• Cases from Group 1: Progressive Trough
• February 20-21, 1979. See 4-panels at T00 and T+12.
• January 17, 1988. See 4-panels at T00 and T+12.
  
• Cases from Group 2: Stationary Southern California/Nevada Closed Low/Trough
• February 8-9, 1976. See 4-panels at T00 and T+12.
• January 4, 1987. See 4-panels at T00 and T+12.
  
• Case from Group 3: Slow-moving, Digging, Negative Tilt, Diffluent Trough
• November 25, 1983 See 4-panels at T00 and T+12.
  
  

Over the years considerable skill has been developed by forecasters at the Salt Lake City WSFO in the forecasting of "major" (6+ inches in 24 hours) snowstorms along the Wasatch Front. Unfortunately, though, the forecast techniques proven effective for northern Utah have not always worked for other areas of the state, especially in the east. One such area is the Price/Castle Valley area. The winter of 1996-97 brought 3 major snowstorms to the area, none of which was forecast well if at all.

1. infrequency of major snowfalls and subsequently a general lack of experience (whereas Salt Lake City receives about 2.5 major snowstorms annually, Price only receives 1 per year),
2. lack of real-time observational feedback (fortunately this is changing with the installation of an ASOS system (not yet commissioned)),
3. dissimilarity to Salt Lake City topographically (in the rain shadow of 11,000 foot mountains instead of at their feet like SLC).

II. BACKGROUND INFORMATION:

• period studied: 1970-1989 (the period of overlap for composite and Price data sets)
• source of data: snowfall data was used from both Price (Price Warehouse, 1970-present) and Helper (UP & L 1981-present). Helper is a smaller town located a mile or two northwest of Price. "Hourly Precipitation" data from Price was used to determine snowfall "onset" times.
• limitations of composite results: as is always the case with composite studies, they do not provide information as to the "false alarm rate"; the frequency with which the key composite signatures occur but the suggested results (heavy snow) never materialize. Owing to this limitation, meteorological charts from 5 of the 19 snowstorms have been included in the study so that the forecaster can take a look at actual cases and draw his own conclusions.
• study objectives: Hopefully this study will provide a basis and beginning for improvements to winter-time precipitation forecasting in the Castle Valley. In addition, it should also suggest keys to snowfall forecasting for other "east-side" locations. Many of the ideas are applications of basic meteorological principles which we sometimes lose track of owing to our "west-side" forecasting myopia/mentality. Following are some questions which hopefully can be answered in time following a closer look at snow forecasting in the Price area. What is the false alarm rate for these patterns? How often does the pattern set-up but no significant precip occur? What makes the difference? Is it moisture? Pressure pattern? Strength of overrunning/warm air advection? Strength of Jet? Storm track?

III. RESULTS:

• Climatological Frequency of Major Snowstorms in Price: As mentioned above, for the 20-year period studied (1970-1989) 19 "major" (6+ inches in 24 hours) snowstorms occurred, about one every year.
  
• Basic Impressions of what to look for to get major snowfall in the Castle Valley: (The charts supplied are from composites of all 19 "major" snowstorms)
  
• strong, moist, overrunning southwest flow (see H5 @ T00 and H5 @ T+12 and H7 @ T00 and H7 @ T+12).
• strong jet with Price under the front left quadrant of the jet. (see Jet @ T00 and Jet @ T+12
• strong southeasterly surface gradient.This gradient holds a dome of cold air in place enhancing the strength of the overrunning. (see SLP @ T00 and SLP @ T+12
• diffluent flow aloft is also common to many of the storms.
  
• Grouping and Compositing of Price Snowstorms: Categorizing Price snowstorms was not easy owing to the variety of patterns which occurred. Test groupings were made then composites run in an attempt to find groupings that yielded distinctly different composites. After a fair amount of trial and error distinctive sets of composites were obtained and the following groupings made:
  
  
  • GROUP 1. PROGRESSIVE TROUGH (10 CASES) See 4-panels at T00 and T+12.
  • GROUP 2. STATIONARY SOUTHERN CALIFORNIA/NEVADA CLOSED LOW/TROUGH (6 CASES)See 4-panels at T00 and T+12.
  • GROUP 3. SLOW MOVING, DIGGING, NEGATIVE TILT, DIFFLUENT TROUGH (3 CASES) See 4-panels at T00 and T+12.
    
