Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 AM 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. Southern Texas may see the highest rainfall totals with this, but there is still considerable uncertainty with how much rain falls onshore versus offshore and how much instability will be in place to enhance rain rates. 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... As the medium range period begins Sunday, a shortwave moving through the northern tier shows some differences between models, made more complex by split flow over the Great Lakes as there are variations in models with where the troughing farther south in the Midwest splits with ridging farther north into southern Canada. Most models seemed reasonably agreeable with this feature now, with the exception of the 12Z UKMET that indicated troughing where other guidance shows ridging. While this was relatively minor, since it was an out of phase difference that could not be blended easily, did leave the UKMET out of the model blend for this reason. Recent model cycles are becoming more agreeable with the trough amplitude and timing as it crosses the East into the Atlantic as the week progresses, where a deterministic and ensemble model blend worked well. Early in the week, shortwave energy will be located over northern Mexico and likely track northeast into the southern U.S. embedded south of the upper ridge by Tuesday-Thursday, with some questions with the timing/track specifics and when the feature erodes. Relatively small differences were seen with the feature initially aloft, but these differences contribute to even more spread for QPF details across the far southern tier than what might be suggested by the comparisons aloft. ECMWF runs have been particularly heavy along and inland from the Gulf Coast while the GFS is most suppressed with the moisture, with the CMC and UKMET in between. By midweek into Thursday, 12/18Z GFS runs had the shortwave disappearing/absorbed while the non-NCEP kept some semblance of it. Favored the latter and fortunately the 00Z GFS came in with keeping the feature existing longer. Regarding the larger-scale flow, for several days models have shown the high likelihood for amplified troughing to develop over the eastern Pacific and affect the West Coast, with a corresponding increase of precipitation over the western U.S. and in particular California toward midweek. However there has been quite a bit of spread and run-to-run variability for details within the overall trough, thus limiting confidence in specifics of precipitation timing/coverage/amounts. These details include the distribution of energy within the trough and potential for upper low(s) formation. But models have at least converged better over the past day overall regarding the axis of the trough, including the ECMWF suite slowing somewhat to be more similar to the other guidance, so a blend of the deterministic and ensemble guidance seemed to provide a good middle ground solution for the trough and the downstream ridge. However, the newer 00Z guidance has overall come in a little faster/farther east with the trough than the 12/18Z suite used for the forecast. Thus the WPC forecast continued the approach of transitioning from a multi-model deterministic blend (18Z GFS, 12Z ECMWF, and 12Z CMC) early in the period toward a mix of models and means (18Z GEFS and 12Z ECens) to downplay some of the less confident 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. continuing into the first half of next week. The highest rainfall totals are likely to be over southern Texas and perhaps shifting into southern Louisiana near a meandering surface low along the front, 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. But on both days, there is considerable uncertainty with how much rain spreads inland, with rain totals maximized over the Gulf. Thus there are no flash flooding risk areas delineated in the Excessive Rainfall Outlook for Sunday and Monday, but this potential rainfall event 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 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 for individual components of the trough. 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 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 the warmest highs (nearing 70F) in the central Plains rather than farther south. 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