Forecast Guidance

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Forecast Guidance
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Forecasting Periods

• Nowcasts
• Short-range forecasts
• Medium-range outlooks
• Long-range outlooks

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Nowcasts

• Nowcasts cover the period from 0 to 6 hours
  into the future and are called Short-Term
  Forecasts by the National Weather service. They
  are supposed to include detailed discussions of
  temperatures, precipitation and other
  meteorological variables.

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Nowcasts (Cont.)

• The Short-Term Forecasts product replaced the
  local radar summaries that were issued by the
  National Weather Service Forecast Offices at 30
  minutes after the hour.

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Short-range Forecasts

• Short-range forecasts cover the period from 6
  to 48 hours into the future. Short-range
  forecasts are called Zone Forecasts by the
  National Weather Service. They include
  predictions of temperature, winds, clouds
  amounts, probability of precipitation and type of
  precipitation.

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Probability of Precipitation

• The Probability of Precipitation (PoP) is the
  statistical likelihood of measurable
  precipitation (0.01 or greater) at a single
  location, based on the current conditions and the
  forecast atmospheric conditions.

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Medium-range outlooks

• Medium-range outlooks cover the period from 3 to
  10 days into the future. They provide
  information on maximum temperatures, minimum
  temperatures and precipitation outlooks.

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Long-range Outlooks

• Long-range outlooks provide general indications
  about the temperatures and precipitation as they
  relate to normal. They tend to be classified as
  above normal, normal or below normal.

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Forecast Model Output

• Forecast model output is displayed graphically or
  in text form in FOUS bulletins.
• The official site for NWS model output is
  http//www.ncep.noaa.gov

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Forecast Model Output (Cont.)

• Many alternate sites also provide forecast model
  output including
• http//twister.sbs.ohio-state.edu
• http//www.ucar.edu

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FOUS Bulletin

• The standard format of a FOUS bulletin for the
  NAM model is shown below.
• FOXXII KWNO DDHHMM
• OUTPUT FROM MOD HHZ MON DD YY
• TTPTTR1R2R3 VVVLI PSDDFF HHT1T3T5

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FOUS Bulletin (Cont.)

• In line 1 FOXXII KWNO DDHHMM
• FO forecast output
• XX region identifier (eg. US)
• II station group identifier
• DD day of the month when forecast issued
• HH hour of the day when forecast issued
• MM minute of the hour when forecast issued

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FOUS Bulletin (Cont.)

• In line 2 OUTPUT FROM MOD HHZ MON DD YY
• MOD model used to produce output
• HH hour when forecast issued
• MON month when forecast issued
• DD day when forecast issued
• YY last two digits of year when forecast issued

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FOUS Bulletin (Cont.)

• In subsequent lines
• TTPTTR1R2R3 VVVLI PSDDFF HT1T3T5
• TT forecast time in hours from when forecast
  was issued
• PTT 6 hour accumulated precipitation in
  hundredths of an inch
• Note TTPTT is replace by NNN// where NNN is a
  three letter station designation.

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FOUS Bulletin (Cont.)

• R1 is the mean relative humidity in the lowest
  model layer in percent.
• R2 is the mean relative humidity in the layer
  from the surface to 500 hPa (mb).
• R3 is the mean relative humidity in the layer
  from 500 hPa (mb) to 200 hPa (mb).

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FOUS Bulletin (Cont.)

• VVV is the vertical velocity, ?, at 700 hPa
  (mb) in tenths of a microbar per second.
• Note although ? lt 0, normally indicates rising
  motion, in the FOUS bulletin a negative sign
  indicates sinking motion.

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FOUS Bulletin (Cont.)

• LI is the Lifted Index in C.
• Note negative numbers are indicated by
  subtracting 100C.
• Thus, 98 indicates
• LI 98C - 100C -2C.

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FOUS Bulletin (Cont.)

• PS is the last two digits of the sea level
  pressure in hPa (mb).
• DD is the wind direction in the lowest model
  layer in tens of degrees.
• FF is the wind speed in the lowest model layer
  in knots.

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FOUS Bulletin (Cont.)

• HH is the 1000-500 hPa (mb) thickness in
  decameters with the first digit omitted.
• T1 is the temperature in the lowest model layer
  in C.
• T3 is the temperature at the 900 hPa (mb) level
  in C.
• T5 is the temperature at the 800 hPa (mb) level
  in C.

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FOUS Bulletin (Cont.)

• Note for T1, T3, and T5, negative numbers are
  indicated by subtracting from 100C.

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FOUS Bulletin (Cont.)
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FOUS Example (Cont.)

• FOUS64 KWNO 261200
• OUTPUT FROM NAM 12Z NOV 26 07
• indicates that the forecast output is from the
  1200 UTC run of the North American Mesoscale
  (NAM) model on November 26, 2007.

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FOUS Example (Cont.)

• 06005969674 01209 150905 50040300
• 06 indicates a 6 hour forecast
• 005 indicates 0.05 inches of precipitation
  forecast during the six hour period
• 96 - indicates a relative humidity of 96 in the
  lowest model level
• 96 indicates an average relative humidity of
  96 between the surface and 500 hPa

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FOUS Example (Cont.)

