Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Sun Oct 27 2019 Valid 12Z Wed Oct 30 2019 - 12Z Sun Nov 03 2019 ...Much below normal temperatures over the West/Plains as of Wed will gradually moderate... ...Developing storm likely to affect the eastern half of the country during the latter half of the week... ...16Z Update... It remains the case that a deep upper low/trough should track across the central U.S. midweek and northeast toward the Great Lakes later in the week, but the details in the evolution of this feature are yet to be resolved. Guidance is in agreement that on day 3/Wed a positively tilted trough will spin off an upper low into the Four Corners region. By day 4/Thu, most model guidance keeps a positively tilted trough moving into the central U.S., except for GFS runs which are aggressive with a cutoff upper low neutrally tilted and centered farther east of other guidance including its mean. Thus once again the GFS was not incorporated into the forecast of the medium range mass fields. After persistently forecasting a separate upper low on Thu, the 00Z ECMWF came in with the trough joined with the main flow. By day 5/Fri, this does lead to a weaker (though still potent) surface low in the 00Z ECMWF, so still some uncertainty with that. The 00Z CMC weakens the energy and keeps it south by Fri, with its surface low appearing too far south compared to other guidance (though the 00Z GEFS mean does have a similar low position). Overall the forecast leaned heavily on the 00Z ECMWF/EC mean, with incorporation of the UKMET throughout its period and the 00Z CMC early in the forecast period. For more details on sensible weather, please see the previous discussion. Tate ...Previous Discussion... ...Overview and Guidance/Uncertainty Assessment... Guidance still shows some broadening of the eastern Pacific mean ridge aloft over the course of the period, ultimately leading to a neutrally tilted mean trough settling into the east-central U.S. in contrast to the positively tilted trough extending into the West early-mid week. While some spread persists for the specifics, there is a steadily improving signal that the last significant bundle of energy aloft ejecting from the West will generate a deepening storm system around Thu-Fri. The surface low should track from the Mississippi Valley/Ohio Valley north-northeast through the Great Lakes into southern Canada and be accompanied by a broad shield of various precipitation types. Depending on how much the storm deepens, strong winds will also be possible as it reaches/passes the Great Lakes. Expect much of the lower 48 to be fairly dry after this storm departs. Maintaining fairly good continuity, the updated forecast emphasized the 12Z ECMWF/UKMET/CMC during the first half of the period. Then the forecast steadily increased 12Z ECMWF mean input toward half weight while holding onto some aspects of the 12Z ECMWF/CMC (plus adding the prior 00Z ECMWF run to account for increasing detail uncertainty). The GFS has been trending slower with the upper low ejecting from the central Rockies but continues to be on the faster side of the spread. Recent verification and trends have steadily favored the slower majority cluster of guidance. Interestingly the past couple GEFS mean runs have aligned quite well with the ECMWF cluster aloft for Wed-Thu but become more open than consensus thereafter. The new 00Z CMC evolves to an open wave aloft as well but remaining models maintain an upper low and in fact become deeper and more concentrated as the system reaches the Great Lakes by Fri. However a weaker trend in recent ECMWF runs does temper confidence in the deepest GFS/UKMET solutions. Behind this system there is decent agreement that cyclonic flow aloft will bring one cold front into the northern tier by Fri (weakening by Sat) with another front approaching by Sun. By next weekend guidance differences are fairly modest outside of details that have low predictability at the days 6-7 time frame, supporting a model/ensemble mean blend. Recent GEFS means have tended to be on the weaker side of the spread for the eastern Pacific ridge aloft so that solution was excluded from the blend. The new 00Z GEFS mean seems to have trended in a favorable direction though. ...Weather Highlights/Threats... The dominant weather focus during the period will be the upper low expected to track from the central Rockies through the Great Lakes and into Canada, with deepening low pressure lifting through the east-central U.S. Thu-Fri. The best potential for meaningful snow in the cold sector should extend from the central Rockies/Plains northeast into the Upper Great Lakes. Ahead of the storm expect a broad area of rainfall and possibly considerable coverage of heavy amounts. Activity may be enhanced by an area of moisture currently over the Caribbean and expected to lift northward across the Gulf of Mexico into the southern states. Some embedded convection could be strong as well. There may be a period of brisk to strong winds over the Great Lakes/Northeast as the deepening surface low reaches and departs from the Great Lakes. Behind this system expect much of the lower 48 to be dry, aside from some lake effect rain/snow and scattered light precipitation with fronts affecting the northern tier as well as mostly light rainfall along the front settling over the Florida Peninsula. The central Rockies/Plains will be in the core of coldest air around midweek with some locations seeing highs 30-40F below normal. A much broader area from the Interior West through central U.S. will see temperatures at least 10-20F below normal. Some daily records for lows/cold highs are likely. Readings will trend warmer from late week into the weekend over the West and into the Plains, with the eastern Pacific upper ridge possibly nudging far enough eastward to bring modestly above normal temperatures to the West Coast states by the weekend. Meanwhile the East will see a cooling trend by the weekend after warm/moist flow leads to a couple days or so of plus 10-20F anomalies for morning lows mid-late week. The cool anomalies over the East by the the weekend will be much less extreme than those forecast farther west early in the period. 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