possible on Wednesday. Single digit to low teens minimum relative humidity is expected for most of the area through Wednesday, with limited humidity recovery overnight. The combination of gusty west winds and low relative humidity values will lead to a few pockets of critical fire weather conditions across far NE Utah this afternoon. An adjacent Red Flag Warning has been issued for SW Wyoming, where winds will be a bit stronger. Lighter winds are forecast Tuesday through Thursday. The previously forecast return of moisture in the late week period appears to be both delayed and slightly less robust than previously forecast. Isolated showers and thunderstorms are now expected to arrive by Friday, with isolated to locally scattered showers and thunderstorms expected through the weekend. Humidity is expected to increase, most notably by Friday -- with similar humidity values expected through the weekend. Minimum relative humidity values are expected to increase into the teens for valleys during this time, and generally into the 25-35 percent range in higher terrain areas and across northern Utah. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...Excessive Heat Warning from noon Tuesday to 9 PM MDT Wednesday for UTZ123-124. WY...Red Flag Warning from noon today to 8 PM MDT this evening for WYZ277. && $$ ADeSmet/Woodward For more information from NOAA's National Weather Service visit... http://weather.gov/saltlakecity For frequently asked questions about the Area Forecast Discussion visit... http://weather.gov/saltlakecity/general/afd_faqs.php