Preliminary Forecasts
Issued by NWS
Issued by NWS
695 FXUS02 KWNH 120714 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 212 AM EST Wed Feb 12 2025 Valid 12Z Sat Feb 15 2025 - 12Z Wed Feb 19 2025 ...Heavy rain and flooding threat in the Tennessee/Ohio Valley and nearby areas especially on Saturday along with another round of snow from the Midwest to Northeast this weekend... ...Overview... The upper trough associated with the wet/snowy western U.S. system late this week should emerge into the central U.S. this weekend and then lift rapidly northeastward. This trough and developing low pressure will spread a broad area of heavy rain from near the south-central Mississippi Valley through the Tennessee Valley into the central Appalachians by Saturday, with the low continuing through the Northeast into the Canadian Maritimes Sunday-Monday. Areas from the 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 first one, confining the rain and higher elevation snow more to the Northwest during the weekend. During the early to middle part of next week, this system will likely spread some rain and mountain snow southeastward into the central Rockies before expanding precipitation across the central/southern Plains and points eastward. Wintry weather will be possible in the northern part of the moisture shield heading into midweek. The Plains will see the most pronounced and persistent cold anomalies during the period and the northern half of the East will also see a significantly colder trend Monday-Wednesday after a brief warmer period ahead of this weekend`s storm. Mean flow from the Pacific will tend to favor moderate temperatures over the West with anomalies in the single digits on either side of normal. ...Guidance/Predictability Assessment... As has been the case lately, most guidance agrees well for the large scale pattern evolution through the period but there are some notable guidance differences for various details. Dynamical/machine learning guidance through the 12Z/18Z cycles favored an operational composite consisting of more 18Z GFS/12Z ECMWF relative to the 12Z CMC/UKMET early-mid period, followed by incorporating some of the 18Z GEFS/12Z ECens means (reaching 50 percent total weight by next Wednesday) while switching the ECMWF component to the old 00Z/11 run by Wednesday. The developing system forecast to reach the upper Ohio Valley/central Appalachians by around early Sunday is exhibiting continued strength/detail differences in response to low- predictability specifics of shortwave dynamics aloft. Recent stronger trends in the GFS have brought it closer to the ECMWF/UKMET (with the ICON/CMC weaker), while machine learning (ML) guidance has so far been running weaker and a bit flatter/slower with the surface low as of 12Z Sunday. However there is an improving signal toward strong development as the system continues northeastward, most likely reaching the 970s mb as it tracks a little east of Maine by early Monday. An average of dynamical guidance provides a reasonable starting point with good continuity. There are no pronounced differences with the system reaching the Northeast by Sunday-Monday. Some typical spread develops as the upper trough continues into the western/central U.S. early-mid week. The 18Z GFS, 12Z ECMWF/CMC, and the ensemble means offered a good clustering into Wednesday while the 12Z GFS was slow. ML models show some variance with the timing and shape of this trough, so some changes are certainly possible in future runs. It will also be worth paying attention to the elongated troughing/possible embedded low over the northern tier U.S./southern Canada. A decent amount of spread develops for this feature and it may ultimately have some influence on the upper trough/surface evolution to the south beyond the end of the forecast period. Finally, guidance shows another system reaching the Northwest around next Wednesday. The 12Z ECMWF/CMC closed off an upper low that results in the surface system tracking southward of other guidance. ML models strongly favored either an open upper trough or an upper low path farther north than those ECMWF/CMC runs, which would take the surface low on a path somewhere north/northwest of Vancouver Island with the trailing front reaching the Pacific Northwest. Thus preference went with the 18Z GFS and old 00Z/11 ECMWF plus the ensemble means. The new 00Z ECMWF/CMC runs appear to have adjusted in that direction. ...Weather/Hazards Highlights... The upper trough emerging from the Rockies at the start of the weekend will support surface low development over the eastern half of the country during the weekend. Guidance has been fairly agreeable and consistent in depicting the potential for a broad axis of heavy rainfall from over/near Arkansas 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 be sensitive due to prior significant rainfall. In the Day 4/Saturday Excessive Rainfall Outlook, a broad Slight Risk area continues to extend from the ArkLaTex region to the central Appalachians. While there is still some uncertainty in the exact location of the max rainfall axis, overall guidance continuity and excessive rainfall first-guess field trends from the past day now offer support for an embedded Moderate Risk area covering portions of Kentucky and Tennessee. There will likely be some nudges to this area based on future guidance runs but overall it signifies the relatively greater potential for flooding issues over this region. The surrounding Marginal Risk area 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. The Day 5/Sunday ERO currently depicts no risk areas over the East as remaining rainfall becomes more progressive. There may still be some locations with runoff issues depending on rainfall distribution/amounts and antecedent conditions, but it may take into the shorter term to resolve the better defined areas of focus. Meanwhile, areas from the Midwest into the Northeast will see the best potential for significant snow extending through the weekend. A transition zone of wintry mix may separate the rain/snow areas. There is still some uncertainty in the precise strength and track of the surface low, affecting the extent of various precipitation types. There may be a period of strong and gusty winds over parts of the East behind this system, along with some lake effect snow. A northeastern Pacific system will likely spread rain and higher elevation snow into the Pacific Northwest on Saturday. In general expect moderate totals on Saturday and somewhat heavier activity on Sunday. Still, current guidance suggests that Sunday`s rainfall will stay below the Marginal threshold for the Day 5 ERO so no area is depicted at this time. Rain and higher elevation snow will progress east/southeast through the Rockies Sunday into the first part of the next work week while tapering off over the Pacific Northwest. Another system could begin to influence the region by next Tuesday-Wednesday with light to moderate amounts. As the upper trough crossing the West early next week heads into the central U.S. by Wednesday, expect precipitation to develop from the central/southern Plains eastward. Wintry weather will be possible in the northern part of the precipitation shield with rain over the South. The northern and central Plains will be most persistently below normal for temperatures during the period, with cold surges reaching farther south around Sunday and Wednesday. Most days will bring the potential for some highs to be 20-30F below normal. Departure of the weekend storm in the East and the trailing upper trough will bring below normal readings eastward early next week, with highs 10-25F below normal from the Great Lakes/Ohio valley into the Northeast Monday-Wednesday. Ahead of the 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 should 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. The Southwest may remain near to slightly above normal into midweek. 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 $$