Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Fri Nov 10 2023 Valid 12Z Mon Nov 13 2023 - 12Z Fri Nov 17 2023 ...Heavy rainfall potential for the west-central Gulf Coast Monday and Tuesday... ...Moisture directed into California could lead to heavy rain and snow Tuesday-Thursday... ...Overview... It overall remains relevent that recent guidance has been showing an 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 may 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 models continuing to trend wetter in this region. 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... Model and ensemble solutions remain reasonably well clustered during the first half of the week with a shortwave exiting the Northeast and energy moving through the southern Plains/Gulf Coast. By Wednesday and especially beyond, details off the West Coast become increasingly murky. The latest trend in guidance suggests more separation between northern stream energy (which slides through the Northern tier of the U.S. Thursday-Friday) and a deepening upper low which parks itself off the coast of California. Model ensemble plots suggest there is still a fair bit of uncertainty in the evolution of West Coast/East Pacific troughing, but the latest 12z GFS, ECMWF, and CMC have continued to trend in this direction this afternoon. This would result in far less precipitation across California than previously thought, and will continue to be monitored. There is also increasing uncertainty in the timing and depth of the northern stream energy through the Dakotas and into the Great Lakes Thursday-Friday/Day 6-7. Todays updated medium range progs utilized a deterministic model blend for days 3-5, which smoothed out any early period small scale differences. Given the increased uncertainty, trended quickly towards a majority ensemble mean blend for days 6 and 7. This generally maintains good continuity with the previous forecast, with some adjustments based on latest trends. ...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 work across the west-central Gulf Coast Monday-Tuesday and potentially even into the Southeast/Florida by later next week, with eastern Pacific and Gulf moisture combining. Limited 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 with rainfall totals along the west-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 relatively low, but there has been an increase in model forecast totals to consider. Accordingly, Marginal threat areas only continue on the WPC Excessive Rainfall Outlook for Monday and Tuesday to capture some threat for coastal communities. 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 may direct moisture plumes into the West, especially California, but overall precipitation totals have trended lighter in recent model runs. 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 and larger scale upper trough progressins and stream phasings. Schichtel 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