A COMPREHENSIVE WINTER
WEATHER FORECAST CHECKLIST
John D. Gordon
National Weather Service Office
There have been a plethora of winter weather papers (Albrecht 1979, Koontz 1986, Weber 1979) written on a wide variety of topics. It is very difficult for an operational forecaster to keep track of the all of these manuscripts. The last major winter weather checklist to be published as a regional technical attachment was nearly 10 years ago (Goetsch 1987).
Furthermore, there are several local techniques and rules of thumb that exist on winter weather that have not been widely circulated to the field. This checklist attempts to fill that void for forecasters. The purpose of this checklist is to help operational forecasters to become more proficient at predicting winter weather and to have a "winter cheat sheet" right at their fingertips.
The following material is an attempt to compile an updated and comprehensive list for forecasting various types of winter weather. In this form, the material can be utilized as a quick and easy reference guide by operational meteorologists during subjective assessment and forecasting of winter weather systems. An in-depth section is included on freezing precipitation, moderate to heavy snow, as well as local snow prediction techniques from across the country. References are included, for further study.
The checklist was initially developed for the staff at NWSO Springfield, MO (SGF). However, with the exception of mountainous regions, it can be utilized almost anywhere across the country. Forecasters across the central and eastern United States can use this checklist as a starting point to handle a variety of winter weather. In applying this information, caution must be exercised.
No one method can be applied by itself without consideration of all other parameters. Thus, it is essential to know how and when to apply such techniques. Considerable experience and knowledge of the atmosphere, local climatology, and model information are crucial.
2. WINTER WEATHER TABLES
First, the forecaster must determine what type of precipitation is expected. Use Table I as a guide to determine whether you will have liquid or frozen precipitation. In addition, you can also use the Freezing Drizzle vs. Snow Checklist (Table II) developed by Headquarters Air Weather Service, Technology Training Division at Scott AFB, IL (1996). It will help you determine whether to forecast freezing or frozen precipitation.
Table I RAIN/SNOW LINES USING THICKNESS
|Critical Thickness||Rain/Snow Line|
|1000-500 mb||5400 m|
|1000-700 mb||2840 m|
|1000-850 mb||1300 m|
|850-700 mb||1540 m|
|850-500 mb||4100 m|
|700-500 mb||2560 m|
Table II CHECKLIST FOR FREEZING DRIZZLE VS. SNOW
***Use this checklist only for an atmosphere completely below freezing***
A. Is a low level moist (T-Td depression 5C) layer (below 700 mb) between 0 C and -15C? Y N
If yes, then freezing drizzle is possible.
B. Is a mid-level dry layer (800-500 mb) present or forecast? Y N
If yes, freezing drizzle or a mixture of snow and freezing drizzle is possible.
C. Is a mid-level dry layer deeper than 5000 ft and have a dew point depression GTE 10C? Y N
If yes, the precipitation may completely change to freezing drizzle or a prolonged period of a mixed snow and freezing drizzle is possible.
D. Is mid-level moisture increasing? Y N
If yes and freezing drizzle is occurring, the precipitation may change to all snow.
E) Is convection occurring or forecast? Y N
If yes, the mid-level dry-layer may erode causing snow to fall instead of freezing drizzle.
Additional information on winter precipitation type can be found in Albrecht (1979), McNulty (1988), and McNulty (1991). When you are positive of the precipitation type, either use Table III for forecasting freezing precipitation or Table IV for forecasting snow.
Table III FORECASTING FREEZING PRECIPITATION
|a. For ZR/ZL to occur exposed surfaces must be LTE 32 F||Y||N||Y||N|
|b. Surface based depth of cold air<1200m? (LTE 0 C). If not...Frozen Precipitation is more likely||
|c. Is there a warm pocket (>0C) aloft and above the surface cold pocket?||
|d. Is the warm pocket aloft at least 400m thick?||Y||N||Y||N|
|The 1000-500mb thickness is:
(5340-5460m = FREEZING RAIN)
(5340-5520m = FREEZING DRIZZLE)
|The 1000-850mb thickness is:
(LTE1310m for FREEZING RAIN)
|The 850-700m thickness is:
(GTE 1555m for FREEZING RAIN )
|The more yes answers you have, the better chance for freezing precipitation. If a. through d. are yes and the thickness value falls within parameters listed above, call and coordinate with surrounding WSFOs immediately for the appropriate watch, advisory or warning.|
TABLE IV FORECASTING SNOW
|a. Is the surface temp <35 F (1.7 C)?||Y||N||Y||N|
|b. Is the freezing level <1200ft (366m)?||Y||N||Y||N|
|c. Is the 850 mb temp <0 C?||Y||N||Y||N|
|d. Is the 700 mb temp <-4C?||Y||N||Y||N|
|e. Is the 1000-500mb thickness <5400m?||Y||N||Y||N|
|f. Is the temp <0 C from 1200ft to 700 mb?||Y||N||Y||N|
|g. Is there a moist layer (T-Td depression 5C from surface to 700mb?||Y||N||Y||N|
|The more yes answers you have, the better chance you have for snow. If a. through g. are yes or forecasted yes, forecast snow!|
If you think the weather situation has the possibility of moderate or heavy snow, continue with Table V. This table has specific parameters and features to look for at mandatory levels from the surface all the way up to 200 mb. This information was gathered from a multitude of sources, the bulk of which came from McNulty (1991), Terry (1995), and Weber (1979). Other sources came from academicians such as Djuric (1994), Moore (1989), and Ucccellini (1990).
