Weather Headlines ( Dec 7-11 )

ALERT For A Heavy To Crippling Fall Of Snow During Sunday ( December 9 )

Snowfall rates during the predawn to mid-day period of Sunday are currently targeted to be most extreme, with many roads likely becoming impassible.  Travel should be avoided and plans to be where you expect to stay Sunday should be made now.

Excessive Snowfall Alert For Mid-Upper Elevations of the High Knob Massif And Tennessee Valley Divide

This now includes the Norton-Wise Area

Prepare for impassible roads and the potential of power outages developing Sunday, especially in locations along and southeast of the VA-KY line, where snow depths of a foot or more will be likely.

ALERT For Blizzard Conditions At Upper Elevations In The High Knob Massif

Strong ESE-ENE winds, with gusts over 40 mph, will generate blizzard conditions and make all roadways impassible at upper elevations.

Forecast Model Discussion

The upcoming storm event will generate huge snowfall gradients, both regionally on the synoptic-scale ( or large scale ) and locally on the meso-scale ( or short distances ).

A general southward shift has been observed in the GFS and European models during the past 24-hours; however, not all ensemble members are so aggressive with pushing it south.  This southward shift is due to stronger High pressure to the north acting to suppress this classic snow storm track.

*The new GFS-FV3 has remained more consistent and farther north with the storm and heavy snow versus the operational GFS model ( as has the GEM noted below and the JMA continues to bring 10-12″ into the Wise area ).

Most forecasting mistakes are made by jumping onto such radical changes, instead of forecasting for the pattern and using past climatology as the reference with past analog storm events (well documented settings).

How models are struggling can be illustrated just today by differences between the 7 AM run of the NAM Model and its latest 1 PM run.

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Observe how snowfall at the Wise gridpoint varied from 2″ on the 7:00 AM run to 17″ on the 1:00 PM run (thus the problem at hand).

Of global-scale models the Canadian remains firm with a heavy hit, but this operational form can not resolve terrain features very well ( that requires the RGEM 48-hour model ).

The 12z European Model group (7AM Friday run) varied from less than 2″ to more than 12″ at the Wise gridpoint, with 5-6″ being the ensemble mean.  It is important to remember that snowfall in Wise tends to be underestimated by this model under the best of circumstances.

Saturday Afternoon Update ( Below )

Mesoscale Snow Depth Forecast

My forecast reflects excessive snow amounts, with a general 1 to 2 feet of snow expected from the Norton-Wise area upward into the high country of the High Knob Massif.  This will also include areas along the Tennessee  Valley Divide ( higher ridges-plateaus from Sandy Ridge and Hazel Mountain to Big A Mountain and the Black Mountains ).

Blowing & drifting snow will complicate this setting  at upper elevations in the High Knob Massif where depths may top 2 feet.

Snow amounts of 6-12″, with locally higher amounts, are expected at elevations below 2000 feet m.s.l. to the north toward Pound and Clintwood.

Snow amounts within the Powell River Valley and the Powell Valley corridor remain most problematic and will be largely dependent upon the development, or not, of a rain-snow transition zone.  If this develops amounts in this corridor will remain lower versus if precipitation remains all snow.

At this time my forecast is based upon downsloping on easterly air flow across the High Knob high country being supportive of a rain-mix transition zone developing over Powell Valley, with a TIM Circulation forming in which cold air will be forced to orographically rise while warming with terrain enhanced subsidence will occur over Powell Valley.

Doppler radar bright-banding, with a notable linear edge against the high country side, will develop if this occurs.

Rain, snow, sleet will all be possible in the transition zone.

By the way, these are the Appa-LATCH-ians not the Appa-LAY-ians as many say ( a disrespectful way to speak about these mountains in the view of many long-time residents and pioneer descendants ).

Based on weak-moderate TIM Circulation development, exclusively identified by Wayne Browning, I expect that a large gradient in snow depths to develop across southern and central portions of Wise County and northern portions of Scott-Lee counties (differences highlighted above are not as great as observed during the January 1998 and December 2009 storm events given this storm is not expected to be as strong as those systems).

It is nearly impossible for the floor of Powell Valley, southwest of the Valley Head, to accumulate significant snow on easterly (E-SE) flow streaming across the high country of the High Knob Massif.  So, if this type of flow develops I expect most of the accumulation toward Big Stone Gap to occur after low-level flow shifts more northerly in direction late Sunday.

Meanwhile, the 7 PM Friday NAM Model group is in and predicts 10″ to 13″ for the Wise gridpoint.

The 12 KM NAM should not be used for mesoscale forecasting, but it can be used for other purposes with the NAM group often being good in complex terrain once weaknesses are learned for any given location and compensated for by forecasters.

Here I am talking about highly detailed local forecasting amid complex terrain (above), as it is obvious by looking at this graphic the model is not accounting for local terrain variations.

A problem with the 3 KM NAM (below) by contrast, is that it tends to over-predict snow in orographic locations near the eastern Continental Divide and to under-predict snowfall in favored lifting zones (like the High Knob Massif) removed from this divide.

Increase the resolution to 3 KM; however, the model becomes much better ( below ).  Note it is predicting more than a foot of snow over Big Cherry Lake basin and the High Knob Massif and as little as 2″ in Powell Valley.  Although this will NOT capture the true, real life gradient it has the right idea versus the broad-brushed depiction shown by the 12 KM NAM.

The latest GFS-FV3 is supporting heavy-excessive snowfall with its 7:00 PM Friday run.

The latest GEM (Canadian) model run continues to predict heavy-excessive snowfall.

Model Runs And Saturday Thoughts

Model runs Saturday have all come in stronger with the storm system, illustrating that the Friday wobble was just that and the pattern is what must always be forecast.  Past such events have hammered the local mountains and there is no current indications to say this will not again happen; although, hopefully not to the extent of January 1998 and December 2009.

The GFS-FV3 is in test mode, being developed to replace the operational GFS package which has major problems related to its mathematical-physical parameterizations of many processes within its atmospheric boundaries.

The 7:00 AM run of the European Model increased its prediction to 14″ for the Wise gridpoint, in line with the NAM Model; although, some ensemble members were higher and some lower ( the 51-member mean was 11″ and well above their Friday forecast ).

The 7:00 PM Saturday NAM Model run is nearly identical to its 1:00 PM forecast, with 16″ to 22″ predicted at the Wise gridpoint.