Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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047 FXUS06 KWBC 191917 PMDMRD Prognostic Discussion for 6 to 10 and 8 to 14 day outlooks NWS Climate Prediction Center College Park, MD 300 PM EDT Thu September 19 2024 6-10 DAY OUTLOOK FOR SEP 25 - 29 2024 A strong anomalous ridge stretching from northeastern Canada southward and 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 weakening. The strength of this ridge will have peaked just prior to this period, but even so, 500-hPa height anomalies are forecast to average almost 24 dm over the mouth of the Davis strait. This is slightly weaker and farther east than yesterday, indicating that this feature will be in the process of deamplification when the forecast period starts. To the east and south, a mid-level trough is expected to be in place over the east-central CONUS, with an axis stretching from approximately the Great Lakes southward through the Lower Mississippi Valley and central Gulf Coast region. Models have trended stronger with this feature over the past several days, and that trend continued today. The Canadian ensemble mean is strongest with this feature, nearly closing off a 500-hPa circulation center over northern Missouri. The other models show lesser negative height anomalies centered slightly farther east, and do not indicate the potential for a closed mid-level low to form. The lower amplitude solution is preferred, as depicted by most of the guidance. Its more consistent with yesterdays models, whereas the Canadian ensemble mean represents a rather dramatic shift. However, with the models trending progressively stronger, the Canadian ensemble mean solution cant be discounted. Farther southeast, models depict an area of near- or below-normal 500-hPa heights over or near the southeastern CONUS. Individual members of the ensembles show the potential for tropical or subtropical cyclone development. Confidence is increasing in the development of a tropical system later this period or during week-2, but confidence in the placement and timing of any system that might develop is low. Also, with the aforementioned trough over the interior eastern CONUS looking stronger, the possibility of some interaction between the two features increases uncertainty further. Meanwhile, farther west, the models are in decent agreement showing a mid-level ridge from the western CONUS into the Canadian Prairies, with the largest height anomalies near the Canadian border, a little stronger than yesterday. Elsewhere, there is a strong mid-level trough with its axis through western Alaska at the start of the forecast period. A significant shortwave moving through the longwave trough is expected to rotate northeastward and weaken while the longwave trough reintensifies near western Alaska, increasing the odds that more than one storm system will affect southeastern Alaska. But the models are very inconsistent regarding the location, strength, and timing of any storms that form after days 6 to 8. Overall, these changes result in a somewhat less amplified pattern across North America by day 10, with the strong mid-level ridge centered in northeastern Canada deamplifying more quickly than shown yesterday. Near normal 500-hPa heights are forecast for Hawaii, with a weak cyclonic circulation west of the state slowly dissipating 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, some additional uncertainty comes from inconsistencies between the raw and bias-corrected temperature and precipitation output on one hand, and the calibrated and reforecast guidance 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 stretching from the Great Basin into the northern Plains under the mid-level ridge axis. Meanwhile, over the South Atlantic region and adjacent areas, the surface high pressure over the northeastern CONUS is not as pronounced as yesterday due to the stronger trough west of the Appalachians. This results in less of a cooler east to northeasterly surface wind, which was expected to keep temperatures close to normal in concert with more unsettled weather than usual. As a result, above-normal temperatures are now slightly favored across this region. The reforecasts are again warmer here than the raw and bias-corrected tools, but less weight has been given to the cooler solutions given the expected surface wind configuration. But more clouds and precipitation than usual over the Southeast is still expected to keep temperatures closer to normal than in most other parts of the CONUS. Across Alaska, considerable uncertainty in the temperature pattern remains, but in Southeast Alaska, the big differences between the colder raw and bias-corrected tools and the much warmer reforecast tools is less pronounced today. Todays somewhat cooler reforecast tools give more confidence to a colder solution in eastern Alaska, so slightly enhanced odds for subnormal temperatures are extended eastward to cover most of the state, with the best odds for below-normal temperature over western Mainland Alaska near the longwave trough axis. Meanwhile, warmer than normal weather is favored over the northwestern half of Hawaii, consistent with the consolidation forecast and ensemble mean output. In the southeastern CONUS, there is a lot of uncertainty regarding the formation and evolution of surface low pressure systems, including the increasing likelihood of one or more tropical or subtropical cyclones. 