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FXUS65 KSLC 091143 AFDSLC

Area Forecast Discussion National Weather Service Salt Lake City UT 443 AM MST Sat Dec 9 2023

SYNOPSIS
High pressure will build into the area for the weekend.
Thereafter, the weather pattern will stay progressive, with additional weak but mostly dry disturbances expected to cross Utah during the first half of next week.

.SHORT TERM (THROUGH 00Z TUESDAY)...Morning satellite water vapor imagery shows that the upper trough that brought the latest round of wintry weather to the area has now shifted east of Utah, the back edge cutting northwest to southeast across the state. Increasing stability has caused most showers to diminish, except south and southeast of the GSL, where steep low-level lapse rates owing to a cold airmass (H7 temperature of -14C) over a still relatively warm lake. Based on an estimated lake temperature near 4C, BUFR soundings indicate around 200-300J/kg of lake-induced CAPE at SLC. Several lake-effect showers have developed overnight, impacting the Oquirrhs and eastern Tooele Valley in a north-northwest flow. This has caused some localized travel impacts due to accumulating snow on area roadways. Otherwise, weaker showers have occasionally drifted across other areas of the Salt Lake Valley, southern Davis County, and the Wasatch Mountains from near Bountiful to the Cottonwoods. This continues to produce very minor accumulations in the mountains but likely no more than a trace of snow in the valleys. The snow showers should taper off for the latter half of the morning with overall stabilization of the atmosphere. Local lake-effect guidance drops from near 40% currently to around 20% by 9am and less than 10% by 2pm.

Meanwhile, some lingering status remains in place, mainly banked along the mountains of northern Utah. Where these clouds exist, temperatures have kept from plummeting too much despite the cold air in place. It has also helped to limit fog development thus far, but patchy fog is not out of the question this morning, mainly in the Wasatch Back and Bear River Valley. Elsewhere, some gusty winds noted, mainly in favored downslope areas such as Castle Country and near Washington County canyons. These winds should decrease through the day as H7 flow diminishes and the surface pressure gradient relaxes. This will all lead to a relatively quiet, though somewhat cold, afternoon. Max temperatures today are forecast to be roughly 5- 12 degrees below climatological normals across the area.

As high pressure builds in from the west tomorrow, H7 warm advection will strengthen across the area. In general, there does not appear to be enough moisture to bring much in the way of precipitation, but keeping some small PoP chances near the Idaho border and in the northern mountains. Some gusty winds can be expected along the far northern Wasatch and Bear River Mountains and in southwest Wyoming late tomorrow morning through tomorrow afternoon, otherwise winds will remain relatively light across the rest of the area. Even with fairly poor mixing, temperatures should trend warmer tomorrow, even in the valleys.

A weak shortwave disturbance is progged to cross Utah on Monday.
With weak dynamics and little in the way of moisture, precipitation is unlikely. However, maintained some small PoPs near the Idaho border since there will be a bit more moisture over Idaho. If nothing else, this weak disturbance will help prevent Utah from settling into a strong inversion.

.LONG TERM (After 00Z Tuesday)...On Tuesday a compact shortwave trough approaching from the PacNW will push into the forecast region. Overall moisture remains limited with this system, and as a result will only see some lower end precipitation chances (~10-20%)
develop for the high terrain from central Utah northward. The low level thermal profile doesn't appear to be overly cold with this system, characterized by 700mb temperatures roughly in the -5C to - 9C range, and will only see a correspondingly modest drop of a few degrees in regards to afternoon surface high temps. Even with the slight cooldown, these high temperatures come in close to climatological normal for early to mid December.

Forecast uncertainty increases from Wednesday onward through the remainder of the forecast period due to the potential evolution of the shortwave. That said, guidance has at least trended slightly towards a scenario in which the shortwave becomes cutoff near or atop the forecast region, eventually progressing eastward out of the area by the weekend. With moisture remaining limited and the airmass still only modestly cool, would generally see pretty minimal day to day variation in the weather pattern. Biggest concern would be the potential for any east-west oriented gradient and some associated threat of downsloping winds dependent on the position of the cutoff low... But would need more pieces to fall into place, and given the aforementioned uncertainty on the cutoff scenario in general, will need to continue to monitor trends moving forward.

AVIATION
KSLC...Quieter weather expected in the wake of departing winter system. Any light southerly winds that develop Saturday morning expected to switch northwesterly by early afternoon, remaining light. Diurnal switch back to southerly drainage flow anticipated thereafter ~01Z-04Z. Cloud coverage decreases through Saturday, with conditions anticipated to remain VFR. Some increase in VFR cloud cover then expected Saturday night into early Sunday.

REST OF UTAH AND SOUTHWEST WYOMING
More benign conditions largely expected as winter system continues to depart the region. Winds are anticipated to remain relatively light at area terminals, with most maintaining VFR conditions. Exception will be potential for fog/mist or low stratus during the early morning hours, with LGU/EVW/HCR the terminals most likely to see any impacts. That said, confidence in timing/coverage is low at this time and trends will bear monitoring.

SLC WATCHES/WARNINGS/ADVISORIES
UT...None.

WY...None.

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