Equinox Storm right on time with dry front moving through Mammoth by early Thursday PM followed by Upper Cut-Off like system Saturday…Some Snowfall Accumulation Likely over Higher Terrain…..
Thursday September 25, 2014
Posted at 10:17 am by Howard
As indicated Friday afternoon, the later guidance continued the trend of the upper center to move directly over the Mammoth area this morning, thereby setting up the area of deformation to the north of us. Areas like Lake Tahoe picked up about 1.25 inches of water EQ over night while only a few hundreds in the Southern Mono County area. Rain and high elevation snow will continue mainly to our north today with showers lingering into Sunday. The best chance of a few inches of snowfall will be later today and tonight in Mammoth as the main upper center shifts east just across the CA/NV border and wrap around moisture gets pulled down from the north into Southern Mono County. We may still get a couple of inches of snow above 8500 to 9K from that circulation later this afternoon in the Mammoth area.
We will dry out Monday with temperatures warming back to the mid 60s by Thursday.
Friday late afternoon…6:00pm…..
This afternoons guidance showed up with the deformation zone further north. Meaning that the main band of precip will probably set up between Northern Mono County northward to Reno/North shore Tahoe. That is the big band with the heaviest precipitation not dependent upon daytime heating or any of the sort. So amounts will likely be less here in Southern Mono County now then what the models showed from last nights or even early this AM runs. Typically, the deformation zone, an area typically 70 to 100 miles wide (Surface Convergence) is located to the north of the center of the upper low. If by chance the upper center verifies some 50 miles or so south of Mammoth, that would enhance possibilities for more precip. Based upon the latest guidance…that is unlikely. So some snowfall yes…possibly a few inches over the higher terrain, but no major amounts as indicated by this afternoons guidance. I will update one more time later tonight when the 00z NAM comes in to see if there is any change…..
Dr Howard and the Dweebs……………………:-)
Friday AM Update: 11:40AM
Everything on track this morning with upper trof off shore now split into two centers of circulation. One west of BC, Canada and the other stronger branch digging SSE with the help of a 110 knot upper jet toward the Central CA coast. Over the next 24 hours a portion of the upper jet in the southern branch will round the base, as at the same time, the northern branch pretty much detaches itself and so that this becomes a quasi-cut off upper low as it moves through Central CA Saturday and Sunday. The track of the center should move pretty much right through Mammoth Lakes by 18Z Saturday.
An area of deformation (surface convergence) will develop on the northern portion of the cut off with precipitation developing to our north then shifting SSE over the next 24 hours. The 12z Friday GFS 700RH has draw of moisture up into the desert southwest then up into NV from the Subtropics. Some of this moisture is actually connected to a tropical system off Baja. This moisture looks likely to get entrained in the larger scheme of things this weekend.
Considering 700 mb temps and 500-1000mb thicknesses, it is possible that Mammoth may have snow levels as low as 7500 to 8000 feet as the upper cut off and associated cold pool moves over us Saturday AM. By Saturday night the upper cut off moves east into Western NV and 700 MB temps begin to rise during the late afternoon Saturday into Sunday. The Snow Level will rise up to about 8500 Saturday Night. It is possible that over the upper elevations above 9,000 to 10,000 feet that some 3 to 6 inches of wet snow may fall. The timing is between, late tonight and Sunday Night, as upslope conditions will likely continue through the weekend.
Remember, that the ground is pretty warm from our long warm Summer and so it will take time for the upper elevation pass roads to cool to freezing. However, those that plan on negotiating Tioga Tonight should get an early start Today, as heavier precipitation is expected to begin about Midnight tonight or thereafter………..
Dr Howard and the Dweebs………………………….:-)
A Major Cool down is in play with winds up over the ridges today, as the first cold front of the new season approaches the Sierra. A FROPA is expected by afternoon today Thursday. Although the cold front itself is expected to be dry, the forecast guidance is also showing a slowing of the upper trof as it becomes nearly cut-off over the Sierra Saturday. This closed upper low is expected to remain in the vicinity of the Central Sierra this weekend.
This will have the effect of providing not only a cold pool aloft at 18,000 feet over the Central Sierra, but a upslope component as well. Because of the aforementioned……Upper elevation hunters, back packers, hikers, etc should be prepared for possible winter like weather, if going into the back country, against hypothermia due to cold, wind and……………….The possibly of several inches of snowfall above the elevations of 8500 up to the crest between Friday night and Saturday Night is a significant possibility.
QPF from HPC: http://www.hpc.ncep.noaa.gov/qpf/fill_98qwbg.gif
Judging from the new HPC graphical information above, about 3/4 of an inch of water EQ is possible by Saturday night over the crest….this is mainly along a line from Mono Lake Northward. Amounts for 2 to 4 inches of snow over the crest in the Mammoth area is possible with 1/2 inch of QPF. The Sierra Crest areas from about Tioga (120) northward to Alpine County 3 to 6 according to ECMWF qpf. If the system comes in a little further south of guidance that would make a difference..
Additionally, those that are planning cross travel over the higher elevation passes, should be prepared for the “Possibility” of a temporary road closure of Tioga Pass as early as Friday night into Saturday….
Dr Howard and the Dweebs……………………..:-)