Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Feb 3 2025 Valid 12Z Thu Feb 06 2025 - 12Z Mon Feb 10 2025 ...Another round of potentially heavy precipitation for California Thursday... ...Wintry weather forecast for the Northeast Thursday and again over the weekend... ...Overview... Guidance continues to show the large scale pattern evolving toward a broad mean trough from northeastern Canada through the western U.S. by the weekend and early next week, in response to a steadily amplifying Pacific through Alaska upper ridge. This evolution follows a period of flat flow extending into late this week. A system reaching California on Thursday and continuing inland thereafter will bring a final surge of rain and higher elevation show into the West before a drier trend across the region. The eastern half of the country will likely see two events with the full range of precipitation types, one lasting into Thursday and then another over the weekend. Each system may produce some areas of moderate to heavy rain and/or snow (possibly separated by a wintry mix) depending on system details. The southern tier will see the most persistent warmth during the period, though individual surface low/frontal systems will briefly pull well above normal temperatures somewhat farther northward, especially on Thursday. The evolving upper pattern and Canadian surface high pressure should promote below normal temperatures from the Northwest into the northern Plains with the coldest anomalies centered over Montana Thursday and perhaps over a similar area again early next week. ...Guidance/Predictability Assessment... Today's models and ensembles remain fairly agreeable for the overall pattern with typical scatter for individual features and shortwave details. Latest runs have converged well enough for the initial Great Lakes/Northeast/southern Canada system and eastern Pacific low pressure moving into the west to favor a 00Z/06Z model composite for this forecast update's starting point early-mid period. From late Saturday onward, Plains through Northeast low pressure clusters decently among the models/means while shortwave details diverge somewhat within the overall Canada/western U.S. upper trough. A transition toward a model/ensemble mean mix (reaching 40 percent mean weight by next Monday) represents the most common ideas of guidance at that time. For the late week Great Lakes/Northeast/southern Canada system, guidance has favorably converged toward compromise ideas favored in recent runs. The GFS has trended northward for its wave as of early Thursday and the ECMWF cluster has trended weaker--as steadily favored by machine learning (ML) guidance, with consensus thereafter emphasizing a deepening low over southern Canada becoming the primary system. Farther west, an intermediate solution resolves ongoing moderate spread for the specifics of eastern Pacific low pressure tracking into California on Thursday. As the system tracks from the Plains through Northeast during the weekend, the average of 00Z/06Z guidance has adjusted a little faster versus the prior cycle but the 06Z/12Z GFS have adjusted slower after a questionably fast 00Z run. ML models are showing less spread versus 24 hours ago and the average depth is comparable to a dynamical model/ensemble mean compromise. Even with some detail/timing differences, the guidance clustering over the Northeast as of Sunday generally appears better than average for a Day 6 forecast. Flow behind this system is agreeable from the large scale perspective but embedded shortwave details have much lower predictability for specifics, recommending the favored model/ensemble mean blend. Teleconnections relative to the upstream Pacific/Alaska upper ridge offer good support for the general orientation and position of the Canada/western U.S. mean trough plus the flat flow forecast over the East with moderately above normal heights. ...Weather/Hazards Highlights... Expect another atmospheric river to take aim at California on Thursday, supported by troughing aloft and surface low pressure moving inland. This AR event should be shorter-lived than the current AR, but some locally heavy rain amounts are possible over areas that will have quite wet antecedent conditions by that time. A Marginal Risk still looks good for the Day 4/Thursday ERO, as faster movement of this AR should temper rain amounts some and preclude an embedded Slight Risk area for now. Thus far the guidance does not show sufficient change in specifics to merit adjustment of Marginal Risk area. Risk upgrades may still be possible as guidance better refines system details in the shorter range. Lighter precipitation should persist into Friday over California but then the pattern evolution will likely shift the best potential for organized (but fairly light) activity into the Pacific Northwest and far northern Rockies during the weekend into next Monday, while California finally dries out. Snow will also be likely from higher elevations of the Cascades and Sierra Nevada across much of the Intermountain West into the northern/central Rockies Thursday into Friday. The heaviest snow amounts are likely in higher elevations, but the northern Great Basin may see snow as well. 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, precipitation is likely in association with a low pressure system located over the Great Lakes by early Thursday. Rain is forecast across the Tennessee Valley to southern/central Appalachians, but rain totals are forecast to decrease by Thursday (compared to Wednesday) and likely stay below thresholds for any excessive rainfall area on Day 4. Will continue to monitor though as instability may still be in place for some locally heavy rain rates, as suggested by the heavier side of the model QPF envelope. Farther north, cold air in place will bring the potential for wintry weather to the Northeast. Probabilities for plowable snow/sleet are increasing across the Adirondacks into northern New England, while south of the greatest snow amounts, a transition zone of sleet and/or freezing rain is likely perhaps around the northern Mid-Atlantic to southern New York/New England. By Friday-Saturday the northern tier will likely see some light to moderate snow as western U.S. low pressure emerges into the Plains. Precipitation will spread into the East once again as this system should reach the Northeast by Sunday. The current forecast has the greatest probabilities for impactful snow across much of Pennsylvania and New York into New England during the weekend. Once again rain is likely farther south across the Ohio and Tennessee Valleys into the central Appalachians, and a band of wintry mix may separate the two primary precipitation types. Colder than average temperatures are forecast for northwestern and north-central parts of the lower 48 through the period. The cold will be especially pronounced across Montana and North Dakota, with single digit highs into Thursday. These areas will likely remain below average but with some moderation Friday into the weekend, but another surge of cold air is likely by next Monday. Meanwhile, areas from the Four Corners states through the East should see above to well above normal temperatures for most of the period. Some areas will likely see one or more days of min/max temperatures reaching 20 or more degrees above normal (perhaps even a narrow band of plus 30F or greater anomalies for morning lows around the Mid-South). These anomalies could set daily records especially on Thursday. Exact temperatures/anomalies vary somewhat at any particular place Friday-Saturday, but by Sunday-Monday there should be a more pronounced cooling trend across the Four Corners and south-central U.S. to the Mid-Atlantic behind a strong cold front. The trend toward greater coverage of below normal temperatures over the western and central U.S. by early next week is consistent with the forecast large scale pattern. Rausch/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