Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Sep 03 2018 Valid 12Z Thu Sep 06 2018 - 12Z Mon Sep 10 2018 ...Pattern Overview and Guidance/Predictability Assessment... Guidance continues to advertise relatively progressive mean flow across the northern contiguous U.S. and southern Canada, downstream from a northeast Pacific trough that exists in response to a very strong Bering Sea ridge. Within this ridge, latest GFS/ECMWF runs suggest potential for heights that could reach 3-4 standard deviations above climatology late in the week. Meanwhile flow around a weakening eastern U.S. ridge aloft should guide Potential Tropical Cyclone Seven into the southern Plains by Thu-Fri, and farther east bring an inverted upper trough from the Bahamas into Florida. Within and downstream from the northeast Pacific trough there has been decent agreement/continuity for synoptic-scale systems over the past couple days. The one exception comes from CMC runs which have varied between being either more open/progressive with northeast Pacific flow or farther westward than consensus with at least one of two likely embedded closed lows. The 12Z CMC mean reflected the open/progressive operational run. The one instability in the GFS is with occasional runs pulling back the second closed low (from energy dropping south across Alaska) a little west of consensus. The majority cluster has one upper low/shortwave well offshore the Northwest at the start of the period Thu. This energy should eject inland by Sat with associated southern Canada low pressure ultimately pushing a trailing front through the Northwest and into the Plains/Midwest. The second closed low should somewhat offshore Vancouver Island by the start of day 7 Mon but a leading frontal system may approach the Pacific Northwest. By late in the period teleconnections relative to the upstream ridge favor flat to slightly cyclonic mean flow across the Northwest versus the ridge that builds into the region in the 12Z-18Z GFS runs. Thus would prefer to downplay the operational GFS runs at that time. Consult National Hurricane Center products for the latest info regarding Potential Tropical Cyclone Seven, expected to track through the Florida Keys and into the central Gulf Coast in the short range time frame and then continue northwestward into the southern/south-central Plains. Thus far GFS runs have been weaker than other guidance with the reflection of this system. The new 00Z UKMET is quite close to the 12Z ECMWF in principle. The 00Z CMC is as well for a time before straying faster with its recurvature. A continuing discrepancy in the guidance involves the surface evolution ahead of shortwave energy expected to migrate across the central U.S. late in the week and then continue northeastward. The CMC with its fast recurvature of Potential T.C. Seven, and GFS with strong concentration of energy aloft after the shortwave exits the Plains, are on the aggressive side of guidance with a possible surface wave tracking through the Great Lakes and into strong downstream surface high pressure. Recent GEFS means have reflected GFS ideas but in a more subdued form. 12Z ECMWF trend toward a little more waviness than its prior run suggests the answer may lie between the two extremes but more likely somewhat closer to the ECMWF given the strength of the leading high pressure. The 00Z UKMET through late Sat is similar to the ECMWF in principle. Ahead of this evolution, there is good agreement regarding the front that drops from the Northeast into Mid-Atlantic and stalls for a time from the East Coast into the Plains. Albeit with uncertain wave details as described above, there is general agreement that this front will lift northeast as a warm front ahead of the system that tracks across Canada during the weekend and early next week. Combining the above considerations, the starting blend for the forecast leaned somewhat more in the direction of the 12Z ECMWF/ECMWF mean relative to the 18Z GFS/GEFS mean with the GEFS mean favored more than the GFS component by days 6-7 Sun-Mon. ...Weather Highlights/Hazards... Expect a period of heavy rainfall potential over parts of the southern Plains and vicinity near the path of Potential Tropical Cyclone Seven. This moisture will feed into a larger scale band of moisture extending from the southern Plains northeastward, interacting with the front that stalls over mid-latitudes of the lower 48 and then lifts northeastward as a warm front. Significant details have yet to be resolved so confidence is not yet as high as desired for pinpointing the best focus for highest rainfall amounts within a favored zone from the southern-central Plains through the Midwest into parts of the northern Mid-Atlantic/Northeast. Mid level energy and a surface trough reaching Florida from the Bahamas may enhance rainfall at times over/near that state, while lingering moisture may aid some activity along portions of the Gulf Coast. The Pacific Northwest should see a period of light-moderate precipitation late week into the weekend with potentially another around the start of next week. Expect very warm and humid conditions over the East late this week ahead of the backdoor front that reaches into the Mid-Atlantic. The best signal for most extreme anomalies (at least plus 10-15F) continues to be for min temps from the Ohio Valley/Appalachians into Mid-Atlantic/Northeast with some readings possibly exceeding daily record warm min values. Clouds/rainfall will temper high temperatures over the East after Fri but keep the lows fairly warm. Meanwhile the coolest anomalies during the period (minus 10F or so) will be over the central Plains late this week. Progression of features across the western half of the northern tier states will lead to a warming trend followed by cooler temperatures. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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