Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Oct 31 2023 Valid 12Z Fri Nov 03 2023 - 12Z Tue Nov 07 2023 ...Synoptic Overview... Latest guidance agrees upon progressive and low amplitude flow aloft from Friday through the weekend, and then a potential transition by next Tuesday to flat western U.S. ridging and an eastern U.S. trough. This pattern will promote multiple episodes of enhanced precipitation from the Pacific Northwest into the northern Rockies. The developing upper trough forecast to reach the East near the end of the period may support an organized low pressure system with an accompanying broad area of precipitation. An expansive surface high will continue to encompass much of the eastern U.S. for the end of the week, with the high slowly moving offshore over the weekend. This airmass will slowly moderate after the widespread frost and freeze conditions earlier in the week. However, cold temperatures will have staying power for the Northern Plains and Upper Midwest through early next week. Locations from the Southwest/southern Great Basin into parts of the central-southern Plains should see the most persistent above normal high temperatures during the period. ...Guidance Evaluation/Predictability Assessment... While there is general consensus for the overall pattern evolution, the progressive and low amplitude nature of the upper flow expected for most of the period normally leads to lower predictability. This in turn keeps confidence in specifics on the lower side of the spectrum. A significant model difference affecting the Pacific Northwest involves Pacific low pressure whose leading frontal system should affect the region during the weekend. A number of ECMWF runs, and especially the 00Z version, track the the low much closer to the coast than most other guidance. The 00Z ECMWF was within the full ensemble spread but 12Z guidance thus far maintains the overwhelming consensus toward a farther offshore track. The new 12Z ECMWF is still an eastern extreme but not quite to the extent of the 00Z run. Farther east, recent GFS runs have been stronger than most other solutions with the leading shortwave emerging from the West after Friday. This leads to much stronger surface development over the Great Lakes by Sunday and then farther eastward progression of the trailing front by Monday-Tuesday. The GEFS mean reflects this wave but in much weaker form, and the new 12Z CMC also has a modest wave along a similar track. It seems plausible for the western shortwave energy to support a frontal wave but most likely a fairly weak one. Then the GFS has been weaker with trailing energy that leads to the consensus eastern U.S. upper trough by next Tuesday. At that time the combination of these two issues in the GFS have led to higher surface pressures over the East versus the guidance majority (including the GEFS/ECens/CMCens means) depicting Lower Great Lakes/Ohio Valley low pressure. The new 12Z GFS has made a favorable trend closer to, but still somewhat east of consensus, for the surface low. A blend of the 00Z/06Z operational model runs provided a reasonable starting point through the first half of the forecast period, leaning away from the least confident aspects of the GFS/ECMWF as described above. With the 00Z/06Z GFS straying away even more from consensus late in the period, the blend phased out the GFS completely by Day 7 Tuesday. Otherwise the forecast rapidly increased 06Z GEFS/00Z ECens mean weight so that it comprised 60 percent of the blend by next Tuesday. The 00Z ECMWF/CMC provided the remaining operational input. ...Weather/Hazards Highlights... Most of the continental U.S. will likely not see any significant rainfall hazards through Saturday. The only region where there will be a risk of excessive rainfall leading to flooding concerns will be in the Pacific Northwest with a period of enhanced onshore flow late this week into Saturday. During the Day 4 time frame (Friday-Friday night) there is a wide spread of guidance for potential QPF maxima especially over northwestern California/far southwestern Oregon, corresponding in part to uncertainty over details of offshore low pressure. The heavy side of the spread (00Z ECMWF/12Z CMC) would be problematic but current preference to maintain no risk area reflects the larger majority of guidance that keeps projected rainfall totals lower. The Day 5 outlook (Saturday) maintains a Marginal Risk of Excessive Rainfall from northwestern California to the Olympic Peninsula in Washington. Locally heavier rainfall on the west facing slopes may just be enough to cause some minor flooding concerns, especially in urban and burn scar areas. There are some mixed signals from guidance for distribution of highest totals (though appearing a little more likely along coastal northern California) while there is some overlap of favorable moisture/instability within the outlook area which could enhanced rainfall intensity at times. Looking beyond Saturday, additional moderate to heavy rain is likely for these same areas as another potential atmospheric river event develops with a few inches of additional rainfall during the Sunday-Monday time period. Some rainfall may continue into Tuesday. Meanwhile some of this moisture will extend into the northern Rockies, with snow over higher elevations. Parts of the northern Plains into Great Lakes may see precipitation during the weekend into early next week, depending on low-confidence details of one or more frontal waves. A stronger surface system that may develop over the eastern U.S. by next Tuesday could produce a broader area of precipitation with higher totals. In both cases the majority of precipitation will be rain, but wintry weather will be possible in the northern fringe of the moisture shield. Temperatures will start off cold Friday morning across much of the southeastern quadrant of the lower 48 with a slow warming trend ensuing over the weekend as the airmass continues to modify. Pleasantly mild conditions will also return to the Rockies and adjacent portions of the central and southern Plains this weekend with some highs reaching up to 10F or so above normal. Warm temperatures should persist over southern areas into early next week. In contrast, the northern Plains and Upper Midwest should remain quite chilly with highs remaining 5-15F below normal. Rausch/Hamrick 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