Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Jan 30 2025 Valid 12Z Sun Feb 02 2025 - 12Z Thu Feb 06 2025 ...Prolonged atmospheric river event expected for the Pacific Northwest this weekend into next week... ...Overview... Guidance shows fairly zonal mean flow aloft across much of the continental U.S. during Sunday-Thursday. Cyclonic flow over Canada will help to push cold air into the northern Rockies/Plains and temperatures will be below normal along the West Coast, but otherwise the low-amplitude flow and potential influence of a Bahamas/Cuba upper ridge on the far southern tier at times will support above normal temperatures over a majority of the country. Meanwhile a slow moving upper level low drifting near/offshore Vancouver Island and the Pacific Northwest along with the fast flow/shortwaves to the south will sustain a prolonged atmospheric river event across northern California and vicinity with potential for heavy rainfall. Moisture streaming inland will produce snow over the northern half of the Rockies. Farther east, southern Canada/northern tier low pressure should spread an area of snow across the Upper Great Lakes and Northeast Sunday into Monday and a lingering front with embedded developing wave in the East may produce another area of precipitation around Wednesday-Thursday. ...Guidance/Predictability Assessment... Models and ensembles continue to show fairly good agreement for the large scale pattern but differ with various details. Of particular interest are the specifics of the upper low and troughing to the south (affecting the exact focus of the West Coast atmospheric river) plus eventual ejection of some overall trough energy, and the relative influence of Canadian cyclonic flow versus lower latitude ridging (determining the position of the surface front over the eastern half of the country). There are also some lingering differences with low pressure tracking across the northern tier/southern Canada around the start of next week. Latest spread for the northern tier/southern Canada system recommends an intermediate solution at this time. Latest machine learning (ML) models are generally in the faster half of the dynamical guidance spread and their average depth is a little weaker than the dynamical runs by early Monday. Lower predictability shortwave details will still take some time to be resolved. ML models support the majority guidance cluster that has the West Coast upper low eventually drifting a little offshore Vancouver Island and the Pacific Northwest, possibly even getting a little farther west than consensus by next Thursday. Among latest solutions, the 12Z UKMET becomes farthest south by Wednesday while the CMC has been most inconsistent--bringing it more inland than other guidance in the 00Z run (though now nearly matched by the new 12Z ECMWF) but holding it farthest offshore in the new 12Z run. Prefer an emphasis on recent GFS/ECMWF runs and their means overall, though with some aspects of other guidance as well for QPF purposes. Farther east, trends over the past couple days have generally been toward a little more influence of lower latitude upper ridging which holds the eastern U.S. surface front somewhat farther northward by next Tuesday. GFS runs have still been leaning on the southern side of the spread for the front at that time but ML models have reflected the trends of most other guidance. On the other hand, CMC runs have been a northern extreme. Toward next Thursday most guidance is signaling that ejecting trough energy upstream should develop a surface wave whose track could be anywhere between the Ohio Valley/Mid-Atlantic and southern Canada. An operational model/ensemble mean blend provides a reasonable starting point for depicting this potential system. ...Weather/Hazards Highlights... The most impactful weather event during this forecast period will be the prolonged atmospheric river that will bring extended periods of moderate to heavy rainfall for northern into central California. An anomalous moisture flux oriented nearly orthogonal to the terrain ahead of a slow moving front will likely support multi-inch rainfall totals during the Day 4 period Sunday, meriting a Slight Risk area in the Excessive Rainfall Outlook over parts of northern California from the coast to the foothills of the northern Sierra Nevada. This pattern continues into Monday (Day 5) over a similar or slightly farther south latitude. After Monday, additional heavy rainfall is likely across the state as the moisture moves very slowly southward but with the focus possibly becoming a little broader/less focused. Guidance continues to show meaningful variations in the exact location of best moisture focus, so continue to monitor forecasts for adjustments in rainfall and ERO products. As for other aspects of the forecast over the West, a broader area of rain and higher elevation snow will extend from the Pacific Northwest into the northern half of the Rockies, with the north- central Intermountain West and Rockies likely seeing the heaviest snow. Early next week the Pacific Northwest may be cold enough to support some snow reaching down to fairly low elevations. Over California, snow levels will initially be rather high but they should fall a little towards midweek with heavy snow becoming more likely for the higher terrain of the Sierra Nevada. The overall pattern may produce areas of strong winds over some areas from the West Coast through the Rockies. There will be two systems of note over the East. Southern Canada/northern tier low pressure will bring an area of light to moderate snow from the northern Great Lakes through the Northeast Sunday into early Monday. The trailing cold front will still over the eastern half of the country by Tuesday-Wednesday, with a potential embedded wave developing and lifting northeastward by Thursday. These features may support an expanding area of precipitation of varying types by Wednesday-Thursday. Best potential for some wintry weather will be over the Northeast, while rainfall of varying intensity will be possible farther south. The primary focus for cold weather during the period will be over the northern tier, especially Montana and the Dakotas. Coldest anomalies of 20-35 degrees below normal are likely during the first half of the week. The Northwest will be below normal as well, but with less extreme anomalies, while the Northeast should start out chilly early Sunday but with near to above normal readings thereafter. 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. Rausch/Hamrick 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