FORECASTING SNOWFALL USING MIXING RATIOS ON AN ISENTROPIC SURFACE: AN UPDATE

 

 

Crispin Garcia Jr.
National Weather Service Office
Sullivan/Milwaukee, Wisconsin

 

 

I. INTRODUCTION

Since the publication of a technique for forecasting snowfall using mixing ratios on an isentropic surface (Garcia 1994), unusual heavy snowfall events during the past five years have pointed out the need for occasional adjustments to the original forecast technique. It has become apparent that the six-year period (1987-1993) of the original study did not include:

This paper briefly examines the Southeast Wisconsin Blizzard of January 2, 1999 and emphasizes the necessity of factoring in the snowfall to meltwater conversion when the "area of concern" is under the influence of a frigid air mass, before and during a heavy snow event. It also addresses how jet streaks and couplets significantly alter the original isentropic snowfall to mixing ratio relationship and suggests a new forecast scale for such events, based on six published papers and informal local studies conducted at WFO Sullivan.

II. THE SOUTHEAST WISCONSIN BLIZZARD - JANUARY 2, 1999

"A powerful blizzard struck the southeast half of Wisconsin January 2-3, 1999 bringing very heavy snow and winds gusting to 60 mph, producing whiteout conditions and drifts to eight feet in some areas. Snowfall amounts ranged from 10 to 20 inches. The combination of wind and snow contributed to numerous traffic accidents and major highway pile-ups with several deaths" (Haase 1999).

On Tuesday, December 29, 1998, four days before this powerful winter storm struck, the WFO in Sullivan (KMKX) had issued a Winter Storm Outlook stating the potential for heavy snow for the southern half of Wisconsin during the New Year holiday weekend. Although the zone forecasts during the remainder of the week continued to highlight the possibility of accumulating snow, the first Winter Storm Watch for the state was not issued until 0415 CST Friday, January 1, 1999. The Winter Storm Warning was issued at 0415 CST Saturday, January 2, 1999, the morning of storm onset, and was followed by a Blizzard Warning at 0930 CST. Differing model guidance during the days preceding the storm and the fact that a strong Arctic air mass had been in place over Wisconsin for quite some time made a difficult forecast situation. The late arrival of the 0000 UTC January 2, 1999 model guidance prevented an earlier issuance of the Winter Storm Warning by the evening public forecaster.

With the receipt of the new upper air data at 0000 UTC January 2, 1999 it became apparent that Wisconsin would not be spared by this Lower Mississippi Valley type storm. The surface low associated with this storm would play only a small role in the production of heavy snow. Instead, the main features would include strong upper level forcing and a 50-60 knot low level jet (LLJ) that would feed in the moisture required to produce up to one foot of snow in less than 12 hours at the General Mitchell International Airport (MKE) in Milwaukee (Figure 1).

 

Figure 1
Figure 1. Observed snowfall totals across Wisconsin from January 1-3, 1999 blizzard.

 

This portion of the study will focus on available mixing ratios on an isentropic surface before and during this heavy snow event. We will look at the isentropic plots (287K) available to the forecasters and show how use of values in the New Snowfall to Estimated Meltwater Conversion Table (DOC 1997) (Table 1) could have led to a better snow accumulation forecast. This table can be used for determining the amount of newly fallen snow based on captured meltwater. Forecasters often use the table as a ratio for snowfall by using expected surface temperatures and inputting a model quantitative precipitation forecast (QPF). Surface temperatures in Milwaukee before and during the blizzard remained in the upper teens to lower 20s. By referring to the table we see that this temperature range coincides with conversion ratios in the 20:1 and 15:1 range.

A. Isentropic and 850mb Analyses

The 0000 UTC 2 January 1999 287K isentropic plot (Figure 2) shows that 1-2 g kg-1 mixing ratios were present in the 700-750 mb layer over southern Wisconsin. Early indications were that the 20-30 knot southerly wind would increase mixing ratios to 2-3 g kg-1 by 1200 UTC 2 January. An overlay of the 0000 UTC 2 January 850 mb chart with the 0400 UTC 2 January NOAA Profiler Network 850 mb wind plot (Figure 3) shows a 40 knot LLJ extending from the Gulf of Mexico to southern Missouri. This 40 knot LLJ had been previously seen on the 1200 UTC 1 January 850 mb chart (figure not shown) across Oklahoma and northern Texas. The 0400 UTC 850 mb profiler plot caught a 60 knot wind maximum over the southeast corner of Missouri. On Figure 3, note that the 850 mb temperature at Green Bay, WI (GRB) was -18°C, while the 0°C to -5°C isotherms were analyzed over southern and central Illinois and southern Indiana. By 0500 UTC 2 January 1999 (Figure 4) the 850 mb profiler plot depicted continued development of the LLJ with a 55 knot wind maximum from Louisiana and Mississippi into southeast Missouri and southern Illinois. It was now evident that the warm air and moisture advection would be faster and stronger than had previously been indicated by the 0000 UTC upper air data.

