Weather Discussion for 21 Jan 2005 1700GMT/1100CST

... Winter storm warning tonight through Saturday evening ...

Today. Cloudy with a chance of flurries. Steadier snow likely toward evening. Highs in the middle 20s. Northeast winds 10 to 15 mph becoming southeast. Chance of snow increasing to 70 percent.

Tonight. Snow. Heavy at times. Accumulating in excess of 6 inches by morning. Lows in the lower 20s. Southeast winds 10 to 15 mph becoming northeast.

Saturday. Occasional snow. Becoming windy with considerable blowing snow. Additional accumulations of 1 to 2 inches. Highs in the lower 20s. Northeast winds becoming north and increasing to 15 to 25 mph with higher gusts.

Saturday night. Mostly cloudy. Snow showers continuing. Especially near lake michigan. Considerable blowing snow. Total snow accumulation 6 to 10 inches. With locally heavier amounts near lake michigan. Lows 10 to 15. North winds 15 to 25 mph with higher gusts.

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Now the odd part of the forecast is for tonight. “Accumulating in excess of 6 inches by morning” ... which is open-ended.
Saturday sees 1-2“ more...and by Saturday night the forecast says ”Total snow accumulation 6 to 10 inches.“ ... now if we’re getting 6+ inches tonight...

The non-public discussions from the Chicago NWS office include the following:

(translated from NWS acronyms/abbreviations into English)

HAVE DECIDED TO GO WITH WINTER STORM WARNING AS OPPOSED TO HEAVY SNOW WARNING DUE
TO LIKELIHOOD THAT BLOWING SNOW MAY BECOME DOMINANT HAZARD ON SAT
WITH HEAVIER SNOWFALL OFF TO E OF REGION...EXCEPT ALONG LAKE MICHIGAN WHERE LAKE
ENHANCEMENT IS EXPECTED TO PROLONG OCCASIONAL HEAVY SNOW SHOWERS.


Oh great...so it *should* be a Heavy Snow Warning...but due to the extended hazard they went with the Winter Storm Warning. Interesting. I’m semi-surprised that it wasn’t a Heavy Snow Warning for tonight and then a Winter Storm Warning for tomorrow/tomorrow evening.

From the NWS Weather Glossary:
Heavy Snow
This generally means...

* snowfall accumulating to 4” or more in depth in 12 hours or less; or
* snowfall accumulating to 6“ or more in depth in 24 hours or less.

In forecasts, snowfall amounts are expressed as a range of values, e.g., ”8 to 12 inches.“ However, in heavy snow situations where there is considerable uncertainty concerning the range of values, more appropriate phrases are used, such as ”...up to 12 inches...“ or alternatively ”...8 inches or more...“.

From the Chicago NWS site:

Winter Weather Definitions:

Winter Storm Watch - Conditions are favorable for hazardous winter weather conditions including heavy snow, blizzard conditions, or significant accumulations of freezing rain or sleet. These watches are usually issued 12 to 36 hours in advance of the event.

Winter Storm Warning - Hazardous winter weather conditions that pose a threat to life and/or property are occurring, imminent or likely. The generic term, winter storm warning, is used for a combination of two or more of the following winter weather events; heavy snow, freezing rain, sleet and strong winds. The following event-specific warnings are issued for a single weather hazard:

* Blizzard Warning - Sustained winds or frequent gusts of 35 mph or greater, considerable falling and/or blowing snow
reducing visibility frequently to 1/4 mile or less for a period of three hours or more. There are no temperature criteria
in the definition of a blizzard but freezing temperatures and 35 mph winds will create single digit wind chills.

* Heavy Snow Warning - Snowfall of 6 inches or more in 12 hours or less, or 8 inches or more in 24 hours or less.

* Lake Effect Snow Warning - Lake effect snowfall of 6 inches or more in 12 hours or less, or 8 inches or more in 24 hours
or less.

* Ice Storm Warning - Accumulations of 1/4 inch or more of freezing rain.

* Heavy Sleet Warning - Accumulations of 1/2 inch or more of sleet.

Winter Weather Advisory - Hazardous winter weather conditions are occurring, imminent or likely. Conditions will cause a significant inconvenience and if caution is not exercised, will result in a potential threat to life and/or property. The generic term, winter weather advisory, is used for a combination of two or more of thefollowing events; snow, freezing rain or drizzle, sleet, blowing snow.

Wind Chill Warning - Extreme wind chills of -30F or colder.

Wind Chill Advisory - Dangerous wind chills of -20F to -30F.

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I guess a Winter Storm Warning is the way to go...but I don’t like it. Winter Storm Warning to me is too vague. Most people hear it and think ”lotsa snow“ but this time it’ll be the wind that really comes into play. Though, in their defense, the new NWS public forecast is highlighting the blowing snow threat tomorrow.

I am still perplexed/confused about one part. Tonight’s forecast calls for 6+ inches of snow. 1-2” tomorrow and more snow Saturday night (no amounts given), but Saturday has a Total Snow Accumulation of 6-10“. Well duh. We’re supposed to get more than 6 tonight and 1-2” tomorrow. That would be at least 8“ to me...possibly more. Seems like it’s a silly total accumulation number. Shouldn’t it get 8-12” or (this is just me looking at the conditions at the surface and aloft) 10-14“.

Well, there’s a long intro post for the weather group!