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U.S. Week-2 Hazards Outlook - Made September 22, 2020 | About the Hazards Outlook
| Composite Map Day 8-14 |
Experimental Probabilistic Outlooks |
 |
Valid Wednesday September 30, 2020 to Tuesday October 06, 2020
US Hazards OutlookNWS Climate Prediction Center College Park MD
300 PM EDT September 22 2020
Synopsis: A transient mid-level flow pattern is forecast to give way to an amplified pattern over North America prior to week-2. Mid-level high (low) pressure is forecast over the western (eastern) contiguous U.S. during the period. Mid-level and surface low pressure favors a continuation of heavy rainfall across southeastern Alaska. After a recent burst of tropical activity across the Atlantic, much quieter conditions are expected heading into week-2.
Hazards
- Moderate risk of much below normal temperatures over the Middle Mississippi Valley, the southwestern Great Lakes region, and the Ohio and Tennessee Valleys, Fri-Sun, Oct 2-4.
- Slight risk of much below normal temperatures over most of the eastern CONUS, Thu-Mon, Oct 1-5.
- Slight risk of much above normal temperatures for California and the Desert Southwest, Wed-Fri, Sep 30-Oct 2.
- Slight risk of heavy precipitation for southeastern Mainland Alaska and the Panhandle, Wed-Tue, Sep 30-Oct 6.
For Friday September 25 - Tuesday September 29: WPC Days 3-7 U.S. Hazards
For Wednesday September 30 - Tuesday October 06: The 500-hPa flow analysis over much of the Western Hemisphere is predicted to transition to a higher amplitude pattern in the few days leading up to week-2. Moderate to strong mean ridging is forecast over western North America and the west-central Atlantic Ocean, and amplified mean troughing (full field; anomalies are moderate) is predicted over Alaska and the eastern CONUS.
The mid-level ridge predicted over the western CONUS favors a slight chance of much above normal temperatures for much of California and the Desert Southwest, Sep 30-Oct 2. Daytime high temperatures are expected to approach 100 degrees F over the San Joaquin and Sacramento Valleys, and 105 to near 110 degrees F across the Desert Southwest. There are dozens of large wildfires burning across the Pacific Coast states and the Northern and Central Rockies, related to the high heat, dryness, wind, and fuel loadings. According to the USDA Forest Service, nearly ten wildland fires in California have burned 100,000 acres or more. By far, the fire that resulted in the most acreage burned to date is the “August Complex” fire in Northern California (Glenn County), which burned nearly 753,000 acres as of Sep 22, and is only 39% contained. One of the largest fires in Los Angeles County history is the "Bobcat Fire", which as of today, Sep 22, has consumed in excess of 106,000 acres and is only 17% contained. As this is the climatological peak of California’s wildfire season (September/October), little relief can be expected.
As the ridge builds across the western CONUS, a trough is expected to deepen over the eastern states, accompanied by a strong cold front. Overnight low temperatures are predicted to drop below 40 degrees F throughout most of this region and increase chances for early frost, with the probable exception of near the Gulf Coast and Southern Atlantic Coast. As a result, there is a slight chance (20%) of much below normal temperatures iposted for most of the eastern CONUS, from Oct 1-5; and a moderate chance (40%) of much below normal temperatures posted for the Middle Mississippi Valley, the southwestern Great Lakes region, and the Ohio and Tennessee Valleys, from Oct 2-4. Most of this highlighted region has not yet experienced sufficiently cold temperatures to end the growing season, so vegetation may be vulnerable to the anomalous cold forecast. Places like Mobile and Pensacola, where some residents are still without electrical power after the passage of Hurricane Sally, can expect temperatures to get down to about 40 degrees F. Climatologically, the first freeze of the season near the Gulf Coast comes in early to mid-November. There are significant differences between the GEFS and ECMWF ensembles regarding the predicted spatial coverage and magnitude of the much below normal temperatures. With the recent predicted amplification of the longwave pattern, it is thought that the dynamical models may be a bit fast in bringing the cold air into the Southeast at this time. The highlighted region represents a compromise between the various model solutions, WPC’s Day 7 temperature forecast, and timing of when this colder air mass is expected to overspread the eastern half of the CONUS.
The 500-hPa trough forecast over western and central Alaska in week-2 supports low-level onshore flow and increased storm activity across the southern Alaska coast from about the Kenai Peninsula to the southern tip of the Southeast Panhandle. Based on this circulation pattern, and the precipitation amounts from the probabilistic extremes tool (both GEFS and ECMWF), a slight risk for heavy precipitation (2-3 inches) is posted for the entirety of week-2.
Forecaster: Anthony Artusa
$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
Resources
Week-2 Probabilistic Extremes Tool
GFS Ensemble Forecasts