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help me understand winter storm warnings vs. regular forecast
February 4, 2014 7:42 AM   Subscribe

Okay so I live north of NYC and commute to work in Manhattan daily. There is currently a winter storm warning in effect for the area that suggests we may be getting 5-8 inches of snow tonight, which will turn into a gross wintry mix in the morning. However, when I just look at the regular forecast (not the winter storm warning), it says 1-3 inches tonight, and wintry mix tomorrow. I'm trying to prepare for my commute tomorrow and I am lost. How do I reconcile these two pieces of information? Is it 1-3 inches, or 5-8 inches?

Similarly, the forecast for the weekend says snow showers on Sunday, accumulation around 1-3 inches again. At the same time, people are work are talking about ANOTHER MAJOR STORM OMG that will bring another 5-8 inches that night. Again, wtf? What should I be preparing for? Why does weather.com tell me the snow won't be so bad and then the national weather service sends out advisories that make me think we're getting a Nor'easter? How am I supposed to make sense of any of this? Can anyone help me interpret what the weather is going to be? I know no one can predict the weather absolutely perfectly and accurately, but I am genuinely confused as to how there can be such a dramatic variability between advisories and forecasts. Help.
posted by thereemix to Grab Bag (8 answers total) 1 user marked this as a favorite
 
The warnings are issued by the national weather service while your local forecast is done by local weather station. They also get a lot of their data from the national weather service. I'd trust the warnings more than local.
posted by AlexiaSky at 7:55 AM on February 4


It also depends on how often things are updated and the area involved. As so much info comes from NWS sometimes it takes a little bit to update. In addition NYC may get 5 to 8 but your town may get less.
posted by AlexiaSky at 7:59 AM on February 4


Personally I like to go to the source data for all the various weather reports: NOAA

In New York City, we are expected to see 1-3 inches tonight, and 1-3 inches tomorrow. (2-6 inches total- annoying but they don't shut down the MTA or anything for that.)

Outside of NYC (about 30 mins) the numbers go up a bit. they expect 3-5inches tonight, and 1-3 inches tomorrow. (4-8 inches- possibly Metro North shuts down some lines if it's towards the higher range of that and icy, but roads still should be mostly cleared by tomorrow night)

Basically if you live north of the city, I'd expect that getting home tonight should be fine. However getting to work tomorrow is going to be a pain in the ass. I'd stock up on batteries, and food since quite frankly the wintery mix is expected to be half ice, and that's usually what takes down powerlines. On the other hand, the lows aren't super low, so things may just remain miserable and sloppy instead of actually freezing and dangerous.
posted by larthegreat at 7:59 AM on February 4 [1 favorite]


Modern weather forecasting relies on complicated numerical modeling of the upper atmosphere, and there are several models that have been developed and are in current use. Generally, the NWS runs a few of these and the forecaster will decide how to weight each one to come up with the forecast. The forecaster will have the experience to say that, under such-and-such conditions, this model tends to be too conservative, this other one tends to overestimate precipitation totals, etc. So the model results get a human touch that causes different forecasters to predict different things.

As far as I understand it, the major weather forecasting outfits (The Weather Channel and its properties, plus the others that supply forecasts to local TV stations and the like) take the NWS forecast and add their own "secret sauce" to the mix. Maybe this means weighting one model that they prefer more heavily than the others, or adding some postprocessing that the NWS hasn't done, or whatever.

If you're interested in learning about how the NWS comes to its conclusions, I really like reading the forecast discussions that are available at weather.gov (here's the one for New York City for the forecast issued at 9:34 am today). Again, stylistically these are highly dependent on who's writing them, but they usually go in to some detail about what models they've run and how the results were blended, plus what the atmosphere is actually doing to cause the weather.

Anecdotally, I've found that wunderground.com (owned by the Weather Channel so probably similar to weather.com) tends to be overly conservative with precipitation estimates, forecast.io tends to overestimate, and the NWS usually falls right in the middle.
posted by backseatpilot at 8:00 AM on February 4 [1 favorite]


Is it 1-3 inches, or 5-8 inches?

