Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Mar 13 2023 Valid 12Z Thu Mar 16 2023 - 12Z Mon Mar 20 2023 ...Deep storm genesis to drive Ark-La-Tex/South strong thunderstorms with an excessive rainfall/flash flooding threat and a heavy snow/wind threat from a cooled central Rockies/Plains to the Upper Midwest/Great Lakes Thursday/Friday... ...Overview... Ejection of southern stream energy from California and the Southwest coupled with an amplifying northern stream upper trough/closed low over the north-central U.S. should favor deep surface low/frontal genesis to focus an excessive rainfall/convective threat across the Ark-La-Tex/South Thursday into Friday. The intensifying storm also offers a concurrent heavy snow/high wind threat in the wintry cold air on the backside of the deepening system from the central Rockies/Plains to the Upper Midwest/Great Lakes. System activity should eject across the East by Saturday. Additional but less certain Pacific system energies reach the West Coast then Intermountain West/Rockies this weekend. ...Guidance/Predictability Assessment... Model and ensemble forecast spread has decreased for Thursday into Saturday, bolstering forecast confidence in the pattern evolution/main weather features. The WPC medium range product suite was primarily derived from a blend of best clustered guidance from 18 UTC GFS/GEFS mean, 12 UTC ECMWF/UKMET/ECMWF ensemble mean along with the 01 UTC National Blend of Models and WPC continuity. Leaned blend weightings in favor of the higher resolution deterministic models in this period for better detail consistent with predictability. Increased blend reliance on the compatible ensemble into Sunday/next Monday to better mitigate growing system and run to run model differences, especially from the Pacific to an unsettled West. New 00 UTC guidance remains overall in line with this forecast plan and continuity. ...Weather/Hazards Highlights... There is a growing guidance signal that favorable cyclo/fronto genesis across the central U.S. and deep pre-frontal moisture and instabiltity return by a strong low level jet into the eastern Plains and Mississippi Valley and onward across the South and East later week will fuel an emerging/expanding area of showers and thunderstorm driven rains. A The WPC experimental Day 4 Excessive Rainfall Outlook (ERO) has been collaborated with local offices to include an upgrade to a "Slight Risk" Thursday from the eastern southern Plains to the lower Mississippi Valley where locally heavy rains may accompany thunderstorms ahead of and along an eastward moving cold front in a region of wet ambient soils. A heavy rain threat and Day 5 "Marginal Risk" ERO should shift to a localized region over the Southeast/Florida Panhandle Friday where access to Gulf moisture remains highest. Otherwise, moderate rainfall chances will overspread the Midwest on Thursday and the East Coast into Friday/Saturday. Additionally, latest guidance preferences have continued to increase wrap-back snow potential from the central Rockies/Plains to the Upper Midwest/Great Lakes Thursday-Friday and eventually the Interior Northeast Saturday-Sunday following the surface low track as per the WPC Winter Weather Outlook (WWO). The highest chances for impactful snowfall currently look to be over the Middle Missouri Valley Thursday and portions of the Upper Great Lakes Thursday-Friday. High temperatures will tend to be cooler than average by 10-20 degrees, especially across most of the central U.S. following the passage of the system cold front and as a Canadian air mass robustly digs in. Upstream, there looks to be a brief reprieve from the recent string of anomalously strong and wet storms over the West after Thursday and into Saturday before a set of uncertain systems approach the region later weekend into early next week, bringing a chance for more moderate lower-elevation coastal/valley rain and higher elevation snow across the broad region. High temperatures look to remain around 5-10 degrees below average in general, with a pocket of colder temperatures 15-20 degrees below average lingering over the Rockies. 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