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U.S. Week-2 Hazards Outlook - Made October 15, 2021 | About the Hazards Outlook
| Composite Map Day 8-14 |
Experimental Probabilistic Outlooks |
 |
Valid Saturday October 23, 2021 to Friday October 29, 2021
US Hazards OutlookNWS Climate Prediction Center College Park MD
300 PM EDT October 15 2021
Synopsis: Mid-level low pressure with associated onshore flow is forecast to affect parts of the West Coast of the contiguous U.S. with periods of precipitation, potentially heavy, and enhanced winds during late October. Surface low pressure associated with the mid-level low pressure is anticipated to be centered over the Gulf of Alaska. However, the orientation of the flow pattern is anticipated to keep the heaviest precipitation south of Alaska. Therefore, hazardous weather is not expected across Alaska at this time.
Hazards
- Moderate risk of heavy precipitation for parts of Oregon and northern California, Sat-Mon, Oct 23-25.
- Slight risk of heavy precipitation for parts of the western U.S., Sat-Fri, Oct 23-29.
- Slight risk of high winds for parts of the western U.S., Sat-Mon, Oct 23-25.
For Monday October 18 - Friday October 22: WPC Days 3-7 U.S. Hazards
For Saturday October 23 - Friday October 29: Ensemble means from multiple models continue to depict an amplified, persistent 500-hPa negatively tilted trough axis across the Northeast Pacific and West Coast of the CONUS during late October. This longwave trough favors an increased chance of atmospheric rivers and heavy precipitation affecting California and the Pacific Northwest. The reforecast model tool guidance remains consistent regarding at least a 20 percent chance of 3-day precipitation amounts exceeding the 85th percentile of the climatological distribution and one inch throughout week-2 for much of the Pacific Northwest and California. Therefore, a slight risk of heavy precipitation is maintained for the region, Oct 23 to 29. A moderate risk of heavy precipitation is maintained across parts of northern California and Oregon from Oct 23 to 25, where the GEFS and ECMWF reforecast tools depict at least a 40% chance of 3-day precipitation amounts exceeding the 85th percentile and 2 inches, liquid equivalent. The GEFS and ECMWF Integrated Vapor Transport (IVT) indicates the greatest likelihood for enhanced values (>250 kg/ms) across the West Coast from the beginning to middle of week-2. Heavy rainfall may trigger flooding and debris flows, especially over burn scarred areas of the West. However, the early season precipitation would be beneficial to drought-stricken areas of the West. According to the U.S. Drought Monitor valid on Oct 12, more than a third of California is designated with severe (D2) to exceptional (D4) drought. No heavy snow hazards are posted due to the expectation that significant snow accumulations are forecast to be limited to the highest elevations of the Cascades and Sierra Nevada Mountains. The amplified trough and increased chance of atmospheric rivers also support a slight risk of high winds for much of the West. Both the reforecast tools indicate the greatest high wind potential at the beginning of the period. Therefore, a slight risk of high winds is posted across parts of the western U.S. based on the anticipated synoptic flow as well as guidance from the deterministic model guidance and reforecast tools.
There is good multi-model agreement indicating the potential for a mid-level shortwave trough forming over the Central CONUS by the middle of week-2, increasing chances for frontal activity in that region. This feature combined with possible strong onshore flow associated with surface high pressure over the southeastern CONUS may increase the likelihood for enhanced rainfall across parts of the Lower Mississippi Valley and eastern Texas. The reforecast tools indicate at least a 20% chance of 3-day rainfall totals exceeding the 85th percentile and one inch across the aforementioned regions Oct 25 to 27. However, recent uncalibrated and deterministic model guidance indicate that disturbances would be relatively progressive and do not support the addition of a related heavy precipitation hazard. The synoptic pattern may also increase chances for high winds across parts of south-central CONUS. The reforecast tools indicate at least a 20% chance of wind speeds exceeding the 85th percentile across the southern and central Plains Oct 25 to 27, but the tools do not indicate wind speeds exceeding 25 mph. Additionally, recent deterministic GFS model runs do not favor wind speeds exceeding 25 mph, precluding a high wind hazard from being posted at this time.
Model guidance continues to indicate a potential amplifying mid-level trough across the eastern third of the CONUS from the end of week-1 into the beginning of week-2, which could support increased chances of the first frost of the season throughout the Corn Belt at the beginning of the period, which would be slightly late for much of that region. The ECMWF reforecast tool depicts a greater spatial coverage of at least a 20% chance of minimum temperatures falling below the 15th percentile and freezing, across many parts of the northeastern quadrant of the CONUS Oct 23 to 25, before shifting more to the Northeast and Appalachians Oct 26. The GEFS reforecast tool, however, limits the greater than 20% chance of exceeding these thresholds to the Corn Belt on Oct 23 to 24. Due to significant model differences indicating the potential for much below normal temperatures, in addition to uncalibrated model guidance and week-2 guidance tools not supporting significantly cold temperatures, and negative anomalies being low in magnitude, a related risk is not designated at this time.
Downstream of a 500-hPa trough, above normal precipitation is only slightly favored across parts of southeastern Alaska and the Alaska Panhandle. However, reforecast tools are unsupportive of precipitation amounts reaching hazard thresholds, as the orientation of the associated surface low over Gulf of Alaska favors the majority of the strong onshore flow further south, primarily to the West Coast of the CONUS.
Forecaster: Melissa Ou
$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
Resources
Week-2 Probabilistic Extremes Tool
GFS Ensemble Forecasts