765 FXUS65 KSLC 251627 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 1027 AM MDT Thu Mar 25 2021 .SYNOPSIS...The next storm system will impact the area Thursday into Friday. High pressure is expected to bring warmer and drier conditions for the upcoming weekend. && .UPDATE...The primary forecast challenge this shift focuses on the overnight tonight into Friday morning period, specifically for areas around the Salt Lake City Metro area, and the central I-15 corridor region. Regarding the latter...the devil remains in the details regarding spin up of an H7 circulation overnight, its placement, and confluent region along its northern/western peripheries namely. The latest 12z HREF continues to come in, but have seen enough to view continued spread in placement of this spin up, with some members remaining Tooele Valley west, while others point closer to the Salt Lake/Tooele Valleys. Will be awaiting the full suite of 12z guidance and coordinate with partners to come up with a plan for the afternoon package, as significant snowfall potential remains...its more of a matter of exactly where within roughly a 50 mile radius. Confidence is higher further south for a period of moderate to heavy snow development tonight into Friday morning from roughly Salina south and through Beaver along the I-15 corridor, as a pivoting mid level axis above modest cold advection and favorable low level northerly flow will allow for solid lift through the mid levels. In large the forecast remains on track and have made no notable updates this morning. Previous discussion below... && .PREVIOUS DISCUSSION...The focus of the short term forecast is the storm system that will move into the region today and linger through Friday, before a clearing and warming trend into the weekend. Ample uncertainty remains with the incoming storm system, which could result in some localized impacts from wintry weather tonight into Friday morning. Picking up early this morning, a trough is digging southward through the Pacific Northwest. This storm will continue to dig into the western Great Basin this afternoon, with a 700 mb low starting to close off over northwest Utah by late afternoon into the evening, along with a surface reflection over western-central Utah. This will pose the first challenge for the forecast. But first, ahead of this, this afternoon, increasing southwesterly flow ahead of the storm system will bring mild temperatures and wind gusts in the 20-30 mph range across much of the area expect northwest Utah where the low will be developing. The exact placement of the 700 mb low will determine where there is potential for heavier precipitation in a band on the northwest side of the low. Forecast models continue to bounce around with the exact location of this pivot point, with the GFS and the hi- res FV3 farther west with this feature, keeping the heavier precipitation mainly west of the Wasatch Front over Dugway and the West Desert. Meanwhile, the 00Z Euro, NAM and more recent HRRR runs are much farther east and brings the heavier precipitation along and axis across the Tooele Valley through near downtown SLC. The EC remains among the most aggressive to the south and east bringing heavier precipitation across the SL Valley and into the Cottonwoods. Why does this matter so much? Thermodynamic profiles are very marginal for precipitation type in the valleys, with 700 mb temperatures only falling to around -7/-8C later tonight, and it is very likely that precipitation rate will play a large role in forcing p-type over to snow if heavy enough. Thus for the valley locations, heavier rates under this band could help push the p-type over to snow and could result in a few inches of accumulations, possibly higher depending on elevation. Also of note, with the flow becoming very light under the 700 mb passing nearly over the northern Utah mountains, orographic enhancement will be much more limited. This is quite possible that if the pivot point sets up north and/or west of the Cottonwoods valley locations could end up with more precipitation overnight than the mountains. Finally, by Friday morning as the low starts to shift southward, light northerly to northwesterly flow will start to develop over northern Utah. This may start to redevelop some typical orographic enhancement, but flow will light initially so this may only help valley to mid- canyon levels with precipitation. Also the GSL will become marginally favorable to support lake enhancement and could result in some high total between the Tooele and Salt Lake Valley. The other location to keep a close eye on for potential impacts is the I-15 corridor in central Utah from around Filmore to north of Beaver. Again, depending on the exact track of the 700 mb low that develops over northern Utah, northerly to northwesterly flow will develop across central Utah. A farther east track like the Euro would favor a more northwesterly flow into this corridor later tonight into Friday morning. Moisture in place with not overly strong northwesterly flow would favor some blocking along the terrain which could locally enhance snowfall