textproduct: Salt Lake City

KEY MESSAGES

- Widespread heavy rainfall is expected to occur across the entirety of the forecast area starting Saturday afternoon/evening as Tropical Storm Rachel transitions to an extratropical system to our southwest and interacts with a strong midlevel trough. Widespread rainfall amounts from 2-4" are expected across much of the forecast area, with localized amounts potentially reaching 6-8" across locations such as higher terrain locations across southwest Utah, the Book Cliffs, and locations within and adjacent to the Bear River Range.

- A wind threat is expected to accompany this system as it begins to take on extratropical characteristics. Initially, advisory level winds will begin to develop across most Utah valley locations starting just after midnight tonight, becoming more widespread Saturday afternoon. Advisory level winds are forecast to remain in place through at least Monday afternoon.

- A more notable high wind threat looks to develop across the valleys and high terrain of southwest Utah as the potential development of a sting jet looks to occur Sunday. This would bring widespread lower elevation winds in excess of 40mph with high terrain and ridgelines seeing gusts potentially exceed 70mph.

- The potential for severe weather exists across southern and eastern Utah Sunday into Monday morning. The primary threat will be severe thunderstorm gusts, though a couple brief tornadoes cannot be ruled out if discrete convection is realized.

DISCUSSION

Currently, broad westerly flow aloft is yielding pleasant conditions across much of the forecast area ahead of Tropical Storm Rachel. This evening, a deep mid-level trough will begin to slide southeast toward the Great Basin with mid-level flow increasing out of the southwest. Additionally, the pressure gradient is forecast to continually tighten overnight increasing winds further. As such, winds will begin to gradually mix to the surface overnight resulting in downslope winds along southwesterly flow favored terrain (such as the Oquirrhs) across northern UT. Into the morning and early afternoon hours, we will continue to see more widespread mixing to the surface, with most valley locations west of and along I-15 seeing gusts through much of the day in excess of 30mph out of the southwest.

Through much of tomorrow, moisture associated with Tropical Storm Rachel will gradually push ahead of the main system as the aforementioned trough slowly moves southeast. With strong ascent in place as the trough moves in, widely scattered showers and thunderstorms are expected to develop along and east of I-15 in the afternoon hours. As ascent continues to strengthen into the evening, lower elevation locations will begin to see more showers and thunderstorms overnight, particularly along a strengthening frontal boundary across northern Utah. This may be the onset of a Predecessor Rainfall Event (PRE), serving to saturate soils markedly ahead of the main rainfall event later on in the forecast period. A PRE is when moisture advects well ahead of the main storm (in this case, Rachel), and interacting with a frontal boundary producing heavy rainfall well ahead of the storm. The potential for flash flooding will gradually increase across southwest Utah later Saturday evening as more robust moisture pushes north overnight. Additionally, some severe threat may develop, though would largely be limited to far southern Utah as instability profiles leave much to be desired.

On Sunday, numerous hazards are expected to develop across much of the state with heavy rainfall continuing across the entirety of Utah, strong winds developing across southwest Utah, and a severe threat across southwest Utah. The midlevel trough will continue to gradually deepen as it progresses southeast with Tropical Storm Rachel pushing inland and phasing into the mean flow, becoming extratropical with time. Flooding is expected to be a major concern through the period with widespread 2-4" of rain in the forecast across much of the state associated with deep tropical moisture. With recent burn scars such as Forsyth, Cottonwood, Widemouth 2, Silver King, and more, flash flooding is a concern primarily across the southern 2/3rds of Utah where the potential for heavier rainfall amounts appear likely. Additionally, southwesterly flow upslope favored terrain such as higher terrain across southwest Utah and the Book Cliffs yield some concern where heavily localized amounts could potentially reach as high as 6-8" with this event. Location remains extremely uncertain regarding highest precipitation amounts, but given the anomalous moisture and forcing in place, these amounts may be realized. Along the northern 1/3rd of Utah where the potential PRE may occur followed by persistent moderate rainfall, widespread areal flooding concerns continue to rise given the large rainfall totals expected. This is particularly interesting across the Bear River Range and areas adjacent, where precipitation totals through the event may also reach 6-8", aided by the PRE. Rainfall is expected to continue through Tuesday across both locations, with an attendant flooding/flash flooding threat each day.

