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Climate Prediction Center

U.S. Hazards Outlook - Made Nov 25, 2014

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Valid Friday, November 28, 2014 to Tuesday, December 09, 2014

Summary of Forecasts and Hazards

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 25 2014

Synopsis: Onshore flow is forecast to shift south along the West Coast during the next week. A strong area of high pressure is expected to progress southeast from Canada into the Great Plains by this weekend. Meanwhile, an upper-level ridge of high pressure is forecast to build across the eastern U.S. at the beginning of December. A surface high is expected to persist across eastern Alaska, while a low pressure system tracks towards the Aleutians this weekend.

Hazards
  • Much below normal temperatures for parts of the northern Rockies and northern Great Plains, Sat-Sun, Nov 29-30.
  • Heavy snow for parts of the Northern Rockies, Fri-Sat, Nov 28-29.
  • Heavy snow for parts of the Cascades, Fri-Sat, Nov 28-29.
  • Heavy snow for the Sierras of California, Sat-Mon, Nov 29-Dec 1.
  • Heavy rain for southwest Oregon and parts of northern/central California, Sat-Mon, Nov 29-Dec 1.
  • Heavy rain for southern California, Mon-Tue, Dec 1-2.
  • Significant river flooding likely for parts of Washington.
  • Significant river flooding possible for parts of New York.
  • A slight risk of much below-normal temperatures for parts of the northern Great Plains, Wed-Thu, Dec 3-4.
  • Severe drought for the Central and Southern Great Plains, Southwest, Southeast, Tennessee Valley, Pacific Northwest, Great Basin, and California.
Detailed Summary

For Friday November 28 - Tuesday December 02: A pattern change is likely by the beginning of December as an upper-level trough develops along the West Coast and 500-hpa heights increase downstream across the southeastern conus. This pattern change is expected to bring heavy rain along coastal areas of California and heavy snow to the Sierras beginning on Saturday. Total precipitation amounts (liquid equivalent) are expected to exceed 5 inches during this period across coastal ranges of northern California and also across the northern Sierras. Although timing differences continue among the models, a shortwave trough with subtropical moisture in the southern stream is expected to bring heavy rainfall to southern California early next week.

Precipitation amounts inland across Arizona next week are less certain due to model differences on the depth and speed of the aforementioned shortwave trough, precluding additional related hazards from being determined.

Meanwhile, across the Cascades and northern Rockies, heavy snow is expected on Friday and Saturday. Snow levels could fall below to 1,500 feet or lower across the southern Washington Cascades and Columbia Gorge by Saturday. Heavy rainfall during the next few days may trigger flooding along the most flood-prone rivers along the western slopes of the northern Washington Cascades.

A strong area of high pressure is expected to shift south from Canada and bring much below-normal temperatures to parts of the northern Rockies and northern Great Plains on Saturday and Sunday. Minimum temperatures could fall below -10 degrees F across the northern high Plains this weekend.

A 1036-hpa surface high is forecast to persist over eastern Alaska through Friday before it shifts southeast. A low pressure system is expected to bring inclement weather to the Aleutians beginning on Sunday, but rainfall amounts and winds are expected to remain below hazards criteria at this time.

For Wednesday December 03 - Tuesday December 09: Model differences increase at the beginning of Week-2 regarding the depth and speed of the southern stream trough that is forecast to cross southern California and the Southwest. The high resolution 0Z ECMWF model solution, which is the slowest and more amplified than other guidance, indicates that heavy rain and high-elevation snow could continue into Week-2 for southern California and the Southwest. However, the latest GFS model runs and many of its ensemble members favor drier weather for these areas.

Multiple days of return flow from the Gulf of Mexico are expected across the east-central U.S. at the beginning of December. An increase in low-level moisture interacting with the upper-level trough moving from the western conus raises the risk of heavy rainfall across the middle/lower Mississippi and Ohio Valleys during Week-2. However, due to uncertainty on timing, a heavy rain hazard is not defined at this time.

A very strong temperature gradient is expected to develop from Canada south into the Great Plains. Since shallow, arctic air may return again spread south during the early part of Week-2, a slight risk of much below normal temperatures is posted for December 3-4 for the Northern Great Plains.

The most recent U.S. drought monitor, released on November 20, indicates a slight decrease in the areal coverage of severe to exceptional drought (D2 to D4) from 17.27 percent to 17.13 percent across the Continental U.S.

Forecaster: Brad Pugh

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Click here to see a display of the GFS Ensemble Forecasts

Please consult local NWS Forecast Offices for short range forecasts and region-specific information.


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