Winter Weather Criteria

Yesterday I posted the criteria for frost and freeze advisories and warnings, now that our growing season is officially over I figured I would post the criteria for winter storm advisories and warnings as the ‘s’ word moves into the forecast.

For watches and warnings:

Blizzard

Visibility frequently below 1/4 mile in falling or blowing snow, and wind speeds (sustained or frequent gusts) at least 35 mph. These conditions are expected to continue for 3 hours or longer.

Winter Storm

Generally defined as a snow accumulation of at least…

• • 8 inches or more in 12+ hours
  • 6 inches or more in 6 to 9 hours

Can be in combination with one of the following: rain, freezing rain, sleet, wind, blowing snow or cold (wind chill).(Note: lesser amount of snow can be forecast for mixed precipitation, blowing, etc.)

Lake Effect Snow

A pure lake effect snow event that is forecast to produce a snow accumulation of at least …

• • • 8 inches or more in 12+ hours
    • 6 inches or more in 6 to 9 hours

(Note: lesser amount of snow can be forecast for mixed precipitation, blowing, etc.)

Ice Storm

A significant, and possibly damaging, accumulation of ice. Normally a coating of at least 1/4-inch, but may be up to 1/2-inch if winds are less than 15 mph.

For warnings only:

Wind Chill

Wind chill values fall to -25°F or colder.

For advisories:

Winter Weather Advisories for…

• Snow

Synoptic scale storm producing snow (average forecast range) of greater than 3 inches in 6 hours, or at least 4 inches but less than warning snowfall.

(Note: lesser amount of snow can be forecast for first snowfall, long duration between snow, etc.)

• Snow and Blowing/Drifting Snow

Situations that cause significant inconveniences, and do not meet warning criteria. Sustained wind or frequent gusts of 25 to 34 mph accompanied by falling and blowing snow, occasionally reducing visibility to less than 1 mile for three hours or more. If caution is not exercised, this could lead to life-threatening situations.

• Winter Weather

A synoptic scale storm producing a combination of winter weather that presents a hazard, and does not meet warning criteria. Snowfall in this case does not have to reach snow advisory criteria.

• Blowing Snow

Blowing snow reducing visibilities to 1 mile or less and winds less than 35 mph.

• Sleet Advisory

Situations that cause significant inconveniences, and do not meet warning criteria. If caution is not used, this could lead to life-threatening situations. Ice accumulation causes driving or walking problems, but no damage to trees or power lines.

• Freezing Rain or Freezing Drizzle

Situations that cause significant inconveniences, and do not meet warning criteria. If caution is not used, this could lead to life-threatening situations. Ice accumulation causes driving or walking problems, but no damage to trees or power lines.

• Lake Effect Snow

Pure lake effect snow event that is forecast to produce snow (average of forecast range) of greater than 3 inches in 6 hours, or at least 4 inches but less than warning snowfall.

• Wind Chill

Wind chill values drop between -15 and -24°F inclusive.

---

NOAA has released its winter weather predictions for the upcoming winter – it is posted here.  They are guessing a warmer and drier winter.  Still not sure about that myself as the variables of the Arctic Oscillation, El Nino formation, eastward movement of tropical storms in the Pacific and north Atlantic oscillation.  A lot of things have to fall in place plus we have Lake Michigan which changes the forecast for the lakeshore counties.  Our atmosphere is a vast ocean of varying currents.  Warmer/drier predictions doesn’t mean we won’t have snow they are just saying we will be below the seasonal norms.

We also have analog forecasting which, simply put, involves examining a day’s forecast scenario and remembering a day in the past when the weather scenario looked very similar.  Other methods include persistent trends and numerical weather prediction.  I will get into these methods in a later post.

---

We will see rain move in to the area later today with maybe a tenth of an inch accumulation through the evening.  Saturday will bring a stronger cold front from the Hudson Bay area – this will produce rain with graupel, small hail and maybe some snow mixed in.  Not expecting much though – rainfall is forecast at another tenth of an inch or so.  This could make the Michigan/Michigan Sate game interesting though….

---

[xyz-ihs snippet=”Seven-Day-Forecast”]