• Key Elements for major Price Snowstorms:

  1. Elements which contrast with Salt Lake City:

     	Price	Salt Lake City
     flow aloft	"moist southwest flow"	"moist northwest flow"
     precip timing	"pre-frontal"	"post-frontal"
     surface pattern	SE surface gradient	NW surface gradient
     track of low	to the north and west	generally overhead
     700 mb temps	-3.0 to -6.5 degrees C.	-7.0 degrees C. or colder

     
     
     
     • Flow Aloft:
       

       Flow aloft almost exclusively exhibits a significant southerly component. There was only one exception, a case in which a stationary baroclinic band set up in strong west-northwest flow. This is in contrast to Wasatch Front snowstorms where "moist northwest flow" is a common rule of thumb.
       

     
     
     
     • Precipitation Timing:
       

       Unlike west-side snowstorms, Price synoptic storm precipitation is almost exclusively PRE-frontal. In a stable pre-frontal pattern warm air overruns cold air and the Price area doesn't experience the drying downslope effects normally experienced in mountain rain shadows. In looking through the individual cases it is observed that precipitation may begin as soon as the 500 mb ridge axis passes, the flow aloft begins to back ahead of an approaching short-wave, and overrunning develops. In most cases, the front/surface low is nowhere to be found when snowfall begins (Group 1 and 2 composites place the low along the West Coast). This is completely opposite that for Utah's west-side upslope locations which generally do not see significant precip UNTIL the front/surface low arrives.

       By contrast, significant precipitation tends to end shortly after the cold front arrives. Once again, this is opposite the tendency for the Wasatch Front. When the cold front arrives, overrunning ends asoverrunning ends, the airmass begins to destabilize, and the southeasterly surface gradient (holding the cold air in place) reverses allowing the cold air dome to be eroded away.

     
     
     • Surface Pressure Pattern:
       

       Major snowstorms in the Castle Valley area are almost always accompanied by a significant southeasterly surface pressure gradient. This strong southeasterly pressure gradient appears to be important in holding cold air in place which enhances the overrunning affect.
       
       Table 1 below lists the surface gradients at the approximate time of onset of snowfall (at T00). Most (12 out of 19) snowstorms exhibited "strong" (5+ mb gjt-slc or gjt-cdc) southerly or easterly surface gradients. A few (4 out of 19) Price snowstorms exhibited a "moderate" (3-4.5 mb gjt-slc or gjt-cdc) surface gradient. Only 3 out of the 19 major snowstorms exhibited a weak or unfavorable surface gradient.
       
       Although southerly gradients lead to a drying downslope wind for the Salt Lake Valley, they are a favorable upslope orientation for the Castle Valley. In addition, the valley appears to exhibit a box canyon type of orographic effect in southeasterly flow with mountains hemming in the valley to the north and west. The author recalls clear air visible satellite pictures illustrating this upslope/box canyon effect. The only valley location in the state receiving snow from this particular event was the Castle Valley. Satellite pictures showed a distinct triangular shaped snowfall pattern up against the mountains to the north and west. Although surface gradients are moderate to strong southeasterly, actual surface winds in these events tend to be light and variable. You will note that, in all but 3 cases, the most favorable gradient is southeasterly (gjt-slc) suggesting that the low usually sets up to the northwest of Price.
       

     
     
     
     • Track of Surface Low
       

       The surface low tends to remain/track north or west of Price (see Table 2 below). Only 3 out of the 19 cases had the low tracking south of the Castle Valley.
       

