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Categorical Outlooks Day 3-7 Day 8-14

Experimental Probabilistic Outlooks (Information) Temperature Hazards Precipitation Hazards

Valid Sunday October 21, 2018 to Thursday November 01, 2018

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT October 18 2018

Synopsis: A front is forecast to remain stationary over the Gulf of Mexico during the 3-7 day period with surface low pressure development along this front on Oct 23 or 24. Concurrently, another cold front is expected to move across the central and eastern CONUS, while a North Pacific storm system is predicted to impact parts of the South Coast of Alaska and the northern Alaska Panhandle. Mid-level low pressure is anticipated to dominate the eastern half of the CONUS, and southwestern Alaska during Week-2, with mid-level high pressure elsewhere.

Hazards

• Heavy rain shifting east from the western Gulf Coast and lower Mississippi Valley to the Southeast, Tue-Thu, Oct 23-25.
• Much below normal temperatures across parts of the eastern U.S., Sun-Mon, Oct 21-22.
• Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sun, Oct 21.
• High winds across portions of the Alaska Panhandle and mainland Alaska, Sun, Oct 21.
• High significant wave heights for coastal portions of mainland Alaska, Sun, Oct 21.
• Flooding possible across portions of the Southern Plains.
• Flooding occurring or imminent across portions of the Southern Plains, the Middle Mississippi Valley, the Mid-Atlantic, the Upper Mississippi Valley, the Southeast, the Great Lakes, and the Ohio Valley.
• Severe drought for parts of the Northeast, the Great Plains, the western U.S., and the Alaska Panhandle.
• Moderate risk of heavy precipitation for parts of the eastern U.S., Fri-Sun, Oct 26-28.
• Slight risk of heavy precipitation for parts of the eastern and south-central CONUS, Fri-Sun, Oct 26-28.
• Slight risk of much below normal temperatures for parts of the south-central and eastern CONUS, Mon-Thu, Oct 29-Nov 1.

Detailed Summary

For Sunday October 21 - Thursday October 25: Several mid-tropospheric shortwave troughs are forecast to move through a broader longwave trough located over the northeastern quarter of the Lower 48 states. Colder air masses are also expected to move across this portion of the country, with much below normal minimum temperatures (8 degrees F or more below normal) depicted on the map on Oct 21-22. Although these predicted temperature anomalies are not that anomalous, the greater concern is for portions of this region to experience the end of their growing season with a killing frost. Frost or a light freeze could extend as far south as the southern Appalachians and adjacent Piedmont areas the Carolinas on Oct 22. Across the Great Lakes region, these colder air masses result in an increased risk of lake-effect snow bands. Most of the expected lake-effect snow will likely be limited to localized parts of western New York by this time period. This is still relatively early in the season for lake-effect snow, and amounts are forecast to be relatively light, but could make for slippery driving conditions.

As a shortwave trough approaches from the west, surface low development is forecast along a stationary front across the Gulf of Mexico. Heavy rainfall (1 to 3 inches) is expected to accompany this low pressure system when it tracks east across the northern Gulf of Mexico from October 23 to 25. Although forecast confidence remains high that this surface low cyclogenesis occurs, there is model spread on its track and how far north the heavier rainfall extends.

A North Pacific storm system is forecast to impact the South Coast of Alaska and the Panhandle region during the 3-7 day period. Dynamical models have had some difficulty in resolving whether this might be a single storm system, or as was predicted yesterday, back-to-back systems. In either case, an extended period of hazardous conditions appears likely for south-central and southeastern Alaska, and the Panhandle region. Periods of heavy precipitation (low elevation rain and mountain snow, 3 inches or more liquid equivalent), strong winds (40 knots or greater) and high significant wave heights (20-30 feet) are favored, as depicted on the map.

The U.S. Drought Monitor, valid October 16, indicates a decrease in severe to exceptional drought (D2 to D4) coverage from 14.4 to 13.51 percent during the past week. Continued improvement occurred across northeast Kansas, Missouri, Texas, and Arizona.

For Friday October 26 - Thursday November 01: The Arctic Oscillation (AO) index is forecast to become negative during late October as positive 500-hpa height anomalies develop at the higher latitudes of the Northern Hemisphere. This negative AO index would be a major change in the pattern since this index has continued to be positive throughout most of the summer and early fall. The evolving longwave pattern with a strong blocking ridge over the north Atlantic increases chances for a stormy end to October along the East Coast. The low pressure system that is expected to emerge from the Gulf Coast later next week is most likely to move north near the East Coast early in Week-2. The moderate risk of heavy precipitation (Oct 26 to 28) from the Gulf Coast north to New England is based on where the 6Z/12Z GFS ensemble mean indicates more than 0.5 inch per 24 hours. If this low pressure system strengthens rapidly, then the risk of other hazards such as high winds and coastal flooding would increase for the mid-Atlantic and Northeast. A broader slight risk area for heavy precipitation captures the uncertainty of its spatial extent.

The GFS and ECMWF ensemble means are in excellent agreement with an amplified 500-hpa trough and corresponding negative height anomalies over the central and eastern CONUS during Week-2. Periods of cold air advection are likely to maintain below normal temperatures across much of the central and eastern U.S. through at least the beginning of November. A slight risk of much below normal temperatures (Oct 29 to Nov 1) is forecast from the southern Great Plains east to the Ohio Valley and parts of the Southeast based on where the GEFS temperature tool indicates minimum temperatures having a 20 percent chance of falling below the 15th percentile and where a frost or freeze could occur.

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