Extended Forecast Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Mon Sep 30 2019 Valid 12Z Thu Oct 03 2019 - 12Z Mon Oct 07 2019 ...Central U.S. heavy rain threat diminishes late this week but could return/extend eastward by the weekend and early next week... ...Record warmth continues for the Southeast into Thursday... ...Overview and Guidance/Uncertainty Assessment... The trend toward less amplified and more seasonable mean flow aloft will continue during the period with height anomalies becoming fairly modest over the lower 48. Within this pattern, an upper low crossing the Great Lakes on day 3 Thu (energy originally associated with the deep trough/low now over the West) will bring fairly vigorous low pressure across the Northeast while the trailing cold front suppresses the heat over the South/East. Then a shortwave progressing eastward from the West Coast will support a front that crosses the western/central states and mostly likely reaches near the Appalachians by next Mon. The greatest uncertainty in the forecast arises by days 6-7 Sun-Mon, with respect to details of eastern Pacific/western North American flow and resulting influence on shortwave energy that comes in behind the initial West Coast trough. Details of this evolution will determine central-eastern U.S. frontal progression, potential waviness, and rainfall extent/intensity during that time. Another question mark will be with the evolution aloft over/near the Gulf of Mexico with guidance not particularly consistent thus far. For days 3-5 Thu-Sat the primary guidance difference involved the 00Z-06Z GFS runs being a little flatter and faster than consensus with the West Coast to central North America trough. An average of operational models looked good for the system crossing the Northeast. Therefore this part of the forecast emphasized operational model input but with somewhat less GFS weight than usual to account for the western trough consensus. By days 6-7 Sun-Mon solutions diverge for the amplitude and longitude of northeastern Pacific troughing, in turn affecting downstream ridging as well as trough energy farther east over the lower 48. The one common theme in the D+8 multi-day mean charts is a strong positive height anomaly center near the central Aleutians. Teleconnections relative to this feature show the northeastern Pacific trough seen in most guidance, but at most just a hint of a western ridge/eastern trough--not nearly as extreme as seen in the 00Z ECMWF (with recent runs not very consistent either). Among remaining guidance the GFS/GEFS mean/CMC mean are somewhat faster than the ECMWF mean/CMC with the northeastern Pacific trough, but solutions do not cluster the same way for the central-eastern U.S. trough. For now the teleconnection guidance recommends a conservative approach to the overall pattern and emphasis on the ensemble means. The GEFS/ECMWF means each have merit based on telecons, the former for bringing some negative height anomalies into western Canada and the latter for its modest western ridge. ...Weather Highlights/Threats... Great Lakes through Northeast low pressure Thu-Thu night will bring focused rainfall to areas near/north of the low track. Rainfall along the trailing front will diminish except for some activity persisting over the southern Rockies/High Plains where the front will stall for a day or so. The system emerging from the West and into the Plains should bring scattered rain/high elevation snow to northern parts of the West late this week and then support some areas of locally moderate to heavy rain over the central U.S. by this weekend. The forecast becomes more uncertain by Sun-Mon given poor agreement regarding upper level flow details that will determine frontal timing/waviness and how much moisture may flow northward from the Gulf of Mexico. At the very least there is the potential for some areas to see significant rainfall but at this time confidence is very low for specifics. Locations along the eastern half of the Gulf Coast may also see periods of enhanced rainfall with moisture around the periphery of Gulf of Mexico low pressure and near a potential upper level weakness/low. Record warmth over and a little north of the Southeast should continue into Thu before temperatures finally moderate. Expect unseasonably cool readings over the Northwest (10-20F below normal) to persist late this week and then trend closer to normal first over the Pacific Northwest and then eventually over the northern Rockies. 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 Hazards: - Heavy rain across portions of the Southern Rockies, the Southern Plains, and the Southwest, Thu-Fri, Oct 3-Oct 4. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Thu, Oct 3. - Flooding possible across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Ohio Valley, and the Upper Mississippi Valley. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, and the Northern Plains. - Flooding likely across portions of the Central Plains, the Middle Mississippi Valley, and the Great Lakes. - Much above normal temperatures across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Thu, Oct 3. - Much above normal temperatures across portions of the Southeast, the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley, Thu-Fri, Oct 3-Oct 4. - Much below normal temperatures across portions of the Central/Northern Rockies and the Central/Northern Plains, Thu, Oct 3. 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