Alpha-Numeric Model Guidance

Two Types of Alpha-Numeric Guidance

• FOUS (Forecast Output United States): Raw model forecast data interpolated to specific geographic points from actual model grid points.– Example: FOUS (NAM Model) bulletins– This raw model guidance is directly translated from the model forecast fields and will

contain all the accuracies and biases inherent in the parent model.

• MOS (Model Output Statistics)– Statistically-altered model forecast data for specific local points.– Examples: MOS for WRF, NAM, and GFS models.– This model guidance is statistically-altered based on historical model performance and

local climatologies.

We will look at FOUS first.But, before we start, let me state a warning:

*** Warning ***• Blindly following model statistics (MOS) can easily lead

to large forecast errors, so do not get in the habit of “MOScasting”. If you have thoroughly analyzed the weather situation and came up with temperatures and precipitation values similar to the model output, you may have a higher degree of confidence in your forecast.

• MOS is computer-derived and easy to obtain. As a professional meteorologist, if all you do is regurgitate MOS for your forecast, you are adding no human value to the forecast. In other words, you may MOS yourself right out of a job.

DECODING FOUS from the NAM

• The NAM FOUS bulletins contain the initial numerical model analyses and forecasts for points in the United States, Canada, and over the adjacent waters. The NAM FOUS will be discussed below:

• The forecasts are provided in 6 hourly intervals from 6 to 48 hours after either 0000 or 1200 GMT (Z) model cycle times. The forecasts are for 6 hour accumulated precipitation, relative humidity, vertical velocity, lifted index, sea level pressure, direction and speed of the mean wind in the boundary layer, 1000 to 500 millibar thickness, and 3 model-layer temperatures.

Example:NAM FOUS for Birmingham

TTPTTR1R2R3 VVVLI PSDDFF HHT1T3T5BHM//521556 -0114 312518 52050605 06000502155 00514 302508 54100605 12000502882 02612 272614 55120706 18000574587 00711 262916 55120806 24000666783 -0409 263014 53110705 30000606656 -0707 252809 53140704 36000614425 -1006 223015 54150704 42000653924 -0107 223514 53120604 48000714927 00408 220405 50060604

The general format:FOXXII KWBC DDTTTTTTPTT R1R2R3 VVVLI PSDDFF HHT1T3T5----------------------------------NNN// R1R2R3 VVVLI PSDDFF HHT1T3T506PTT R1R2R3 VVVLI PSDDFF HHT1T3T512PTT R1R2R3 VVVLI PSDDFF HHT1T3T518PTT R1R2R3 VVVLI PSDDFF HHT1T3T524PTT R1R2R3 VVVLI PSDDFF HHT1T3T530PTT R1R2R3 VVVLI PSDDFF HHT1T3T536PTT R1R2R3 VVVLI PSDDFF HHT1T3T542PTT R1R2R3 VVVLI PSDDFF HHT1T3T548PTT R1R2R3 VVVLI PSDDFF HHT1T3T5

CODE EXPLANATION===== ==================XX Region identifier.

II Station group number.

DD Day of the month forecast was issued.TTTT Greenwich time of forecast cycle on which the data is

based.

NNN Forecast station three letter identifier.

PTT 6 hour accumulated precipitation in hundredths of inches.

R1 Mean relative humidity of the lowest model layer (lowest 35 mb), in percent.

R2 Mean relative humidity of model layers 2 through 9 (up to 500 mb), in percent.

R3 Mean relative humidity of model layers 10 through 13 (500 to 200 mb), in percent.

VVV Vertical velocity at 700 mb, in tenths of a microbar per second, weighted average of three hourly values at forecast time, one hour before, and one hour after (double weighted at forecast time). Minus sign represents downward motion.

LI Lifted index in degrees Celsius. Negative values are designated by subtracting from 100; e.g. -4= 96. Taken from the lowest (most unstable) of four possible values. The values derived from lifting parcels from the four lowest model layers up to 500 mb.

PS Sea level pressure calculated from lowest sigma level (based on the contour base map).

DD Direction in tens of degrees of the mean wind in the lowest model layer (35 mb).

FF Wind speed in knots of the lowest model layer (lowest 35 mb).

HH 1000-500 mb thickness in decameters with the first digit omitted.

T1 Temperature in model layer 1 (lowest 35 mb) in degrees Celsius.

T3 Temperature in model layer 3 (approximately 900 mb).

T5 Temperature in model layer 5 (approximately 800 mb).

Numerical Models

• FOUS data for a particular location should agree very closely with that model’s raw graphics output for similar variables.– For instance, Sea Level Pressure from the current

NAM FOUS forecast should agree with the current NAM Sea Level Pressure forecast graphics from the NAM for particular locations.

Model Output Statistics

I. What is MOS?• MOS - Model Output Statistics - a statistical

interpretation of output from numerical weather prediction models

• MOS techniques develop optimal statistical relationships (using multiple linear regression and other techniques) between past model forecasts and "verifying" weather elements (e.g., wind speed, temperature, etc...). For example, statistics could relate model 850 mb temperatures to maximum surface temperatures associated with previously forecast 850 mb temperatures.

