Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Wed Mar 5 2025 Valid 12Z Sat Mar 08 2025 - 12Z Wed Mar 12 2025 ...Overview... Guidance shows a moderately progressive pattern from the weekend into the middle of next week, with three significant systems of note. An upper low ejecting from the southern Rockies/High Plains early Saturday may produce a small area of High Plains snow and then open up and develop a surface wave along with Southeast U.S. enhanced rainfall. A trailing upper ridge building into the West should weaken as it reaches the eastern states. Then the focus will turn to a couple lows developing over the East Pacific, one nearing the West Coast next Monday (reaching the central U.S. Wednesday) and the second around Wednesday. Both will likely bring some focused rain/mountain snow to the West with the latter storm expected to bring the potential for heavier totals. Meanwhile, Great Lakes/Northeast upper troughing should flatten after Monday while a wavy mean frontal boundary over southern Canada and the Northeast may dip into the northern tier as the first Pacific system moves inland. ...Guidance/Predictability Assessment... Dynamical and machine learning (ML) guidance agree fairly well with the large-scale aspects of the pattern, but there are some medium to smaller scale details that are up for debate. Among the 00Z/06Z runs, the CMC was the slow extreme with the weekend into Monday southern tier shortwave with the ML models varied somewhat for timing but also not supporting the slow CMC. Farther west, guidance clustering is improving for the idea of amplifying eastern Pacific trough energy to close off an upper low offshore California by early Monday. However the 00Z UKMET strayed fast/open (improving in the new 12Z run) while the 06Z GFS wobbled northwest of consensus for a time (00Z/12Z runs better). The upper system should reach the Rockies/Plains by next Wednesday, with typical spread for exact timing. A model/mean blend yields a Plains/Midwest surface low depth close to the average of ML models. There is still a decent amount of spread for the potential Wednesday system near the West Coast. Some dynamical models show a rather vigorous surface low, while CMC runs are a few hours later to develop their strongest low. ML models are generally weaker than the dynamical models at the surface (but agree reasonably well for the alignment of the upper trough) with varied ideas for surface details. A model/ensemble mean blend maintains good continuity for track and an intermediate depth of the surface low while awaiting better detail agreement. Guidance comparisons led to starting with mostly 00Z/06Z GFS and 00Z ECMWF emphasis, with a little 00Z UKMET, early-mid period. The GFS component switched entirely to the 00Z run around Monday- Tuesday due to poor agreement near the West Coast in the 06Z run. The rest of the forecast incorporated some of the 06Z GEFS/00Z ECens means with the GFS/ECMWF runs. ...Weather/Hazards Highlights... The upper low forecast to eject from the southern Rockies/High Plains on Saturday may initially produce a narrow band of meaningful snow. Probabilities of 0.25 inch liquid in the form of snow are currently about 10-35 percent over northeastern New Mexico and far northwestern Texas. Then showers and thunderstorms will increase in coverage over the South as the upper system opens up and moves eastward, supporting a surface wave. The Day 4/Saturday Excessive Rainfall Outlook maintains a Marginal Risk area over parts of the Lower Mississippi Valley, Gulf Coast, and Southeast. Moisture anomalies reach up to 2-2.5 standard deviations above normal and the synoptic setup appears to support potential for some training of convection. The only adjustment in the afternoon update was to nudge the area a bit southward, per ECMWF/ECens/ICON guidance leaning on that side of the envelope. ML guidance falls between the ECMWF and farther north GFS, while support for the northern side of the spread has waned. By Day 5/Sunday, guidance remains split regarding the potential for some continuation of training/heavy rainfall over northern Florida. Prefer to maintain no risk area for now but one may be needed if future runs gravitate more to the heavier side of the current spread. Farther north, there will be strong and gusty winds lingering across the Northeast on Saturday behind eastern Canada low pressure. Waves along a front may bring one or more episodes of mostly light snow to the northern Great Lakes/Northeast from the weekend into next week. Eastern Pacific upper troughing during the weekend should initially focus an atmospheric river into Vancouver Island with some precipitation extending into northwestern Washington state. With an upper low possibly closing off near California by early Monday (with associated surface development), it may take until early next week for an atmospheric river event to bring heavier rain to portions of central and southern California, along with heavy snow for the Sierra and eventually moving inland across the higher terrain of the Intermountain West. This looks to set up a particularly windy pattern for parts of the West as well. With uncertain details, another significant system may affect the West Coast by Wednesday. Most guidance suggests that this storm could produce heavier precipitation and locally stronger winds than the first one. The southern Rockies/High Plains upper low will support well below normal highs (by 15-20F or so) underneath it on Saturday. Above average temperatures will expand southward/eastward from the northern Plains from the weekend into next week. The northern Plains should return closer to normal by next Wednesday as a front sags south from Canada. Locations from the northern-central Plains through the Midwest and eventually into the northern Mid-Atlantic should see one or more days with highs reaching 15-30F above early March averages. This warmth may push highs well into the 60s from the Dakotas into Iowa and into the 70s as far north as the Middle Mississippi Valley and parts of the Mid-Atlantic. Rausch/Campbell 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