Extended Forecast Discussion NWS Weather Prediction Center College Park MD 229 PM EST Thu Nov 09 2023 Valid 12Z Sun Nov 12 2023 - 12Z Thu Nov 16 2023 ...Moisture directed into California could lead to heavy rain and snow Tuesday-Thursday... ...Overview... Guidance generally shows the upper pattern transitioning from fairly low amplitude mean flow with progressive shortwaves toward a more amplified regime consisting of a deep eastern Pacific trough, a broad ridge aligned over the central U.S., and a trough reaching the western Atlantic. This evolution will favor dry weather over a majority of the lower 48 and a trend to well above normal temperatures over the northern two-thirds of the central U.S. into Midwest. The most likely areas of precipitation focus during the period will be along the West Coast, with gradually increasing coverage and intensity ahead of the amplifying Pacific trough that should eventually direct an atmospheric river into California in particular, and across the southern tier/Gulf Coast states where moisture should overrun a slow-moving front over the Gulf of Mexico. A southern stream shortwave/upper low ejecting from northwestern Mexico after Sunday should contribute to this activity, with the region trending wetter over the past day. Along the Eastern Seaboard, one or more shortwaves pushing through the northern tier of the country early next week and forming the midweek Atlantic trough will lead to cooler than average temperatures before a moderating trend as the week progresses. ...Guidance Evaluation/Predictability Assessment... Guidance indicates the medium-range period will feature amplifying mean long-wave ridging in the northern stream, shifting west to east across the CONUS, as a southern stream shortwave passes across the South and a deep trough digs southward off the West Coast. Recent runs of the GFS and ECMWF have shown timing and strength differences with an initial shortwave over the Upper Midwest and the embedded southern stream wave over northwestern Mexico beneath ridging over the western U.S. However, the most recent 00Z/06Z runs have at least converged towards a similar solution even as confidence remains a bit low given the run-to-run variability. The northern wave should shift eastward through the early-mid period as a subsequent surface system traverses the Upper Great Lakes/Northeast, while the southern wave passes slowly over the south-central and then southeastern U.S. helping to encourage rainfall chances north of a quasi-stationary boundary in the Gulf. All of the guidance looks agreeable on the northern wave evolution while the 12Z CMC differs in strength/phase with the southern wave. The 00Z ECens mean also offers support for the clustered deterministic solutions of the southern wave while the GEFS mean notably lacks any signal entirely. On a broader scale, the 00Z CMC/ECMWF are more amplified with the expanding longwave ridging over the western then central U.S., as recent runs of the GFS have been shifting less amplified, though reverse a bit with the 06Z run. Attention in the mid-late period turns to the West Coast as energy over the northeastern Pacific drops southward with continued notable run-to-run differences with regards to all aspects of the system evolution, including timing of embedded shortwave energy, closed/open wave, and phasing as the mean troughing either pushes closer towards or lingers off the West Coast. This will continue to have impacts on subsequent precipitation chances. The 00Z/06Z ECMWF/CMC/GFS have a general consensus early on with a slow southward progression as an initial surge of upper-level energy from the northeastern Pacific drops southward off the West Coast, while the 00Z UKMET has a stronger closed low in comparison. Later, the ECMWF has been faster and more aggressive with this first wave of upper-level energy, deepening troughing along southern California, while the mean trough axis in the GFS/CMC lingers further off the coast. There are also subsequent differences across the guidance with respect to additional surges of energy dropping southward, with the 00Z ECMWF indicating a secondary wave will reinforce troughing off the West Coast while this energy remains to the north in the most recent 06Z run of the GFS. The 06Z GFS also trends back towards a closed low cutting off and lingering further off the West Coast given the lack of reinforcing energy dropping southward, and the 12Z GFS update continued this trend. The 00Z ECens mean offers a general compromise between solutions in terms of the axis of mean troughing. The 00Z GEFS mean is similar here as well, though the lack of agreement further east with the southern stream wave complicates finding a broad general consensus. The updated WPC forecast blend features a composite of the deterministic guidance early in the period given good overall agreement with respect to all systems. The contribution from the UKMET is removed by the mid-period as the evolution of the western system diverges, with a steadily increasing contribution from the ECens mean given its fit within the deterministic solutions with respect to both the southern stream wave and the western system. Variable contribution reductions are made with the other deterministic guidance depending on their divergence from ECens mean as a compromise solution, and a lower contribution from the GEFS mean is included due to differences with respect to the southern wave, which now appears to linger over the southeastern U.S. longer than prior forecasts. Given the noted run-to-run variability opted to also include a bit of continuity mid-late period. ...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. continuing into next week. The highest rainfall totals are likely to shift from southern Texas Sunday eastward along the central Gulf Coast Monday-Tuesday and potentially even into the Southeast/Florida by later next week, with eastern Pacific and Gulf moisture combining. Not much instability will be in place through early next week since the region will be in a post-frontal airmass, so rain rates should not be too high. By Monday though, there may be more support for convection from Texas into the Lower Mississippi Valley as mid-upper level energy approaches and possibly increases instability, and the potential for heavier rain totals has trended upward as the guidance indicates this mid-upper energy may be stronger and track slower over the region. Some of the deterministic guidance has become particularly aggressive wtih rainfall totals along the central Gulf Coast around Monday. However, with the heaviest rainfall expected to remain along the Gulf Coast, and antecedent conditions already very dry, the risk for flash flooding should remain very low unless there is a substantial increase in forecast totals, and no areas are delineated in the Excessive Rainfall Outlook for Sunday and Monday. Rounds of frontal systems and shortwaves pushing through the Northwest will maintain precipitation chances across the Pacific Northwest and the northern Rockies through early next week. Moderate to heavy amounts may 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-Thursday, 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. While confidence is somewhat above average in a period of enhanced precipitation associated with the overall upper trough, it is much lower in terms of specifics (timing, coverage, amounts over particular locations) due to model spread and variability of individual shortwave energies. On Sunday, much of the East will be under a post-frontal cool high pressure system, with temperatures near to below average, but warming closer to normal Tuesday-Wednesday and perhaps reaching above normal by Thursday. The exception will be the Florida Peninsula, with highs well into the 80s on Sunday moderating gradually closer to normal after 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 above average pockets extending back into the Rockies and Great Basin as well. Meanwhile, below normal highs are expected in the south-central U.S. early next week with lower heights overhead and clouds and rain possible, before moderating midweek. Putnam/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