US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
344 PM EST Thu Dec 05 2019
 
Valid Sunday December 08 2019 - Thursday December 12 2019
 
Hazards:
- Heavy precipitation across much of the Washington Cascades, Wed-Thu, Dec 11-Dec 12.
- Heavy rain across portions of the Olympic Peninsula, Tue-Wed, Dec 10-Dec 11.
- Heavy rain across portions of coastal Maine, down across southern New England into the northern 
Mid-Atlantic, as well as parts of the interior Southeast, Sun-Mon, Dec 8-Dec 9.
- Heavy snow across portions of the higher elevations of the northern and central Rockies, Sun, Dec 
8.
- Heavy snow across portions of the central Great Lakes into northern Wisconsin, Mon, Dec 9.
- Heavy snow across the central Great Lakes, Mon-Wed, Dec 9-Dec 11.
- Flooding occurring or imminent across portions of the Northern Plains.
- Much below normal temperatures across portions of the Great Lakes, the Upper Mississippi Valley, 
and the Northern Plains, Mon-Wed, Dec 9-Dec 11.
- Heavy precipitation across portions of southern Alaska, Sun-Tue, Dec 8-Dec 10.
- High significant wave heights for coastal portions of mainland Alaska, Sun, Dec 8.
 
Detailed Summary:

Much of the U.S. mainland should see relatively weak weather systems to start off the Day 3-7 
period (Dec 8-12).  However, a large low pressure system moving into the western U.S. during the 
weekend will likely interact with arctic air from Canada to bring a period of active weather across 
the eastern half of the country early next week.  This will be followed by frigid temperatures 
toward the middle of next week for the northern U.S.

Starting in the West, precipitation associated with the large low pressure system should bring a 
good chance of heavy snow across portions of the higher elevations of the northern and central 
Rockies on Sunday.  The remaining snows across the Sierra Nevada and Shasta ranges should be 
tapering off by Sunday evening.  Thereafter, models show very good agreement that an outbreak of 
arctic air from Canada will interact with a low pressure system in the central Plains on Monday.  
The low pressure system is expected to intensify and track across the Great Lakes Monday night.  
There are notable model differences regarding the intensity and speed of motion of the low center.  
Nevertheless, a swath of wintry precipitation can be expected across the northern Plains to the 
Great Lakes on Monday, with the upper Midwest to the central Great Lakes region most likely seeing 
the best chance of heavy snow.  Arctic air will then surge into the Plains and the Great Lakes 
behind the departing storm.  This will promote lake-effect snows to linger over much of the Great 
Lakes into Tuesday, likely into Wednesday for the upper Great Lakes.

Ahead of the low pressure system, mild air will overspread much the eastern U.S. Monday and 
Tuesday.  Rainfall amounts of 1 to 2 inches are forecast for the interior portion of the Southeast, 
including the southern Appalachians on Monday into early Tuesday ahead of a strong cold front.  
Similar rainfall amounts are also forecast from the northern Mid-Atlantic to coastal Maine during 
this period.

The arctic air behind the low pressure system will bring sub-zero high temperatures down from 
Canada.  But the much colder than normal conditions are forecast to be confined across the northern 
Plains into the upper Midwest, and for the upper to central Great Lakes early to middle of next 
week.  By next Thursday, the GFS shows a reinforcing shot of arctic air from Canada.  The ECMWF, 
however, disagrees with this scenario.  The WPC forecast for next Thursday is based mainly on the 
ECMWF solution.

In Alaska, a highly amplified synoptic pattern will bring moisture all the way up from the 
sub-tropics toward southern Alaska early next week.  Very heavy precipitation together with winds 
occasionally reaching storm-force are expected to impact the southern coast of Alaska into the 
mountainous areas just inland ahead of a low pressure center tracking up a sharp front.  The main 
threat will be across the Chugach mountains, Kenai Peninsula and Alaska range this weekend into 
early next week, with heavy snow across higher elevations and rainfall of potentially several 
inches per day at lower elevations.  Another significant low pressure system will likely approach 
the eastern Aleutians with increasing winds and rain by the middle of next week.

Kong