 

Figure 2
Figure 2. 287k isentropic surface for 0000 UTC 2 January 1999. Thick dashed lines indicate pressure surfaces for 700 mb and 750 mb. Thin solid lines are mixing ratios analyzed every g kg-1. Arrows indicate areas of warm and cold advection.

 

 

Figure 3
Figure 3. 850 mb analysis for 0000 UTC 2 January 1999 combined with 0400 UTC 850 mb profiler network plot. Thin dashed lines indicate isotherm analysis for 0°C and -5°C. The heavier dash/dotted line indicates the low level jet.

 

 

Figure 4
Figure 4. 850 mb profiler network plot for 0500 UTC 2 January 1999. The dashed/dotted area indicates the low level jet.

 

The 1200 UTC 2 January 1999 287K isentropic plot (Figure 5) shows that the 3 g kg-1 mixing ratio isohume has reached the far southern portion of Lake Michigan with the 700-750 mb layer positioned over the southern half of Wisconsin. The analysis of the 1200 UTC 2 January 1999 850 mb chart (Figure 6) shows another very crucial feature, a 50-70 knot LLJ extending from central Tennessee across Indiana and wrapping westward across southern Wisconsin (50 knots at the Blue River, WI profiler). The LLJ shows up on the 287K isentropic plot as a 65 knot wind over central Tennessee. In addition, the arrival of a 55 knot southeast wind by 1200 UTC at the 5000 foot elevation on the MKX WSR-88D VAD display, confirmed that the LLJ extended into southeast Wisconsin.

 

Figure 5

Figure 5. 287K isentropic surface for 1200 UTC 2 January 1999. Thick dashed lines indicate pressure surfaces for 700 and 750 mg. Thin solid lines are mixing ratios analyzed every g kg-1. The heavier dashed/dotted line indicates the low level jet.

 

Figure 6
Figure 6. 850 mb upper air plot for1200 UTC 2 January 1999 combined with 1200 UTC 850 mb profiler network plot. Thin dashed lines indicated isotherms analysis for 0°C and -5°C. The heavier dash/dotted line indicates low level jet.

 

The 1200 UTC 2 January 850 mb analysis shows how well entrenched the cold air mass was over Wisconsin and Michigan. Since 0000 UTC 2 January, the 850 mb temperatures had warmed only slightly to -16°C at GRB while cooling to -17°C at Detroit, MI (DTX). At the same time the -5°C isotherm region, the favored area for heavy snow in a more classic storm scenario (Browne and Younkin 1970), remained well to the south of Wisconsin. At 1200 UTC 2 January a surface low (Figure 7) was located over extreme northeast Arkansas, too far south to be a factor during the heaviest snow accumulations in southeast Wisconsin. However, its movement toward and across Lower Michigan helped to spread snow across the entire state, prolonging snow accumulations.

 

Figure 7
Figure 7. Track of surface low associated with January 1-3, 1999 snowstorm. MKX post-mortem report (Haase 1999).

 

B. Isentropic Snow Scale and Snowfall to Meltwater Table

Referring to the isentropic snow scale (Appendix A) and using 3-4 g kg-1 as the effective mixing ratio range for this snow event would have led to a 6-8 inch snowfall forecast for the next 12 hours. However, since Milwaukee ASOS observation temperatures ranged from 19°F to 21°F during the event, incorporating the snowfall to estimated meltwater conversion table (Table 1) into the forecast would have led to significantly higher totals. Adjusted snowfall forecast ranges could have been either 9 to 12 inches (15:1) or 12 to16 inches (20:1) for the ensuing 12 hour period.

