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Categorical Outlooks Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards

Valid Thursday September 26, 2019 to Wednesday October 02, 2019

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 18 2019

Synopsis: Mid-level high pressure is favored for the eastern half of the lower 48 states and Alaska. Mid-level low pressure is predicted for the western half of the contiguous U.S., potentially bringing anomalously cool temperatures to the area. A strong storm is predicted to form over the Bering Sea at the beginning of Week-2, which may impact the west coast of mainland Alaska.

Hazards

• Slight risk of much below normal temperatures across parts of the western half of the CONUS, Fri-Mon, Sep 27-30.
• Slight risk of heavy precipitation for portions of the West Coast states, Thu-Sat, Sep 26-28.
• Slight risk of heavy precipitation across parts of the Central Plains and Upper and Middle Mississippi Valley, Thu-Mon, Sep 26-30.
• Slight risk of high winds across parts of the Great Plains, Thu-Sat, Sep 26-28.
• Slight risk of high winds across parts of the Central and Southern Plains Plains, Sat-Mon, Sep 28-30.
• Slight risk of high winds for coastal parts of western mainland Alaska, Wed-Fri, Sep 25-27.

Detailed Summary

For Saturday September 21 - Wednesday September 25: WPC Days 3-7 U.S. Hazards

For Thursday September 26 - Wednesday October 02: There are significant differences between the 6Z GEFS and 0Z ECMWF, especially during the beginning half of the period. The 6z GEFS indicates mid-level ridging building across the eastern third of the CONUS at the beginning of Week-2, whereas the 0Z ECMWF ensemble shows amplified troughing. Due to consistency in the model solutions over the past several days, the forecast favors and reflects the 0Z ECMWF ensemble solution. A slight risk of heavy precipitation is introduced today for parts of the West Coast states, Sep 26 to 28, where the ECMWF reforecast tool indicates a 20 percent chance or greater of 3-day rainfall totals reaching or exceeding the upper 85th percentile and one inch. Amplified troughing across the West in combination with the potential of moisture surges associated with Tropical Storms Mario and Lorena in the Eastern Pacific Ocean may support localized heavy rainfall across parts of the Southwest at the beginning of Week-2. Given the high model uncertainty regarding the evolution of the mid-level trough and whether these tropical storms will track northward enough to support a moisture surge, a related heavy precipitation hazard is not highlighted at this time but will be monitored in the upcoming days.

The aforementioned mid-level amplified troughing across the West is anticipated to continue throughout much of Week-2. This pattern is expected to support a slight risk of much below normal temperatures across parts of the western half of the CONUS, Sep 27 to 30. The ECMWF reforecast tool indicates a 20 percent chance or greater of some areas falling to the lowest 15th percentile and freezing. This could be the first freeze event of the fall for some areas in the West.

There is a potential for mid-level ridging to build across parts of the eastern CONUS at the beginning of Week-2. Both the ECMWF and GEFS reforecast tools show a 20 percent chance or greater for areas across the Eastern Seaboard, Southeast, and Southern Plains reaching the uppper 85th percentile. However, since the expected temperatures do not reach the NWS heat advisory criteria, additional excessive heat risk areas are not designated at this time.

Surface low pressure is anticipated to form across the Rockies and shift eastward during Week-2. The 0Z ECMWF indicates significantly stronger and widespread low pressure across the central CONUS associated the with the mid-level trough, compared to the 6Z GEFS. Despite the model disagreement, there has been good day to day model consistency in both the GEFS and ECMWF indicating increased chances for heavy preicpitation across the Mid-West during the Week-2 period. A slight risk of heavy precipitation (3-day rainfall totals reaching or exceeding the upper 85th percentile and one inch) is highlighted across parts of the Central Plains and Upper and Middle Mississippi Valley for Sep 26 to 30. The expected heavy rainfall may exacerbate or prolong flood conditions in the Missouri River basin. Parts of the Missouri River basin is already at 200 percent or greater of their normal rainfall for the last 30 days according to gauge amounts.

In addition to the forecast heavy rainfall across parts of the Mid-West, surface low pressure over the Rockies and Great Plains supports a slight risk of high winds across parts of the Great Plains, Sep 26 to 28. The slight risk of high winds is isolated to the Central and Southern Plains Sep 28 to 30. Sustained wind speeds in the highlighted areas may reach the upper 85th percentile and 20 mph.

There is good model agreement indicating the possible formation of a strong storm system in the Bering Sea and tracking eastward, approachnig the west coast of mainland Alaska by the beginning of Week-2. A slight risk area is highlighted across the west coast of mainland Alaska, Sep 25 to 27. The 6Z deterministic GFS shows the potential for some areas to experience sustained wind speeds of 35 mph or greater. This storm may also support strong southerly onshore flow to parts of the South Coast of Alaska, leading to localized heavy precipitation. However, due to the high model uncertainty regarding associated precipitation impacts, a heavy precipitation risk is not designated for Alaska at this time.

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