Winter On The Way-Major Snowstorm?

The weather is going to change and change dramatically starting late Friday with winds ahead of the artic front heralding in much colder air. The precipitation appears to be spotty initially but a number of the weather models are suggesting that the low will set up off of the northern California coast which would allow Pacific moisture to betapped into creating a possible major and prolonged snowstorm for the Lake Tahoe basin and the entire Sierra. Snow easily may fall to the hills around the Bay Area and definately should coat the Nevada valleys as well.

Keep you fingers crossed that the low does indeed setup off of the coast and taps the Pacific moisture but whatever happens it will be cold enough for at least 5-7 days for possible around the clock snowmaking with a bit of mother natures natural stuff thrown in for good measure.

Here is the weather synopsis for December 10:

Area forecast discussion
National Weather Service Reno Nevada
235 am PST Wednesday Dec 10 2008 

Short term... 

Quiet weather will continue through Thursday before significant
changes develop by Friday. A high pressure ridge will remain along
the West Coast today keeping Valley High temperatures a few degrees
above normal. Middle and upper slopes will remain warm as inversions
remain in place. A weak upper level disturbance will pass over the
area Thursday evening which will retrograde the ridge and bring some
cloud cover and slight cooling to the northern half of the County Warning Area. A few
light showers cannot be ruled out Thursday night over northern Nevada
associated with this disturbance...but the chance of measurable
precipitation is too low at this time to include in the forecast. 

A dramatic change will occur beginning on Friday. A strong cold
front will approach the area from the north on Friday. Strong
pressure and thermal gradients ahead of the front will bring breezy
conditions to the area by Friday afternoon with stronger winds
expected Friday night into Saturday morning...especially in the
Sierra and along Highway 395. This will break any inversions and
bring the warmest weather of the week to many areas over the southern
half of the area. Model soundings and cross sections also indicate a
critical layer developing near ridge top with very stable air above
the critical layer. This indicates impressive downslope wind
potential developing to the Lee of the mountains...especially Friday
night. Wind gusts during the day on Friday will likely approach
30-35 miles per hour especially north of Highway 50. By Friday night...winds in
excess of 50 miles per hour will likely develop in some lower elevations with
locally stronger winds. The main question is how widespread these
stronger winds will be. 700 mb winds increase to 50-60 kts by Friday
night which will likely cause winds along the Sierra ridges to peak
above 90 miles per hour. Therefore wind statements of some kind will be needed
in some areas for Friday night. Later shifts can analyze this
potential and issue statements as needed. 

Good frontogenesis will also develop along the front by Friday
evening over northern zones. Probability of precipitation were increased over the far north as a
moderate area of precipitation will likely be located right along
the front as stability decreases. Snow levels will drop quickly as
the front passes with a quick change from rain to snow expected.

Long term /Saturday through Tuesday/... 

Really no big changes in the synoptic pattern from the 00z model
solutions again. A large and cold trough will be over the west
most of the period with additional energy dropping south along the
Pacific coast. 

Arctic cold front will be pushing across the area Saturday bringing
a good chance for at least advisory level winds ahead of it. Front
looks drier today than yesterday. Expect a chance of rain ahead of
the front and snow behind it. Temperatures in the Sierra will be
below freezing for several days...and the Nevada valleys high may
struggle to reach 35-40 degrees Sunday through Tuesday. 

Model agreement is better Sunday through Tuesday with somewhat low
spread in the ensemble data. Only big concern is slightly
different speeds in the models on how fast the energy will drop
south. 00z GFS is a bit slower than the European model (ecmwf) which has the low
clearing the area late Tuesday night/early Wednesday. The GFS keeps
the low over the Sierra until Wednesday. Even most ensemble members
show a low or trough slowly moving south along the coast Sunday
through Tuesday. 

The biggest concern for this period is the moist onshore flow Sunday
through Tuesday...with favorable forcing and instability in the
model data. The pattern immediately brings the concern for a multi
day snow event in the Sierra. Have increased probability of precipitation Sunday and Monday
to likely in the Sierra due to the improved model agreement and
lower ensemble spreads. Snow ratios will be high...on the order of
20-30 to 1 for the Sierra peaks and 15 to 1 for areas down to 6000
feet including the Tahoe basin. So even quantitative precipitation forecast totals as low as 0.50
inches would bring significant powdery accumulations to the Sierra. 

Snow amounts may also be a concern for western Nevada Monday and
Tuesday. Based on the instability and lift in the 00z runs...snow
should easily push over the Sierra. 

Still several days to go until the event arrives so there could be
changes in the model solutions. For now they seem to be trending
toward a better chance for a winter storm.


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