Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 AM 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... Latest models and ensembles continue to show nearly zonal flow across most of the lower 48 into late week, followed by a trend toward broad mean troughing from northeastern Canada into the western U.S. over the weekend as Alaska upper ridging regains amplitude. This trend should finally lead to a drier pattern for California after a system brings another episode of significant rain and snow around Thursday. Moisture farther inland will support precipitation across the Great Basin and Rockies, with significant snow possible for higher elevations. 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 latitude and 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 again early next week. ...Guidance/Predictability Assessment... Most guidance continues to show good agreement for the large scale pattern evolution through much of the period, while models slowly come into better agreement on some smaller scale features as the period begins Thursday. The California AR on Thursday is supported by a surface low tracking from the eastern Pacific toward the state. The 12Z ECMWF was slower than consensus with this surface low, which led to less precipitation on Thursday compared to other guidance, but the newer 00Z run has trended faster. Meanwhile the other feature of note Thursday is a shortwave supporting a Great Lakes surface low or two that have shown spread and placement and depth. GFS runs have been favoring a surface low developing over the Eastern Seaboard that becomes the dominant feature, but the 00Z GFS finally became better aligned with other guidance favoring the Great Lakes lows as they track northeastward Thursday-Friday. By late week, most models show reasonable agreement for the Pacific upper trough to pivot eastward into the West and quickly into the central U.S. over the weekend. The exception was the 12Z CMC, which had its trough axis slower/farther out into the eastern Pacific, but the 00Z CMC has trended favorably toward other guidance. Surface low pressure is forecast to develop in the south- central U.S. by Saturday and move quickly northeastward. There is once again spread for the low track and GFS runs are farther east/faster than the 12Z EC, CMC, and many of the AI/ML models. The 00Z EC and CMC are still farther west than the GFS but have jumped farther north, around the Lower Great Lakes rather than the central Appalachians. So these details will take additional time to resolve. Upstream, Pacific and northwestern Canadian energies complicate the general pattern across the West into early next week as models have varying ideas on how to combine and absorb the vort maxes into the mean flow. It is difficult to determine clear outliers in this pattern, but the new 00Z GFS did seem deeper than other guidance with its West trough and also with a closed low west of Haida Gwaii early Monday. Generally preferred to use the ensemble means for an intermediate solution. The WPC forecast used a multi-model deterministic blend early in the forecast, without too much emphasis on any particular model given the various issues of each. As the period progressed, increased the proportion of ensemble means to near/over half by Days 6-7 as model spread continued to increase. ...Weather/Hazards Highlights... Another atmospheric river is forecast 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. 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 during the weekend, 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. 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 some light to moderate snow is forecast for the northern tier, but as a low pressure system organizes over the weekend, increasing precipitation will spread to the east-central and eastern U.S. again. 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. Tate/Rausch 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