FXUS06 KWBC 181957 PMDMRD Prognostic Discussion for 6 to 10 and 8 to 14 day outlooks NWS Climate Prediction Center College Park, MD 300 PM EDT Wed September 18 2024 6-10 DAY OUTLOOK FOR SEP 24 - 28 2024 A strong anomalous ridge stretching from northeastern Canada southward to southwestward into much of the central and eastern Contiguous United States (CONUS) is expected to remain approximately in place through the 6 to 10 day period while gradually weakening. The strength of this ridge will have peaked just prior to this period, but 500-hPa height anomalies are forecast to average at least +27 dm over southern Baffin Island, and the Canadian operational model shows a 576 dm closed 500-hPa height circulation on day 6. To the east and south, an area of near- or below-normal 500-hPa heights is forecast over and near the southeastern CONUS. Individual members of the ensembles show the potential for tropical or subtropical cyclone development, but confidence in the placement and timing of any system that might develop is low, with a broad array of possibilities shown. Meanwhile, a mid-level trough over east-central North America should drift eastward. Today’s guidance continues the trend of keeping this feature intact for longer, tracking it into the eastern CONUS by the end of the period. The ensemble means are in slightly better agreement than yesterday on the evolution of this weak mid-level trough, and a compromise of the guidance was preferred. Farther west, the models are in better agreement than yesterday as well, although substantive differences remain. The GEFS and European ensemble means both depict a shortwave through moving into western Canada, then weakening as it rotates northeastward, while the longwave trough reintensifies near western Alaska. The Canadian ensemble mean is different, keeping the initial shortwave farther west and deamplifying the trough more quickly. The majority solution is preferred, pulling an initial shortwave farther east than anticipated yesterday, then reloading the trough farther west. The initial shortwave amplifies a mid-level ridge to its southeast over west-central North America which weakens but remains identifiable through the forecast period before settling over the Rockies or Plains. Overall, these changes result in a somewhat less amplified pattern across North America by day 10, but with strong positive height anomalies still centered in northeastern Canada, a mid-level trough affecting Alaska, flat ridging with above-normal 500-hPa heights in the central and western CONUS, and a very weak mid-level trough farther east. Near-normal 500-hPa heights are forecast for Hawaii, with a weak cyclonic circulation situated west of the state that slowly dissipates through the period. Despite uncertainties in the evolution of unsettled weather in the Southeast, the tools derived from the ensemble means are in fair to good agreement, but with some notable exceptions. In addition to differences in the model solutions, additional uncertainty comes from inconsistencies between the raw and bias-corrected temperature and precipitation output on one hand, and the calibrated and reforecast data on the other. There is general agreement that above-normal temperatures will prevail across most of the CONUS, with the best chances for above-normal temperatures across parts of the Great Lakes and interior Northeast. Significantly enhanced odds for above-normal temperatures also cover parts of the interior West, consistent with the forecast for a stronger and longer-lasting mid-level ridge than anticipated in yesterday’s forecast. Meanwhile, near-normal temperatures are favored in parts of the Eastern Seaboard, which is consistent with the presence of surface high pressure over southeastern Canada or the Northeast bringing in mild, maritime air on persistent easterly winds. The reforecasts are warmer here than the raw and bias-corrected tools, but the cooler solutions are more consistent with the mid-level pattern and the forecast location of surface high pressure. In addition, clouds and precipitation in and near the southeastern CONUS have the potential to keep temperatures down. There is also considerable uncertainty across Alaska, where raw and bias-corrected tools show colder than normal conditions covering most or all of the state while the reforecast and calibrated outputs lean toward warmer than normal weather in southeastern Alaska and the eastern Mainland. With good agreement between the GEFS and European ensembles, there is increased confidence in a scenario which brings one storm system across southeastern Alaska early in the period, followed by another system approaching from the west, spun up by the reloading mid-level trough. This would favor a colder solution in central and western Alaska, with variable temperatures averaging near normal farther east. Meanwhile, warmer than normal weather is favored over the northwestern half of Hawaii, consistent with the consolidation forecast. The European ensemble mean shows cooler weather across the islands, but this is not consistent with the expected mid-level pattern. In the southeastern CONUS, there is a lot of uncertainty regarding the formation and evolution of surface low pressure systems, including the possibility for tropical or subtropical cyclone development. However, most potential scenarios would result in heavy rain affecting at least part of the Southeast. Chances for abnormally wet weather exceed 50 percent in the South Atlantic region, where guidance is in good agreement showing unusually heavy rain; however, it should be emphasized that there is a large degree of uncertainty regarding the development and track of any tropical or subtropical cyclones that form. The likelihood of unusually heavy precipitation also exceeds 60 percent in southeastern Alaska, where two impactful storm systems are expected during the period. Elsewhere, enhanced chances for surplus precipitation extend northward up the Atlantic Seaboard into lower New England, across the Ohio, Tennessee, and Middle and Lower MIssissippi Valleys, and the south-central Plains, consistent with most of the derived precipitation tools. Farther west, the mid-level ridge pushing into the northwestern and eventually the north-central CONUS should inhibit precipitation development, resulting in a large area with increased chances for subnormal precipitation extending as far south as the southern Rockies. The western half of the Gulf Coast is an area of uncertainty. Raw ensemble mean outputs from the GEFS and European models favor lighter than normal rainfall, particularly over Texas, while reforecast tools call for wetter conditions. Since this area could be affected with a more westward track of potential tropical systems, the forecast splits the difference, favoring above-normal precipitation in the Lower Mississippi Valley, and neither precipitation extreme farther west. Meanwhile, weak cyclonic mid-level flow increases the odds for surplus precipitation over western Hawaii while the consolidated forecast along and many deterministic models lean toward drier than normal conditions in the Big Island. The official 6-10 day 500-hPa height blend consists of 15% of Today's 6z GFS Ensemble Mean centered on Day 8, 25% of Today's GFS Superensemble Mean centered on Day 8, 10% of Today's operational 6z GFS centered on Day 8, 25% of Today's 0z European Ensemble Mean centered on Day 8, 10% of Today's operational 0z ECMWF centered on Day 8, and 15% of Today's 0z Canadian Ensemble Mean centered on Day 8 FORECAST CONFIDENCE FOR THE 6-10 DAY PERIOD: Average, 3 out of 5, with fair agreement on the evolution of the mid-level pattern and areas of where raw and derived temperature and precipitation tools are in poor agreement. 8-14 DAY OUTLOOK FOR SEP 26 - OCT 02, 2024 Deamplification of the mid-level pattern is expected during week-2 is expected, with mostly a zonal pattern covering the North Pacific Ocean and North America by the end of the period. The moderately amplified pattern in place at the start of week-2 should evolve into broadly near- or above-normal 500-hPa heights, with weak mid-level ridging over northern and western North America while a very weak mid-level trough settles into the eastern CONUS. But these features will be subtle, with 500-hPa heights much closer to normal than at the start of the 6- to 10-day period. Still, the favorable set-up for storm development will continue at least through the beginning of week-2, including possible tropical or subtropical cyclone development. This pattern should keep above-normal temperatures over most of the CONUS outside the South Atlantic region and adjacent areas, where surface high pressure to the north and unsettled weather are expected to keep daytime temperatures closer to normal. Meanwhile, slightly enhanced odds for warmer than normal conditions continue over westernmost Hawaii, and the dissipating mid-level trough also increases the odds for warmer than normal weather in northeastern Alaska. The only areas with increased chances for subnormal temperatures are western and south-central Alaska, where subnormal 500-hPa heights will persist the longest. Enhanced chances for wet weather continue in southeastern Alaska initially downstream from a mid-level trough, but the relaxing 500-hPa flow will lower expected precipitation totals later in the period, so the odds of above-normal precipitation for week-2 as a whole are lower than in recent forecasts. However, tools continue to depict unsettled weather, including the potential development of a tropical or subtropical cyclone, over the southeastern CONUS. Therefore, enhanced chances for above-normal rainfall still exceed 50 percent in the South Atlantic region from the central Carolinas southward. Precipitation guidance derived from the deterministic models bring increased chances for surplus precipitation farther north today, so the area where wetter than normal weather is favored extends northward into the Northeast and lower New England. Guidance remains inconsistent in the central Gulf Coast region, but odds lean toward near- or above-normal precipitation in the official forecast because there is a nontrivial chance that a more westward track of storms forecast across the Southeast, may influence conditions over the central and western Gulf of Mexico as well. Meanwhile, drier than normal weather