Anatomy
of a Forecast
The National Weather Service
issues forecasts for sky condition, temperature, wind, and precipitation
probability on a routine basis. Because the weather is always changing, the
terminology used in these forecasts is also quite variable. Listed below are
descriptions of regularly used weather terms that should give you a better
understanding of each forecast.
SKY
CONDITION
The sky condition describes the predominant/average sky condition based
on eighths of the sky covered by opaque (not transparent) clouds. If a high
probability of precipitation (60% or greater) is expected, then the sky condition
may be omitted since it is inferred from the precipitation forecast.
| Sky
Condition |
Opaque
Cloud Coverage |
|
| Cloudy |
7/8
to 8/8 |
| Mostly
Cloudy / Considerable Cloudiness |
5/8
to 7/8 |
| Partly
Cloudy / Partly Sunny |
3/8
to 5/8 |
| Mostly
Clear / Mostly Sunny |
1/8
to 3/8 |
| Clear
/ Sunny |
1/8
or less |
|
TEMPERATURE
The temperature in a forecast is used to describe the forecast maximum and minimum temperature,
or in some cases, the temperature expected at a specific time.
| Temperature
Description |
Forecast
Meaning |
|
| Around
40 / Near 40 / About 40 |
A range
of temperatures from 38 to 42 |
| Lower
40s |
Temperatures
of 40, 41, 42, 43, 44 |
| Middle
40s |
Temperatures
of 43, 44, 45, 46, 47 |
| Upper
40s |
Temperatures
of 46, 47, 48, 49 |
| 40s |
A range
of temperatures from 40 to 49 |
| 40
to 45 |
Temperatures
of 40, 41, 42, 43, 44, 45 |
|
WIND
The wind describes the prevailing direction
from which the wind is blowing, with speeds in miles per hour. Wind velocity
is normally included in the first five periods of a zone forecast, although
descriptive terms may be used in all periods.
| Sustained
Wind Speed |
Descriptive
Term |
|
| 0-5
mph |
Light /
Light & variable wind |
| 5-10
mph / 10-15 mph / 10-20 mph |
None |
| 15-25
mph |
Breezy
(mild weather)
Brisk or
Blustery (cold weather) |
| 20-30
mph |
Windy |
| 30-40
mph |
Very
windy |
| 40-73
mph |
Strong,
dangerous, high, damaging
(High
Wind Warning criteria) |
| 74
mph or greater |
Hurricane
force |
|
PRECIPITATION
PROBABILITY
The Probability of Precipitation (POP)
is defined as the likelihood of occurrence (expressed
as a percent) of a measurable amount of liquid precipitation (or the
water equivalent of frozen precipitation) during a specified period of time
at any given point in the forecast area. Measurable precipitation
is equal to or greater than 0.01”. Unless specified otherwise, the time
period is normally 12 hours. The forecast area, or zone, is generally considered
to be a county. NWS forecasters use such categorical terms as occasional,
intermittent, or periods of to describe a precipitation event
that has a high probability of occurrence (80% +) but is expected to be of an
“on and off” nature.
| POP
Percent Expression of Uncertainty |
Equivalent
Area Qualifier (convective use only) |
|
| 10
percent |
None
used (Isolated / Few) |
| 20
percent |
Slight
chance (Widely scattered) |
| 30,
40, & 50 percent |
Chance
(Scattered) |
| 60
& 70 percent |
Likely
(Numerous or none used) |
| 80,
90, & 100 percent |
None
used |
|
TIME
PERIODS
| Forecast
Time Period |
NWS
Definition (Local Standard Time) |
|
| Today |
6AM-6PM |
| This Morning |
6AM-Noon |
| This Afternoon |
Noon-6PM |
| This Evening |
6PM-Midnight |
| Overnight |
Midnight-6AM |
| Tonight |
6PM-6AM |
|
|
