Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sat Mar 22 2025 Valid 12Z Tue Mar 25 2025 - 12Z Sat Mar 29 2025 ...Overview... An amplified western ridge/eastern trough upper pattern as of Tuesday will gradually progress eastward while trending flatter, as dynamics from a strong midweek upper trough (supporting a storm system off the Pacific Northwest coast) push into the western half of the lower 48. The mean ridge should reach the East by Friday or Saturday. Expect very warm temperatures under the western upper ridge through midweek with some daily records possible. Anomalous warmth will then move into the Plains, followed by moderation farther east. Upper dynamics and the leading front arriving from the Pacific will bring rain and high elevation snow to the Northwest mid-late week. A southern tier shortwave will support the threat of locally heavy rainfall over southern parts of Texas late in the week, with this moisture and additional Gulf inflow potentially combining to spread a broader area of mostly rain over parts of the Plains and east-central U.S. by the end of the week. A weak Midwest wave on Tuesday may produce a band of mostly light precipitation near its track that may extend offshore the Mid- Atlantic/New England coast by Wednesday. ...Guidance/Predictability Assessment... Most guidance is fairly agreeable with the forecast during Tuesday- Wednesday. The primary exception is the CMC that has been persistently digging eastern U.S. shortwave energy more slowly and sharply to produce a surface wave that is west of other guidance. The GFS/ECMWF/UKMET and the GEFS/ECens means offer a reasonable cluster for the upper dynamics and weak Midwest to western Atlantic surface wave. Machine learning (ML) guidance agrees with this cluster in principle as well. By Thursday guidance continues to diverge, for the storm forecast to track offshore the Pacific Northwest and amplitude of the upper trough nearing the West Coast. Both dynamical and ML models show latitude variation for the eastern Pacific storm with shifting trends, in particular the CMC adjusting northward after being on the southern side earlier versus the new 00Z ECMWF trending southward. In addition GFS runs continue to be flatter with the upper trough than most other guidance. This developing spread near the West Coast expands inland across the western half of the country during Friday-Saturday. GFS runs remain on the high side of the spread for heights aloft. Most other guidance shows better defined shortwave energy but with increasing spread for details. This spread extends to the ML models as well. Not surprisingly, GEFS ensembles generally lean toward higher heights than CMC/ECMWF members, though at least the latest GEFS means temper the questionable aspects of the operational GFS runs. This spread tempers confidence in the exact details of low pressure that should reach the central U.S. toward the end of the week. Also of note, the differences over the West lead the GFS to lean on the slower side of the spread for the southern stream shortwave forecast to reach into the southern Plains by Friday. Guidance comparisons led to favoring a 12Z/18Z model blend of the GFS/ECMWF/UKMET early in the period, followed by phasing out the GFS and steadily increasing ensemble mean input (18Z GEFS and 12Z ECens/CMCens). This led to forecast consisting of 75 percent ensemble means and 25 percent 12Z ECMWF by Day 7 Saturday. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks covering the period from Tuesday through Wednesday night currently depict no risk areas but there are a couple regions of interest for continued monitoring. During Day 4 there are some guidance signals for localized heavy rainfall potential over and west/northwest of the ArkLaTex region, focused by a couple frontal boundaries and with increasing afternoon instability. Drier than average ground conditions and small-scale nature of this activity have favored no risk are thus far, but improved clustering on the heavier side of the range could eventually merit a risk area. Then on Day 5 the Pacific system affecting the West Coast may bring a brief period of enhanced rainfall to the Pacific Northwest and northern California coast. There is continued spread with magnitude and coverage (though with most guidance suggesting higher amounts extending farther south then GFS runs) while first-guess guidance is fairly muted with its excessive rainfall potential for the time being. Again will monitor guidance for improved clustering here. Expect the southern tier shortwave nearing the southern Plains after midweek to start generating rainfall near the Rio Grande Valley on Wednesday with heavier activity likely reaching eastward more into Texas by Thursday. Precipitation should push across the West late in the week, with leading Gulf inflow helping to increase the coverage and intensity of precipitation over the Plains/east- central U.S. by Friday-Saturday. Most of this precipitation should be rain but some wintry weather will be possible near the Canadian border. Earlier in the period, a weak wave that should track from the Midwest to offshore the Mid-Atlantic/New England coast Tuesday- Wednesday may produce a band of mostly light precipitation along its path. The majority of this activity should be rain, though a little snow is possible on the northern fringe of the moisture band and in the Appalachians. The strong upper ridge over the West early-mid week will bring well above normal temperatures to the region, with a broad area of highs 10-20F above normal with locally higher anomalies. There will also be potential for numerous daily records for highs/warm lows, especially during Tuesday-Wednesday. Some places over the Desert Southwest could get close to 100 degrees. As the upper ridge shifts eastward with time, the West will cool down while already above normal anomalies over the central U.S. will increase. Locations from the northern-central Plains into Midwest should see one or two days of highs reaching 20-25F or so above normal by Thursday- Friday. The East will see increasing coverage of highs 10-20F above normal by Friday-Saturday after near to slightly below normal readings Tuesday-Thursday. 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