Friday, April 6, 2018

A Saturday Soaker That Is Simply INCONCEIVABLE!!!

My meteorological excitement factor is relatively elevated today compared to where it has been much of the winter, mainly because of expectations of a strong frontal system pushing across the western US through tomorrow.

Right now we are getting just a bit of an appetizer.  The analysis below is for 1200 UTC (0600 AM) MDT and shows a weak 700-mb trough approaching Utah (brown dashed line).  With this system is a slug of moisture, valley rain showers, and upper-elevation snow showers.

After the remarkably low dewpoints earlier this week, the humidity just hit me in the face and felt so good when I walked out the door this morning.  The meteogram below shows how dewpoints (blue line) have climbed from the low single digits on Tuesday to 46 this morning.  Good for the skin!

It's warm, with temperatures of 34˚F at the base of Alta (8560 ft) and 36˚F at Snowbasin Middle Bowl (7402 ft).  The 8000 ft snow cam at Snowbasin shows about 1.5" of wet snow this morning.  I suspect it is raining at the base.  It's a garbage bag morning if ever there was one.  

Precipitation will taper off after 1800 UTC (Noon MDT).  Then, overnight, Mother Nature revs up the system for Saturday.  At 0600 UTC (Midnight MDT), a deep, 975 mb low is located off the Pacific Northwest coast.  It's a powerful, well developed system.  Ahead of the cold front, northern California, especially the mountains, get considerable rain.  At this time, the warmth of the storm is remarkable with 700-mb temperatures near +4C.  Perhaps only the upper elevations of Mt. Shasta will be seeing rain snow at this time.  Such a waste of water.   

The cold front then pushes inland and redevelops over Nevada.  At 1800 UTC (noon) Saturday, it's forecast to be located over eastern Nevada.  Note the disortion by the Sierra Nevada, as is common, and the lack of precipitation over southern Nevada and southern Utah.  The action is instead concentrated over far northern Utah where the remnant warm front can do its thing.  

The cold front pushes across northern Utah Saturday afternoon [forecast for 2100 UTC (3 PM MDT) below].  I expect for the valleys, the front will bring a pretty good deluge, with a possibility of thunderstorms and locally heavy rain.  It is, however, a fairly mobile feature and moves quickly across the state with precipitation winding down in the evening in Salt Lake.  

The forecast above is from the 0600 UTC initialized NAM.  Let's look more braodly at the possibilities based on our downscaled precipitation estimates from the SREF.  For the Salt Lake City airport, one sees this morning's precipitation, then the afternoon and overnight break, with a couple of members producing showers here or there.  Precipitation chances increase tomorrow morning and all members produce rain tomorrow afternoon.  Mean accumulations with the front are about a half an inch, which is a pretty good for a fast moving front in this part of the world.  The box-and-whisker plot below shows the period of strongest precipitation from about noon to evening (although timing could shift some between now and then).  
Shifting to the mountains, let's first talk about water totals.  In my view, the the pre-frontal period is perhaps the biggest wildcard.  Much will depend on the moisture plume, flow direction, and intensity of the forcing.  The northern Wasatch are likely to get more from that period than the southern.  Everyone should get a pounding with the front.  At Alta-Collins, the SREF mean puts out a bit over an inch of water.  The range, after deducting today's totals, ranges from about 0.5 to 1.7 inches.  The latter would be a big water total for about a 12-hour period.  I would not be surprised to see an hourly accumulation of more than 0.3 inches of water at Alta-Collins tomorrow afternoon.  

This is, however, going to be a warm storm.  Remarkably warm.  The NAM forecast for 2100 UTC (1500 MDT) tomorrow shows us sitting at +4˚C.  I'm not sure if I've ever seen heavy frontal rain in early April with such a high 700-mb temperature.  Someone should dig through the records as my memory is not as sharp as it used to be.

If we look at the numbers from the NAM at Alta, the wet-bulb-zero level tomorrow is over 10000 feet, reaching as high as about 12,000 feet from 11-3 PM.  

If that forecast verifies, we are going to see heavy rain perhaps all the way to 11,000 feet at some point tomorrow afternoon.  

I'd like to see some analysis of the past records to see if my gut reaction to these numbers is accurate, but this forecast makes me feel like Vizzini in the Princess Bride, "INCONCEIVABLE."

The GFS is a bit cooler, but I think we need to expect heavy rainfall tomorrow to elevations at least as high as 10,000 feet.  This is the definition of an outlier event.  Expect the unexpected.

Addendum @ 9:15 AM Friday

I feel the need to add a bit more to this post.  The forecast sounding from the NAM below for 2100 UTC (3 PM MDT) Saturday is simply unbelievable.  Fully saturated through the entire troposphere.  A precipitable water of 25 mm (about an inch).  That's monsoon-like moisture with a midlatitude front the likes of which appear unprecedented.  
Let me show you just how just out unprecedented such moisture would be, the yellow dot indicates where that sounding would sit in the sounding climatology for Salt Lake City.  We have never observed a precipitable water above 0.8" in April.  The forecast is for 1.0"!

It's a good thing this will be a short-lived event.  Even still, the potential for heavy valley rain is real and in the mountains I would expect the unexpected.  This is a Cascadian rain storm coming to Utah.  

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