Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Wednesday November 06, 2024 to Tuesday November 12, 2024

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT October 29 2024

Synopsis: A frontal boundary near the middle of the contiguous United States (CONUS) may serve as a focus for heavy precipitation early in week-2 from the Southern Great Plains through much of the Mississippi Valley before drifting eastward and weakening. Cold surface high pressure in the north-central CONUS is forecast to interact with a mid-level low developing in the Southwest to create the potential for significant snowfall over parts of the central and southern Rockies and the adjacent High Plains. The pressure gradient between the surface high pressure and lower pressures farther east along the front could produce high winds over parts of the eastern Rockies and Plains. Meanwhile, a potent storm system potentially merging with a tropical cyclone may bring heavy precipitation and high winds to the southern tier of Alaska, with windy conditions possibly extending westward across the Aleutians.

Hazards

• Slight risk of heavy precipitation in portions of the Mississippi Valley, lower Ohio Valley, and Great Plains Wed-Thu, Nov 6-7.
• Slight risk of high winds for portions of the eastern Rockies and Great Plains, Wed, Nov 6.
• Slight risk of heavy snow for portions of the central and southern Rockies and adjacent High Plains Wed-Thu, Nov 6-7.
• Slight risk of heavy precipitation in south-central and southeastern Alaska, Wed-Fri, Nov 6-8.
• Slight risk of high winds for southern Alaska, including southwestern Alaska and the Aleutians, Wed-Fri, Nov 6-8.

Detailed Summary

For Friday November 01 - Tuesday November 05: WPC Days 3-7 U.S. Hazards

For Wednesday November 06 - Tuesday November 12: As week-2 begins, a mid-level trough should be draped across the Rockies with mid-level ridging farther east over the eastern CONUS. Early in week-2, the trough is expected to lift eastward and de-amplify. This set-up should establish a significant frontal boundary over or near the Plains which could serve as a focus for heavy precipitation. This storminess should be ongoing as week-2 begins, with activity slowly diminishing as the mid-level pattern de-amplifies and the frontal boundary weakens during the first two days of the period. As opposed to yesterday, most models now bring a second piece of vorticity southward into the western CONUS trough that cuts off a mid-level low over the Southwest. This is the favored solution today, which should slow down the frontal system somewhat relative to yesterday’s forecast and prolong the threat for heavy precipitation. The Probabilistic Extremes Tools (PETs) for the European ensembles show a better than 40 percent chance for precipitation amounts exceeding the 85th percentile early in week-2 in Texas and adjacent New Mexico, while the GEFS and Canadian ensemble PETs are less robust. But the operational European and Canadian models, and to a lesser extent the GFS, show bands of heavy precipitation - considerably more than shown by the ensemble means - persisting over parts of the region into mid-week. As a result, the potential for absolute totals exceeding an inch are only depicted in the European ensemble PET with relatively low chances, but one to several inches is shown in smaller parts of the region in the operational models. For these reasons, a slight risk of heavy precipitation is posted, Nov 6-7. High winds are also possible on the backside of the frontal boundary, supporting a slight risk for high winds for portions of the eastern Rockies and The Plains for the first day of week-2 (Nov 6).

More of the tools now show a strong mid-level vorticity maximum dropping into the Southwest behind the one lifting into the Plains early week-2. The GFS, operational European, and operational Canadian models all show some reflection of this feature, with the GFS less amplified than the others. The European ensemble mean shows a relatively weak reflection of this feature while the GEFS and Canadian ensemble means do not show a cut-off mid-level low. This implies that, while the development of a cut-off mid-level low seems more likely than it did yesterday, there is still considerable uncertainty. But even with a somewhat weaker scenario than shown in the operational models, this would serve to draw moisture westward from the frontal boundary into an area where high surface pressure to the north is bringing in colder air. This set-up is favorable for moderate to heavy snow over portions of the central and southern Rockies and adjacent High Plains. The operational European and Canadian models show small areas where over a foot of snow is possible, but the operational 6z GFS brings in little or no snowfall despite it showing some reflection of a cut-off mid-level low in the Southwest. Meanwhile, the GEFS PET focuses the best odds for heavy snow a little farther east extending into the High Plains as far east as the Texas and Oklahoma Panhandles. Given all the uncertainty, a slight risk for heavy snowfall is posted for areas at risk according to either the GEFS PET or operational European and Canadian models, extending from southern Wyoming through most of Colorado and eastern New Mexico, as far west as eastern Arizona and Utah, and as far east as the southwestern Great Plains for Nov 6-7.

Strong mid-level low pressure and associated surface storm system is expected over or near the Aleutians at the start of week-2. This system could be enhanced as the mid-level low merges with the remnants of Typhoon Kong-Rey following extratropical transition. The 6z and 12z operational GFS runs start week-2 with a 950- to 955-hPa surface low over the central Aleutians. The operational European and Canadian models are a bit weaker with this system, with pressures bottoming out in the low 970’s and tracking a little farther away from the southern Alaskan coast. In any case, this set-up has the potential to bring unusually stormy weather into the southern tier of Alaska. Heavy precipitation would cover south-central and southeastern Alaska near and ahead of the surface low pressure center, while high winds could extend farther west through much of the Aleutians, especially if the central pressure drops as low as the operational GFS shows. But due to the uncertainties in the development and track of the storm system, including its potential interaction with the remnants of Typhoon Kong-Rey, only slight risks for heavy precipitation and high winds are posted Nov 6-8. With most models showing a slower-moving system than yesterday, this hazard has been extended for an additional day relative to yesterday’s forecast. In addition, there are signs that this storm could be followed by additional storm systems, which would extend the risk for an even longer period.

Very dry conditions have prevailed across the southeastern CONUS, the mid-Atlantic region, and portions of the Northeast for the past few weeks. In Philadelphia, PA, October 29 is the 31st consecutive day without measurable precipitation, besting the prior record of 29 days established in 1874. Signs of surface moisture stress have been increasing, as reflected in the U.S. Drought Monitor, but rapid-onset drought is not anticipated due to autumnal temperatures and lower moisture demand compared to summer.

A tropical cyclone may form in the eastern North Pacific late week-1. This system is not expected to become a strong hurricane, but it has the potential to bring some gusty winds, high surf, and heavy precipitation to portions of Hawaii if it tracks near the state. No hazards are posted in association with this system, but it will continue to be monitored.

Forecaster: Rich Tinker

$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts