Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EDT Mon Oct 28 2024 Valid 12Z Thu Oct 31 2024 - 12Z Mon Nov 04 2024 ...General Overview... An initially energetic upper trough and moderate surface low will lift northeastward from the Midwest through the Northeast Thursday/Friday overtop a warm ambient upper ridge over the eastern-southeastern U.S.. An associated cold front will swing through the east-central to eastern U.S. to focus lingering showers and thunderstorms and usher in moderating post-frontal high pressure. The trailing portion of the front is forecast to slow/dissipate, but remain a convective focus to eminate out from the south-central U.S. with return flow. Meanwhile upstream, robust Pacific upper trough energy digs sharply into an unsettled West, with moderate rains and some elevation snows over the Northwest, and more spotty activity further inland and down the coast across California. This pattern may lead to closed trough/low development near southern California/Baja. Warm and amplified subtropical ridging over the Southeast look to remain in place through the period. ...Guidance/Predictability Assessment... Deterministic guidance is in good agreement for the early part of the period (late week/early weekend) depicting an initial upper- level shortwave over the Midwest passing over the East Coast by Saturday. Upper-level ridging will build northward in its wake ahead of upstream energies significantly deepening an upper-level trough over the West. The main uncertainty is in the latter part of the period early next week with respect to the finer details on the evolution of the upper-trough over the West as well as the degree of amplification/strength of the eastern upper-level ridge and displacement of northern stream flow. The deterministic and ensemble guidance differ on how upstream energies digging southward in the West may lead to the development of a deep closed low over the Baja/northwest Mexico. The past few runs of the GFS through 00Z have shown a bit more amplified pattern with a sharpening upper- trough and northward building ridge to the east, while the ECMWF depicts the development of this closed cut-off low over Baja Mexico with a strong but less amplified area of high pressure to the east. The 06Z GFS did trend slightly closer to the more recent runs of the ECMWF, suggesting the development of a weaker closed low that does not cutoff from the mean flow, before the 12Z trended back to showing varying northern/southern stream energies within the broad trough but no trend to develop a closed low on the southern end. Most of the guidance in the suite of AI models from the EC actually tend to favor the pattern shown in the GFS. The GEFS/ECens ensemble means are remarkably similar in placement with a deep upper trough while the CMC ensemble mean develops a closed low more similar to the ECMWF. The CMC ensemble is also less amplified with the ridging over the east compared to the GEFS/ECens and continues more zonal flow over the northern tier of the country. The updated WPC forecast uses a composite blend of the deterministic guidance through the early to mid-period given good agreement. Opted to include a contribution from the ECens and GEFS means for the latter part of the period which, when the GFS remains included, does not offer as much detail over the Baja/northwest Mexico, but rather a compromise solution given uncertainties in the evolution and lack of support from the EC AI guidance. While the lingering uncertainties don't have a significant effect on the broad overall pattern that will favor cooler temperatures in the West, warmer temperatures in the East, and heavy precipitation chances where the upper-jet/surface frontal systems remain in place over the center of the country, varying evolutions/embedded energy timing specifically over the West show larger discrepancies in precipitation chances. For the QPF forecast, the means were relied on more significantly in the West compared to central portions of the country. ...Weather/Hazards Highlights... Ahead of a sharpening front, a long fetch of return flow from the Gulf of Mexico appears to initiate and support a lingering threat of moderate to heavy rain over the Lower Mississippi Valley and vicinity along with a possible severe weather threat holding into Thursday. Maintained a WPC Excessive Rainfall Outlook Marginal Risk for this area, though the antecedent conditions are quite dry and most of the rain will hopefully be beneficial. The trailing portion of the front is then forecast to stall over the southern Plains where moderate to heavy rainfall could redevelop with ejecting impulses and instabilities. Accordingly, a WPC ERO Marginal Risk has been introduced over the southern High Plains to address potential. Expect activity to expand and focus along a northeast-southwest axis ahead of additional frontal boundaries Sunday and into early next week as robust upper trough/closed low energy digs upstream into the West Coast to re- amplify the flow over the Lower 48 and vicinity. Multiple days of southerly, moist Gulf flow look to increase the chances of more widespread, heavy rainfall with flooding concerns despite dry antecedent conditions, especially over the central/southern Plains into the Middle Mississippi Valley. Precipitation associated with the next Pacific system and long- fetch moisture is forecast to re-focus into the West Coast later in the week and into next weekend, with high-elevation snow reaching into the northern Rockies and modest rains reaching down through California. Here, rainfall could be heavier (per recent ECMWF AIFS trends) should a slower/stronger solution verify. Regardless, the cold frontal surge and possible closed low development at the southern base of the main upper trough will bring in cooler temperatures to much of the West to west-central U.S.. Some lingering frost/freeze concerns will remain for southern areas that have yet to reach freezing this season. Upper ridging together with a pre-frontal long fetch of strong and gusty southerly winds will transport warmth from the South through the Northeast Thursday before moderating into the weekend post- frontal passage. Temperatures will return to above average early next week. Putnam/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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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