Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 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... In the guidance through the 12Z/18Z cycle, there were still discrepancies with the ultimate timing of the upper low progressing through the southern Rockies and then continuing northeastward. Most of these differences corresponded to differences in the position and orientation of the Pacific jet coming into North America. GFS runs tended to be on the slower side of the spectrum for the upper low due to being more westerly/northward with the Pacific jet. On the other hand the 12Z ECMWF strayed to the fast side after late Saturday due to a farther south Pacific jet with a bit more of a northwesterly tilt. Recent GEFS means have at least been suggesting those operational GFS runs could be too slow with the upper low. In order to maintain the best continuity possible until guidance shifts enough to suggest a more confident change for the upper low and incoming Pacific flow, the updated forecast transitioned toward a blend of the 12Z CMC and 00Z/30 ECMWF (which was slower than the 12Z run, more in line with the 12Z CMC) along with the 18Z GEFS/12Z ECMWF means by days 6-7 Sunday-Monday. The first half of the period featured good enough clustering to allow for an operational model composite. The new 00Z GFS has trended favorably faster for the upper low in response to adjusting closer to consensus for incoming Pacific flow. The CMC has adjusted somewhat faster than its previous run after Saturday while the ECMWF remains the fastest solution (with a deep Upper Midwest storm in the process). This yields a guidance average that is now somewhat faster than prior preference. Predictability of western U.S. flow details would appear to start declining appreciably late in the period and thereafter, as guidance diverges on specifics of upper ridging that builds over/near Alaska. As seen in previous cases, issues regarding whether the Alaska ridge closes off and its ultimate path if it does, can cause havoc with the forecast over the eastern Pacific and downstream into the lower 48. ...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 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. The experimental Day 5 Excessive Rainfall Outlook currently depicts a Slight Risk area from portions of southern Kansas into northern Texas for the early Friday into early Saturday time frame. This event may feature a combination of flash flood threats 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 preciptation 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 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. 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, 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