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U.S. Week-2 Hazards Outlook - Made February 26, 2021 | About the Hazards Outlook
| Composite Map Day 8-14 |
Experimental Probabilistic Outlooks |
 |
Valid Saturday March 06, 2021 to Friday March 12, 2021
US Hazards OutlookNWS Climate Prediction Center College Park MD
300 PM EST February 26 2021
Synopsis: Mid-level low pressure is forecast to shift east to the West Coast from March 6 to 8, likely resulting in rain and high-elevation snow across the Pacific Northwest and northern California. As this area of mid-level low pressure continues to progress eastward, there is an increasing chance of a stormy pattern developing for the north-central U.S. by mid-March. Multiple low pressure systems are forecast to track eastward across the Bering Sea which favors above normal precipitation throughout Alaska during week-2.
Hazards
- Slight risk of heavy precipitation (rain and high-elevation snow) for parts of northern California and the Sierra Nevada Mountains, Sat-Mon, Mar 6-8.
- Slight risk of heavy snow for the Sierra Nevada Mountains, Sat-Mon, Mar 6-8.
- Slight risk of heavy precipitation for parts of the Great Plains, Middle to Upper Mississippi Valley, Great Lakes, and Ohio Valley, Tue-Fri, Mar 9-12.
- Slight risk of much below normal temperatures for parts of the western U.S., Wed-Fri, Mar 10-12.
For Monday March 01 - Friday March 05: WPC Days 3-7 U.S. Hazards
For Saturday March 06 - Friday March 12: An amplified 500-hPa trough is expected to progress east to the West Coast which favors near to above normal precipitation for the Pacific Northwest and California during week-2. Based on model continuity and precipitation tools, the most likely area and timing of heavy precipitation (rain and high-elevation snow) exists across northern California and the Sierra Nevada Mountains from March 6 to 8. Therefore, a slight risk of heavy precipitation (24-hour amounts of near 1 inch, liquid equivalent) is posted for these areas early in week-2. As 500-hPa heights decrease, snow levels are likely to lower and a slight risk of heavy snow (more than a foot per day) is designated for the Sierra Nevada Mountains, also from March 6 to 8. As of February 25, snow water equivalent for this time of year are: 66% (Northern Sierra), 71% (Central Sierra), and 49% (Southern Sierra) of normal. Although no snow hazards are currently posted for the Great Basin or Rockies, a continued eastward shift of the 500-hPa would likely bring at least occasional snowfall to those areas during early to mid-March. Later in week-2 (Mar 10-12), a slight risk of much below normal temperatures is posted for areas across the West where the GEFS and ECMWF reforecast tools depict that minimum temperatures have a 20 percent chance of falling below the 15th percentile of the climatological distribution and below 40 degrees F. These tools indicate that there is at least a 20 percent chance of a frost occurring across the Central Valley of California.
As an amplified 500-hPa trough axis becomes centered over the interior West during the second week of March, lee side cyclogenesis is probable across the High Plains. During early March, the pattern favors at least several days of return flow from the Gulf of Mexico which is likely to lead to an increase in low-level moisture across the central U.S. This evolving pattern and precipitation tools support a slight risk of heavy precipitation, beginning on March 9, from the Great Plains east to the Ohio Valley and Great Lakes. This slight risk area coincides with a typical mid-latitude storm track given the longwave pattern over North America and is also consistent with the week-2 analog tool. To the northwest of any surface low track, there would be an elevated risk of snowfall across the northern/central Great Plains northeast to the Upper Mississippi Valley.
Forecast confidence for any week-2 precipitation hazards along the East Coast remains low due to poor model consistency and agreement. The deterministic GFS model and many of its ensemble members are trending towards a closed 500-hPa low developing over the Southeast by March 6 with a slow-moving surface low near the Mid-Atlantic. However, there is little to no support from the 0Z ECMWF model as it features a progressive full-latitude trough. Due to these large model differences, no precipitation hazards are posted for the eastern U.S.
A longwave trough over the Bering Sea and multiple low pressure systems favor above normal precipitation throughout the Aleutians and Mainland Alaska during week-2. Given that temperatures are also likely to average below normal, snowfall amounts are expected to be enhanced, especially across southwestern Mainland Alaska. The surface lows, crossing the Bering Sea, are forecast to increase wind speeds along the coast of the southwestern Mainland, but wind speeds are expected to remain below hazards thresholds.
Forecaster: Brad Pugh
$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
Resources
Week-2 Probabilistic Extremes Tool
GFS Ensemble Forecasts