With winter weather lasting 3 or 4 months in most locations, forecasters tend to forget what to look for at various levels. The following information will ease that memory loss, and allow operational forecasters quick and easy reference, to better forecast major snowstorms.
Table V HINTS FOR FORECASTING MODERATE TO HEAVY SNOW (S+)
2. 850 mb
3. 700 mb
4. 500 mb
5. 300 mb
6. 200 mb
The next step, after you've finished looking at the various levels of the atmosphere, is to forecast snow amounts. Table VI will help you come up with the accumulations. There are four methods listed: Lemo Technique, Magic Chart, Garcia's Method, and Cook Method. Each method keys in on different parameters and levels to come up with accumulations. However, each method has been used quite effectively to better forecast snowfall amounts.
The Lemo Technique was developed by a forecaster several years ago at WSFO CHI. The other three techniques are more widely known and have had numerous papers and studies written about them. The method that seems to have worked best on major snowstorms over the NWSO SGF county warning area is Garcia's Method.
Table VI SNOW ACCUMULATION FORECASTING TECHNIQUES
A. Lemo Technique (Used by WSFO CHI)
MS = (Va - 10) x 30/s MS -total maximum snowfall in inches
Va - absolute vorticity (interpreted from progs)
S - estimated speed of vorticity max in knots
1) Va is found by taking a straight line from the center point of the estimated area of max snowfall and extending a line perpendicular until intersecting the estimated path of 500 mb vorticity max. This point is then interpolated from your model of choice.
2) Estimate the speed of vorticity max in knots and then plug in the values.
1) Clipper systems (NW-SE) reduce LEMO estimate by 25%
2) Increase LEMO estimate for thundersnow situations
3) Technique is only as good as model output is
B. Magic Chart (Sangster 1985 and Chaston 1989)
1) Call up 12-hr Net Vertical Displacement (NVD in mb, AFOS graphic 7WG, over a 12hr period ending 24hrs after data time for parcels arriving at 700 mb.
2) Overlay 12hr or 24hr 850 mb temperature prog, AFOS graphic 82T or 84T, from the NGM.
3) Overlay the NGM MRH for 12hr or 24hr, AFOS graphic I2D or I4D.
4) Where the greatest NVD overlays the temperature region of -3C to -5 C and the MRH of GTE 70% is where the heaviest snow is likely to fall.
|20 to 40 mb||2 to 4 in|
|40 mb||4 in|
|60 mb||6 in|
|80 mb||8 in|
|100 mb||10 in|
1) Does not work well with very cold systems.
2) Only as good as model data.
C. Garcia's Method (Garcia 1994)
1) Make a cross section (XSCT) over the area of concern and pick the theta isentropic surface that intersects the 700-750 mb surface.
2) Analyze the isentropic surface for pressure (every 50 mb) and mixing ratio (every 1 g/kg). Then select forecast hour (either 12 hour or 24 hour point)
3) Determine the mixing ratio over the area of concern.
4) Multiply the average wind speed by time increment (12 or 24 hours). This computed distance must be followed using the direction of the prevailing wind flow to locate the highest advected mixing ratio that could move into the area of concern.
5) Average the mixing ratio over the area of concern with the mixing ratio that could be advected into this area. Compare this number with the table below:
|MIXING RATIO AVERAGE||FORECAST SNOWFALL AMOUNTS|
|1-2 g/kg||2-4 inches|
|2-3 g/kg||4-6 inches|
|3-4 g/kg||6-8 inches|
|4-5 g/kg||8-10 inches|
|5-6 g/kg||10-12 inches|
|6-7 g/kg||12-14 inches|
D. Cook Method (Cook 1980)
1) 200 mb warm pocket coincides with 500 mb vorticity center (vorticity center moves toward 200 mb cold pocket, with movement parallel to line connecting 200 mb warm and cold pockets). Coldest 200 mb temperature downstream from a warm pocket is area of heavy snow in the following 24 hrs
2) Snow Index: Average snowfall (in) for next 24hrs = to one half the maximum WAA expected at 200 mb (maximum WAA of 840 NM in 24 hrs)
There are additional beneficial winter weather products and charts to use at your disposal (Table VII). Skew-T information is critical in forecasting winter weather. The Sharp Workstation enables the forecaster to manipulate the sounding, and better predict what weather will move into your area. AFOS charts K0L, K0S, and K08 can be extremely helpful in winter. They all contain specific critical thicknesses in their plot, and are manually computed twice a day at 12Z and 00Z. Table VII is not complete, but it gives the forecaster additional winter weather information to better forecast the winter type precipitation.