 

TABLE 1
New Snowfall to Estimated Meltwater Conversion Table
MELTWATER EQUIVALENT
(INCHES)
NEW SNOWFALL (INCHES)
Temperature (°F)
34 to 28 27 to 20 19 to 15 14 to 10 9 to 0 -1 to -20 -21 to -40
trace trace 0.1 0.2 0.3 0.4 0.5 1.0
.01 0.1 0.2 0.2 0.3 0.4 0.5 1.0
.02 0.2 0.3 0.4 0.6 0.8 1.0 2.0
.03 0.3 0.5 0.6 0.9 1.2 1.5 3.0
.04 0.4 0.6 0.8 1.2 1.6 2.0 4.0
.05 0.5 0.8 1.0 1.5 2.0 2.5 5.0
 
.06 0.6 0.9 1.2 1.8 2.4 3.0 6.0
.07 0.7 1.1 1.4 2.1 2.8 3.5 7.0
.08 0.8 1.2 1.6 2.4 3.2 4.0 8.0
.09 0.9 1.4 1.8 2.7 3.6 4.5 9.0
.10 1.0 1.5 2.0 3.0 4.0 5.0 10.0
 
.11 1.1 1.7 2.2 3.3 4.4 5.5 11.0
.12 1.2 1.8 2.4 3.6 4.8 6.0 12.0
.13 1.3 2.0 2.6 3.9 5.2 6.5 13.0
.14 1.4 2.1 2.8 4.2 5.6 7.0 14.0
.15 1.5 2.3 3.0 4.5 6.0 7.5 15.0
 
.16 1.6 2.4 3.2 4.8 6.4 8.0 16.0
.17 1.7 2.6 3.4 5.1 6.8 8.5 17.0
.18 1.8 2.7 3.6 5.4 7.2 9.0 18.0
.19 1.9 2.9 3.8 5.7 7.6 9.5 19.0
.20 2.0 3.0 4.0 6.0 8.0 10.0 20.0
 
.21 2.1 3.1 4.2 6.3 8.4 10.5 21.0
.22 2.2 3.3 4.4 6.6 8.8 11.0 22.0
.23 2.3 3.4 4.6 6.9 9.2 11.5 23.0
.24 2.4 3.6 4.8 7.2 9.6 12.0 24.0
.25 2.5 3.8 5.0 7.5 10.0 12.5 25.0
 
.30 3.0 4.5 6.0 9.0 12.0 15.0 30.0
.35 3.5 5.3 7.0 10.5 14.0 17.5 35.0
.40 4.0 6.0 8.0 12.0 16.0 20.0 40.0
.45 4.5 6.8 9.0 13.5 18.0 22.5 45.0
.50 5.0 7.5 10.0 15.0 20.0 25.0 50.0
 
.60 6.0 9.0 12.0 18.0 24.0 30.0 60.0
.70 7.0 10.5 14.0 21.0 28.0 35.0 70.0
.80 8.0 12.0 16.0 24.0 32.0 40.0 80.0
.90 9.0 13.5 18.0 27.0 36.0 45.0 90.0
1.00 10.0 15.0 20.0 30.0 40.0 50.0 100.0
 
2.00 20.0 30.0 40.0 60.0 80.0 100.0 200.0
3.00 30.0 45.0 60.0 90.0 120.0 150.0 300.0
This table can be used only for determining amounts of newly fallen snow. It cannot be used for determining the water equivalency (933RRR) of "old snow". Packing and melting/refreezing have substantial effects on the density of the snow

 

Accurately measuring the snowfall during the blizzard was an almost impossible task due to the dry character of the snow and the extremely strong easterly winds (Table 2). Snowfall measurements across southern Wisconsin were at best observational estimates. As an example, the operational MKE ASOS measuring gauge did not register any precipitation until 0700 CST. However, the FAA contract observer was reporting five inches of new snow on the ground by this time. The observer's location on the west side of the airport (adjacent to an expanse of open ground and runways) made that site very susceptible to blowing snow and snow drifts. The visibilities reported by ASOS and the contract observer (M1/4SM and 1/16 mile respectively) differed significantly from the tower visibility (1/4 mile) during the period of heaviest snowfall. The blowing snow, in effect, hindered the contract observer's ability to objectively judge the rate of snowfall (Nouhan 1999). In spite of these observational difficulties, it is still likely that the snowfall rates were averaging one inch per hour, occasionally approaching two inches per hour during the peak of the storm.

