Extended Forecast Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Mon Oct 31 2022 Valid 12Z Thu Nov 03 2022 - 12Z Mon Nov 07 2022 ...Western-central U.S. system to bring heavy mountain snow to parts of the Great Basin/Rockies and heavy rain potential to portions of the Plains... ...Late week atmospheric river event likely to bring an episode of heavy precipitation to parts of the Northwest... ...Overview... Expect an amplified pattern to prevail during late this week into the early weekend as a deep trough carries an embedded upper low across southern parts of the West and strong ridging builds over the East. Meanwhile strong North Pacific flow will initially extend into southwestern Canada and then sag southward into the Northwest U.S. Once this Pacific jet reaches far enough south after early Saturday, it should accelerate the upper low progressing eastward/northeastward across and beyond the Plains. By late weekend into early next week the pattern should take on a more zonal or broadly cyclonic look as east-west troughing evolves over the western half of Canada in response to strong Alaska ridging while the East Coast ridge moves into the Atlantic. The leading western system will initially support meaningful rain/mountain snow over parts of the Four Corners states and then a heavy rain threat should emerge over the south-central Plains, while the Northwest atmospheric river event should be most pronounced around Friday-Friday night with a gradually lighter trend thereafter. ...Guidance and Predictability Assessment... The latest guidance shows good enough agreement days 3 and 4 for a purely deterministic model blend. Timing and intensity differences really start to emerge by day 5/Saturday regarding the closed low as it lifts from the southern Plains into the Midwest and eventually the upper Great Lakes, with some notable shifts in the guidance with the new 12z guidance (which was available after forecast generation time). The 00z ECMWF was the quickest to lift the low out of the southern Plains after Saturday and phase it with a northern stream shortwave across the northern Plains, and the new 12z run maintains that (although is even quicker to weaken the low over the Midwest). The GFS on the other hand is slower and much more amplified than the ECMWF, maintaining and well defined closed low over the Midwest through Sunday. The 12z maintains this stronger depiction, but is a little faster than its 6z run (though still slower than the ECMWF). The CMC was also stronger than the GFS and both its 00z and 12z runs from today are pretty close to the new 12z run from the GFS. The ensemble means were of course weaker than the deterministic runs, but at least kept a fairly middle ground on placement. Without a clear direction, the WPC forecast opted to lean closer to continuity which favored something more like the GFS. For days 6 and especially 7, blending the means with continuity and the 06z GFS seemed to give a reasonable solution for now. It does appear given the latest trends in the 12z guidance, there is support for a little bit faster progression of this system than even the current WPC forecast depicts, but these trends will need to be monitored to see if they continue in subsequent cycles. The details and predictability of energy into the Pacific Northwest seemed to decline appreciably by days 6 and 7 and so a trend towards the ensemble means was favorable there as well. ...Weather/Hazard Highlights... The initial western U.S. upper trough and embedded low tracking across southern parts of the West will initially bring mountain snow and lower elevation rain to the Great Basin and central Rockies, with some precipitation extending farther south as well. WPC Winter Weather Outlook probabilities reflect areas with the best potential for meaningful to heavy mountain snow over this region. Expect rain to increase in coverage and intensity over the central U.S. late in the week and persist into the weekend as the western system approaches. Some rain may become heavy and strong convection is possible as well, as per the latest outlooks from the Storm Prediction Center. The experimental Day 5 Excessive Rainfall Outlook currently depicts a Slight Risk area from portions of eastern Kansas into northern Texas for the early Friday into early Saturday time frame. This event may feature a combination of flash flood threats (in mainly urbanized/metro regions) where the heaviest rain occurs but also beneficial rainfall given the long-term drought over the region. Monitor Storm Prediction Center outlooks for latest information on severe potential. Rainfall should trend lighter and more progressive from late weekend into early next week as the supporting upper low accelerates northeastward. The other significant precipitation focus will be over the Northwest late this week into the weekend, due to an atmospheric river event associated with strong westerly flow reaching into the region. A period of heavy rain/mountain snow appears likely with highest totals tending to be during Friday-Friday night. Some significant precipitation should continue into the weekend but the southward progression of the Pacific jet (likewise pushing the precipitation shield gradually farther south) should be sufficient to allow for a lighter trend. A period of strong winds will be possible in this event as well. Somewhat less extreme but still significant precipitation will reach into the northern Rockies. The West will see a broad area of daytime highs 10-20F or so below normal under the forecast upper trough/low through Friday. Some areas over or near southern California may see record cold highs. Double-digit anomalies should become much more scattered during the weekend as temperatures moderate but most areas should remain near to below normal for highs. The evolving upper pattern should support a push of cold air reaching the northern High Plains/eastern slopes of the Rockies by next Monday, possibly bringing highs down to 20-30F below normal over Montana. On the other hand, well above normal temperatures will prevail over most areas east of the Rockies through about Thursday and then the Plains will likely trend cooler while the Mississippi Valley and especially the East remain warm. During Thursday-early Friday the warmest anomalies (plus 10-25F) should be from the Upper Midwest into central Plains while areas from the Great Lakes/Ohio Valley into Northeast should see multiple days with temperatures 10-20F above normal during the period. Santorelli/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 Hazards: - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Fri-Sun, Nov 4-Nov 6. - Heavy precipitation across portions of the Northern Rockies and the Northern Great Basin, Sat-Sun, Nov 5-Nov 6. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, and the Upper Mississippi Valley, Fri-Sat, Nov 4-Nov 5. - Severe weather across portions of the Central Plains and the Southern Plains, Thu, Nov 3. - Severe weather across portions of the Southern Plains, Fri, Nov 4. - Flooding occurring or imminent across portions of the Southeast. - High winds across portions of the Northern Plains and the Northern Rockies, Fri-Sat, Nov 4-Nov 5. - High winds across portions of mainland Alaska, Thu-Sat, Nov 3-Nov 5. - High winds across portions of the Alaska Panhandle, Fri-Mon, Nov 4-Nov 7. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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