Extended Forecast Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Mon Mar 20 2023 Valid 12Z Thu Mar 23 2023 - 12Z Mon Mar 27 2023 ...Heavy rainfall threat over parts of the east-central U.S. after midweek... ...Overview and Guidance/Predictability Assessment... A series of shortwaves are expected to traverse the mid-section of the country before a deep upper low arrives along the West Coast late next weekend. Guidance is in good agreement regarding the mid-late-week system moving through the Plains and Midwest. The 06z GFS is notably deeper and faster with the northern stream energy as well as the amplified southern stream trough. Therefore, a general model blend consisting of the 00z EC/CMC/UKMET and 06z GFS, with added weighting toward the EC, were utilized through day 5. A number of shortwaves propagate through the northern tier of the country this weekend, but guidance seems to be all over the place with respect to the details of these waves, therefore the 00z ECE and 06z GEFS were introduced on day 5 and continued through day 7. There's a lot of spread regarding a potentially impactful day 7 upper low in the East Pacific. ...Weather/Hazards Highlights... The leading system crossing the northeastern U.S. and southeastern Canada will spread some light to moderate precipitation across parts of the Northeast, including some snow over northern New England. Then the focus will turn to a potential heavy rain event over the east-central U.S. during Thursday-Friday, as an upper trough initially over the West progresses eastward while already-established moist flow from the Gulf of Mexico persists into late week and interacts with a wavy front left behind by the Northeast system. Even with wide array of solutions for specifics at the surface and aloft over recent days, there was a consistent signal for this event in principle, and now some of these details are starting to get better refined--but some additional adjustments are still likely. The new day 4 (12Z Thursday-12Z Friday) Excessive Rainfall Outlook will be quite similar to the previous day 5 issuance, reflecting only minor adjustments for latest guidance with respect for a Slight Risk area from eastern Oklahoma and vicinity northeast into western Ohio. The day 5 (12Z Friday-12Z Saturday) will follow along with the southeastward progression of heaviest rainfall, with a Slight Risk area from parts of Arkansas/northern Louisiana through much of Kentucky. Strengthening of low pressure later in the day 5 period should eventually lead to some acceleration of rainfall by early Saturday. Continue to monitor Storm Prediction Center outlooks for the latest information regarding severe weather threats from the southern Plains eastward. As the system continues northeastward, interior portions of the Northeast may see meaningful snowfall depending on the exact low track. The initial upper trough crossing the West followed by upstream energy will support periods of rain and mountain snow from the northern/central West Coast into the Rockies. Most amounts should be in the light to moderate range but localized enhancement will be possible. Once this trailing energy departs from the West, another system may bring some precipitation of varying intensity to portions of the central/eastern U.S. by Sunday-Monday. Also in that time frame, the upper trough most likely amplifying just off the West Coast may start to increase precipitation totals along the central coastal areas based on the consensus trough position. The large scale pattern will keep much of the West and parts of the northern Plains quite chilly through the period, with broad coverage of highs 10-20F below normal each day. The Great Basin and central Rockies could see highs as cold as 20-25F below normal during the weekend. Some record cold highs may be possible. Temperatures over the Northwest should be less extreme but still be mostly below normal. In contrast, an area of well above normal temperatures will extend from the central-southern Plains through much of the East during the latter half of the week. Thursday-Friday will be the warmest days for highs with plus 10-20F anomalies and some morning lows that could be as much as 20-25F above normal Thursday into early Saturday. Frontal passage across these areas will bring a cooling trend to near or slightly above normal levels. Kebede/Rausch Hazards: - Heavy rain across portions of the Southern Plains, the Middle/Lower Mississippi Valley, the Ohio Valley, and the Lower Great Lakes, Thu, Mar 23. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, and the Ohio Valley, Fri, Mar 24. - Heavy snow across portions of the Pacific Northwest, Thu-Fri, Mar 23-Mar 24. - Severe weather across portions of the Southern Plains, Thu, Mar 23. - Severe weather across portions of the Lower Mississippi Valley and Tennessee Valley, Fri, Mar 24. - Heavy rain across portions of California and the Pacific Northwest, Mon, Mar 27. - Heavy snow across portions of California, Mon, Mar 27. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. 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