[--MAIN HOME--] [--ALL HABYHINTS--] [--FACEBOOK PAGE--]

 SHORT RANGE FORECASTING OF WINTER PRECIPITATION

METEOROLOGIST JEFF HABY

A. OVERVIEW OF ANALYSIS CHARTS

1. Examine latest analysis charts to get a feel for the weather pattern you will be dealing with today. DIFAX can take too much time to load. On the Internet, a basic chart for each level is adequate. Websites for each of these charts:

 SFC 925 850 700 500 250

B. GRAPHICAL SYNOPTIC MODELS

1. Understanding the value and limitations of the forecast models is critical. For a comprehensive discussion on the interpretation of the graphical forecast models go to the following website:

2. One of the first steps is a general overview of the synoptic short term (12 to 48 hr) graphical synoptic models. The most widely used graphical models are the ETA, meso-ETA, NGM, AVN, CANADIAN, and RUC. A location to view the most recent output from each of these models is given below

Here are some important questions to answer while putting together your forecast:

1. What types of thermal advection are taking place at the various levels in the atmosphere?

Discussion: Thermal advection at various levels will change the temperature structure of the atmosphere over the forecast region. This is important to a precipitation type forecast.

2. Is the atmosphere likely to produce precipitation that will reach the ground over the forecast region?

Discussion: See the precipitation progs from the various models and compare them. Infer what the risk is of precipitation evaporating before reaching the ground (can occur if UVV is small and PBL is dry).

3. Are surface temperatures below freezing during precipitation event?

Discussion: Keep in mind the precip progs show accumulated precipitation over a 12 hour period. Since the temperature occurs at the valid time but precipitation is cumulative, freezing temperatures at the valid time does not mean the precipitation that occurred during the previous 12 hours has to be of the frozen variety. Use the surface and 1000-mb progs for low level temperatures and compare the models.

4. Will the precipitation fall as all snow?

Discussion: If sfc and 850 temperatures are below freezing, snow is the most likely precipitation. Compare models.

Discussion: 850 temperatures of 4 C or more above freezing but with 1000 and sfc temperatures below freezing are an indication freezing rain or sleet is more likely than rain or snow. Freezing rain and sleet events occur when a shallow polar air mass differentially advects under warmer air or when warmer air differentially advection over a subfreezing boundary layer. Sleet also occurs in the transition between rain and snow. The depth of the PBL subfreezing temperatures differentiates freezing rain from sleet. Compare models.

6. What is evaporation cooling potential?

Discussion: Precipitation into a dry layer will result in a cooling and moistening of that layer.

7. What is the timing of the precipitation event with the thermal structure of the atmosphere over the forecast region?

Discussion: Timing of precipitation with the vertical temperature structure of the atmosphere is critical because it determines the precipitation type that will fall. Compare models.

8. What are some problems the models may have?

Discussion: Surface snow/ice cover and lack of data over ocean regions can distort model output. See if the models are moving toward a consensus or are diverging. If they are diverging, deduce what important process is complicating the weather situation.

C. NUMERICAL OUTPUT

1. GFS(MAV) / NAM(MET) MOS

Online location: GFS(MAV) / NAM(MET) MOS

Examine forecasted temperature, dewpoint and wind trends. See how closely the actual observations are matching to the MOS numbers as the forecast period goes by.

2. From the MOS notice the similarity and differences in precipitation type, precipitation chance, precipitation amount, temperature trend, dewpoint trend, and other relevant factors.

D. THICKNESS MODEL PROGS

Thickness values between various pressure surfaces is used to forecast the most likely precipitation type. Output can be compared from the various forecast models. The website below has the various thickness values a forecaster examines on each model prog. See "Winter Critical Thickness" for each of the models (NAM, NGM, and GFS).

Winter Critical Thicknesses: http://www.wxcaster.com/conus_0012_us_models.htm

The website below has interpretation of each of the commonly analyzed thickness values

http://www.theweatherprediction.com/winterwx/thicknesscriteria/

E. BUFKIT

1. BUFKIT is sounding software that takes model data and predicts the precipitation type and amount that will fall over a SPECIFIC forecast location. This software works best with predicting 12 to 24 hour parameters.

Location of software on Internet: http://www.wbuf.noaa.gov/bufkit/bufkit.html

2. If you do not currently have the software you will need or have someone download the program onto your computer.

Once the program is downloaded, data for ingestion into the software is available at:

http://www.wbuf.noaa.gov/bufkit/prof.htm

and

http://www.meteo.psu.edu/~koval/map.html

Once this data is ingested into BUFKIT, it gives the following: forecast sounding in one hour intervals from most recent to 48 hours into the future, precipitation type that will fall in one hours increments, total accumulation of precipitation during the event, and plenty of other specific data.

Keep in mind that the reliability of the BUFKIT soundings depend on the accuracy of the model data. Therefore, it is important to do a thorough comparative analysis of each of the graphical forecast models before interpreting the BUFKIT output for a particular model.