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U.S. Week-2 Hazards Outlook - Made November 30, 2021 | About the Hazards Outlook
| Composite Map Day 8-14 |
Experimental Probabilistic Outlooks |
 |
Valid Wednesday December 08, 2021 to Tuesday December 14, 2021
US Hazards OutlookNWS Climate Prediction Center College Park MD
300 PM EST November 30 2021
Synopsis: Mid-level low pressure is forecast to move across Alaska early in week-2, resulting in a period of below normal temperatures across the western Mainland. This feature is likely to result in enhanced Pacific flow across the Northwest contiguous U.S., bringing rounds of heavy precipitation (coastal rain and high elevation snow) to parts of the Pacific Northwest and Northern California. There is considerable uncertainty in the dynamical guidance in terms of the mid-level height pattern evolution across the eastern U.S. The 0z ECMWF ensemble depicts stronger and more progressive mid-level low pressure moving through the East compared to the 0z and 6z GEFS ensembles. While the ECMWF solution would favor a colder and drier pattern across portions of the Midwest and Northeast, the slower GEFS supports a frontal system moving through the eastern CONUS early in the period increasing precipitation chances.
Hazards
- Slight risk of heavy precipitation across parts of the Pacific Northwest and Northern California, Wed-Fri, Dec 8-10.
- Slight risk of heavy snow across parts of the Northern Rockies and Northern Great Basin, Wed-Thu, Dec 8-9.
- Slight risk of much below normal temperatures across much of western Mainland Alaska and the Alaska Peninsula, Wed-Thu, Dec 8-9.
For Friday December 03 - Tuesday December 07: WPC Days 3-7 U.S. Hazards
For Wednesday December 08 - Tuesday December 14: Longwave troughing is forecast across north-central Canada and the southern Arctic during week-2 associated with negative 500-hPa height anomalies over most of Canada and extending through the North Atlantic. Further south across the CONUS, the mid-level height pattern is predicted to be more tranquil, with near to above normal heights generally forecast for most areas for the period as a whole, especially over the Southeast. The greatest uncertainty is related to the propagation of a shortwave trough out of the mean longwave trough over Canada early in week-2. The 0z ECMWF ensemble is fairly robust with this potential, showing an area of negative 500-hPa height anomalies shifting from the Upper Midwest late in week-1 to the Northeast by day-9 (12/9). The ECMWF solution would also result in a period of anomalously cold temperatures across the Great Lakes, Ohio Valley, and Northeast on the backside of the trough. Conversely, the 0z and 6z GEFS are weaker and slower with this shortwave feature, but still indicate the potential for a cold front to push through the eastern U.S. early in week-2. The GEFS reforecast tool indicates a broad area of the East with probabilities exceeding 20% for 3-day precipitation exceeding the 85th percentile, and some of this would likely be snow across parts of the Great Lakes and New England. The ECMWF reforecast tool does not depict these enhanced precipitation probabilities, likely due to its more progressive solution with any precipitation impacts more likely to occur prior to the start of week-2, and only lingering chances existing across the Northeast on day-8 (12/8). Although the GEFS indicates some anomalously cold temperatures across portions of the Upper Midwest and Great Lakes behind the front, the Eastern Seaboard is spared from these colder temperatures. Due to the large amount of uncertainty between the guidance, no related temperature or precipitation hazards are highlighted across the East in today’s outlook.
There is greater model consensus upstream, where troughing is forecast to move across Alaska and the Northeast Pacific, and eventually into the West Coast of the CONUS toward the middle of the period. This trough supports enhanced moisture flow over the northwestern CONUS early in week-2, with the GEFS and ECMWF ensembles indicating elevated probabilities (40-60%) of integrated vapor transport exceeding 250 kg/m/s. This would favor enhanced chances of heavy precipitation across the Pacific Northwest and northern California, supporting a slight risk of heavy precipitation over these areas from 12/8 to 12/10. The GEFS and ECMWF reforecast tools indicate at least a 20% chance of 3-day liquid equivalent totals exceeding the 85h percentile and 2 inches over parts of the highlighted region. Heavy rainfall is anticipated for lower elevation areas, whereas higher elevation areas may receive heavy snow. As the trough moves closer to the West Coast, surface low development over the Rockies may increase chances for heavy snow over the higher elevations of the Northern Great Basin and Northern Rockies, and a corresponding slight risk of heavy snow is issued for 12/8 and 12/9.
As the aforementioned troughing moves across Alaska early in the period, enhanced northerly mid-level flow on its backside is likely to result in a period of below normal temperatures over western Mainland and the Alaska Peninsula, with minimum temperature anomalies reaching -10 deg F in some areas according to the uncalibrated ECMWF and GEFS guidance. The 0z and 6z deterministic GFS runs are particularly aggressive with the cold depicting some areas having temperatures fall below -20 deg F. Therefore, a slight risk of much below normal temperatures is maintained across portions of western Mainland Alaska and the Alaska Peninsula for 12/8 and 12/9. The GEFS is more amplified with ridging over the Aleutians and southwestern Mainland compared to the ECMWF ensemble, and would favor a moderation in temperatures over the western Mainland by the middle of week-2. The ECMWF maintains higher probabilities of below normal temperatures throughout the period. Anomalously cold air temperatures are also forecast to develop over the Panhandle later in week-2 as the downstream troughing continues to progress to the east.
Forecaster: Thomas Collow
$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
Resources
Week-2 Probabilistic Extremes Tool
GFS Ensemble Forecasts