Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Tue Feb 11 2025 Valid 12Z Fri Feb 14 2025 - 12Z Tue Feb 18 2025 ...Rainfall trending lighter for coastal California by Friday while heavy heavy snow continues from the Sierra Nevada inland... ...Another round of snow from the Midwest to Northeast Friday- Sunday with heavy rain likely in the Tennessee Valley and nearby areas especially on Saturday... ...Overview... The very wet and snowy system reaching the West Coast on Thursday will initially spread rain and heavy mountain snow over the West on Friday. Then expect a broad area of heavy rain to develop from near the south-central Mississippi Valley through the Tennessee Valley into central Appalachians by Saturday as developing low pressure tracks northeastward, with the low continuing through the Northeast into the Canadian Maritimes Sunday-Monday. Areas from the Upper Midwest through the Northeast will see the best potential for significant wintry weather with this system. The next Pacific system will track farther north than the leading one, confining the rain and higher elevation snow more to the Northwest during the weekend. This system should spread some rain and mountain snow southeastward into the central Rockies by early next week. The northern-central Plains will see the most pronounced and persistent cold anomalies, with a modified form of this cold reaching the East early next week. Increasing Pacific influence should help the West to moderate from its chilly temperatures leading into Friday. The Southeast will tend to see above normal temperatures during the weekend, with some of this warmth briefly extending northward. ...Guidance/Predictability Assessment... Broadly speaking, latest guidance has displayed better than average agreement and continuity for most aspects of the forecast, but as usual there are some embedded detail uncertainties along with one solution or another differing from consensus at times. Of particular interest is the eastern U.S. system during the weekend. A multi-run average of the ECMWF/ECens mean have been particularly stable with an early Sunday surface low position over the central Appalachians/northern Mid-Atlantic, with other solutions waffling somewhat north or south depending on exact details of upper dynamics. Thereafter, most models show much stronger development (even if not quite to the degree of the ECMWF) than recent GFS runs as the system continues into the Canadian Maritimes. This may be related to the GFS being a little slower to pull an initial Hudson Bay/southeastern Canada upper low eastward. Machine learning (ML) models are generally not quite as deep as the dynamical model average with this system Sunday-Monday but can sometimes be a little slow to reflect a deeper consensus a few days out in time. A model/ensemble mean blend looks good for the next upper trough reaching the West early next week, with reasonable agreement from the ML models. The 18Z GFS got questionably sharp with its shortwave by next Tuesday. The updated forecast started with more 18Z GFS/12Z ECMWF emphasis relative to the 12Z UKMET/CMC early-mid period, reflecting consensus/continuity for significant features. Then by Monday- Tuesday the blend incorporated 30-40 percent total weight of the 18Z GEFS/12Z ECens means while switching GFS input from the 18Z run to 12Z run. ...Weather/Hazards Highlights... As of early Friday, upper troughing will be starting into move into the West while the associated surface front passes through far southern California. The Day 4 Excessive Rainfall Outlook maintains a Marginal Risk area along the far southern coast, with lingering moisture along the front early in the day and the upper trough providing some possible instability over burn scar areas that would be most sensitive to additional showers. Continued progression of this system will spread areas of rain and heavy mountain snow farther inland, eastward to the Rockies. The upper trough will likely support surface low development over the eastern half of the country during the weekend. Guidance has been fairly agreeable and consistent in depicting potential for a broad axis of heavy rainfall from the south-central Mississippi Valley northeastward into the central Appalachians on Saturday, with anomalous moisture representing a combination of Pacific and Gulf origins. Some instability should help to enhance rain rates over at least southern areas as well. In addition, a large portion of this region will have already experienced heavy rainfall in the shorter term. Reflecting the combination of forecast rainfall and likely wet ground conditions, the Day 5/Saturday ERO starts out with a broad Slight Risk area from the south-central Mississippi Valley into the central Appalachians. A decent portion of this area would be considered a "higher end" Slight Risk at this time, with close evaluation for potential upgrade at some point depending on future guidance behavior/clustering. The surrounding Marginal Risk extends into the northern Mid-Atlantic where guidance depicts less heavy but still significant rainfall in combination with what will likely be wet ground from short-term snow and rain. There is a modest signal that activity could already get locally heavy over parts of the Mississippi Valley late in the Day 4 time frame but agreement is not good enough yet for a risk area then. Meanwhile, areas from the Midwest into the Northeast will see the best potential for significant snow Friday-Sunday. A transition zone of wintry mix may separate the rain/snow areas. There is still some uncertainty in the precise strength of the surface low, affecting the extent of various precipitation types. There may be a period of gusty winds over parts of the East behind this system. A northeastern Pacific system will likely spread rain and higher elevation snow into the Pacific Northwest on Saturday. There is some uncertainty over initial intensity, though overall expect moderate totals. Sunday's precipitation will likely be somewhat heavier while moisture begins to spread farther eastward into the northern Rockies. Rain and higher elevation snow will progress east/southeast through the Rockies early next week while tapering off over the Pacific Northwest. The eastern periphery of another system could begin to influence the region by next Tuesday but with low confidence in details at this time. The northern and central Plains will be most persistently below normal for temperatures during the period. Coldest anomalies up to 25-30F below normal for highs are possible over the northern High Plains on Friday and expanding southward during the weekend. Departure of the weekend storm in the East and the trailing upper trough will bring below normal readings eastward early next week, with highs at least 10-20F below normal over the Great Lakes/Ohio valley into the Northeast. Ahead of this storm, expect a brief surge of warmer air over the East Saturday into Sunday. Some highs over the Florida Peninsula could challenge daily records, especially on Saturday. The Northwest may stay a little below normal for most of the period, but the remainder of the West should trend toward normal by Sunday and reach modestly above normal next Monday. A cold front will likely confine the warmth to far southern areas by Tuesday (and extending into southern Texas). 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