Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EST Mon Jan 30 2023 Valid 12Z Thu Feb 02 2023 - 12Z Mon Feb 06 2023 ...Heavy rain could cause flash flooding in the Lower Mississippi Valley to Southeast Thursday... ...Dangerously cold temperatures and wind chills are forecast across the north-central U.S. late week spreading into the Northeast for the weekend... 19Z Update: The latest 12Z model guidance suite maintains above average agreement through this forecast period on the synoptic scale. It is worth noting that the GFS/GEFS is a bit more progressive with the shortwave crossing the south-central U.S. late in the week. The next shortwave crossing the Rockies Saturday and then the Plains by early Sunday is depicted more strongly with the CMC, which supports a closed low solution over the southern Plains, and is slower once it reaches the eastern U.S. early next week. The third in a series of disturbances is also depicted well by the guidance, although the GFS is a little slower. More substantial differences become apparent across the northeast Pacific going into Monday and beyond. The WPC fronts/pressures forecast was primarily based on a multi-deterministic blend through Saturday, and then up to 40% of the ensemble means was incorporated by Monday. The Slight Risk area for excessive rainfall on Day 4 has been maintained with only minor revisions. The previous forecast discussion is appended below for reference. /Hamrick ------------------------- ...Overview... The overall upper-level pattern during the medium range period looks to be fairly wavy and progressive, with northern stream troughing extending from a persistent Hudson Bay closed low shifting from the Midwest/Great Lakes into the Northeast late this week producing significantly cold temperatures and wind chills. Meanwhile an initially southern stream upper trough will shift from the Southern Plains across the Southeast as it phases back with the northern stream, and is expected to provide support for heavy rainfall potentially causing flash flooding from the Lower Mississippi Valley into the Southeast through Thursday. Meanwhile, ridging over the West as the period begins will give way to a couple of rounds of troughing coming in from the Pacific and lead to a wetter pattern for the West, but nothing too extreme there. ...Guidance/Predictability Assessment... As the period begins Thursday, model guidance is in good agreement for the overall pattern, including the northern and southern stream troughs in the central U.S. tracking east and phasing through late week. The 12Z UKMET was a bit on the slower side of the rest of the well clustered guidance. Farther west, there remained some spread in 12/18Z model cycle with the timing of troughing coming in from the Pacific by around Friday, with the ECMWF and UKMET slower while the CMC clusters better with the faster GFS runs. A middle ground in between these seemed most likely, as the ECMWF and UKMET appeared to be too slow compared to the trough axes in the ensemble means. The 12Z ECMWF stays slow while the 12Z UKMET seemed to flatten this trough too fast by day 5 while other guidance tracks the trough going all the way into the central and eastern U.S.--which the 12Z CMC was perhaps too deep with the trough compared to the ensemble members once it reaches the East by Sunday. Given the minor to moderate UKMET issues, the early part of the WPC forecast blend was based on a blend of the 12/18Z GFS and 12Z ECMWF and CMC. The pattern on the large scale with these troughs and a next trough coming into the West early next week are all reasonably agreeable given the forecast lead time, but transitioned the blend to include the GEFS and EC ensemble means to minimize any small scale differences in the individual models. Good continuity was maintained with the previous forecast, and the incoming 00Z models seem to be consistent as well. ...Weather/Hazard Highlights... The southern stream upper trough/low tracking across the southern U.S. and the broader northern stream trough will provide good upper-level support for frontal boundaries with waves of low pressure through the south-central to southeastern U.S., which combined with Gulf moisture streaming in will lead to widespread precipitation. By Thursday most of the precipitation looks to be rain, and heavy rain is forecast from parts of the Lower Mississippi Valley into the Southeast and perhaps the southern Appalachians into the Carolinas. Instances of flooding are possible especially where there is sufficient instability for higher rain rates, and there is a Slight Risk in place in WPC's experimental Excessive Rainfall Outlook for much of the southeastern U.S. Thursday. Rain may continue into Friday for the Southeast and the Florida Peninsula, with drier conditions Saturday before some moisture from the Atlantic may stream into the Eastern Seaboard Sunday. Farther north, lake effect snow showers are possible, and light snow may expand in coverage across the Midwest/Great Lakes/Northeast by the weekend with a clipper system. Meanwhile in the West, increasing precipitation chances are expected late this week into early next week near and ahead of a couple of upper troughs. The Pacific Northwest into northern California will be the most likely to get multiple rounds of lower elevation rain and higher elevation snow, along with the northern Rockies. Precipitation amounts will be more typical of a normal cool season event and nothing like those in the parade of atmospheric river events about a month ago. Another round of very cold conditions is likely for the north-central U.S. Thursday-Friday underneath cold upper troughing. Highs in the single digits are likely for North Dakota into northern Minnesota while bitterly cold lows combined with wind will produce dangerous wind chills. Much below normal temperatures then shift into the Northeast by late week, with the possibility of highs below zero in northern New England by Saturday, and bitterly cold lows potentially setting daily records. On the other hand, the Southeast into the Florida Peninsula should see above average temperatures Thursday into Friday, and temperatures are forecast to warm to above average across the central U.S. in particular by the weekend into early next week. Tate 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 Hazards: - Heavy rain across portions of California and the Pacific Northwest, Sun, Feb 5. - Heavy rain across portions of the Southeast, the southern Mid-Atlantic, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Thu, Feb 2. - Heavy snow across portions of California, the Central Great Basin, and the Pacific Northwest, Sun, Feb 5. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Fri-Sat, Feb 3-Feb 4. - Flooding possible across portions of the Lower Mississippi Valley and the Southern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Much below normal temperatures across portions of the Northern Plains, Thu, Feb 2. - Much below normal temperatures across portions of the Northeast, the Upper/Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Fri, Feb 3. - Much below normal temperatures across portions of the Great Lakes, the northern Mid-Atlantic, the Northeast, and the Central Appalachians, Fri-Sat, Feb 3-Feb 4. 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