 

TABLE 2
Abbreviated Milwaukee ASOS observations for 2 January 1999 showing visibility in statue miles, weather elements, temperatures in F and winds in mph.
MKE ASOS OBSERVATIONS
Time CST Visibility Weather Temperature Winds
0258 AM 3/4 -SN BR 19 11029G36
0358AM 1 -SN BR 19 10029G38
0458AM 1/2 SN 19 10032G39
0558AM M1/4 +SN 19 09030G44
0658AM M1/4 +SN 19 10041G50
0758AM M1/4 +SN BLSN 21 10036G52
0858AM M1/4 +SN BLSN 21 09041G52
0959AM M1/4 +SN BLSN 21 09039G46
1058AM M1/4 +SN BLSN 21 08040G50
1158AM M1/4 +SN BLSN 21 08040G50
1258PM M1/4 +SN BLSN 19 08040G48
1358PM M1/4 +SN BLSN 19 08039G52
1458PM M1/4 +SN BLSN 19 07035G45
1558PM M1/4 +SN BLSN 19 07032G41
1658PM 1/4 +SN BLSN 19 07033G43
1758PM 1/4 SN BLSN 21 07030G38

 

III. COUPLED JET AND JET STREAK EVENTS

The technique described in Garcia (1994) was originally thought to be able to cope with jet stream induced snow events. However, this assumption was based on a few short-lived cases and subsequent studies have proved this conclusion to be premature. There have been several papers written during the past five years dealing with the unusually strong and localized forcing produced by coupled jets and jet streaks. These forcing mechanisms can produce snowfall rates of up to four inches per hour with accumulations of two feet or more, often in less than six hours. These events are usually accompanied by upright or slantwise convection. Thunder is often observed and the precipitation will take the form of convective bands on radar.

Based on six independently published papers and informal local studies, a new forecast scale (Table 3) for jet induced and convective snowfall was developed. A 4 to 1 scale of snowfall to mixing ratio is more applicable to these convective events than is the 2 to 1 ratio intended for synoptic scale systems. Since these dynamic features are fast moving and concentrated over smaller areas, the time scale for these snow events is usually much less than 12 hours.

These following papers have been summarized with regard to snowfall, isentropic lift, and other forcing mechanisms. Look for the 4 to 1 snowfall to mixing ratio relationship in each example:

 

TABLE 3
Mixing ratio to convective snowfall relationship. The mixing ratios shown on this scale should be considered to be readily available moisture in the 700-750 mb layer. Snowfall amounts should be considered to be the maximum snowfall for the period.
CONVECTIVE SNOWFALL SCALE
(Less than 12 hours)
1-2 g kg-1 4-8 inches
2-3 g kg-1 8-12 inches
3-4 g kg-1 12-16 inches
4-5 g kg-1 16-20 inches
5-6 g kg-1 20-24 inches

 

IV. SUMMARY

The January 2, 1999 blizzard event in southeast Wisconsin demonstrated that it may be necessary at times to use the New Snowfall to Estimated Meltwater Conversion Table to increase the isentropic snowfall forecast for unusual, non-classic snowstorms involving a dry Arctic air mass. However, the 2 to 1 snowfall to mixing ratio relationship outlined in Garcia (1994) is still applicable to the majority of snowstorms that occur during the winter season. A built-in advantage of the original method is its self-adjusting characteristics. Warm and cold air masses will generally show correspondingly higher and lower mixing ratio values, making use of the table unnecessary, except as noted here.

An inherent shortcoming of the original snowfall to mixing ratio scale was its inability to handle convectively driven snowfall events. After studying several coupled jet and jet streak heavy snowfall papers, plus informal local studies during the past five years, a convective snowfall scale was developed. This new scale establishes a 4 to 1 snowfall to mixing ratio relationship that is more suited to the tremendous lift and brief nature of these convective events.

V. REFERENCES

Albright, W.F., and H.D. Cobb, 1995: The 8 February 1995 Heavy Snow Event over Northeastern North Carolina. NOAA, NWS Technical Attachment ER 95-8A. DOC, NOAA, NWS Eastern Region Headquarters, Scientific Services Division, Bohemia, NY, 10pp.

Browne, R.F., and R.J. Younkin, 1970: Some Relationships Between 850-millibar Lows and Heavy Snow Occurrences over the Central and Eastern United States.Mon. Wea. Rev., 98, 399-401.

