Soft launch into Fall

850 mb temp anomalies 9/1-9/7

We have finally crossed into meteorological fall and are only two weeks away from the astronomical equivalent. After a rather dry and moderately hot June-August, the last tendrils of summer have finally loosed their grip. Large portions of the U.S have already gotten a taste or at least a Lacroix flavor of fall the past couple of weeks, particularly east of the Continental Divide (right figure). But for the most part, Utah has only seen a very gradual shift away from blasting heat and a sputtering monsoon to what I like to call Utah "window weather". 

Now to avoid confusion, there is the Icelandic definition of "window weather" which is when weather looks pleasant from your window but is actually unpleasant when experienced outside. However, I am not referring to this definition. I define "window weather" as when the weather looks nice from your window and is actually nice. We have been dipping our toes "window weather" here in northern Utah, but should see an even more pronounced shift to these conditions by the end of the workweek. 

Over this past weekend, a bit of heat and floundering monsoonal pushes of moisture kept conditions mildly summer like. We saw temperatures largely stay below 90F at KSLC (figure left/bottom) and had a few days of scattered/hit or miss showers/storms. I think the most noticeable yet subtle changes were overnight lows consistently dipping well in the 60s in the Salt Lake Valley, which was a nice respite from 70+ F lows observed during July and August. There were also a few scattered showers and storms across the state, with mountainous areas of southern and central Utah picking up a bit of liquid (figure left/top). A few storms even made there way into SLC, but with not much measurable precipitation.

By this time of year, the sun angle is decreasing at a significant rate with solar heating following suite. This means that with every bit of cooling it is increasingly more difficult for temperatures to warm back to previous level. With conditions expected to change going this weekend, we are one step closer to consistent "window weather".

At the moment, a slow moving closed upper level low is spinning westward into the west coast of the CONUS. Utah now sits to the east of the trough axis (figure below/right), with increasing SW upper level winds and some high clouds associated with mid to upper level moisture.

This low will progress across Utah in the next few days, bringing some gusty southerly winds Wednesday and the opportunity for a few scattered showers and thunderstorms through Saturday (figure below). There will be some upper level lift associated with a jet streak passing over the intermountain region, but in large the energy and moisture from this system is expected be be largely broad and unfocused. This means much of the precipitation development will likely be aided by topography or daytime heating. The forecast ECMWF Sat 00Z forecast period (6pm MDT) is shown below for upper and mid levels.   

500 mb heights, winds, and temp 9/9

GOES 18 true color satellite imagery Sept. 9 

Forecasted Sat 00Z 500 mb heights, winds, and vorticity (left) and 700 mb heights, winds, and moisture

Temperatures will likely top out by Wednesday afternoon before cooler air, associated with the upper level low, moves in and knocks temps down 10-15 F (figure right). More importantly, overnight lows look to drop down into the 50s, which means shutting off the AC and opening the windows overnight. 

How long will this spurt of milder weather last? Probably at least through the weekend. There are hints of some marginally active conditions the third week of September, but that period is still out in forecasting fantasy land and nothing could be said with confidence.

Left: Forecasted 6hr precip at 00Z 9/13; Right: High/Low temps at KSLC