Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Wed Nov 08 2023 Valid 12Z Sat Nov 11 2023 - 12Z Wed Nov 15 2023 ...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 the region, 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 during the period 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 this weekend into early next week and forming the midweek Atlantic trough will lead to cooler than average temperatures before a moderating trend next Tuesday-Wednesday. ...Guidance Evaluation/Predictability Assessment... Over the West Coast and eastern Pacific, models and ensembles have been showing a strong signal for amplified troughing to develop 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. In general the ECMWF and some ECMWF ensembles have been fastest to bring leading height falls into the West, including one shortwave which the 00Z ECMWF brings into the Northwest around late Monday-Tuesday and farther south ahead of the larger scale trough (and in particular its opening upper low nearing California early next Wednesday). Recent slower trend for height falls/QPF arrival among consecutive operational ECMWF runs, along with the majority of GFS/GEFS/CMC/CMCens guidance being slower than the ECMWF, would argue for some degree of compromise at the very least. Predictability tends to decrease by 6-7 days out for details of how embedded upper low(s) and upstream energy may ultimately interact. A blend approach by that time depicts the best potential (on a relative basis) for an embedded low to be offshore the Pacific Northwest. The new 12Z ECMWF has trended slower with leading height falls for both aspects of the forecast described above. There is a moderate degree of spread for details of the upper low/trough ejecting out of northwestern Mexico and into the southern Plains, with the 00Z ECMWF/12Z UKMET leaning on the faster side relative to the GFS/12Z CMC. Confidence in specifics would tend to decrease as the feature becomes embedded within/underneath the larger scale upper ridge building over the central U.S. 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. The CMC (heavier over southern Texas) and UKMET (some enhanced totals near the central Gulf Coast) offer varying intermediate ideas. The details continue to be variable for pieces of shortwave energy tracking across the northern tier and then reaching into the eastern U.S./western Atlantic. 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. Operational model runs through the 00Z/06Z cycles (used for the updated forecast) plus the new 12Z cycle seem to be waffling with respect to overall trough amplitude near the East Coast, favoring a multi-model and eventually model/ensemble mean blend. The WPC forecast continued the approach of transitioning from a multi-model deterministic blend (06Z GFS and 00Z ECMWF/CMC/UKMET) early in the period toward a mix of models and means (06Z GEFS and 00Z 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. 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 risk 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, but model guidance continues to diverge significantly for specifics. 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 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. 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 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 which will actually be close to daily records. 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 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 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. Rausch/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