II. What MOS does:

• Accounts for systematic model bias • Accounts for systematic model timing errors• Accounts for some regional or local effects• Accounts for regional climatic conditions• Predicts weather elements that are not

explicitly simulated by numerical models• Provides reliable probability forecasts

III. What MOS doesn't do:

• Doesn't account for random numerical model errors• Doesn't handle extreme climatic conditions well (e.g.,

record highs and lows)• Only accounts for some numerical model biases that

occur with specific synoptic situations• Doesn't account for changes in the modeling system– (This is important for NAM and GFS MOS construction)

• Doesn't account for the full impact of local circulations and orographic effects

IV. Using MOS for weather forecasting • Can be used for time management: Forecaster can focus

on "mission critical" forecast issues.• Can be used as a forecast "check”• Can be used as a first guess• Remember that NAM, WRF, and GFS MOS come from

their respective models and may disagree with the picture presented by other models.

• Forecasters can improve significantly on MOS by:– identifying rare or extreme events, accurately choosing the

model of the day– considering mesoscale or local effects not accounted for by

MOS– looking for dynamical consistency in graphical model

forecast panels– being aware of common model biases and weaknesses

• Keep in mind these things:• Often, MOS and FOUS forecasts from the same

model will be different.• Keep in mind that MOS approaches continue to

outperform raw model forecasts or "Perfect Prog" techniques that relate model variables to observed weather. Such raw model forecast techniques thus don't include the effects of systematic timing errors, biases, etc...

INTERPRNAMTION OF THE NAM BASED MOS FORECAST MESSAGE (FWC - FOUS14)

FOUS14 KWBC 060357 - Bulletin Header.

DCA ESC NAM MOS GUIDANCE 3/06/91 0000 UTC - Message ID [Station, Routing, Model, Initial Date and Time (UTC)].

DAY /MAR 6 /MAR 7 /MAR 8 - Valid Month and Day (UTC).

HOUR 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 - Valid Hour (UTC).

MX/MN 59 39 54 24 - Maximum or minimum temperature for

daytime/nighttime period (F).

TEMP 37 34 33 38 45 53 52 49 46 43 40 42 47 51 42 39 35 30 24

- Temperature at specified time (F).

DEWPT 27 28 28 30 32 36 40 38 41 41 37 33 28 27 25 21 20 19 19

- Dew point temperature at specified time (F).

CLDS OV OV OV OV OV OV OV OV OV OV BK BK BK SC SC SC CL CL CL

- Opaque cloud cover forecast for specified time (see below).

CLDS CL - Clear SC - Scattered BK - Broken OV - Overcast

WDIR 26 18 08 12 14 14 15 18 24 27 28 29 29 29 29 33 01 02 00 - Wind direction ( x 10 = compass direction) for specified

time.

WSPD 01 04 06 10 11 12 16 18 13 15 12 20 24 22 14 12 14 08 00 - Wind speed (kts) for specified time.

POP06 4 9 46 85 62 3 7 12 8 - Probability of precipitation for 6-h period ending at

specified time.

POP12 49 91 8 19 - Probability of precipitation for 12-h period ending at

specified time.

QPF 0/ 0/ 1/1 3/ 2/4 0/ 0/0 0/ 0/0 - Precipitation amount forecast for 6- and 12-h periods (see

below).

QPF format "A/B" A - Value for 6-h period / B - Value for 12-h period 0 = no precipitation 0 = no precipitation 1 = 0.01 - 0.09 inches 1 = 0.01 - 0.09 inches 2 = 0.10 - 0.24 inches 2 = 0.10 - 0.24 inches 3 = 0.25 - 0.49 inches 3 = 0.25 - 0.49 inches 4 = 0.50 - 0.99 inches 4 = 0.50 - 0.99 inches 5 = > 1.00 inches 5 = 1.00 - 1.99 inches 6 = > 2.00 inches

TSV06 2/ 0 3/ 0 4/ 1 5/ 1 6/ 2 16/ 3 11/ 1 8/ 0 0/ 0 - Thunderstorm/conditional severe T-storm

probabilities for 6-h periods ending at time.

TSV12 4/ 0 8/ 1 21/ 4 9/ - Thunderstorm/conditional severe T-storm

probabilities for 12-h periods ending at time.

PTYPE S S S S S R R R R R R R R S Z

- Conditional precipitation type forecast for specified time (see below).

PTYPE. R - Liquid Z - Freezing or Ice pellets S - Snow

POZP 8 10 12 6 0 0 0 0 0 1 3 0 2 24 35

- Conditional probability of freezing precip. or ice pellet, if precipitating.

POSN 65 67 70 48 41 14 11 13 15 16 20 9 16 50 42

- Conditional probability of snow for specified time, if precipitating.

SNOW 0/ 0/ 0/1 0/ 0/0 0/ 0/0 0/ 0/0 - Snow amount for 6- and 12-h periods (see below).

SNOW format "A/B" A - Value for 6-h period B - Value for 12-h period

0 = no snow 0 = no snow 1 = trace - 2 inches 1 = trace - < 2 inches 2 = > 2 Inches 2 = 2 - < 4 inches 4 = 4 - < 6 inches

6 = > 6 inches

CIG 4 5 4 4 5 6 7 6 3 2 1 5 6 - Ceiling height forecast for specified time

(see below).

CIG1 = < 200 ft.2 = 200 - 400 ft.3 = 500 - 900 ft.4 = 1000 - 3000 ft.5 = 3100 - 6500 ft.6 = 6600 - 12000 ft.7 = > 12000 ft.

VIS 3 4 3 5 5 5 5 4 2 2 1 3 4 - Visibility forecast for specified time

(see below).

VIS1 = < 1/2 mile2 = 1/2 - 7/8 mlle3 = 1 - 2 3/4 miles4 = 3 - 5 miles5 = > 5 miles

OBVIS H H H N N N N F F F F H N

- Obstruction to vision forecast for specified time (see below).

OBVISH - HazeF - FogN - No haze, or fog