Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
927 FXUS02 KWBC 220719 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 219 AM EST Fri Nov 22 2024 Valid 12Z Mon Nov 25 2024 - 12Z Fri Nov 29 2024 ...Messy Thanksgiving Day travel possible in the eastern half of the U.S., including potential northern tier winter weather... ...Overview... The general upper-level pattern next week will start with quasi- zonal flow atop the lower 48 with embedded shortwaves at various times and regions, transitioning to a broad deepening trough pattern by late week. Some moist inflow, though much weaker than the ongoing AR event, will continue to spread into parts of the West and produce precipitation early next week. Precipitation is likely to spread across the central Great Basin/Intermountain West/central Rockies including potential for heavy snow in higher elevations. A forming low pressure/frontal system is forecast to help focus moisture farther east by midweek and beyond. Currently expect moderate to heavy precipitation across the Tennessee and Lower Ohio Valleys into the Appalachians by Wednesday and Thursday, and some precipitation may be in the form of snow in higher elevations and farther north across the Great Lakes and interior Northeast, with questionable precipitation types in the Mid- Mississippi and Upper Ohio Valleys. Increasing coverage of colder than average temperatures is likely as the week progresses underneath the deepening upper troughing and cold surface high pressure. ...Guidance/Predictability Assessment... Model guidance is in fairly good agreement regarding the overall pattern next week but varies with the details of smaller scale features. A shortwave in the north-central U.S./south-central Canada is one example. The 12Z ECMWF was more aggressive with a secondary shortwave (behind a reasonably agreeable one progressing along the U.S./Canada border from northern Minnesota toward Lake Superior and eastward) dropping southeast toward North Dakota Tuesday-Wednesday than other deterministic models and AI/ML guidance, with some impacts on the surface low track. The more recent 00Z run of the ECMWF seems to be in better alignment. Farther west, the 12Z UKMET was on its own holding onto a shortwave just offshore northern California into Wednesday morning while other models brought that energy farther inland into the West. How much that energy digs, as well as its interactions with northern stream shortwaves, will impact the depth and positioning of the deepening broad trough into later week. The 12Z CMC held troughing back into the central Great Basin into Thursday unlike other guidance that was more progressive. The forming surface low and frontal system on the southern/eastern side of the upper trough show some notable spread, but the 00Z ECMWF and CMC happened to agree on a low track across the Southeast Thursday and turning north, reaching offshore of NYC or so by Friday morning. Uncertainties with flow upstream from this trough will also have to be resolved in future forecasts. The WPC forecast was based on a blend favoring the deterministic 12Z EC/CMC and 18Z GFS with lesser proportions of the GEFS and EC ensemble means and the UKMET early on. Gradually reduced the proportion of deterministic models in favor of the ensemble means, with the latter over half by Days 6-7 given variations with the individual models. ...Weather/Hazards Highlights... An upper trough/possible closed low and a surface low offshore of the Pacific Northwest could continue to direct moisture into the West Coast early next week, though much weaker than the current atmospheric river. By Monday the highest moisture anomalies look to focus into southern California, with generally modest rain expected near coastal areas there, but do not currently expect flooding concerns with this rain since this area has not been impacted as much by recent heavy rain. However, some precipitation is likely into northern California too, which will have much wetter antecedent conditions after the past few days. Rain amounts of 1" to locally 2" of rain would not typically cause concerns in northern California/far southwestern Oregon, but there may be a chance for some flooding because of saturated soils and above normal streamflows, so have added a Marginal Risk to the Day 4/Monday ERO for those areas. Meanwhile snow is likely to accumulate across the Oregon Cascades and especially the Sierra Nevada Monday-Tuesday, spreading east across the Intermountain West higher elevations including the Wasatch and into the central Rockies. Heavy snow amounts could pile up especially in the Sierra (1 to 3 feet) and in the Colorado Rockies. Precipitation should finally abate in the West for the latter part of next week. Farther east, a round of light to moderate precipitation is forecast to move through the east-central U.S. Monday ahead of a cold front as a relatively weak surface low moves through the Great Lakes region. Most precipitation should be rain other than some northern tier snow. Lake effect rain/snow showers should continue into Tuesday behind the low. Then as another low pressure/frontal system develops in the south-central Plains and tracks eastward as the week progresses, moisture will stream into the eastern half of the CONUS. Currently the heaviest rainfall axis looks to be around the Mid-South/Tennessee Valley/Lower Ohio Valley, though with uncertainty in placement. North of the low track and frontal boundary, snow or ice may be possible. Some wintry weather may affect the Middle to Upper Mississippi and Ohio Valleys along with the Great Lakes region Wednesday-Thursday, while currently the highest probabilities of notable snow are into the central/northern Appalachians into northern Pennsylvania and the interior Northeast by Thanksgiving Day, possibly impacting travel. Continue to monitor forecasts as details are likely to change. Temperatures are forecast to be warmer than average in the southern tier stretching into the east-central U.S. on Monday ahead of a cold front. This front will cool the Ohio Valley by Tuesday and then the East by Wednesday, leading to highs a few degrees below normal. A couple of surges of Arctic air look to impact the northern Plains early in the week and again later week, with highs in the teens and lows below 0F in some areas. The second surge of cold air is forecast to make it through much of the lower 48 (in moderated fashion) by later next week, even cooling the southern tier to near or below normal after a warm early week period with upper 80s in southern Texas. 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 (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 $$