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Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EST Sun Feb 28 2021 30-DAY OUTLOOK DISCUSSION FOR MARCH 2021 The updated March 2021 temperature and precipitation outlooks are not too significantly changed from the previous mid-month outlook. The updated outlooks utilized the latest short-range, medium-range, extended-range and subseasonal dynamical model forecast guidance for some adjustments. In addition, the most recent assessment of boundary conditions (anomalous soil moisture and snow cover), the MJO state and the AO state are also considered in making the update. The uncertainty whether short-range forecasts of a change to a positive AO phase would persist and be supported by changes in stratosphere troposphere interaction have been removed as the extended range forecasts of the AO support continuation of the current positive AO phase through at least mid-month. The MJO influence described at mid-month, however, remains in play as predictions of the MJO signal (i.e., RMM) are forecast to shift from Phase 5/6 (Indonesia/far western Pacific) to potentially Phase 8/1 over the next two weeks. This increases uncertainty in the temperature outlook some for the Great Lakes, Ohio Valley and Northeast as the odds for below-normal temperatures would increase for the 3rd and 4th weeks of March. This is strongly at adds with the latest Week 3-4 dynamical model guidance which continues persistence of favored above-normal temperatures for the eastern two-thirds of the CONUS from Week-2. Anomalous snow cover and depth remains strongly negative across the northern plains as noted in the mid-month outlook. Soil moisture anomalies are likely to become increasingly positive for areas in the lower Mississippi Valley, the Tennessee Valley and lower Ohio Valley as a combination of antecedent conditions from recent snowmelt and heavy rains in some areas prior to the start of the month. Based on the above factors, the updated March 2021 temperature outlook keeps elevated odds for monthly mean below-normal temperatures for southern Alaska, and the Far West - which is a more narrow area than the mid-month outlook depiction. Above-normal monthly mean temperatures remain favored for the eastern two-thirds of the CONUS, however, there are some changes. First, above-normal temperatures in the Southwest were shifted eastward in the updated outlook and probabilities across the northern Plains and Great Lakes are increased. Lower probabilities are located in a zonal swath for the Tennessee and lower Ohio Valley's due to possible impact from above-normal surface wetness and also along the eastern seaboard due to cooler temperatures forecast early in the month and also due to the potential impact from the MJO, albeit highly uncertain. For precipitation, the latest dynamical model guidance, especially in the short term help better focus favored above-normal precipitation in the Tennessee Valley at the start of the month of March. This region and areas northward to the Great Lakes are slightly modified from the half month lead outlook. Below-normal precipitation is highlighted from the interior West and Southwest eastward to the central and southern Plains and along the Gulf Coast and Florida. Changes in these regions from the earlier outlook include removal of the slightly elevated odds for above-normal precipitation in the northern Rockies, removal or reducing of chances for below normal precipitation for parts of California and the central and southern Great Basin and Rockies. A strong jet forecast to impinge western Alaska through at least the first half of the month, favors elevated odds for above-normal precipitation for much of Alaska with removal of the small region of elevated odds for below-normal precipitation along the Alaska south coast in the mid-month outlook. **************************************************************************** *** Previous mid-month discussion below *** **************************************************************************** As we approach the month of March, the pattern across the greater North America region appears to be in the process of changing from the last few weeks. Week 2 dynamical model forecast guidance supports a change to anomalous ridging over the north central Pacific with the central North America mean trough shifted westward and deepening over portions of western North America including into western CONUS. Downstream of this, more amplified ridging and positive 500-hPa height departures are forecast for much of the southeast U.S. favoring a more southwesterly-westerly mean flow for the eastern half of the CONUS then in recent weeks. The most recent Week 3-4 guidance from the CFS, ECMWF and extended GEFS support a continuation of this change, in the mean, for approximately the first two weeks of March. March CFS forecasts as well as those from the majority of the NMME and Copernicus model suites also favor this pattern and associated anomalous temperature and precipitation patterns which are consistent with typical La Nina conditions in many ways during this point in the seasonal cycle. Other factors to consider in addition to a review of the available dynamical model guidance across time scales, is the state and forecast evolution of the MJO, the AO and most recent anomalous snow cover. The MJO has been active during January and February resulting in both constructive and destructive interference with La Nina impacting anomalous tropical rainfall, circulation and other variables in the Pacific basin. Forecast information suggests that the MJO is likely to weaken or become less coherent entering March and is not strongly utilized in the March outlook due to this as well as indications that much of recent mid-latitude temperature and precipitation anomalies have most likely been driven by high latitude subseasonal variability. Predictions in the short-term (next 10 days) indicate the phase of the AO (which has returned to near zero) to remain near zero or weakly positive in the mean as compared to the strongly negative phase of the last couple months. The Sudden Stratospheric Warming (SSW) of early-mid January would continue to elevate odds for a negative AO phase into the month of March resulting in the continuation of potential cold air outbreaks into the mid-latitudes. There is high uncertainty, however, in whether the negative phase of the AO will return and persist and if so where troughing and so favored below-normal temperatures are most likely to develop at forecast lead times well into the month of March. Finally, substantially below-normal snow cover and snow depth exist across parts of the northern Plains and upper Mississippi Valley. Although, early in the seasonal cycle, taken alone, this is likely to favor above-normal temperatures in these areas. After consideration of the factors described above, the March temperature outlook favors monthly mean below-normal temperatures for a region stretching from south-central Alaska southeastward to include northern California, the Pacific Northwest and the northern Rockies. This is strongly supported by dynamical model guidance across time scales and is consistent with La Nina conditions. Elevated probabilities for above-normal temperatures are depicted for parts of the north and west coast of Alaska based on negative sea ice and positive ocean surface temperature trends especially later in the month of March. Dynamical model guidance and in some cas es typical La Nina conditions and positive temperature trends favor above-normal temperatures for a region from the Southwest eastward to include the southern CONUS and northward to encompass the central Mississippi Valley, eastern Great Lakes, Ohio Valley and eastern seaboard. Only slightly tilted odds of above-normal temperatures or Equal-Chances (EC) are depicted for the northern and central Plains, central and upper Mississippi Valley and parts of the Ohio Valley, Great Lakes and Tennessee Valley due to the uncertainty and conflicting indicators described above with respect to the phase of the AO, anomalous snow cover and snow depth and dynamical model guidance. For precipitation, below-normal monthly total precipitation amounts are most likely for the southern tier of the CONUS with some expansion of this highlighted area to the central Great Plains, primarily based on dynamical model guidance which is consistent with La Nina typical impacts. For similar reasons, elevated odds for above-normal precipitation is forecast for a small region in the northern Rockies and for the Ohio Valley and Great Lakes region. For Alaska, elevated odds for above-normal precipitation is depicted for parts of the north and west coasts associated with anticipated more open and warmer waters in nearby areas as well as typical La Nina impacts. Below-normal precipitation is most likely for the south-central coast and far northern Alaska Panhandle. FORECASTER: Jon Gottschalck The climatic normals are based on conditions between 1981 and 2010, following the World Meteorological Organization convention of using the most recent 3 complete decades as the climate reference period. The probability anomalies for temperature and precipitation based on these new normals better represent shorter term climatic anomalies than the forecasts based on older normals. The next monthly outlook...for Apr ... will be issued on Thu Mar 18 2021 These outlooks are based on departures from the 1981-2010 base period. $$
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