• 74 indicates an average relative humidity of
  74 between 500 and 200 hPa
• 012 indicates a vertical velocity at 700 hPa of
  1.2 µb per second (rising motion).
• 09 indicates a Lifted Index of 9C

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FOUS Example (Cont.)

• 15 indicates a sea level pressure of 1015 hPa
• 09 indicates a wind direction of 090 in the
  lowest model layer
• 05 indicates a wind speed of 5 kts in the
  lowest model layer

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FOUS Example (Cont.)

• 50 indicates a 1000-500 hPa (mb) thickness of
  5500 meters
• 04 indicates a temperature of 4C in the lowest
  model layer
• 03 indicates a temperature of 3C at the 900
  hPa (mb) level
• 00 indicates a temperature of 0C at the 800
  hPa (mb) level.

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MOS Format

• DT indicates the date
• HR indicates the time in UTC
• N/X indicates the minimum and maximum
  temperatures for the 12 hours periods ending at
  1200 UTC and 0000 UTC, respectively
• TMP indicates the temperatures in F

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MOS Format (Cont.)

• DPT indicates the dew point temperature in F.
• CLD indicates the sky cover
• CL clear (0 octas)
• FW few (0 to 2 octas)
• SC scattered (2 to 4 octas)
• BK broken (4 to 7 octas)
• OV overcast (8 octas)

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MOS Format (Cont.)

• WDR is the wind direction in tens of degrees
• WSP is the wind speed in knots
• P06 is the probability of precipitation during
  the prior six hours
• P12 is the probability of precipitation during
  the prior 12 hours

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MOS Format (Cont.)

• Q06 is the quantitative precipitation forecast
  for the prior 6 hours
• 0 0.00 inches
• 1 0.01-0.09 inches
• 2 010-0.24 inches
• 3 0.25-0.49 inches
• 4 0.50-0.99 inches
• 5 1.00-1.99 inches
• 6 2.00 inches

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MOS Format (Cont.)

• Q12 is the quantitative precipitation forecast
  for the prior 12 hours
• 0 0.00 inches
• 1 0.01-0.0 inches
• 2 010-0.24 inches
• 3 0.25-0.49 inches
• 4 0.50-0.99 inches
• 5 1.00-1.99 inches
• 6 2.00 inches

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MOS Format (Cont.)

• T06 indicates the probability of a thunderstorm
  during the next 6 hours and the conditional
  probability that if a thunderstorm occurs that it
  will be severe.

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Conditional Probability

• The conditional probability is the probability
  that if event A occurs, then event B will occur.
• The conditional probability of a severe
  thunderstorm is the probability that a
  thunderstorm will be severe (event B), if a
  thunderstorm occurs (event A).

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MOS Format (Cont.)

• The thunderstorm probabilities have the format
• TS/SV
• where
• TS indicates the probability of a thunderstorm
• SV indicates the conditional probability of a
  severe thunderstorm

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MOS Format (Cont.)

• T12 indicates the probability of a thunderstorm
  during the next 12 hours and the conditional
  probability that if a thunderstorm occurs that it
  will be severe.

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MOS Format (Cont.)

• POZ is the conditional probability of freezing
  precipitation (i.e. freezing rain, freezing
  drizzle, ice pellets, or any mixture containing
  those types of precipitation) given that
  precipitation is occurring.
• POS is the conditional probability of snow given
  that precipitation is occurring.

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MOS Format (Cont.)

• TYP indicates the type of precipitation that is
  forecast to fall
• R indicates rain
• S indicates snow
• Z indicates freezing precipitation

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MOS Format (Cont.)

• SNW indicates the amount of snow that is
  forecast
• 0 no snow
• 1 trace to 2 inches
• 2 2-4 inches
• 4 4-6 inches
• 6 6-8 inches
• 8 gt 8 inches

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MOS Format (Cont.)

• CIG indicates the ceiling height forecasts
• 1 less than 200 feet
• 2 200-400 feet
• 3 500-900 feet
• 4 1000-1900 feet
• 5 2000-3000 feet
• 6 3100-6500 feet
• 7 6600-12,000 feet
• 8 greater than 12,000 feet

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MOS Format (Cont.)

• VIS indicates the visibility categorical
  forecasts
• 1 less than 0.5 miles
• 2 0.5-1.0 miles
• 3 1.0-2.0 miles
• 4 2.0-3.0 miles
• 5 3.0-5.0 miles
• 6 6 miles
• 7 greater than 6 miles

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MOS Format (Cont.)

• OBV indicates the type of obstruction to
  visibility that is forecast
• N none
• HZ haze, smoke, dust
• BR mist (fog with visibility 5/8 mile)
• FG fog or ground fog (visibility lt 5/8 mile)
• BL blowing dust, sand, snow.

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MOS Availability

• The official National Weather Service site for
  MOS forecasts is at
• http//www.nws.noaa.gov/mdl/synop/products.shtml