Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1249 AM 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 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... Model and ensemble solutions are reasonably well clustered valid early next week, and a composite solution along with the 01 UTC National Blend of Models seems to provide a steady starting point for the forecast. That said, the most noteable difference relating to threat messaging is whether the heaviest axis of rainfall works more into the west-central Gulf Coast or hold more offshore into the Gulf. The 06 UTC GFS and 00 UTC Canadian/ECMWF and ECMWF ensembles show more deep moisture and inland rainfall potential Monday and Tueaday than the 06 UTC GEFS and 00 UTC UKMET. Guidance trends are overall favorable for some runoff issues into coastal areas given possible duration and rising QPF amounts, especially for urban flooding despite generally limited intensities and considering antecedent conditions. Forecast spread then becomes increasingly problematic Tuesday onward next week along/off the West Coast with the extent of digging/phasing of upper trough energies. A solution closer to the nicely compatable 06 UTC GEFS/ 00 UTC ECMWF ensemble means offers a seemingly middle of the road mean upper trough solution just off the West Coast that despite the lack of embedded feature depiction seems most prudent given the lack of a common model sigmal nor clear upstream pattern to favor any model camp at this time. ...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, small Marginal threat areas have been introduced in 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 direct moisture plumes/atmospheric rivers into the West, especially California. Expect heavy lower elevation rain and higher elevation snow, especially for favored slopes of the Sierra. 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