DOC, NOAA, NWS, 1997: Observing Handbook Number 7. Surface Observations, Part IV, Supplementary Observations, Table 2-14(New Snowfall to Estimated Meltwater Conversion Table), 440pp.

Funk, T.W., C.W. Hayes, M.B. Scholz, and K.A. Kostura, 1994: Vertical Motion Forcing Mechanisms Responsible for the Production of a Mesoscale Very Heavy Snow Band. Preprint 14th Conf. on Wea. Analysis and Forecasting, Dallas, TX, AMS (Boston), 176-181.

Garcia, C., Jr., 1994: Forecasting Snowfall Using Mixing Ratios on an Isentropic Surface - an Empirical Study. NOAA Technical Memorandum NWS CR-105. DOC, NOAA, NWS, Central Region Headquarters, Scientific Services Division, Kansas City, MO, 31pp.

Haase, J.D., 1999: Summary for Second Significant Winter Event. MKX Post-Mortem Report. National Weather Service Office, Sullivan, WI.

Holsten, T.E., and T. Hendricks, 1997: A Forecast Overview of the 26 January 1996 Blizzard Across Eastern Iowa and Northwest Illinois Using PCGRIDDS and Non-standard Forecast Techniques. NWS CR Applied Research Papers 18-09. DOC, NOAA, NWS, Central Region Headquarters, Scientific Services Division, Kansas City, MO, 75-90.

Nouhan, V.J., 1999: Measuring Snowfall, Depth and Liquid Equivalent During and after Extreme Blizzard Events. NWS, CR Applied Research Papers 20-05. DOC, NOAA, NWS, Central Region Headquarters, Scientific Services Division, Kansas City, MO, 1-14.

Reiter, S.M., and T.D. Kleffman, 1997: Re-analysis of the Gridded Model Output to Forecast the South Dakota Heavy Snow Event of March 4, 1995. NWS, CR Applied Research Papers 18-09. DOC, NOAA, NWS, Central Region Headquarters, Scientific Services Division, Kansas City, MO, 67-74.

Riddle, D., J. Gordon, S. Lindenberg, S. Shumway, and M. Sutton, 1995: Southwest Missouri Snowstorm of 18-19 January 1995. Postprints, 4th Winter Weather Workshop, DOC, NOAA, NWS, Central Region Headquarters, Scientific Services Division, Kansas City, MO, 26, 1-9.

Shea, T.J., T. Schroeder, R.W. Przybylinski, J.T. Moore, and P.S. Market, 1995: The March 8-9, 1994 Winter Storm over Southern Missouri: A Challenging Operational Forecasting Problem. Postprints, 4th Winter Weather Workshop, DOC, NOAA, NWS, Central Region Headquarters, Scientific Services Division, Kansas City, MO, 18, 1-21.

 

APPENDIX A

Mixing ratio to snowfall relationship. Condensed and modified from Garcia (1994).

 

PROCEDURE
(modified for the AWIPS era)

 

Determine the geographic location (area of concern) for the expected snowfall.

Determine what isentropic surface (K) best intersects the 700-750 mb layer over the area of concern.

The isentropic surface should be analyzed for pressure (every 50 mb) and for mixing ratios (every g kg-1).

Determine the mixing ratio value over the area of concern and from the isentropic wind field approximate the mixing ratio to be advected in during the next 12 hours. From these two values calculate an average mixing ratio for the 12 hour period.

Determine if the dynamic forcing (lift) necessary for maximum snowfall will exist for most or just part of the 12 hour period. Often this will be a subjective interpretation based partially or wholly on model guidance.

Empirical observation has shown that a 2 to 1 relationship exists between the maximum snowfall amount and the average mixing ratio. This relationship has been confirmed many times in case studies and in actual forecast situations over the past six winter seasons.

SNOW SCALE
(12 hour period)
1-2 g kg-1 2-4 inches
2-3 g kg-1 4-6 inches
3-4 g kg-1 6-8 inches
4-5 g kg-1 8-10 inches
5-6 g kg-1 10-12 inches
6-7 g kg-1 12-14 inches

NOTE: The mixing ratios shown on this scale should be considered to beaverage mixing ratios reaching the 700-750 mb level. The snowfall amounts should be considered maximum snowfalls for a 12 hour period.

 


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