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 westward to near the central Gulf Coast, but the high degree of uncertainty, especially regarding any tropical development, cant be overemphasized. The likelihood of unusually heavy precipitation also exceeds 50 percent in southeastern Alaska, where multiple impactful storm systems are expected during the period. Elsewhere, with the stronger mid-level trough west of the Appalachians, unusually moist air should be steered farther north than expected yesterday, enhancing chances for surplus precipitation northward up the Atlantic Seaboard into lower New England. Farther west, the mid-level ridge over the western to 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 Gulf Coast is an area of uncertainty due to some indications that unsettled weather, including potential tropical systems, could be pulled farther west than expected at this time. Meanwhile, weak cyclonic mid-level flow increases the odds for surplus precipitation over western Hawaii while the consolidated forecast along with most deterministic models lean toward drier than normal conditions in the Big Island. The official 6-10 day 500-hPa height blend consists of 10% of Today`s 0z GFS Ensemble Mean centered on Day 8, 10% of Today`s 6z GFS Ensemble Mean centered on Day 8, 10% of Today`s GFS Superensemble Mean centered on Day 8, 45% of Today`s 0z European Ensemble Mean centered on Day 8, and 25% of Today`s 0z Canadian Ensemble Mean centered on Day 8 FORECAST CONFIDENCE FOR THE 6-10 DAY PERIOD: Below average, 2 out of 5, with fair agreement on the mid-level pattern offset by increased uncertainty later in the period as the mid-level features deamplify, along with the high degree of uncertainty regarding tropical development in or near the Southeast and storm systems affecting Alaska later in the period. Also, there are areas where raw and derived temperature and precipitation tools are in poor agreement. 8-14 DAY OUTLOOK FOR SEP 27 - OCT 03, 2024 Deamplification of the mid-level pattern is expected during week-2, with mostly a zonal pattern covering the North Pacific Ocean and North America by the end of the period. This expectation is a little tentative today, as there are some models showing an amplified 500-hPa pattern toward the end of the period, but there is little agreement on the strength and location of individual features. Given broad inconsistencies, more zonal mid-level flow is favored, though with a little more uncertainty than yesterday given the increased disagreement between model solutions. Presently it looks like 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. The mid-level trough over or near Alaska is expected to linger a little longer than yesterday, consistent with the ensemble means, but this doesnt create significant changes to the forecast mean temperatures and precipitation there. However, most features across North America and adjacent areas are expected to be subtle at this time, with resulting 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 in the Southeast, including possible tropical or subtropical development. One or more significant storms are also expected to affect south-central and southeastern Alaska, but the evolution of individual systems is highly uncertain. This pattern should keep above-normal temperatures over most of the CONUS, especially across the west-central to north-central CONUS under a weak but persistent mid-level ridge. Meanwhile, slightly enhanced odds for warmer than normal conditions continue over westernmost Hawaii. The only areas with increased chances for subnormal temperatures is the Alaskan Mainland, especially in western parts of the state where subnormal 500-hPa heights will persist the longest. Enhanced chances for wet weather continue in southeastern Alaska downstream from a variable mid-level trough. This favors more storminess than normal across that region, but the evolution of individual storm systems is highly uncertain at this time. Similarly, tools maintain unsettled weather over the southeastern CONUS, including the potential of tropical cyclone development, but the day-to-day evolution of individual features is highly uncertain. Enhanced chances for above-normal rainfall still exceed 50 percent in the South Atlantic region. Precipitation guidance derived from the deterministic models bring increased chances for surplus precipitation northward, so the area with enhanced chances for wet weather 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 are some indicators raising the possibility that at least some of the storminess expected in the Southeast could extend farther west. Meanwhile, drier than normal weather is favored across a large area covering most of the central and western CONUS under weakening mid-level ridging. Slightly increased odds for above normal rainfall remain in place over westernmost Hawaii, but deterministic model output shows more rainfall than yesterday in eastern parts of the state, thus the enhanced odds for dryness have been removed. The official 8-14 day 500-hPa height blend consists of: 30% of Today`s GFS Superensemble Mean centered on Day 11, 40% of Today`s 0z European Ensemble Mean centered on Day 11, and 30% 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, the potential for a more amplified solution with high uncertainty in the nature and location of individual features, some areas of disagreement among the temperature and precipitation tools, significant uncertainty on the evolution of the wet pattern in the Southeast and perhaps farther west along the Gulf Coast, and uncertainties in the daily evolution of the wet pattern expected in southeastern Alaska. 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 October 17. 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 - 19880919 - 19831001 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: 20040919 - 20070922 - 20050905 - 19830930 - 19850916 6-10 DAY OUTLOOK TABLE Outlook for Sep 25 - 29 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 N OKLAHOMA A N N TEXAS A N S TEXAS A N W TEXAS A N MINNESOTA A B IOWA A B MISSOURI A N ARKANSAS A A LOUISIANA A A WISCONSIN A B ILLINOIS A N MISSISSIPPI N A MICHIGAN A N INDIANA A N OHIO A A KENTUCKY A A TENNESSEE A A ALABAMA A A NEW YORK A A VERMONT A A NEW HAMP A A MAINE A A MASS A A CONN A A RHODE IS A A PENN A A NEW JERSEY A A W VIRGINIA A A MARYLAND A A DELAWARE A A VIRGINIA A A N CAROLINA A A S CAROLINA A A GEORGIA A A FL PNHDL A A FL PENIN A A AK N SLOPE B A AK ALEUTIAN B N AK WESTERN B N AK INT BSN B A AK S INT B A AK SO COAST B A AK PNHDL B A 8-14 DAY OUTLOOK TABLE Outlook for Sep 27 - Oct 03, 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 B S TEXAS A B 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 N A MICHIGAN A B INDIANA A N OHIO A N KENTUCKY A A TENNESSEE N A ALABAMA N A NEW YORK A N VERMONT A N NEW HAMP A N MAINE A N MASS A N CONN A N RHODE IS A N PENN A N NEW JERSEY A N W VIRGINIA A A MARYLAND A A DELAWARE A A VIRGINIA A A N CAROLINA N A S CAROLINA N A GEORGIA N A FL PNHDL A A FL PENIN A A AK N SLOPE N A AK ALEUTIAN B N AK WESTERN B N AK INT BSN B A AK S INT B 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. $$