Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Apr 03 2023 Valid 12Z Thu Apr 06 2023 - 12Z Mon Apr 10 2023 ...Heavy rainfall to focus over the western Gulf Coast region... ...Very cold conditions behind a winter storm across the northern Plains on Thursday will be in stark contrast to summer-like heat along the East Coast... ...Model Guidance/Predictability Assessment... With the midweek departure of the northern Plains/Upper Midwest blizzard into Ontario, the upper flow pattern over the CONUS becomes rather zonal through late week into the weekend with a parade of inherently low predictable shortwave troughs and impulses before re-amplifying into next week. Nevertheless, this overall flow transitions are reasonably depicted among the latest guidance through medium range time scales and preference is for a general blend of best-clustered guidance from the ECMWF/GFS/CMC/UKMET and their ensemble means. The most uncertain aspect in the forecasts appears to be the translation speed of a wave that is forecast to move across the Carolinas and then offshore Sunday to Monday. ...Overview and Weather/Hazards Highlights... The center of a deep storm should be moving across James Bay, Canada as the medium-range period begins Thursday morning, ending a widespread winter weather event across the northern Plains with mainly ground blizzard conditions lingering over the upper Midwest/Great Lakes. In stark contrast to wintry weather in this part of the country, strong upper ridging along the Eastern Seaboard ahead of the deep storm will shift offshore Thursday but will manage to bring exceptional warmth up the East Coast as the cold front passes through the region. Cold air rushing in behind the front will drop temperatures rapidly by as much as 40 degrees Thursday night. The cold post-frontal airmass will moderate as it spreads from the central to eastern states. When the trailing portion of this cold front settles near/along the Gulf Coast Thursday, the anomalous warmth and abundant moisture from the Gulf of Mexico will trigger widespread showers and strong to severe thunderstorms across the south-central to eastern U.S. Thunderstorms and flash flooding issues appear most likely across the western Gulf Coast region on Thursday, where a low pressure wave is forecast to form near the front just offshore. Accordingly, the experimental Days 4 WPC Excessive Rainfall Outlook (ERO) offers a "Slight Risk" threat area from the Texas coastal Plain to southwestern Louisiana. The stalling of the front near the Gulf Coast along with less certain frontal wave enhancements with the right entrance region of the subtropical jet should continue to focus convergence/convection and additional potential runoff/flash flooding issues through the weekend. The WPC product suite still tends to limit the northern lift of later week/weekend activity given overall flow. However, increasing northern and southern stream interaction into early next week offers renewed potential for more northward spread along with moderate coastal low development off the Southeast but with uncertainties on its timing. Out West, the switch to a more zonal flow pattern opens the Pacific Northwest to repeating shortwave troughs and onshore flow. These energetic Pacific systems should favor moderate to locally heavy precipitation across the Pacific Northwest Thursday/Friday but set to weaken/lift north over the weekend when a strong upper ridge builds up the West Coast which also ushers in warmer conditions. There has been a generally southward and higher QPFs trend late next week along the coast of Washington, Oregon, and down into northern California. A marginal excessive rainfall risk has been introduced for the coastal section near the Oregon-California border later on Thursday to Friday morning. Cold conditions in the wake of the blizzard persist Thursday over the northern Rockies/Plains with some areas seeing highs and lows 25F below normal, but moderate into Friday. A few record low mins are possible Thursday morning above a fresh snowpack in the Dakotas. The exceptional warmth under the ridge along the Eastern Seaboard should result in dozens of high min records from southern New England to north Florida Thursday morning. Kong/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