Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Oct 28 2019 Valid 12Z Thu Oct 31 2019 - 12Z Mon Nov 04 2019 ...Much below normal temperatures over the Rockies/Plains will gradually moderate... ...Developing storm likely to affect the eastern U.S. Thu-Fri... ...Overview and Guidance/Uncertainty Assessment... Models and ensemble means agree that a persistent eastern Pacific ridge will promote broad cyclonic flow downstream, with a mean trough axis tending to align over the east-central states. Guidance consensus and continuity provide fairly high confidence in the large scale pattern but individual embedded features have lower predictability. By early next week there are also some aspects to the forecast that become more uncertain near the West Coast and into western Canada. In terms of sensible weather effects the greatest forecast problem continues to involve the upper trough expected to be over the central U.S. as of early Thu and eject northeastward thereafter, supporting development of surface low pressure that tracks from near the Ohio Valley northeastward into eastern Canada. Over the past day there has been some notable trending in the guidance as a whole toward faster progression (likely due in part to a stronger trailing upper trough dropping southeastward from Canada) and a weakening of the surface low among the prior strongest solutions (some ECMWF/GFS/UKMET runs). The past couple CMC runs are in the minority with a more phased/open system aloft leading a faster and less wound-up wave. Spread and trends in the guidance as well less than usual clustering among individual ensemble members for the time frame involved keep confidence in any specific solution lower than average. Based on guidance available through the 18Z cycle the updated forecast was based on a blend among the 12Z and 00Z/27 ECMWF runs along with the 18Z GFS and 12Z UKMET. This solution kept GFS weight low enough to yield minimal influence on day 3 Thu when that model is farther south and/or east than most other guidance aloft. After that time the GFS fell more in line, and in fact was slower than the ECMWF/ECMWF mean by early Fri. By Fri some of the new 00Z guidance continues to trim away the western/slower side of the prior model envelope for the surface low--but in a significant exception the 00Z ECMWF has adjusted slower. At that time the manual forecast falls between the new and previous ECMWF runs. Behind the eastern system the aforementioned model blend represented consensus well for the upstream trough aloft amplifying into the lower 48 along with the associated cold front which should weaken by Sat. Given the time frame involved there is decent agreement and consistency with the next frontal system expected to reach the northern tier/Great Lakes by days 6-7 Sun-Mon. The scale and progression of shortwave energy aloft suggest there could be more variability in future runs though. Late in the period GFS/GEFS mean runs develop lower heights aloft near/offshore California than most other guidance. In addition the GEFS mean remains among the weakest guidance with the Pacific ridge and the 12-18Z GFS runs gradually shifted the ridge-trough pattern a little east of consensus (while the 00Z GFS looks better in that regard). As a result the forecast adjusted toward primary emphasis of recent ECMWF runs and the 12Z ECMWF mean. The 18Z GEFS mean did compare acceptably for the overall North America trough to merit a small minority weight in the forecast. ...Weather Highlights/Threats... Confidence remains lower than desired for exact details of the developing system likely to track from the Ohio Valley through the eastern Great Lakes and into eastern Canada during Thu-Fri. However there is still a common theme of a widespread precipitation shield across the eastern U.S. with some snow possible in the northwestern periphery of the system (Midwest to Upper Great Lakes) and areas of heavy rainfall along with pockets of strong convection in the warm sector. Currently expect highest rainfall totals from early Thu onward to extend from along and just west of the Appalachians into the Northeast. A brief period of fairly strong winds will be possible over parts of the Great Lakes/Northeast but details will require better agreement on the depth/track of the surface low. Behind this system expect periods of lake effect rain/snow with periodic light precipitation over other parts of the northern tier in association with a couple fronts dropping south from Canada. The front trailing from the late week eastern storm will settle over the Florida Peninsula and could provide a focus for light rain into early next week. The Rockies and Plains will remain quite chilly into late week with a fairly broad area seeing temperatures 10-30F below normal. These areas should trend much closer to average by Sun-Mon while locations closer to the West Coast should see highs up to 5-10F above normal from the weekend onward as the Pacific ridge aloft extends its influence a little farther eastward. Meanwhile frontal passage over the East will replace initial warmth (plus 10-20F anomalies for morning lows and moderately warm highs on Thu) with cooler readings generally 5-15F below normal. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml