Saturday, September 3, 2016

Frontal Flirtations

Weak fronts can be extremely difficult to pin down at night and in the morning when the nocturnal inversion that forms overnight tends to obscure larger-scale wind and temperature contrasts.

Nevertheless, evidence of a weak front moving over the Salt Lake Valley can be seen in the meteograms at the Salt Lake City airport overnight.  Note how winds shift from southeasterly to northwesterly from about 2300 to 0100 MDT.  Without larger-scale forcing, the preferred flow direction at the airport overnight is southeasterly, so northwest flow is a dead give away that something is up.

Source: MesoWest
This morning, the MesoWest surface analysis at 0836 MDT shows northerly or northwesterly flow across much of the western Salt lake Valley, easterly flow near the University of Utah (consistent with overnight drainage and canyon flows).  Temperatures are generally in the low to mid 60s in that area.  In contrast, a number of stations in Utah Valley show southerly flow and temperatures in the 70s, suggesting that the weak front has pushed to near Point of the Mountain.

Source: MesoWest
I'm no expert in this area, but I'm guessing that the front took one look into Utah County, saw the Y on the hill, and decided it was better off hanging around the U than to spending time today at BYU.  

Forecasts for today show a somewhat disorganized pattern with the front weakening and another push of colder air moving into the state later in the day.  The HRRR forecast for 0000 UTC (1800 MDT), for example, shows strong southerly flow pushing into southern Salt Lake County, where it meets an apparent lake breeze or perhaps remnants of last night's front.  A stronger windshift lies over the northwest corner of Utah.  

The net impact of this pattern will be a gradual cooling trend with temperatures today in the high 80s along the valley floor and mid 80s tomorrow.   With a weak frontal boundary, there's a chance of a shower or thunderstorm, but really, given what we've been through the past few weeks, the day should be quite pleasant.  Mountain biking this morning was simply splendid.

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