Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Sep 30 2023 Valid 12Z Tue Oct 03 2023 - 12Z Sat Oct 07 2023 ...Heavy rainfall potential over parts of the central U.S. next week... ...Overview... Latest guidance continues to show an amplified upper pattern next week. A mean trough over the Rockies on Tuesday should progress into the east-central U.S. by next Saturday, while an eastern Pacific ridge drifts toward the West Coast and initial eastern U.S. ridge continues onward into the Atlantic. Moist and unstable inflow just ahead of the upper trough and an associated wavy cold front will likely produce an emerging convective pattern for much of the Plains Tuesday and focused in the southern Plains by Wednesday, with some of this rainfall likely to be heavy. Expect mostly lighter rainfall to push into the eastern U.S. later in the week. Cooler than normal early-mid week temperatures over the West under the upper trough will likely moderate and push into the central U.S. as the week progresses, though with potential for a reinforcing cold front to bring cooler air east of the Rockies toward the end of the week. Warmer than normal temperatures across the northeastern quadrant of the CONUS will become more limited in area by late week while the upper ridge nearing the West Coast will bring warmer temperatures to that region. ...Guidance Evaluation/Predictability Assessment... Operational models and ensemble means agree on the general idea of gradual eastward progression of the amplified ridge-trough-ridge configuration but there are notable detail uncertainties that complicate the forecast of sensible weather over some regions. Particular issues of note involve Tuesday-Wednesday frontal wave(s) lifting from the northern Plains into Canada as shortwave energy ejects northeastward from the Rockies, what happens with energy in the southern Rockies part of the initial trough Wednesday-Thursday, and specifics of eastern Pacific-West Coast ridging/peripheral shortwave energy impacting the evolution of eastern North American troughing late next week. Fairly small-scale (thus having lower predictability) detail and timing differences are affecting the precise evolution of northern Plains into southern Canada low pressure during Tuesday-Wednesday. In guidance through the 00Z-06Z cycles, the GFS/GEFS mean were generally faster and more consolidated with the surface system (fully over southern Canada by early Wednesday) while to varying degrees the 00Z ECMWF/CMC/UKMET depicted a second trailing wave. The ECens/CMCens means hinted at the ECMWF cluster. Overall preference was to lean away from the slowest side of the spread, and new 12Z model runs strongly favor a faster trailing wave if it exists. Regarding the southern part of the upper trough, 00Z and new 12Z CMC/UKMET runs pull a notable amount of energy into the southwestern U.S., in contrast to ECMWF runs that remain phased and progressive. Interestingly, the GFS has trended from pulling off some energy in the 00Z/06Z runs to being more like the ECMWF in the 12Z run. The full ensemble spread includes all of these possibilities, but the CMC/UKMET scenario is sufficiently in the minority for the ensemble means to be more like the ECMWF and 12Z GFS. Thus the preference for this aspect of the forecast was closest to the ensemble means/ECMWF. Differences continue with how energy over and upstream from the Gulf of Alaska feeds into the mean trough based on the character of the eastern Pacific ridge drifting toward the West Coast. The 06Z GFS and 12Z UKMET show some brief dampening of the northern periphery of the ridge leading to somewhat broader downstream troughing. Shortwave differences in the 06Z GFS get to be significant enough by Day 7 Saturday to result in a central U.S. surface pattern that becomes out of sync with most other models and the ensemble means. 12Z guidance as a whole is starting to look a little more similar though, hopefully providing a little more stability for specifics of the leading cold front reaching the East Coast and trailing front expected to drop into the northern tier by late week. An operational model blend was reasonable for Day 3 Tuesday but then guidance preferences led to steady reduction of CMC/UKMET input (replaced by 06Z GEFS/00Z ECens means), with the ensemble means reaching 60 percent total weight by Days 6-7 Friday-Saturday. Lingering operational model input at that time leaned more to the ECMWF than GFS, and among GFS runs more to the 00Z run versus the 06Z version. ...Weather/Hazards Highlights... The upper trough and leading cold front emerging into the Plains will support widespread rain and thunderstorms in the central U.S. on Tuesday. A broad Marginal Risk in the Day 4 Excessive Rainfall Outlook covers most of the region. Model and ensemble guidance has recently narrowed in on an axis of relatively heavier QPF along the surface low track in the northern Plains. The frontal low along with a potent shortwave and some instability in the vicinity should provide the ingredients for heavy rainfall, and a Slight Risk area has been maintained--centered mostly over eastern North Dakota and far northwestern Minnesota. Slightly faster 12Z model trends are lowering confidence in some of the higher totals previously forecast, but for now there is at least some persistence of a relative maximum over this area. Then by Wednesday, the northern side of the cold front may be moving more progressively through the Midwest for some lighter rain, while heavy rain and thunderstorms become more widespread in the southern Plains where the right entrance region of the jet will provide ample lift and plenty of instability is in place just ahead of the front. Thus a Slight Risk is in place there in the Day 5/Wednesday ERO, as despite the drought conditions, high rain rates that could overcome the dryness are likely. Latest guidance maintains support for this Slight Risk area. Uncertainty over what happens to energy in the southern part of the upper trough leads to some question marks over the longitude of heaviest rainfall, with seemingly lower-probability UKMET/CMC solutions leading to a farther west location. The western periphery of the Marginal Risk area accounts for this possibility. The cold front should gradually push through the south-central U.S. Thursday and Friday, pressing rain chances southward with it. Meanwhile showers will likely track across the Ohio/Tennessee Valleys around Thursday and into the East by Friday along and ahead of the front. Depending on the specifics of upper troughing by Saturday, the cold front could begin to tap Atlantic moisture within southerly flow to increase rainfall over parts of the Northeast. Not too much precipitation looks to occur elsewhere, though there could be some lingering snow over higher elevations of the West in association with the upper trough crossing the region through midweek. Some rain in the western Atlantic could provide a few showers to eastern Florida, but otherwise expect the rain to stay mainly offshore. In terms of temperatures, the amplified trough will lead to cooler temperatures than average while the ridge to the east will support above normal temperatures. The Interior West should see cooler temperatures than average especially through midweek, with with lows 5-10F below normal but highs around 10-20F below average. This puts high temperatures generally in the 80s for the Southwest and the 50s and 60s for the Great Basin through around midweek, warming to near normal for the latter part of the week. Meanwhile warmer than average temperatures by around 15-20F above normal are forecast for the Midwest into the Great Lakes and the Interior Northeast, with highs in the upper 70s to low 80s. Some daily records for highs/warm lows may be possible Tuesday-Wednesday. As the cold front and upper trough march east into the latter half of the week, high temperatures will cool to below average across the central and then east-central U.S., limiting the above normal temperatures to the Northeast. A trailing front may reinforce the cooling trend by Friday-Saturday. Approach/arrival of the eastern Pacific upper ridge should raise temperatures along the West Coast to above average, by as much as 5-15F, and some record warm lows may be possible. 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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