It's 1-3 inches today, and 5-8 inches total, for today and tomorrow. Weather forecasts are quite often per day, which can be confusing, since Mother Nature does not respect human timekeeping conventions.
posted by 1970s Antihero at 8:04 AM on February 4 [1 favorite]


Oh, and straight from the horse's mouth (which is difficult to directly link to due to the layout of the site...):

Advisory
(Abbrev. ADVY)- Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.

Watch
A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location, and/or timing is still uncertain. It is intended to provide enough lead time so that those who need to set their plans in motion can do so.

Warning
A warning is issued when a hazardous weather or hydrologic event is occurring, is imminent, or has a very high probability of occurring. A warning is used for conditions posing a threat to life or property.


There are specific snowfall amounts and wind speeds that the NWS uses to issue winter weather warnings, but except for Blizzard warnings they vary locally:

Blizzard Warning
Issued for winter storms with sustained or frequent winds of 35 mph or higher with considerable falling and/or blowing snow that frequently reduces visibility to 1/4 of a mile or less. These conditions are expected to prevail for a minimum of 3 hours.

Winter Storm Warning
This product is issued by the National Weather Service when a winter storm is producing or is forecast to produce heavy snow or significant ice accumulations. The criteria for this warning can vary from place to place.

Winter Storm Watch
This product is issued by the National Weather Service when there is a potential for heavy snow or significant ice accumulations, usually at least 24 to 36 hours in advance. The criteria for this watch can vary from place to place.

Winter Weather Advisory
This product is issued by the National Weather Service when a low pressure system produces a combination of winter weather (snow, freezing rain, sleet, etc.) that present a hazard, but does not meet warning criteria.

posted by backseatpilot at 8:06 AM on February 4 [2 favorites]


The previous answers have pretty much covered what's going on but I wanted to add a couple of things.

You will often see quite a variation in the snowfall forecasts with coastal storms. That's because the band of precipitation is typically narrow and there is not much distance separating the warmer, rainy part of the storm from the colder, snowy portion. Tonight's storm is expected to pass closer to the coast than yesterday's, which means more snow inland and that the city will be closer to the rain/snow line.

The local National Weather Service Forecast Office for NYC also has a very good Facebook page, where they often post informal updates and discussion about their forecasts.

If you read through the NWS discussions you'll see that they don't yet have much confidence in what's going to happen with the storm expected this weekend. As mentioned above, forecasters use several models to shape their predictions. The models differ in that some are meant to look at larger scale features, while others provide more detail, some are better at short-term prediction, while others are more appropriate for the longer-term. A forecaster has a lot more confidence in a prediction when the models agree, and less confidence when the model output is scattered. The meteorologist's skill and experience come into play in the latter case, because they are familiar with the biases in the models -which models to rely on and which to downplay.

Late last week one run of one model predicted a whopper of a storm for this coming weekend (10 days in advance!). Someone, not the NWS and I don't really know who, started blathering about a major storm with ridiculous amounts of snow. That's a silly and irresponsible forecast, but it seems to have created a lot of buzz and rumors, which might be what your co-workers are responding to. As of now the NWS and the Weather Channel predict a chance of snow this weekend and only AccuWeather bumps that up to "heavy snow possible".
posted by plastic_animals at 11:06 AM on February 4


Thanks so much to everyone who answered! I've learned several new things today, thanks to you all. :)



For anyone who lives in the NYC Metro/Hudson Valley area who finds this later, one of my colleagues just introduced me to hudsonvalleyweather.com, which is an awesome website run by a HV weather-wonk who provides really detailed analyses of weather patterns using four different models (including the European model, which is apparently touted as the most accurate frequently). In the past few days I've found that this fellow's predictions are dead-nuts perfect for any weather happenings in the area. He will be my go-to source for the weather from now on.
posted by thereemix at 7:39 AM on February 13


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