Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Mar 01 2020 Valid 12Z Wed Mar 04 2020 - 12Z Sun Mar 08 2020 ...Heavy rainfall threat over the South through Wednesday-Wednesday night... ...Overview and Guidance/Predictability Assessment... Guidance shows a significant pattern change taking place during the period. An initial Gulf of Alaska/Northeast Pacific upper trough will steadily amplify mid-late week and then upstream energy will drop southeastward, most likely closing off an upper low well offshore the West Coast (130-140W longitude) next weekend. Farther to the east a building upper ridge will cross the West/Rockies late this week and the Plains/east-central U.S. during the weekend. This strengthening ridge will encourage amplification of shortwave energy streaming into western North America as of early Wed, ultimately leading to some interaction with the southern stream system expected to produce heavy rainfall over parts of the South into midweek. The question by Fri into the weekend becomes exactly how the deep upper trough reaching the eastern U.S. and western Atlantic evolves, with some potential for a strong western Atlantic storm that could bring a period of precipitation and strong winds to parts of the Northeast. As for forecast specifics, the updated forecast based on 00Z/06Z data continued to lean toward various ideas of the ECMWF/UKMET/CMC and GEFS mean with minimal input from the GFS. There is a robust consensus depicting slower progression than the GFS for the upper low crossing Texas on day 3 Wed and opening up thereafter. The GEFS mean reflects the farther south GFS track for the associated surface low but the 00Z ECMWF/CMC/UKMET and the ECMWF/CMC means all favor a somewhat northward adjustment for this system. These differences become significant for western Atlantic development by Fri-Sat as the earlier-period GFS track along the Gulf Coast leads to a farther offshore path versus most other guidance including the 06Z GEFS mean. On the other hand within the amplifying eastern U.S. upper trough the 00Z ECMWF closes off a deeper/more concentrated upper low than other solutions, ultimately leading to a very deep surface low off Cape Cod by early Sat--on the western side of the full ensemble spread. While uncertainty is too great to gravitate to that scenario at this point, the past two CMC runs and 12Z UKMET are only about 50-150 miles to the east of the 00Z ECMWF at 12Z Sat and the overall guidance average has trended somewhat westward. Thus the current forecast does reflect some of this general trend. 12Z ECMWF trends highlight the ongoing uncertainty in the forecast--weakening the southern upper low a bit faster to yield a farther south surface low track, ultimately leading to a more GFS-like scenario over the western Atlantic. Farther west the primary forecast consideration is that latest GFS runs are more aggressive than most other guidance to bring leading eastern Pacific trough energy through the West and into the Plains from about Fri night through the weekend. It seems possible for this leading energy to eject inland to some degree--perhaps more than the ECMWF/ECMWF mean--but more likely not to the extent of the GFS. The GEFS mean appears more palatable than the operational GFS runs, hinting at some energy ejecting inland but without the more extreme height falls of the GFS. Incoming 12Z guidance continues to suggest doubt in the GFS scenario. Thus far there is better than average agreement for the trailing upper low forecast to drop into the eastern Pacific next weekend. ...Weather/Hazard Highlights... Even with some differences for upper low/trough timing and surface low track, guidance appears to be stabilizing somewhat regarding the axis of heaviest rainfall with the system affecting the South during Wed-Thu, most likely to extend from Texas eastward through the Southeast. Some timing questions remain and these would affect rainfall totals. Currently expect some multi-inch accumulations with best potential for maximum values from early Wed onward to be centered over or near central Alabama-Georgia. Northward extent of some of this moisture into the southern Mid-Atlantic will be very sensitive to storm track. Some areas across the South have seen much above normal rainfall over the past month, which could enhance the excessive rainfall/flooding threat with this system. The Storm Prediction Center is also highlighting some severe weather potential over parts of the Gulf Coast/Southeast Wed-Thu. Check latest SPC products for updated information regarding severe threats. The system supported by the deepening upper trough reaching the East late this week will brush parts of the Upper Great Lakes with some rain/snow and then trailing cold cyclonic flow should promote a period of lake-effect and terrain-enhanced snow between the central Appalachians and New England. Uncertainty is too great at this time to specify effects from deepening Atlantic low pressure Fri-Sat. There is greater confidence in a period of brisk to strong winds in the gradient between the low and surface high to the west, versus coverage/intensity of any snow, given the current spread for low track. Over the western states precipitation will initially focus over the Northwest, first with a brief episode over northern areas on Wed associated with fast moving shortwave energy and then ahead of the front anchored by low pressure that develops off British Columbia. A broader moisture shield will likely spread across the West by next weekend as some eastern Pacific upper trough energy ejects inland. Highest totals should align with favored terrain from the Sierra Nevada into the northern-central Rockies. A large portion of the lower 48 should see above normal temperatures for the overall Wed-Sun period. Expect the highest warm anomalies to be over the northern-central Plains Fri into the weekend as the pattern becomes favorable for some areas to reach at least 15-25F above normal. Much of the West will see readings 5-15F above normal and locally higher, though clouds/precip should keep highs moderately below normal over the Pacific Northwest. The South will see morning lows up to 10-20F above normal around midweek in the warm/moist flow ahead of the storm system affecting the region while northern parts of the East will likely see min temps 10F or more above normal mid-late week. Cool air behind the southern system may keep highs below normal especially over western Texas on Wed. From Fri into the weekend the deep upper trough crossing the East and western Atlantic storm system should bring a day or so of moderately below normal temperatures to the eastern states. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml