WXRISK.COM's  SNOW FORECASTING  TECHNIQUES.

Below are several snow forecasting techniques that  many many others forecasters have used for years and years -- in some cases Decades. I do not present the MAGIC chart or the GARCIA techniques... those are rather well known in the business. The idea here is present and preserve some of the older forecasting schemes and techniques used by the older timers.

I have tested and used these snow forecasting schemes over the last few years and they have worked quite well. Ideally, when I get the time I will use some recent cases  of major snow events across the US  during the past 3 or 4 years and present these forecasting schemes to see how well or poorly they worked, worked vs. the model guidance and worked vs. what actually fell or occurred.

When I get the time...

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A GRAPHICAL METHOD USING THICKNESS PARAMETERS SR/MR

Knowing the 1000/850 thickness and 850/700 thickness you can get a very good handle on winter precipitation types… rain vs. snow vs. sleet

The chart below is best used EAST of 100 degrees Longitude and moderate to heavy snow

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THE HILLWORTH METHOD FOR FORECASTING RAIN vs. SNOW

Best used South of 50 N latitude and East of 100 West Longitude

1. Determine the 1000mb (or surface) Wet bulb on the table below.
2. Determine the 1000mb (or Surface) Wet Bulb Temp Tw on the table below determine the 850 Mb Wet Bulb (Tw) on the next graph

2. Now determine the 850 Mb Wet Bulb (Tw) on the next graph

Now cross-reference the 1000-Mb and 850 wet bulb numbers on the graph below to determine the precipitation type.

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MOVEMENT OF PRECIPITATION AREA: (SR-MR)

• If the 700-Mb trough axis is WEST of 100 degree meridian use 850 winds to extrapolate precipitation area for movement. Using 850mb winds, expand the precipitation shield by 1-degree latitude as it moves eastward in a 12-hour period. The western edge only gets moved as the 700 trough axis moves.

• If 700 Mb EAST of 100^th Meridian use 700 MB winds If you are using 700mb winds move the precipitation shield edge whatever speed & direction the 700mb winds say.

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A GOOD ESTIMATE FOR QPF AMOUNTS FOR NEXT 12 HOURS     (SR)

Best used when 850 trough is East of 110^th longitude.

1. Delineate the area under consideration…. Let’s say it’s a forecast area (FA).
2. Draw a line showing where the max WAA at 850 mb is going to occur in the FA
3. Measure the difference in surface pressure between the entry point of the 850 line
   and the area it exits.
4. Enter the values I supplied in this graph used in a 12-24 hour forecast of QPF amounts when 850 trough is east of Meridian
   850 MB Temps plus surface Td

For a 12-24 hour forecast of QPF amounts when 850 trough is East of 110^th longitude

1. Delineate the area under consideration…. Let’s call the forecast area (FA).
2. Forecast the max 850 WAA in the 12-24 hr period in the FA
3. Measure the 850-mb CAA in the FA; differences in surface pressure between the entry point of the 850 line and the area it exits.

Remember that on these graphs the 850 WAA and CAA is determined by Advection not actual number at a given time. So lets say that at the FA under consideration the WAA lifts 850 Tt from +3 to +12… the WAA would be 9. I mean it seems pretty elementary to me but…

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SOME SNOW RULES/GUIDELINES THAT ARE OFTEN OVERLOOKED     (SR- MR)

• The Western edge of the moderate to heavy snow shield ends at the 700-mb trough line in 90% of cases east of 100 longitude, regardless of what the model says.
• The Light snow ends at the 500-mb trough axis.
• In synoptic situations Moderate snow usually needs a 14 Vort max. Heavy snow are usually associated with a 20+ vort.
• The average 500 mb temp with 3 degrees latitude of the Vort max should be –24 to
  –37C
• The favorite area of moderate/heavy snow is in an area of 7 degrees latitude downstream of the Vort max AND within 3 degrees to the Left of the Vort max.
• Another good place for moderate and heavy snow to fall is under closed 500 low and/ never further east than the inflection point where the 500 flow turns from cyclonic to anti-cyclonic .

• The moderate/heavy snow usually occurs about 90 miles to the left of the 850 low (if the cold air mass is arctic then the distance is 50 miles) and 120-140 miles north of the surface low (east of 100 longitude). In the Southeastern U.S. -below 35-north latitude and east of the Mississippi river- the snow band to surface low distance is more like 200 miles.

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BASIC  500 MB VORTICITY &  2840 DM 1000-700  MB THICKNESS

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850 and 2840 rules (SR - MR)

These rules involving the 850 low track and the 2840 meter line may be one of the most often overlooked yet EASIEST winter precipitation forecast rules that a forecaster can use.

1. Find and outline or display the 2840 meter line.
2. Estimate an 850 track… either your best guess or use model guidance.
3.  

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Tracking the movement of the mean RH, 2840 thickness line.

This technique is useful when using the "Garcia" method. One of the problems that forecasters have in using the "Garcia’ method is in estimating what the water vapor is going to be in 12, 24, or 36 hours. Using the model is not an answer since we are seeking an independent way of a figuring water vapor Advection.

RH & The Geostrophic wind.  (SR)

The rate of Advection regarding the 2840 meter line is 30% of the speed of the geostrophic wind if there is NO precipitation falling over the area. If there is precipitation then the rate of advection at the 2840 line is 15% of the geostrophic wind.

RH & 850 MB winds  (SR-MR)

• 200+ miles North of the Warm front on synoptic scale systems, RH and water vapor will get advected at 80% of the 850 wind speed

• 100-200 miles North of the Warm front on synoptic scale systems, RH and water vapor will get advected at 85% of the 850 wind speed.

• 50-100 miles North of the Warm front on synoptic scale systems, RH and water vapor will get advected at 90% of the 850 wind speed

• Within 50 miles of the Warm front on synoptic scale systems, RH and water vapor will get advected at nearly 100% of the 850 wind speed

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THE VORTICITY METHOD

The Vorticity method is an old method for snow forecasting amounts but it is not that well known, which is a shame in my opinion since it works quite well… as least as well as the magic chart.

1. Calculate SURFACE VORTICITY

• Draw a circle from the surface low’s (either actual or projected) position about 200 miles radii.
• Add the MSLP on the circle in a North, east, south, and west direction. Divide this number by 4 to get average pressure.
• Subtract the central MSLP of the surface low from the average pressure. Let’s call this number X.
• Multiply X by the following
• At 30 degree North, multiply X by 3.5
• At 35 degree North, multiply X by 3.0
• At 40 degree North, multiply X by 2.8
• At 45 degree North, multiply X by 2.5

This figure is referred to as the SURFACE VORTICITY of the storm.

Next let's find the RELATIVE VORTICITY...

RELATIVE VORTICITY

1. Determine the 500 mb Relative Vorticity… the progs generally show absolute vorticity so use this table below to make the correction.

• At 30 degree North, subtract   7.3 from absolute Vorticity
• At 35 degree North, subtract   8.3 from absolute Vorticity
• At 40 degree North, subtract   9.3 from absolute Vorticity
• At 45 degree North, subtract 10.3 from absolute Vorticity

      2. Using the SURFACE ( vertical column of LEFT side)   and RELATIVE  vorticity ( horizontal  row on top),  cross-reference on the chart below.

500  mb    RELATIVE   VORTICITY
	0-4	5-6	7-8	9-10	11-12	13-14	15-16	17-18	19-20
2- 6 SFC VORT	Up to 2 inches	2-3 inches	4 inches	5 inches	6 inches	7 inches	8 inches	9 inches	10 inches
7-10 SFC VORT	2-3 inches	4 inches	5 inches	6 inches	7 inches	8 inches	9 inches	10 inches	12 inches
11-14 SFC VORT	3-4 inches	5 inches	6 inches	7 inches	8 inches	9 inches	10 inches	12 inches	14 inches
15-18 SFC VORT	3-4 inches	6 inches	7 inches	8 inches	9 inches	10 inches	11inches	13 inches	15 inches
19-22 SFC VORT	5 inches	6 inches	8 inches	9 inches	9 inches	10 inches	12 inches	14inches	16 inches
23-26 SFC VORT	5 inches	7 inches	8 inches	9 inches	10 inches	11 inches	13 inches	15 inches	17 inches
27-30 SFC VORT	6 inches	8 inches	9 inches	10 inches	11 inches	12 inches	14 inches	16 inches	18 inches

Some rules to keep in mind using this table

• Snow Vorticity forecasting works best with SURFACE AND 850 LOW is moving from 180 to 290 degrees to a Easterly Direction..
• The 500 mb wind speed must be at least 25 knots near the 2840 line
• The 500 mb vort max must be within 500 mile of the 2840 meter thickness line to use this table.

1. If the vort max is 20 or less the Surface low must be within 250 miles of the 2840 line
2. If the vort max is 21-30, the Surface low must be within 250-300 miles of the 2840 line
3. If the vort max is 31 or more, the Surface low must be within 300 miles of the 2840 line

If neither of the conditions of  A, B, C, are NOT met,    then a closed 850 mb Low within 150 miles of the 2840 line must exist to use the snow vorticity method

• If the surface low is expected forward speed is  15 or less  knots increase snow amounts on the chart by 30%
• The snow vorticity chart is based on the idea of snow occurring no more than 300 miles downstream from the surface low.  But this is not always the case so for each 100 miles downstream increase snow amounts by 1-2 inches depending on rate of snowfall
• If 850 temperatures > than –11 C and RH is dry, reduce all snow amounts by 50%. If 850 temps are > -6 to –10 C reduce snow amounts by 30%
• When model guidance shows the dry slot moves within 200 miles of the surface Low,   for those areas,  reduce snow amounts on the table above by 30%.

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THE 200 MB SNOW FORECASTING RULE

Another well known but forgotten rule for snows forecasting locations and amounts, and it is easy to use too.

Guidelines:

1. Only use the 200-mb technique between October15th and March 15^th.

• There must be other indications of precipitation at lower levels - say at 700 or 850 mb -Warm air advection.
• If there is WAA at 700 mb into the area, total snow accumulation will be .50 of the indicated WAA from the warmest into the coldest air masses at 200 mb.
• If there is CAA at 700 mb into the area, total snow accumulation will be .25 of the indicated WAA from the warmest into the coldest air masses at 200 mb.

1. Limit the WAA to within 15 degrees latitude upstream regarding the 200 mb trough axis
2. If the WAA is within 6 degrees of 200 mb trough axis snow period will be short

Location using 200 mb snow rule.

1. The southwest end of the moderate to heavy snow is just downstream of the max WAA while the eastern limit to any possible heavy snow is the eastern edge of the cool pool.
2. For a 24 hour forecast the Northeast limit of the moderate to heavy snow should be limited within 14 degrees latitude downstream of max WAA.
3. For a 36 hour forecast the Northeast limit of the moderate to heavy snow should be limited within 21 degrees latitude downstream of max WAA.
4.  

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