The second threat to develop Sunday afternoon is the potential for strong wind gusts across southwestern Utah. As Tropical Storm Rachel transitions to an extratropical system, it will begin to broaden in size and expand its wind field as well as strengthening aloft. Additionally, hi-res guidance hints at drying occurring in the midlevels where winds look to strengthen the most. This may indicate the potential for the development of a sting jet, which would translate strong wind gusts to the surface in excess of 40-50mph, with ridgelines potentially seeing gusts in excess of 70mph. This solution would be particularly concerning given heavy rainfall totals resulting in saturated soils which may result in a locally widespread tree blowdown event. As such, a high wind watch has been issued for portions of southwestern Utah.

Lastly, a severe weather threat looks to develop primarily across southern Utah and potentially east-central Utah. This setup remains largely conditional on cloud breaks occurring in order to support destabilization, though multiple CAM suites hint at this with CAPE values reaching 1000-1500J/kg across much of the area in question. In the afternoon hours, a strong shortwave impulse (the main remnants of Tropical Storm Rachel) will eject northeast as it begins to phase into the mean flow. Midlevel winds in excess of 60-70kts with strong surface winds will yield large, looping hodographs across much of southern Utah with effective shear values ranging from 40-60kts. Additionally, a few pieces of guidance continue to indicate the presence of drier air in the midlevels which would support more isolated convection developing rather than a persistent stratiform rainfall event, with some concern for the development of low topped supercells across southern Utah in the afternoon hours. The drier air pushing in will assist with increasing DCAPE values to over 1000J/kg, and with strong gradient winds ongoing at the time, will yield a convectively enhanced downburst threat with these storms. A conditional threat for a couple tornadoes exists as well given the shear and instability in place, though some caveats to this exist. In particular, the tornado threat is heavily dependent on afternoon clearing in order to preserve lapse rates greater than 7C/km. Additionally, with the potential for highly sheared low- topped supercells, or "mini spinnies", there is potential that the higher DCAPE may result in RFD winds shredding the storm apart leading toward a more pronounced wind threat. However, storm- relative flow to around 30-40kts may assist with balancing the inflow/RFD with these storms, which may result in a couple tornadoes, potentially brief in nature. Terrain interactions may also assist this process as well, though locations regarding this are highly uncertain.

Come Monday, the remnants of Rachel will continue ejecting northeast away from Utah as it continues to phase into the mid-level trough. The midlevel trough will meander across the Great Basin through at least Tuesday providing ample ascent with moisture remaining in place bringing even more rainfall to the area. This may result in an extension of any Flood/Flash Flood products issued shortly. Additionally, this will keep the wind threat in place throughout Monday across much of the state.

Come Wednesday, the mid-level trough will push east with a cold front sweeping across the forecast area ushering in cooler temperatures statewide. With this frontal boundary pushing through and moisture remaining in place, high elevation snow is expected across primarily northern Utah. Amounts are not forecast to be high given snow ratios on the order of 4:1, but hopefully this is a good sign for the winter season moving forward. Lastly, this frontal passage will bring temperatures statewide to near-normal temperatures for this time of year.

AVIATION

KSLC...VFR conditions will continue at the KSLC terminal through the TAF period. Winds are expected to remain southerly through the day today, although there is a 15 percent chance of a brief shift to northwest within the 21-00z timeframe. A few high-based showers and thunderstorms may develop in the vicinity of the terminal after 21Z with some increase in coverage in the overnight hours.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions will prevail across all regional terminals through the TAF period. Diurnal winds this morning will trend more southerly this afternoon. Isolated showers and thunderstorms will develop over the higher terrain this afternoon, with more focus on northern Utah overnight. These storms could bring some gusty and erratic winds, lightning, and brief mountain obscurations.

FIRE WEATHER

Ahead of Tropical Storm Rachel, a system will begin to push southeast with widespread gusty winds developing this evening through early next week. Through this weekend, widespread wetting rain is expected across the entirety of the state as the remnants of Rachel move into the area. Following Rachels departure, a cool and dry pattern is forecast to develop around midweek next week.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Wind Advisory from 3 AM Saturday to 6 AM MDT Sunday for UTZ101>109-115-116-118-119-122-126-127.

Flood Watch from late Saturday night through Monday afternoon for UTZ101>112-114-115.

Flood Watch from Saturday afternoon through Monday afternoon for UTZ113-116>131.

High Wind Watch from Sunday morning through late Sunday night for UTZ115>117-119-122-125>127.

WY...None.