     
     
     
     • 700 mb temperatures
       

       10,000 foot temperatures tend to be warmer than those for SLC snowstorms. Whereas major SLC snowstorms occur with 700 mb temperatures of -7 degrees C or colder, Price 700 temperatures are generally warmer than -7 degrees C.. This is due to their warm overrunning nature.
       

  
  
  
  2. Other Elements:
     
  
  
  • left front quad of jet: polar side of jet...left front quadrant...and, in the case of group 3 snowstorms, a coupled jet (right rear and left front). unfavorable jet dynamics were exhibited by the time snowfall had ended...left rear quadrant with jet core moved well southeast of area.
    
  • diffluent flow aloft (500 mb and above)
    
    

• Cases from Group 1: Progressive Trough
• February 20-21, 1979. See 4-panels at T00 and T+12.
• January 17, 1988. See 4-panels at T00 and T+12.
  
• Cases from Group 2: Stationary Southern California/Nevada Closed Low/Trough
• February 8-9, 1976. See 4-panels at T00 and T+12.
• January 4, 1987. See 4-panels at T00 and T+12.
  
• Case from Group 3: Slow-moving, Digging, Negative Tilt, Diffluent Trough
• November 25, 1983 See 4-panels at T00 and T+12.
  
  
TABLE 1: surface gradient strength

ONSET TIMES    GRADIENT IN MILLIBARS
OF SNOWSTORMS  GJT-SLC   CDC-SLC   GJT-CDC   GRADIENT STRENGTH

70 01 17 00    4                             moderate
75 02 10 00              6                   strong
76 02 09 00    2                             weak
78 01 10 12    3.5                           moderate
78 01 15 12    4.5                           moderate
78 02 10 12    6                             strong
79 02 21 00    5 @ T+12                      strong
80 01 29 12                        5         strong
81 12 27 00    4                             moderate
82 01 04 12    6                             strong
82 11 30 12    6.5                           strong
83 11 25 12    6.5                           strong
83 12 03 12    8 @ T+12                      strong
84 12 16 12    6                             strong
84 12 20 00    -3                            negative
84 12 27 12    1                             very weak
87 01 04 12    7                             strong
87 12 22 12    6                             strong
88 01 17 12                        6 @ T+12  strong  

"Weak/No" Gradient = <3
"Moderate" Gradient = 3-4.5
"Strong" Gradient = 5+

* gradients are derived from gridpoint data charts *
     
** T+12 times were used in a couple of cases if the snowstorm was
of extended duration (24+ hours)**

Most (12 out of 19) snowstorms exhibited "strong" southerly or easterly surface gradients.

A few (4 out of 19) Price snowstorms exhibited only a "moderate" surface gradient.

The remaining few (3 out of 19) exhibited little or no favorable surface gradient. Each of these 3 exceptions were from Group 2 storms (stationary southern California/Nevada low/trough) which suggests that other dynamics come into play in such cases.

TABLE 2: Track of the Surface Low

               LOCATION OF LOW AT 12 HR INCRE-
ONSET TIME     MENTS FOR DURATION OF STORM        TRACK 

70 01 17 00    s id...wy                          north
75 02 10 00    s id...wy                          north
76 02 09 00    ca...ca...wy                       north
78 01 10 12    ca...ca                       remains well to the w
78 01 15 12    off Oregon cst                remains well to the w
78 02 10 12    s ca...rno...env...bce...elp       south
79 02 21 00    off or cst...lkv...env...n co      north
80 01 29 12    tph...cny                          south
81 12 27 00    boi...se co                        north
82 01 04 12    or...id...wy...co                  north
82 11 30 12    vci...env...evw                    north
83 11 25 12    vci...wy...co                      north
83 12 03 12    off sfo...wmc...co/wy              north
84 12 16 12    env...nw co                        north
84 12 20 00    mtr...bce                          south
84 12 27 12    baja & sea...baja lws         remains well to the w
87 01 04 12    ca/nv...s ca...s nv           remains well to the w
87 12 22 12    env...co                      roughly overhead
88 01 17 12    off sfo...lax...bce...nm           south