is favored across a large area covering most of the central and western CONUS under weakening mid-level ridging and nondescript 500-hPa flow. Slightly increased odds for near normal rainfall remain in place over westernmost Hawaii, but deterministic model output at the consolidation forecast lean toward drier than normal weather in the eastern islands, especially on the southeastern tier of the Big Island. The official 8-14 day 500-hPa height blend consists of: 40% of Today's 6z GFS Ensemble Mean centered on Day 11, 40% of Today's 0z European Ensemble Mean centered on Day 11, and 20% of Today's 0z Canadian Ensemble Mean centered on Day 11 FORECAST CONFIDENCE FOR THE 8-14 DAY PERIOD: Below average, 2 out of 5, due the inherent uncertainties involved in forecasting a mid-level pattern change, some areas of disagreement among the temperature and precipitation tools, and significant uncertainty on the evolution of the wet pattern in the Southeast and perhaps farther west along the Gulf Coast. FORECASTER: Rich Tinker Notes: Automated forecasts are issued on Saturday and Sunday. Occasionally manual intervention is necessary to address quality control and consistency issues. In these cases, forecasts are manually drawn but a full discussion is not issued. The notation for the categorical forecast indicated on the maps is the same as that in the tables: A-above N-near normal B-below The temperature map shows regions with > 33% chance of being warmer (orange, "A"), colder (blue, "B"), or close to (unshaded, "N"). Historical average values for the calendar period of the forecast (dashes, "f"). Labels on the shaded lines give the probability (> 33%) of the more likely category (B or A). Probability of N is always < 40%. The precipitation map shows regions with > 33% chance of being wetter (green, "A"), drier (tan, "B"), or close to (unshaded, "n"). Historical median values for the calendar period of the forecast (dashes, "inches"). Labels on the shaded lines give the probability (> 33%) of the more likely category (B or A). Probability of N is always < 40%. In the southwest and other climatologically dry regions - there will be a greater than 33.3% chance of no precipitation and occasionally even a normal (i.e. Median) value of zero - especially during the dry seasons. In such cases a forecast of near normal is effectively a forecast of little or no precipitation. The climate prediction center uses 1991-2020 base period means as reference in the climate outlooks. The next set of long-lead monthly and seasonal outlooks will be released on September 19. Analogs to the 5 day mean observed pattern centered 3 days ago (D-3) for the region from 20N to 70N latitude and 175E to 60W longitude include the 5 day periods centered on the following dates: 20050905 - 20040920 - 20070923 - 20070929 - 19880919 Analogs to the 7 day mean observed pattern centered 4 days ago (D-4) for the region from 20N to 70N latitude and 175E to 60W longitude include the 7 day periods centered on the following dates: 20070922 - 20070927 - 20040919 - 19830930 - 20050905 6-10 DAY OUTLOOK TABLE Outlook for Sep 24 - 28 2024 STATE TEMP PCPN STATE TEMP PCPN STATE TEMP PCPN WASHINGTON A B OREGON A B NRN CALIF A B SRN CALIF A N IDAHO A B NEVADA A B W MONTANA A B E MONTANA A B WYOMING A B UTAH A B ARIZONA A B COLORADO A N NEW MEXICO A N N DAKOTA A B S DAKOTA A B NEBRASKA A N KANSAS A A OKLAHOMA A A N TEXAS A N S TEXAS A N W TEXAS A N MINNESOTA A B IOWA A N MISSOURI A A ARKANSAS A A LOUISIANA A A WISCONSIN A N ILLINOIS A A MISSISSIPPI A A MICHIGAN A N INDIANA A A OHIO A A KENTUCKY A A TENNESSEE A A ALABAMA A A NEW YORK A A VERMONT A N NEW HAMP A N MAINE A N MASS A A CONN A A RHODE IS N A PENN A A NEW JERSEY N A W VIRGINIA A A MARYLAND N A DELAWARE N A VIRGINIA N A N CAROLINA N A S CAROLINA N A GEORGIA N A FL PNHDL N A FL PENIN A A AK N SLOPE B A AK ALEUTIAN B A AK WESTERN B N AK INT BSN B A AK S INT B A AK SO COAST B A AK PNHDL N A 8-14 DAY OUTLOOK TABLE Outlook for Sep 26 - Oct 02, 2024 STATE TEMP PCPN STATE TEMP PCPN STATE TEMP PCPN WASHINGTON A B OREGON A B NRN CALIF A B SRN CALIF A N IDAHO A B NEVADA A B W MONTANA A B E MONTANA A B WYOMING A B UTAH A B ARIZONA A B COLORADO A B NEW MEXICO A B N DAKOTA A B S DAKOTA A B NEBRASKA A B KANSAS A B OKLAHOMA A B N TEXAS A N S TEXAS A N W TEXAS A B MINNESOTA A B IOWA A B MISSOURI A B ARKANSAS A N LOUISIANA A A WISCONSIN A B ILLINOIS A B MISSISSIPPI A A MICHIGAN A B INDIANA A N OHIO A N KENTUCKY A A TENNESSEE A A ALABAMA N A NEW YORK A N VERMONT A N NEW HAMP A N MAINE A N MASS A A CONN A A RHODE IS A A PENN A A NEW JERSEY A A W VIRGINIA A A MARYLAND N A DELAWARE N A VIRGINIA N A N CAROLINA N A S CAROLINA N A GEORGIA N A FL PNHDL N A FL PENIN A A AK N SLOPE N A AK ALEUTIAN B N AK WESTERN B N AK INT BSN N A AK S INT N A AK SO COAST B A AK PNHDL B A LEGEND TEMPS WITH RESPECT TO NORMAL PCPN WITH RESPECT TO MEDIAN A - ABOVE N - NEAR NORMAL A - ABOVE N - NEAR MEDIAN B - BELOW B - BELOW THE FORECAST CLASSES REPRESENT AVERAGES FOR EACH STATE. NORMAL VALUES - WHICH MAY VARY WIDELY ACROSS SOME STATES - ARE AVAILABLE FROM YOUR LOCAL WEATHER SERVICE FORECAST OFFICE. FOR ADDITIONAL INFORMATION SEE MESSAGE FXUS06 KWBC - ON AWIPS AS PMDMRD. $$