Table VII AFOS CHARTS/PRODUCTS
Use the following products and tools to help you better forecast winter weather.
1) Look at surrounding SHARP Skew-Ts
2) Look at the 925 mb chart for temperatures and warm air advection
3) Analyze K0L, K0S, K08 (Issued by NWSFO TOP via AFOS, explanation of charts is given below)
K0L...LOW LEVEL THICKNESS/MEAN WIND CHART. The upper and lower left numbers are the 850 mb temp and dew point respectively. The upper right number is the 1000-850 mb thickness. Wind plot is mean wind from the surface to 5000 ft.
K0S...RAIN/SNOW THICKNESS CHART. The Upper left number is the 850-700mb thickness; the Lower left number is the 1000-850 mb thickness. On the Right side is the 1000-700 mb thickness with the wind plot being the 850 mb level.
K08...COMPOSITE STABILITY/THICKNESS CHART. The upper left number on plot is the K Index; the lower left number is the Total Totals, and the lower right number is SSI. The upper right number is the 850-500 mb thickness. Wind plot is the mean from 5000-10000 ft.
4) Look at model thickness guidance, graphic products are listed below:
|12 hr||KCK||K23||K2K **|
* K1K 06 hr fcst ** K3K 18 hr fcst
5) Look at the NGM FOUS Guidance which has 1000-500 mb thickness in HH column. The HH is two numbers with the 4 or 5 omitted in decameters. Example 66 is 566 dm
6) Run PCGRIDDS macros on: (See Przybklinski 1994 for more information)
I am very grateful to David Gaede, SOO at NWSO Springfield for all of his help in preparing this lengthy checklist, especially for all of his computer assistance.
Albrecht, L.F., 1979: A study of Freezing Precipitation Parameters. WR Technical Attachment 79-2. 4 pp.
Chaston P. R., 1989: The Magic Chart for Forecasting Snow Amounts. Nat Wea Dig., 14, 20-22.
Cook, B.J.,1980: A Snow Index Using 200 mb Warm Advection. Nat. Wea. Dig., 5, 29-40.
Djuric, D., 1994: Weather Analysis. Prentice Hall, 304 pp.
Garcia, C., Jr. 1994: Forecasting Snowfall Using Mixing ratios on an Isentropic Surface - An Empirical Study. NOAA Technical Memorandum NWS CR-105, 28 pp.
Goetsch, E. H., 1987: Checklist of Significant Winter Weather Forecasting Techniques-
A Summary of Some Long Established Methods. CR Technical Attachment 87-30, 5 pp.
Headquarters Air Weather Service, Technology Training Division Scott AFB, IL: A Technique for Forecasting Freezing Drizzle. FYI February 1996 Number 32, 14 pp.
Koontz, G., 1986: Heavy Snow Forecasting Aids, CR Technical Attachment 86-26. 3 pp.
McNulty, R.P., 1988: Winter Precipitation Type, CR Technical Attachment 88-4. 9 pp.
McNulty, R.P., 1991: Heavy Snow, NWS Training Center, 9 pp.
McNulty, R.P., 1991: Precipitation Type, NWS Training Center, 11 pp.
Moore, J. T., 1989 Isentropic Analysis and Interpretation, NWS Training Center, 84 pp.
Przybylinski, R.W. 1994: New PCGRIDDS Command Files, PCG Note 94-1 (Memorandum to WSFO St Louis Staff), 9 pp.
Sangster, W. E., and E. Jagler, 1985: The (7WG,8WT) Magic Chart, CR Technical Attachment 85-1, 5 pp.
Shea, T.J., and R.W. Przybylinski, 1995: Forecasting the Northern Extent of Significant
Snowfall in a Major Winter Storm: An Operational Forecasting Problem, 14th Conference on Weather and Forecasting, 6pp.
Terry, B., 1995: Heavy Snow Forecasting at the NMC (Lab sessions A & D), Fourth NWS Winter Weather Workshop, September 19-22, 1995, 10 pp.
Uccellini, L.W., and P.J. Kocin, 1990: The Interaction of Jet Streak Circulation During Heavy Snow Events Along the East Coast of the United States, Weather and Forecasting, 2, 298-308.
Weber, E.M., 1979: Major Midwest Snowstorms. USAF 3WW Technical Note 79-2, Offutt AFB, NE, 95 pp.