Extended Forecast Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Tue Sep 29 2020 Valid 12Z Fri Oct 02 2020 - 12Z Tue Oct 06 2020 ...Hot and dry conditions persist for the West... ...Heavy rainfall threat for South Florida... ...Overview and Guidance/Predictability Assessment... The highly amplified upper-level pattern of a western U.S. ridge and central/eastern U.S. trough should persist into the medium range, causing warmth in the West and generally cooler than normal temperatures farther east. Model guidance was in good agreement regarding the large-scale pattern, and a multi-model deterministic blend sufficed for the mass fields early in the period. As the days progressed, more and more weight was placed on the GEFS and ECMWF ensemble means (which were highly agreeable) to minimize differences in the smaller-scale flow. In particular, one uncertain part of the forecast was the timing and placement of embedded shortwaves coming through the mid/upper-level northwest flow over the northwestern/north-central U.S., and it did not seem prudent to focus on any particular model solution with that. ...Weather/Hazard Highlights... An amplified upper ridge over the West offers hot and dry conditions and a renewed fire risk. Expect temperatures to reach 5-15 degrees F above normal (isolated higher values) from California/Arizona/Nevada through the Pacific Northwest. Some record-breaking values will be possible, mainly over the Desert Southwest into California, but perhaps into Oregon/Washington as well. A front with multiple but uncertain waves will reach the East Coast later in the week and may provide another focus for rainfall over portions of the Eastern Seaboard along with unsettled maritime conditions. Rainfall amounts will depend on how far offshore in the Atlantic the frontal boundaries stall, which is still uncertain, but coastal areas of the Mid-Atlantic in particular could see some heightened rainfall totals. Fronts will tend to decelerate and possibly stall near southern Florida Peninsula and the Keys, so periods of heavy rain are possible through the period. The amplification of the east-central upper trough favors deep tropical moisture influx into south Florida associated with a western Caribbean wave/disturbance that could fuel lingering heavy downpours. Cold air aloft will meanwhile provide instability for scattered showers across the Midwest/Great Lakes/Appalachians under the upper trough, including some Great Lake enhancement. A wave expected to drop into the central then east-central U.S. Sat-Mon may produce light to locally moderate rain from the Plains to the Great Lakes and Ohio Valley. Cold air surges will support temperatures of 5-15 degrees F below normal for much of the central to eastern U.S., and there is a chance for a first frost/freeze of the season for the Northern Plains and Upper Mississippi Valley late week. Tate/Schichtel Hazards: - Heavy rain across portions of the Florida Peninsula, Fri-Tue, Oct 2-Oct 6. - Heavy rain across portions of the Mid-Atlantic coast, Sun-Mon, Oct 4-Oct 5. - Flooding occurring or imminent across portions of Florida and Louisiana. - Much above normal temperatures across portions of northern California to southern Oregon, Fri-Tue, Oct 2-Oct 6. - Much above normal temperatures across portions of central California, Fri, Oct 2. - Much below normal temperatures across portions of the central Plains to the west of the Middle Mississippi Valley, Fri, Oct 2. 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