Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 PM EST Mon Feb 10 2025 Valid 12Z Thu Feb 13 2025 - 12Z Mon Feb 17 2025 ...Heavy rain and flash flooding possible for coastal California and heavy snow from the Sierra Nevada inland Thursday-Friday... ...Another round of snow from the Midwest to Northeast Friday- Sunday with heavy rain likely in the Tennessee Valley again especially Saturday... ...Overview... The medium range period will feature a series of systems producing areas of significant rainfall and winter weather. The first system should linger in the Northeast on Thursday with snow for northern New England at the same time a wet/snowy system reaches the West Coast, with heaviest precipitation focused over California. Continued progression of the West Coast system's upper trough will likely support low pressure reaching the East by Sunday and the Canadian Maritimes by Monday. This low should spread wintry weather from the Upper Midwest through the Northeast, with heavy rain farther south over some areas that may be affected by prior heavy rain, like the Tennessee Valley. The next Pacific system will track farther north than the first one, confining the rain and higher elevation snow more to the Northwest from the weekend into early next week. Parts of the Plains will see the most pronounced cold anomalies, especially on Thursday and the weekend, while gradually increasing Pacific influence should help the West to moderate from its cold temperatures as of Thursday. The Southeast will tend to see above normal temperatures from late week through the weekend. ...Guidance/Predictability Assessment... Model guidance is reasonably agreeable for most large-scale systems through the period. A trough axis moving through the northern tier will support a low pressure system with good agreement over the Lower Great Lakes region at the start of the period 12Z Thursday quickly tracking across the Northeast, and a multi-model blend worked well for that. Meanwhile, upper and surface lows over the eastern Pacific Thursday will push into the West Coast by Friday, with the remaining trough moving across much of the lower 48 over the weekend-Monday. More model differences gradually arise into the East, especially with the resulting surface low. Low pressure should redevelop over the south-central Plains Saturday but as it tracks quickly northeastward, there is variability with its placement by Sunday in the East, perhaps made more complicated by the potential for a double-barrelled low. Recent models cluster a main low somewhere around the central Appalachians/Mid-Atlantic but some models remain well north of there, including the 00Z ECMWF and GFS. This low should strengthen as it moves quickly northward into eastern Canada by Monday. Upstream, the general idea of troughing moving into the Pacific Northwest by next Monday is in guidance but with differences in the strength and timing, including a slightly faster trend in the new 12Z GFS and CMC. Given the reasonably good agreement overall, the WPC forecast was based on a blend of the 00/06Z deterministic models early in the period, with gradually increasing proportions of the ensemble means to 40 percent by Day 7 to minimize differences from individual models. ...Weather/Hazards Highlights... The system tracking through the Northeast on Thursday will bring meaningful snow to northern New England, while the trailing cold front will cross the Southeast. Recent guidance runs have been narrowing the spread for this front and its associated axis of moisture. Models show some instability with this front and potential for some locally heavy rainfall, mainly over parts of northern Florida into Georgia. However this region has dry antecedent conditions with very high flash flood guidance values. Thus the Day 4/Thursday Excessive Rainfall Outlook continues to depict no risk area for this activity. Will continue to monitor for any trends toward broader organized heavy rainfall on the higher end of the envelope. With fairly strong low pressure and associated frontal system reaching the West Coast Thursday into early Friday, guidance has maintained good continuity over the past day regarding favorable moisture anomalies/enhanced IVT values focusing heaviest rainfall and higher elevation snow over California. The inherited Slight Risk area for Day 4/Thursday over a majority of coastal California and extending into the Sierra Nevada foothills still looks good. Expect coastal areas from the Bay Area south into the Los Angeles metro including recent burn scars to have relatively higher probabilities of seeing flooding impacts, and this is considered a high-end Slight Risk. Especially as shorter term guidance refines the best focus of heaviest rainfall and what the rain rates will be, it is possible an eventual upgrade to a Moderate Risk may be needed across southern California and in particular the burn scar areas. The Day 5/Friday ERO continues to depict a Marginal Risk area along the far southern coast, with lingering moisture early in the day and the upper trough passing overhead to provide 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. South-central areas that will already be sensitive due to prior heavy rainfall can expect another episode of heavy rain, particularly Saturday in the Tennessee Valley. 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 may be a period of gusty winds over parts of the East behind this system. A farther northward track of the next Pacific system will favor the Northwest for the next round of rain and higher elevation snow from the weekend into early next week. Amounts should start out on the light to moderate side on Saturday but may trend somewhat heavier on Sunday. Expect moisture to spread farther eastward/southeastward into the Rockies by Monday. Parts of the central U.S. will see well below normal temperatures for most of the period, with some central-southern Plains locations 20-30F below normal for highs on Thursday followed by another cold surge bringing similar anomalies into the far northern Plains on Friday and extending farther southward during the weekend. Progression of an upper trough into the East will spread a modified form of this cold air eastward by next Monday with most of the northeastern quadrant of the lower 48 seeing highs 10-20F below normal. Cold air will also extend into the Northwest through late week but the West as a whole will gradually trend warmer as Pacific flow gains influence. The Four Corners states and vicinity may achieve above normal highs by the start of next week. Systems forecast to reach the East around Thursday and Sunday should pull some above normal temperatures northward from the Gulf Coast/Southeast. A few locations over the Florida Peninsula could challenge record highs on one or more days. Even the Southeast could cool to a bit below normal by next Monday behind a 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