Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Mar 10 2025 Valid 12Z Thu Mar 13 2025 - 12Z Mon Mar 17 2025 ...Potent system brings precipitation to the West into Thursday, the central U.S. Friday, and toward the east-central/eastern U.S. next weekend... ...Overview... The primary weather feature of concern later this week will be a deep and phased upper trough near the West Coast by early Thursday that closes off an embedded upper low over the Plains on Friday while spinning up a strong surface low. The upper trough will bring widespread rain and higher elevation snow across the West on Thursday. Then hazards associated with the surface low pressure system will include heavy rain and severe thunderstorms in the Mississippi Valley beginning Friday and into the Southeast and Tennessee/Lower Ohio Valleys Saturday. Convection should reach the Eastern Seaboard Sunday. Snow is possible on the backside of the low in the northern half of the Plains into the Upper Midwest Friday-Saturday. Meanwhile areas of strong winds are likely, which could cause fire danger especially in the southern Plains. Behind that system, another upper trough is forecast to approach the West this weekend and spread additional precipitation there. The leading Pacific trough crossing the West will bring below normal highs to that region while the leading ridge (and possibly a weaker one farther east) should promote above normal temperatures over the central/eastern U.S., gradually lessening in scope with time. ...Guidance/Predictability Assessment... Most dynamical guidance agrees reasonably well for the large scale pattern evolution, with modest spread for the deep Plains/Upper Midwest storm and associated upper trough along with acceptable clustering for the next upper trough nearing the West Coast by next Monday. Some guidance guidance differs more significantly for smaller scale frontal wave details over the eastern half of the country during the weekend as well as for specifics of the surface system nearing the West Coast around Sunday ahead of the second upper trough. The early-mid part of the forecast maintained good continuity by way of a 06Z/00Z model composite with more GFS/ECMWF emphasis relative to the UKMET/CMC. Days 6-7 Sunday-Monday introduced 20-40 percent total weight of the 06Z GEFS/00Z ECens means, leading to modest detail adjustments within the more persistent general evolution of forecast systems. For the central High Plains through Upper Midwest storm Friday- Saturday, guidance as a whole has been fairly steady over the past day with some lingering typical spread for depth/track/timing. As of early Friday over the High Plains, the members from most ensemble systems favor a depth in the 980s mb (some GEFS members leaning a bit deeper). Then by Saturday ensemble members concentrate in the 970s mb. Operational models generally fit within this envelope. Ahead of this system by early Saturday, there is some divergence in guidance for how quickly the front near the East Coast may return north as a warm front. In particular there are some machine learning (ML) models that suggest potential for surface ridging to be more persistent north of the front than seen in the dynamical guidance, delaying northward progress. Then by Saturday into Sunday latest guidance is showing more of a signal today that the front anchored by the central U.S. storm may contain a western Gulf Coast into Great Lakes wave. Thus far the dynamical models say that the parent low will continue northeast into Canada with the frontal wave lagging, but a number of recent ML models have been showing slow enough progression of the main low for the frontal wave to merge with it over the Great Lakes by early Sunday. Will monitor for any dynamical model trends, but for now the forecast does not account for the alternative ML model scenario. Heading into next Monday there is also spread for frontal timing and possible additional embedded waviness due to differences in how energy is distributed within the overall upper trough. This leads to significant differences for existence/magnitude of rainfall along the East Coast. Possibly due in part to that alternative ML evolution, ML frontal timing leans slower than the dynamical consensus by Monday, though the 00Z/06Z average has nudged a bit slower versus continuity. The 12Z ECMWF has trended slower as well. For the system that may approach the Pacific Northwest/Vancouver Island by next Sunday, there was still a lot of spread for system specifics through the 00Z/06Z cycle given dependence on small-scale evolution of supporting dynamics. This spread includes a broad envelope for track and depth among ensemble members and separate clusters depicted by the ML models. However there was very little support for the suppressed structure in the past couple ECMWF runs. Thus preference favored the depiction non- ECMWF models and the ensemble means, still leaning on the weaker side due to depth uncertainty (though a few mb deeper than continuity). Incoming 12Z guidance suggests growing confidence in a stronger system, with the new ECMWF now looking more similar in shape to other dynamical models. An average of guidance looks good for resolving timing/detail differences with the upper trough and associated surface reflection over the West by next Monday. ...Weather/Hazards Highlights... On Thursday, the tail end of a weak atmospheric river (with precipitable water values briefly around 2-3 standard deviations above normal very early in the day) and possible convective showers behind it could bring moderate to heavy precipitation to southern California, and a Marginal Risk remains in place in the Day 4 ERO with latest guidance maintaining support. More broadly, areas of rain and higher elevation snow are forecast across much of the West into Thursday. Higher elevation snow could be heavy across the Sierra Nevada and into the terrain of the central Great Basin and Mogollon Rim. Widespread high winds will also be a concern in the West on Thursday. Ahead of this system, an initial shortwave will produce some light to moderate rain across the Southeast on Thursday. Precipitation remains more questionable farther north, with latest guidance becoming lighter and more scattered with any Great Lakes/Ohio Valley into Northeast activity that does occur. As the primary upper trough moves into the central U.S., there is high confidence in surface low development/consolidation in the central Plains by Friday with strong fronts and northeastward progression of the system thereafter. Potential exists for this low to challenge some sea level pressure records for March from eastern Nebraska into parts of Minnesota/Wisconsin. On the backside of the low, snow could spread across the central High Plains and into the Dakotas on Friday and into the Upper Midwest on Saturday. Some of these areas may initially see rain in the warm sector, and a transition zone of ice is also possible. Best potential of meaningful snow waits until Saturday, when probabilities for 0.25 inch liquid in the form of snow maximize in the 30 to slightly more than 50 percent range over western Minnesota and eastern Dakotas. Rain and thunderstorm chances will emerge ahead of the cold front across the Mississippi Valley on Friday. The Storm Prediction Center is highlighting the best severe potential from parts of the Midwest southward almost to the west-central Gulf Coast on Friday. Most guidance (minus the GFS/GEFS) shows heavy rain rates beginning late Friday into Friday night that could cause flooding concerns. The updated Day 5 ERO for Friday maintains a Marginal Risk for the Lower/Middle Mississippi Valley and Mid- South with minimal adjustment from continuity. Rain totals are forecast to become even heavier by Saturday centered in the Tennessee Valley, and SPC indicates severe potential for the Tennessee Valley and Southeast. During Friday-Saturday SPC shows a relatively greater severe threat from parts of Missouri/Arkansas into Mississippi and Alabama. As the upper trough and surface fronts move east, rain and thunderstorms will overspread the East on Sunday. Winds are another possible concern near the deep low track, as well as in the Plains behind the cold front. Currently the guidance shows the potential for highest wind speeds over the southern Rockies/High Plains around Friday, likely producing fire weather concerns. In addition southerly winds in the warm sector could get fairly strong over/near the Appalachians and East Coast next weekend. Additional rounds of precipitation are likely in the West late this week into the weekend, with one frontal system forecast to arrive around Friday-Saturday and another storm/frontal system likely arriving by Sunday. Rain amounts in the Pacific Northwest to northern California look to stay below thresholds for any excessive rainfall risk through Friday, but chances for heavy precipitation (lower elevation rain and higher elevation snow) could increase by the weekend for the Pacific Northwest into California. The upper trough supporting the latter system should reach at least as far east as the West Coast by Monday, leading to a decrease in precipitation intensity near the coast and eastward expansion of the moisture shield over parts of the Interior West and Rockies. Above average temperatures are likely over most areas east of the Rockies to start the period. Temperature anomalies of 20 to as much as 35 degrees above normal are forecast Thursday for the northern Plains to Midwest as highs reach the 60s and 70s. Farther south, temperatures in Texas and Louisiana could reach the 80s while localized areas like far South Texas can expect 90s, even approaching 100F. Warmer than normal temperatures will gradually shrink in area late week into the weekend as possibly multiple cold fronts track east. The Midwest and Great Lakes can expect plus 20-30F anomalies on Friday but more limited to the Great Lakes on Saturday, and warmer than normal temperatures by 10-20F are likely in the Northeast over the weekend. Daily record highs look fairly isolated across the central to eastern U.S., but there could be more widespread record warm lows from the Midwest through the East Friday-Sunday if the morning mins hold for the calendar day. In the West, the upper troughing along with clouds and precipitation will promote below normal temperatures spreading across the region mid- late week. Highs are forecast to be generally below average by 15-20 degrees for the central Great Basin and Southwest, while lows of 5-15 degrees below normal are likely. Temperatures should moderate somewhat over the weekend as the trough pulls away, but another trough approaching could lead to another bout of cool temperatures there early next week. 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