amounts along this portion of the I-15 corridor and near the I-70 interchange. However, a farther west track like the GFS would keep the better precipitation potential out across more rural areas of west- central Utah. During the day Friday, the main 500 mb low will dive into northern Arizona, leaving the forecast area under a baggy 500 mb trough axis with cool, moist northwesterly flow in place, but with drier air moving in across northwest Utah in the wake of the trough passage. Thus precipitation lingering into Friday will turn showery, and shift over and east of the higher terrain by Friday afternoon. High pressure builds in quickly for Saturday, with cool northwesterly flow in place. Temperatures will start to moderate but only get to near normal values for afternoon highs, but with dry and mostly sunny conditions just in time for the weekend. The majority of the long term forecast discussion will focus on a potent cold front that will bring sharp changes to Utah. The discussion will focus the cold front arrival time differences and the potency of the cold air between the models and how this could effect temperatures across northern Utah for Monday Tuesday. Another impact with this approaching trough axis and cold front will be the development of gusty winds both ahead and behind the cold front. The discussion will have a section dedicated to each element and both will be discussed below. Cold Front Temperature Impacts: On Sunday we will be monitoring and area of low pressure that originally originated in Siberia regions that has tapped into extremely cold H7 temperatures around -30C. This low will migrate south through the Gulf of Alaska and approach the Pacific Northwest coast by Sunday night. The airmass will have modified, but GFS/GEFS outputs are showing H7 temperatures associated with the trough in the -17 to -19C range as it moves ashore along the Washington coast. The ECMWF is the coldest solution and has H7 temperatures at this time a degree or two cooler than the GFS. Ahead of the cold front, warm H7 temperatures will be advecting across much of Utah backed by a stout southwesterly flow (more on the wind impacts below). Prior to the arrival of the cold front, H7 temperatures max out around 4 to 6C across the majority of Utah, except for the extreme northwestern portion where the cold front will be passing through. Timing differences will create a large spread in expected maximum temperatures across northern Utah. A faster front will cut into maximum heating potential and with a sharp temperature gradient along the front, temperatures are expected to fall quickly behind the front. A slower front will allow for those warm H7 temperatures and maximized day time heating to push temperatures warmer. Given that some uncertainties exist trended the maximum temperatures across Utah on the warm side but kept them from accelerating to rapidly given the proximity of the front. Temperatures will quickly fall behind the front as the warm H7 temperatures are replaced by -10 to -13C temperatures, and it is not out of the realm of possibility that we could see a 30-40F drop from the day time high to the overnight low across northern Utah. The extent of the plunge will depend on how high we get during the day. Current forecast for the SLC valley is calling for a high temperature of 68F on Monday with an overnight low dropping to 29! With the cold air still settling over Utah, highs will struggle to get out of the 40s across much of central and northern Utah on Tuesday. Moisture availability with this storm looks sparse but some light valley and mountain precipitation will be possible but expect little to no accumulation. Wind Impacts: In advance of the trough and associated cold front, a tightening pressure gradient will increase the southwesterly flow across Utah as H7 winds increase to 40-45 knots during the afternoon. Winds will begin to gusty across western Utah and eventually spread into the I-15 urban corridor. As the front moves through expect gusty northwest winds to develop quickly behind the front. These will be the strongest winds with a 6-12hr of wind gusts ranging from 30 to 40 mph will be possible across western Utah and the I-15 corridor from Monday night through Tuesday morning. This will be monitored as for any potential wind highlights as we approach the event. The rest of the long term from Wednesday onward is quiet with temperatures rebounding into the 50s Wednesday and into the 60s Thursday as ridging builds across the west. && .AVIATION...VFR conditions expected through this afternoon as CIGs drop to 5 kft AGL and VCSH develop. MVFR conditions are expected to develop around or shortly after 00Z as areas of rain and snow develop and CIGs drop to 2-3 kft AGL. There is a 40% chance of periods of IFR as any heavier precipitation will fall as snow overnight, particularly 03-09Z. 1-2" of snowfall expected, mainly on grassy surfaces. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...None. WY...None. && $$ UPDATE/AVIATION...Merrill/Burghardt PREVIOUS DISCUSSION...Church/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