Forecasting Guide

Rules of Thumb
 
FAIR weather is likely when one or more of the following occurs:
  • the wind blows from the west or northwest
  • the barometric pressure remains steady or rises slowly
  • fair weather cumulus are present
  • early morning fog is burned off by midday
  • the barometric pressure rises
  • south winds shift west
  • the cloud ceiling begins to lift
RAIN or SNOW is likely when one or more of the following occurs:
  • the barometric pressure falls
  • cumulus clouds begin to develop vertical columns
  • a halo appears around the moon
  • cirrus clouds begin to thicken and the cloud ceiling lowers
  • the sky darkens
  • a south wind increases in speed
  • the wind shifts counterclockwise, which indicates a low (prime example: from N to W to S)
COLD temperatures are likely when one or more of the following occurs:
  • on a clear night with no cloud cover and virtually no wind
  • the barometric pressure rises during winter or any time in front of an oncoming system of clouds or a cold front
Using the Clouds

Cirrus uncinus indicate an approaching warm or occluded front as well as precipitation and are often called mares’ tails.

image

Mares’ tails—photo by Fir0002 on Wikipedia 

Cloud cover that darkens, beginning with cirrostratus, then altostratus and finally nimbostratus over a 24-hour period indicates an approaching warm front and and rain. Commonly, the longer the sequence of cloud formations and darkening in the sky, the longer the storm will be.
The rapid formation of altocumulus followed in quick succession by stratocumulus and cumulonimbus usually indicates an incoming cold front.
Cumulus Clouds
 
Cirrocumulus indicate unstable air and may be the harbinger to precipitation within 10 hours when accompanied by cirrus and cirrostratus.
image
Cirrocumulus—photo by King of Hearts on Wikipedia
Altocumulus indicate thunderstorms or precipitation within 24 hours if seen during the early morning.
Cumulonimbus are associated with severe thunderstorms.
Cumulus humilis or fair-weather cumulus indicate a stable airmass being warmed by the earth’s surface, but development into cumulus mediocris can indicate an atmosphere with increasing instability.
Cumulus mediocris or swelling cumulus indicate an unstable airmass. Their appearance in the late morning and early afternoon can indicate significant instability and the possibility of storms later in the day.
Cumulus congestus indicate an unstable airmass and pending thunderstorms.
image
Cumulus congestus—photo by Carptrash on Wikipedia
 
Stratus Clouds
Cirrostratus with halos indicate a lowering cloud ceiling and possible precipitation.
Altostratus indicate possible precipitation within 48 hours if the cloud cover darkens.
image
Altostratus undulatus, stratus fractus in lower left—photo by The Great Cloud Watcher

image
Altostratus translucidus—photo by Earth100 on Wikipedia
Nimbostratus are a sign of steady, dreary precipitation.
See also Wikipedia’s comprehensive list of cloud types.
Detecting Fronts
 
Warm Fronts
 
Cloud cover that darkens, beginning with cirrostratus, then altostratus and finally nimbostratus over a 24-hour period indicates an approaching warm front and and rain. Commonly, the longer the sequence of cloud formations and darkening in the sky, the longer the storm will be.
Incoming storms: Barometric pressure falls steadily. Wind from SE or NE and increases in speed. Cirrus turns to altostratus and nimbostratus. Clouds thicken. Temperature gradually increases.
Outgoing storms: Barometric pressure levels. Wind from S or NW. Nimbostratus turns to stratocumulus.
Cold Fronts
 
The rapid formation of altocumulus followed in quick succession by cumulostratus and cumulonimbus usually indicates an incoming cold front.
Incoming storms: Barometric pressure falls slowly at first, then more rapidly as the storm approaches. Wind from S or NE. Cumulus turns to cumulonimbus.
Outgoing storms: Barometric pressure rises sharply. Wind from the N or NW and gusty. Cumulonimbus break up to reveal partial clearing and intermittent storm before quick total clearing and drop in temperature.
Occluded Fronts
 
Incoming storms: Barometric pressure falls steadily. Winds from E or NE, sometimes SE. Cirrus turns to cirrostratus, then altostratus, then nimbostratus. Temperature slowly rises.
Outgoing storms: Barometric pressure rises steadily. Winds from SW or N and calming. Stratocumulus turn to altocumulus with slow clearing. Temperature slowly falls.
Using the Wind
 
Winds from the southwest to northwest ↗︎↘︎
  • If the barometric sea-level pressure is above 30.10 and holding steady, it indicates fair weather with little change for the next 24 hours.
  • If the barometric sea-level pressure is around 30.10 and rising quickly, it should remain fair for about 24 hours, but rain and a warm front are on the way within 48 hours.
  • If the barometric sea-level pressure is around 30.20 and falling slowly, then it should remain fair for the next 24 hours, and temperatures will likely rise.
Winds from the south to east ↑←
  • If the barometric sea-level pressure is around 30.20 and falling quickly, rain is likely within 24 hours, maybe as soon as the next 12 hours.
  • If the barometric sea-level pressure is around 29.80 and falling quickly, a severe weather front is moving in, followed by a drop in temperature.
Winds from southeast to northeast ↖︎↙︎
  • If the barometric sea-level pressure is around 30.10 and falling slowly, rain is likely within 12 hours.
  • If the barometric sea-level pressure is around 30.10 and falling slowly, rain and increasing wind are likely over the next 12 hours.
  • If the barometric sea-level pressure is around 30.00 and falling slowly, rain lasting up to 48 hours is likely.
  • If the barometric sea-level pressure is around 30.00 and falling quickly, rain and high winds are likely over the next 24 hours.
Winds from the east to northeast ←↙︎
  • If the barometric sea-level pressure is around 30.10 and falling slowly, rain is likely within 24 hours during winter, and during summer, rain with light winds are likely within 48 hours.
  • If the barometric sea-level pressure is around 30.10 and falling rapidly, rain with increasing winds is likely over the next 24 hours.
Winds from east to north ←↓
  • If the barometric sea-level pressure is around 29.80 and falling rapidly, severe weather with heavy rain and high winds is likely.
Winds blowing toward the west ←
  • If the barometric sea-level pressure is around 29.80 and rising rapidly, colder temperatures and clearing skies are likely.

SOURCES

  • Buckley, Hopkins, Whitaker. Weather: A Visual Guide.
  • Falcon Guides. Weather Forecasting.

This post is intended for personal reference and education only, and large portions of it are lifted directly from the second source. It was reorganized and some common information (such as cloud genus and species names) was added for clarity and ease of access. Copyright infringement is not intended.

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