Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM EST Sat Feb 04 2023 Valid 12Z Tue Feb 07 2023 - 12Z Sat Feb 11 2023 ...Guidance/Predictability Assessment... Despite reasonably similar larger scale pattern transition from split flow to amplified flow next week over the country, recent models and ensembles have been offering less than stellar forecast spread with the weather focusing embedded systems. This is evident starting early in the week with northern stream timing differences and the extent of phasing with a main weather focusing southern stream closed low once slated to eject out from the Southwest through the east-central U.S. with some fanfare, but there is a growing signal for later next week flow amplification and slowing with development of potent upper troughs off the West Coast and the east-central U.S., and amplified upper ridges over the west-central U.S. and western Atlantic. Two main forecast scenarios continue to exist in guidance associated with the emergence of a separated southern stream within emerging split flow over the nation. In this flow, a closed upper low develops in the Southwest/northern Mexico Tuesday-Wednesday. Recent GFS/Canadian runs and to a lesser extent the GEFS mean have been slower to eject this feature downstream through the east-central U.S. next week than recent ECMWF/UKMET runs and to a lesser extent the ECMWF ensemble mean. Ensemble members by and large showed similar progression trends as their parent models with the GEFS/Canadian/NAEFS ensemble means less progressive than the ECMWF ensemble mean, but the progression differences are less than as in the models. This remains a difficult emerging split flow forecast with low predictability, but still suspect that a solution on the less progressive side of the full envelope of guidance makes more sense given much guidance does develop a separated southern stream system of which often are on the slow side to eject. Models and ensembles are slowly converging on more common progression more in line with ensemble means with the 00 UTC GFS/CMC trending slightly faster and the 00 UTC ECMWF/UKMET trending slightly slower, but they still have some way to go to finally reach a common solution. Accordingly and overall, the WPC medium range product suite through medium range time scales was primarily derived from a composite blend of the 18 UTC GEFS mean and 12 UTC ECMWF ensemble mean given uncertainty of model details along with WPC continuity and the 01 UTC National Blend of Models. ...Weather/Hazard Highlights... Early next week rounds of light precipitation will pass over portions of the Great lakes and the Northeast region with shortwave passages and the approach of a lead low/frontal system. There may be lake enhancement of the snowfall for the favored areas downwind of the Great Lakes which could result in higher amounts than surrounding locations. Moderate precipitation is expected to overspread the West Coast and inland to the Intermountain West and Rockies region early next week. For most of lower coastal areas the precipitation will likely fall as rain and then transition to snow for the mid-base to highest elevations; with typical terrain enhancement across the coastal ranges and the Sierra Nevada. As the low pressure system advances eastward the precipitation ahead of it could overspread the central U.S. The details of this precipitation including placement and precipitation type remain uncertain given the model differences. The current forecast shows enhanced rainfall amounts centered across the Southern Plains and Lower Mississippi Valley and surrounding areas by Tuesday-Wednesday with enhanced low/frontal system induced return Gulf of Mexico moisture inflow and instability. WPC has identified an area across the eastern Southern Plains and parts of the Lower Mississippi Valley into the Tennessee Valley as having the potential for excessive rainfall with best moisture convergence and favorable right entrance region upper jet dynamics. WPC experimental Marginal Risk areas are to be issued for that area Tuesday into Thursday. The northern edge of the precipitation shield has the potential for a swath of snow/ice across portions of the Southern Plains to Midwest and interior Northeast next week. Upstream eastern Pacific system approach Tuesday/Wednesday should offer modest system amplitude and associated moisture/rainfall potential across the Pacific Northwest along with terrain/mountain snows inland from the Northwest through the northern Great Basin/Rockies. There is though an aforementioned and growing guidance signal that this leads to the amplification of the oevrall flow significantly into later next week across the nation and adjoining oceans, but with limited precipitation re-emergence later next week to monitor. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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