Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Nov 08 2023 Valid 12Z Sat Nov 11 2023 - 12Z Wed Nov 15 2023 ...Overview... Over the weekend, a shortwave or two will push through the northern tier of the country, developing into troughing over the Eastern Seaboard early next week and leading to cooler than average temperatures. Farther south, moisture should overrun a slow-moving front over the Gulf of Mexico and spread rain across parts of the southern and eastern U.S. Southern Texas may see the best chance for heavy rain especially into next week when upper-level energy comes through, but there is still considerable uncertainty with how much rain falls onshore versus offshore and how much instability will be in place that would increase rain rates. Meanwhile, rounds of precipitation are possible across the Pacific Northwest for the weekend into early next week, but precipitation looks to increase considerably for the West Coast (most likely maximized over California) by Tuesday-Wednesday as deep eastern Pacific upper troughing likely directs an atmospheric river into the region. Downstream, upper ridging in the north-central U.S. should lead to above average temperatures for the northern/central Plains into the Midwest. ...Guidance Evaluation/Predictability Assessment... The details of shortwaves tracking across the northern tier remain uncertain this weekend early in the medium range period. Split flow over the Great Lakes around Sunday complicates matters too, with variations in models with where the troughing farther south in the Midwest splits with ridging farther north into southern Canada. With the 12/18Z suite of guidance there did not seem to be particular outliers through the weekend, so utilized a multi-model deterministic blend favoring the 18Z GFS and 12Z ECMWF. Though into the workweek as energy combines to form troughing over the East, the ECMWF seems to stray faster than consensus with the eastward movement of the trough into the Atlantic. The newer 00Z ECMWF seems slower but also flatter with the trough. Fortunately the upper pattern becomes more dominated by synoptic-scale longwave troughs and ridges into next week. Flow on this scale is usually better handled by the model guidance, though some model differences remain particularly in terms of the timing of the features. Overall models are agreeable that eastern Pacific troughing will deepen and edge toward the West Coast, while ridging amplifies farther east over the central U.S. and troughing over the East tracks into the Atlantic by midweek. The Pacific trough has shown some notable differences in its timing over the past couple of days. The ECMWF suite has been faster in bringing the trough and likely embedded upper low and thus the QPF inland compared to the other guidance. The newer 00Z ECMWF run has slowed down though and looks more consistent with consensus. Another consideration is energy atop northwestern Mexico early in the week that should track northeast across the south-central U.S. around Monday-Wednesday. Models generally agree on this track but even minor differences can cause discrepancies with the sensible weather such as the QPF--the 12Z ECMWF was quite high with its QPF and farther inland compared to other guidance. The newer 00Z ECMWF seems still farther inland and heavy compared to other guidance, but at least not to the same extreme as the previous run. The WPC forecast transitioned from a multi-model deterministic blend of the 18Z GFS and 12Z ECMWF/CMC/UKMET to incorporate the GEFS and EC ensemble means in the blend, with half models/half means composing the forecast blend by Days 6-7. This minimized individual model differences while capturing the overall pattern. ...Weather/Hazards Highlights... Moisture overrunning the cold front that is forecast to stall over the northwestern Gulf (but track a bit more progressively through Florida) will lead to some rain across the south-central to southeastern quadrant of the U.S. for the weekend into early next week. The highest rainfall totals are likely to be over southern Texas near a meandering surface low along the front, with eastern Pacific and Gulf moisture combining. Not much instability is likely to be in place since this will be after the frontal passage, so rain rates should not be too high. There is also considerable uncertainty with how much rain spreads inland, with rain totals maximized over the Gulf. Thus through the weekend, there are no flash flooding areas delineated in the Excessive Rainfall Outlook. Into Monday and beyond though, there may be more support for convection from Texas into the Lower Mississippi Valley as mid-upper level energy approaches, so this will continue to be monitored. Rounds of frontal systems and shortwaves pushing through the Northwest will maintain precipitation chances across the Pacific Northwest and the northern Rockies over the weekend. Moderate to heavy amounts are likely to be directed into the northern parts of the Olympics and Cascades, with lower elevation rain and higher elevation snow. Starting Monday and especially by Tuesday-Wednesday, upper troughing will deepen over the eastern Pacific and direct moisture plumes/atmospheric rivers into the West, especially California. Expect heavy lower elevation rain and higher elevation snow. On Saturday, most of the lower 48 will be under a post-frontal cool high pressure system, with temperatures near to below average. The exception will be the Florida Peninsula, with highs well into the 80s actually close to daily record highs. Temperatures over Florida should gradually moderate to near normal early next week as the cold front presses through. As upper ridging builds across the north-central U.S. early next week, above normal temperatures by around 10-20F will overspread the central and northern Plains into the Midwest, with some above average pockets back into the Great Basin as well. Meanwhile below normal highs should linger in the south-central U.S. with lower heights overhead and clouds and rain possible, and interestingly lead to similar highs from Texas all the way north to South Dakota or so (in the upper 50s and 60s). Near to cooler than normal temperatures over the Eastern Seaboard this weekend should gradually moderate through the early part of next week. Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw