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U.S. Week-2 Hazards Outlook - Made January 19, 2021 | About the Hazards Outlook
| Composite Map Day 8-14 |
Experimental Probabilistic Outlooks |
 |
Valid Wednesday January 27, 2021 to Tuesday February 02, 2021
US Hazards OutlookNWS Climate Prediction Center College Park MD
300 PM EST January 19 2021
Synopsis: Mid-level low pressure is forecast west of the Rockies at the start of the forecast period, which is anticipated to shift west-northwestward with time. This evolution would result in some potential for cold over parts of the West early in the period that times off as Pacific flow increases, while the onshore flow could yield precipitation concerns over Northern California and the Pacific Northwest. A windstorm may be in progress over the Bering Sea near the beginning of the period that may track southeastward to the North Pacific. Strong northerly winds on the backside of this feature could introduce cold-related concerns over portions of southwestern Alaska.
Hazards
- Slight risk of much below-normal temperatures for most areas west of the Rocky Mountains, Wed, Jan 27.
- Slight risk of heavy precipitation for portions of the Pacific Northwest, Northern California, and Nevada, Wed-Tue, Jan 27-Feb 2.
- Slight risk of high winds for portions of the Pacific Northwest, Northern California, and Nevada, Wed-Tue, Jan 27-Feb 2.
- Slight risk of much below-normal temperatures across the Aleutians and parts of southwestern mainland Alaska, Wed-Tue, Jan 27-Feb 2.
- Slight risk of high winds for portions of the Aleutians and southwestern mainland Alaska. Wed-Sun, Jan 27-31.
For Friday January 22 - Tuesday January 26: WPC Days 3-7 U.S. Hazards
For Wednesday January 27 - Tuesday February 02: Anomalous 500-hPa troughing is forecast to build over the North Pacific during Week-2, tied to the phasing of a shortwave digging south from the Bering Sea and another stationary shortwave off the California coastline. This is likely to limit cold hazard potential over the CONUS due to increased flow of mild Pacific air. Nevertheless, some cold may linger early in the period across areas west of the Rockies, where a slight risk for much below-normal temperatures is indicated on the 27th in areas that have at least a 20% chance of dropping below freezing before the anticipated moderation of temperatures. As Pacific flow increases to moderate temperatures, there also comes increased atmospheric river potential. A slight risk for heavy precipitation is posted across portions of Northern California, Nevada, and the Pacific Northwest for the duration of Week-2 where model guidance exhibits the most consistent atmospheric river signals. This precipitation is likely to be predominantly rain aside from high elevations, with anticipated temperatures being close to climatology for the event. Due to the lack of anomalous cold, no accompanying heavy snow hazard is introduced. As always, the greatest concerns lie with any heavy rain falling on burn scars from the antecedent fire season that could result in landslide or flash flooding concerns. A slight risk for high winds throughout the period stretching from Northern California through the Pacific Northwest accompanies this atmospheric river threat.
Model guidance indicates a wind storm may be over the Bering Sea at the start of the forecast period, resulting in a slight risk of high winds over the Aleutians and portions of the southwestern mainland extending from January 27th through the end of the month. Coastal flooding concerns from this system appear to be limited. As this system crosses from the Bering Sea to the Gulf of Alaska, strong northerly flow is likely to drive cold-air advection across the Aleutians and southwestern mainland Alaska. A slight risk of much below-normal temperatures is posted across region stretching from the Aleutians through approximately King Salmon and Unalakleet, where air temperatures could reach -35 degrees F. Of secondary concern is increasing easterly flow out of the Yukon into mainland Alaska, where there is a 20% chance that parts of the interior see temperatures of -40 or below. These values are marginal in terms of Alaskan hazards criteria, but will need to be monitored in subsequent outlooks.
Much to the chagrin of cold and snow lovers in the East, the North Atlantic Oscillation (NAO) pattern that has been persistently in its negative phase since late December appears primed to continue its failure of yielding sensible weather impacts. As noted earlier, cold air over the Yukon is likely to encroach on Alaska, rather than descend over the Lower 48 states. Model guidance remains on board with this likely evolution in terms of the CONUS experiencing generally near- to above-normal temperatures. Model guidance is also finally trending toward the NAO- weakening with time, suggesting that many of the big cities up and down the Mid-Atlantic and Northeast may miss out on this typically favorable setup for heavy snow potential. Some glimmers of hope for snow lovers can be found in today's 0Z deterministic runs of the Canadian (coastal low off the Carolinas at 240 hours) and ECMWF model (positively tilted-trough over the Ohio Valley at 240 hours). The latest ECMWF ensembles support the 10-hPa westerlies to recover through March and be near climatological values by mid-February. A handful of GEFS and ECMWF ensemble members produce a third stratospheric warming event in early February, but these tend to be an outlier. Even if this warming were to occur, composites of weak polar vortex events reveal insignificant temperature responses across the CONUS and Alaska which underscores the frailness of causality hypotheses related to weakening of the polar vortex and North American sensible weather.
Forecaster: Daniel Harnos
$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
Resources
Week-2 Probabilistic Extremes Tool
GFS Ensemble Forecasts