Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 PM EST Fri Jan 31 2025 Valid 12Z Mon Feb 03 2025 - 12Z Fri Feb 07 2025 ...Prolonged atmospheric river event will last into next week for California... ...Overview... Quasi-zonal flow is forecast across much of the lower 48 next week. Subtropical ridging near the Bahamas/Cuba will promote above normal temperatures across a majority of the country, while broad cyclonic flow allows Arctic air to ooze southward across the northern tier. Meanwhile, a slow moving upper low drifting near/offshore Vancouver Island and the Pacific Northwest will sustain a prolonged atmospheric river across northern California Monday into Tuesday before gradually drifting south later in the week. Snow is likely farther inland along with the potential for significant snowfall accumulations, particularly across the higher elevations. Persistent moisture will likely lead to additional rounds of precipitation for much of the Intermountain West through most of next week. Farther east, moisture return and a strengthening low pressure system will lead to increasing precipitation chances across the eastern third of the Continental U.S. mid to lake week. ...Guidance/Predictability Assessment... The synoptic evolution from Monday into Tuesday remains in good model agreement amongst the latest guidance, with deterministic solutions having a solid handle on the ongoing pattern. As such, a general model blend featuring the 00Z ECMWF/GFS/CMC was favored with more weight placed on the ECMWF and the GFS. From Wednesday onward, the aforementioned deterministic blend shifted to a blend that was heavily weighted towards the 00Z EPS and 06Z GEFS ensemble means and the 00Z ECMWF given stark differences in timing and strength of synoptic features, particularly across the western half of the Continental U.S. The 00Z Canadian and especially the 06Z GFS was given minimal consideration, mainly due to how those runs handled the evolution of the upper low off the Pacific Northwest, phasing with a mid-latitude wave farther west and developing a much stronger surface low off the coast of California later in the period. That being said, a subset of the latest machine learning (ML) models at least hinted at this evolution being a possible outcome. So uncertainty remains high and a more stable solution anchored by the ensemble means and the similar deterministic run of the ECWMF is a good medium ground solution for now. Across the East, model spread regarding the position of a strengthening low pressure system continues to shrink, with most guidance showing a location across the east-central Great Lakes by Day 6 on Thursday. The remaining uncertainty with this system is now mostly confined to the strength of the low pressure, with the latest 12Z UKMET now sub-990mb, the ECMWF and CMC sub-1000mb, and the the GFS and ICON were weaker and above 1000mb. ...Weather/Hazards Highlights... A strong and long-lasting atmospheric river taking aim at northern California will be ongoing as the period begins Monday. After a few very wet days in a row, multi-inch rainfall totals will again be a concern. As a result, a Slight Risk remains delineated in the Day 4 Excessive Rainfall Outlook from the coast into the Sierra Nevada foothills. Depending on how much rain falls in previous days in similar locations, this may be considered a higher-end Slight Risk and future upgrades are not out of the question. By Tuesday, the atmospheric river finally starts to shift southward and thus the heaviest rain will follow suit across California. The Day 5/Tuesday ERO maintains a Marginal Risk across central California given a focus for heavier rains there during this period. While that aspect of the forecast hasn't changed, did decide to nudge the Marginal Risk farther south along the southern California coast, which now includes the northwestern Los Angeles suburbs. Given a multi-model signal for heavier rains over sensitive terrain from recent burn scars, confidence was high enough to make this change on Day 5. It's also possible that an expansion even farther south to include all of Los Angeles is warranted with future updates. Snow will also be likely across much of the Intermountain West with this system. Early next week, heaviest snow amounts are forecast across the Sierra Nevada (as snow levels come down after the weekend) into the northern Rockies, while the northern Great Basin also receives snow. Some snow should extend into the Wasatch and central Rockies for midweek and beyond while some moderate snows last in the Sierra. The pattern will also promote strong winds especially on mountain ridges like the Sierra and central Rockies, but some gusty conditions may reach lower elevations of the Great Basin and Four Corners too. Meanwhile farther east, some lingering rain/snow is possible across the Interior Northeast on Monday as a frontal system exits. Then moisture is forecast to return for Tuesday and especially Wednesday-Thursday in the east-central U.S. as strengthening low pressure system and its attendant warm front lifts northward. As this system evolves, an expanding area of precipitation will develop, particularly across the Ohio Valley where there's a growing signal for widespread moderate to perhaps heavy rainfall. The best potential for some wintry weather will be over the Northeast where the cold air hangs on the longest. Precipitation potential then shifts south and eastward on Friday as a cold front pushes into the region. Colder than average temperatures are forecast for northwestern and north-central parts of the lower 48 next week. Montana to the Dakotas will be particularly cold, with anomalies of 20-35 degrees below normal and highs barely reaching 0F in some locations early in the week. These areas will likely remain below average but with some moderation later next week. Meanwhile, other areas from the Four Corners states through the East should see above to well above normal temperatures during the period. Some areas will likely see one or more days of min/max temperatures reaching 20 or more degrees above normal and some daily records will be possible next week. A cold front coming through the central U.S. will cause temperatures to moderate there by late week. Miller/Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw