Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Feb 2 2025 Valid 12Z Wed Feb 05 2025 - 12Z Sun Feb 09 2025 ...Another round of potentially heavy precipitation for California around Thursday... ...Overview... Latest models and ensembles continue to show nearly zonal flow across most of the lower 48 mid-late week, followed by a trend toward broad mean troughing from northeastern Canada into the western U.S. next 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 around Wednesday-Thursday and then another next 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. Individual waves will briefly pull well above normal temperatures somewhat farther northward, especially the Wednesday-Thursday one. 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 around midweek. ...Guidance/Predictability Assessment... Most guidance continues to show good agreement for the large scale pattern evolution through the period. However there are ongoing meaningful differences for the eastern U.S. system Wednesday- Thursday while solutions are still working out specifics of the system forecast to reach California around Thursday. As eastern Pacific/West Coast dynamics progress inland late week through the weekend and broad troughing takes shape over the West by next weekend, typical differences arise for eastern U.S. low pressure by next Sunday while some spread develops for the character of flow over the Northwest U.S. and western Canada. The Wednesday-Thursday system features two persistent clusters, one consisting of the weak/suppressed GFS and multiple runs of most machine learning (ML) models and the other composed of ECMWF/UKMET/CMC runs and to a fair degree the ICON. These differences are a result of fairly modest differences in strength of the upper level shortwave. Continued favoring of the weak side of the spread in the ML guidance leads to keeping the forecast close to continuity by way of a compromise that reflects a somewhat more inland wave than the GFS but with a depth weaker than the other models. Guidance continues to improve its clustering for the system tracking into California around Thursday, though with some continued differences for track/depth/timing. The 06Z GFS looked a little fast/weak while latest ECMWF runs are a bit on the slow side. The 12Z UKMET/CMC are deepest. An intermediate solution looks good at this time. As this system continues eastward, the ensemble means achieve remarkably good agreement near the East Coast by next Sunday. Most operational models are fairly close as well but the ML models show a lot more spread, suggesting potential for more divergence in future model runs. The faster trend in the 12Z GFS is an initial example of this. There was minimal support for the stronger ECMWF/UKMET, with the new 12Z ECMWF adjusting weaker. The 00Z ECMWF/CMC suggested potential for some westward elongation of upper flow over/offshore southwestern Canada by next Sunday. Most ML models did not support that idea and the new 12Z ECMWF/CMC have reverted back to more northwesterly flow as recommended by most other models/means. Preferences in the early-middle part of the period led to starting the forecast with 60 percent GFS (split between the 00Z and 06Z runs) with the remainder comprising 20 percent each of the 00Z ECMWF and CMC. The forecast next weekend incorporated 30-50 percent of the ensemble means (06Z GEFS/00Z ECens) while removing the 00Z GFS (strayed slow with the western trough). Throughout the forecast this approach kept significant systems close to continuity while awaiting any more pronounced trends. ...Weather/Hazards Highlights... The long-lasting atmospheric river (AR) taking aim at California will finally weaken and move south by Wednesday, leading to reduced rain amounts. Light to moderate showers could linger over southern California, but these look to stay under any flooding thresholds even for the sensitive burn scars in that region. Thus the Day 4/Wednesday ERO does not depict any risk area, but will continue to monitor guidance for any trends. Meanwhile lower elevation rain/higher elevation snow could reach Oregon into northern California Wednesday and increase in magnitude in California on Thursday as another AR strengthens with the approach/arrival of low pressure. This AR event should be shorter-lived than the near term/short range AR, but some locally heavy rain amounts are possible over areas that will have quite wet antecedent conditions by that time. Latest guidance provides good support for a Day 5/Thursday Marginal Risk area very close to that proposed in the previous cycle for northern/central California. Guidance still suggests that faster movement of the AR should temper amounts some, precluding 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. 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. The heaviest snow amounts are likely in higher elevations, but the northern Great Basin may see snow as well. The Thursday AR may produce a better defined period of enhanced snowfall in the Sierra Nevada. 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, significant moisture (with precipitable water values at least 2-3 standard deviations above climatology) will be present by Wednesday-Thursday in the east-central U.S. as a developing frontal wave should reach the Ohio Valley/Great Lakes by early Thursday. An expanding area of precipitation will develop, particularly across or somewhat north or south of the Ohio Valley. There continues to be some threat for flooding somewhere within the Ohio Valley/central or south-central Appalachians as instability could reach sufficiently far north. Also, depending on surface wave strength (especially on the weaker side of the spread) there could be strong westerly flow at low levels just above the surface to promote some training and terrain enhancement. There is still significant spread for surface details and QPF placement with this system. However it seems that enough guidance clustering exists (including recent ML model runs and first-guess ERO fields) over areas of wet antecedent conditions to favor the introduction of a Marginal Risk area centered mainly over West Virginia and the eastern two-thirds of Kentucky in the Day 4/Wednesday ERO. This Wednesday-Thursday system will also bring the potential for wintry weather to the Northeast and northern Mid-Atlantic. A transition zone of sleet and/or freezing rain may exist between the snow over far northern areas and the rain to the south. Energy ejecting from the West next weekend will likely produce another surface system reaching the East by early next Sunday, spreading snow across northern latitudes and rain farther south. As with the prior system, 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. Montana to the Dakotas will be particularly cold, with anomalies of 20-35 degrees below normal and highs barely reaching 0F in some locations into Wednesday. These areas will likely remain below average but with some moderation later this week. Meanwhile, other 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 over south-central areas) and some daily records are likely to be set. Latest forecasts continue to show the most numerous daily records around Wednesday- Thursday. Some slight cooling is possible into Friday-Saturday but temperatures should remain well above average across the southern tier. Upper troughing and a cold front will likely finally cool the Four Corners to parts of the south-central U.S. by next Sunday. 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