Potentially Historic West Coast Precipitation Event

There is the potential for a historic atmospheric river (AR) precipitation event between 1 - 4 December 2012 along the U.S. West Coast. The impact guidance precipitation products associated with the top 15 most similar historic events (located here) based on the 20121128/0000F096 GFS forecast show very high chances for another extreme event (potentially more extreme) in this area.

VERIFICATION - Modest Lake-Effect Snow Event: 23-24 November 2012

A modest lake-effect snow (LES) event occurred this past weekend across the Great Lakes. The 72-h snowfall ending 25 November 2012 at 1200 UTC (image below) shows the COOP and CoCoRaHS snowfall reports in this area. For the most part amounts were relatively light, however the favored LES areas across the Upper Peninsula (UP) of Michigan, northern Lower Michigan, and southwestern New York received more than 6" of snowfall.

VERIFICATION - PAC NW Heavy Precip Event: 19-21 November 2012

A week and a half ago, a blog post that illustrated how historical events can be used in the forecast process (located here) highlighted the potential for a high-impact heavy-precipitation event for the Pacific Northwest for early Thanksgiving week. The precipitation map for the 72-h period ending 21 November 2012 at 1200 UTC shows that coastal areas of Oregon and Washington received widespread >5"(with some locations greater than 7") of precipitation.

Modest Lake-Effect Snow Event: 23-24 November 2012

A modest lake effect snow event is looking more likely after Thanksgiving across the Great Lakes. However, the mean analog pattern and impact guidance products based on the 20121120/0000 96-h GFS forecast are indicating the potential for a modest event. Here is an excerpt from last night's NWS Area Forecast Discussion from Gaylord, MI for this coming Friday.

Impact Guidance - Aviation Products

A new set of aviation products have been added to the Impact Guidance that are based on surface observations...they include:

Probability of >3 h MVFR or worse

Probability of >3 h IFR or worse

PAC NW Heavy Precip Event: 19-21 November 2012

Models are coming into agreement that a heavy precipitation episode will impact the Pacific Northwest during Thanksgiving week. This is due to a quasi-stationary midlevel trough over the Gulf of Alaska and a strong low-level impulse driven by a 70 kts low-level jet and relatively broad atmospheric river (AR).

Northern Plains Heavy Snow: 9-10 November 2012

Over the next couple days, a major winter storm will exit the northern Rockies and move into the northern Plains. Numerous winter storm watches and warnings are in effect from now through 10 November (see image below).

7-8 November 2012: Importance of Analyzing Individual Analogs

This mornings analog run of the GFS 20121105/1200F060 for the East Coast cyclone was similar to the run on this blog post. There were low probabilities for >4" of snowfall across the higher elevations of the Appalachians and interior New England. This threat is shown in the probability guidance map below.

Northern Rockies Heavy Snow: 9-10 November 2012

While the EC storm at the end of the week exits, a new one will be taking shape over the northern Rockies and into the northern Plains. The Billings, MT NWS Forecast Office highlighted the storm for the end of the week in their Area Forecast Discussion from last night.

Early EC Winter Storm: 7-8 November 2012

Models have been coming into agreement that an early-season winter storm may affect the East Coast from North Carolina to Maine in the next 4-5 days. The GFS 96-h forecast valid on Wednesday morning 7 November shows the cyclone off Cape May, New Jersey with a track north-northeast just off the East Coast.