Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Tue Apr 16 2024 Valid 12Z Fri Apr 19 2024 - 12Z Tue Apr 23 2024 ...Overview... The pattern during the medium range period should transition towards more progressive flow after the start of the period on Friday, as an Omega block over the northeast Pacific into western North America and the northern U.S. tier begins to break down. A cold front through the East late this week will be accompanied by some showers and thunderstorms, with the Northeast drying out thereafter. From the Southern Plains into the Southeast, a slow moving cold front and shortwave energy will increase the heavy rainfall potential, particularly from central Texas into the Lower Mississippi Valley on Saturday. By early next week, general troughing will be present across both the West and East coasts, with upper ridging in between, and generally fairly dry conditions across the CONUS. ...Guidance/Predictability Assessment... Latest guidance shows good agreement Friday and Saturday as an upper low shifts into southeast Canada and energy out of a deep low in the Gulf of Alaska shifts towards the Northwest U.S.. By Sunday, there remains quite a bit of variability on the strength and timing of the energy as it moves inland and towards the Northern Plains. The GFS and CMC are more progressive/weaker with the system vs yesterdays 12z ECMWF which showed a weak closed low moving slowly eastward. The new 00z run tonight has trended faster and more in line with the GFS and CMC. The ensembles and the ECMWF- initialized AI/ML models also seem to show better support for a more progressive system, but with continued variability. After this, there are some differences in the details of energy rounding the base of the Eastern U.S. trough, and even greater uncertainty by Monday-Tuesday with another trough moving towards the West Coast. The WPC forecast today used a blend of the deterministic models for Days 3-4, gradually transitioning towards the ensemble means, along with the GFS and CMC for some added system definition. ...Weather/Hazards Highlights... A cold front moving through the Northeast Friday-Saturday will be accompanied by some generally light precipitation and storms, with the southern portion slower to move or possibly stalling briefly into the weekend across the Southern Plains-Southeast region. Anomalous moisture and instability present in the vicinity of the wavy front favors a low-end Marginal Risk area on Friday from near the confluence of the Ohio/Mississippi Rivers southwest through parts of Texas. This overall area should have some potential for storm training with higher rain rate potential, though guidance signals do not show great coherence for details yet. Also most of this area has been rather dry as of late, which should inhibit the overall flash flood threat. Favorable flow into the Central Rockies should work to enhance precipitation there too, with some snow in the highest elevations. By the weekend, weak shortwave energy reaching into the Southern Plains should help to further enhance precipitation across Texas and vicinity on Saturday with increasing heavy rain potential. Given how wet the southern TX to southern AR region has been lately, opted to introduce a slight risk on the Day 5 Excessive Rainfall Outlook across this area, with a broader marginal risk encompassing the greater Southern Plains/Lower Mississippi Valley/into parts of the lower Tennessee Valley. The system currently expected to brush the Northwest during the weekend may bring some light to locally moderate precipitation. By Friday and beyond, much of the Northeast should gradually cool behind a cold front. Meanwhile, the upper trough over the northern tier states and eventually into the East will support below normal temperatures (with greater anomalies for highs than lows) for most east of the Rockies but Florida by next weekend. Above normal temperatures across the Southwest should gradually expand northward underneath a building Western U.S. ridge and progress eastward into the Rockies early next week. Northern parts of the West may stay near normal due to the upper shortwave/cold front crossing the region this weekend. Santorelli 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw