Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Sep 26 2019 Valid 12Z Sun Sep 29 2019 - 12Z Thu Oct 03 2019 ...Much below normal temperatures over the West/northern Plains with heavy snow possible in the northern Rockies... ...Much above normal temperatures over the East, extending for a time as far west as parts of the Plains... ...Heavy rainfall potential from the southern High Plains into Great Lakes... ...Pattern Overview and Guidance/Uncertainty Assessment... From Sun into Mon the models and ensemble means remain agreeable and consistent with the forecast of a very amplified pattern consisting of a western trough containing an upper low over the Northwest and eastern ridge whose upper high center reaches near the Lower Mississippi Valley. The first guidance discrepancy arises as some combination of ejecting western trough energy and southern Canada flow propels northern Plains low pressure on Mon northeast into Canada. After Tue consensus shows a fairly rapid flattening of the mean pattern as a northeastern Pacific trough drops southeast toward the West Coast, ejecting the initial western trough. There has been some spread/variability with how quickly the initial trough departs (though perhaps a little less than could be the case in such situations) and with details of height falls reaching the West Coast by next Thu. Based on guidance through the 12Z-18Z cycles, an operational model blend represented consensus well for days 3-4 Sun-Mon. Even into day 5 Tue a model blend with a modest introduction of the 18Z GEFS/12Z ECMWF means provided a reasonable compromise for timing of the northern Plains into Canada surface low. The 12Z ECMWF and 12-18Z GFS runs jumped faster than some previous runs and 12Z-00Z CMC/UKMET runs and the new 00Z GFS is even faster. Ensemble mean signals have been mixed as well with the ECMWF mean supporting the ECMWF/GFS versus the slower CMC/GEFS means. Over a multi-day time frame the timing has varied and important upper details are small enough in scale to have relatively lower predictability--favoring an intermediate approach. Over the past couple days the operational models and ensembles have shown fairly significant spread for specific details of the northeast Pacific trough dropping toward the West Coast. The primary guidance consideration was to phase out the 12Z ECMWF from the model/ensemble mean blend after day 6 Wed due to its faster trough arrival versus most other solutions. The new 00Z GFS/CMC/GEFS mean maintain support for the trough axis remaining off the West Coast into Thu. ...Weather Highlights/Threats... The upper low over the Northwest and strong low level easterly flow north of surface low pressure over the north-central Rockies will support continued heavy precipitation over the northern Rockies through the weekend. The greatest snow totals should be over higher elevations of western Montana with some snow possibly extending to somewhat lower elevations to the east or northeast. The surface low will likely bring areas of locally heavy rainfall across the extreme northern Plains into the Upper Mississippi Valley/Upper Great Lakes Sun into Mon. At the same time flow around the western side of the eastern ridge aloft should begin to bring rainfall-enhancing moisture from the tropical eastern Pacific and Mexico into the southern High Plains. This flow may persist through midweek, leading to multiple episodes of locally heavy rainfall over the region. The interaction of this moisture and wavy front trailing from the northern Plains/Canada low will raise the potential for a heavy rainfall event from the central Plains northeast into the Great Lakes (centered over Iowa). The signal for this event has persisted over the past couple days and the possibility that the front moves slowly for a day or two would allow for significant rainfall totals due to training/repeat activity. To a lesser degree some moisture should extend into New England. Deep-layer easterly flow will promote an increase in rain/showers over Florida. The flow may ultimately contain some moisture associated with Tropical Storm Karen, but Karen itself is currently forecast to weaken slowly to a tropical depression and then a post-tropical remnant low in the coming days. Monitor NHC forecasts for the latest information regarding Karen. The most extreme temperature anomalies over the lower 48 will exist from Sun into early next week. The western U.S. into northern Plains will see highs 10-25F below normal with localized areas in Montana seeing highs at least 30F below normal. Daily records for cold highs are most likely on Sun but some are possible beyond then. The east will see a broad area of plus 10-20F anomalies for highs with numerous daily records. The northern Plains low pressure early in the week will also bring a surge of warmth into parts of the central U.S. with morning lows particularly warm (plus 20F or greater anomalies) from the central Plains into Great Lakes. As the pattern aloft begins to trend less amplified expect a gradual moderation of chilly western U.S./northern Plains temperatures while warm to hot readings become more confined to the East--with some record highs/warm lows continuing to be possible into Wed-Thu. 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