Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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 across much of Texas on Saturday. Into next week, fairly low amplitude troughing will be present across both the West and East coasts, with upper ridging in between, and generally fairly quiet conditions across the CONUS. ...Guidance/Predictability Assessment... Latest guidance shows good agreement Friday and Saturday as an upper low shifts into southeast Canada, bringing low amplitude troughing to the northeastern U.S., while energy out of a deep low in the Gulf of Alaska shifts through the eastern Pacific toward the Northwest. By Sunday, there remains quite a bit of variability on the strength and timing of the western energy as it moves inland and towards the Northern Plains. GFS runs have consistently been on the weaker side with this shortwave while CMC and ECMWF runs are a bit deeper and slower (though the 00Z and new 12Z ECMWF runs are faster than the slow outlier of the 12Z 4/15 run). Considering typical model biases, generally preferred the non- NCEP models though balanced somewhat with the GEFS mean for a middle ground. 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 transitioned toward using more GEFS and NAEFS ensemble means through the period to 60 percent by day 7. The EC ensemble mean could not be used due to ongoing data issues but it looked to be in good alignment. This maintained good continuity with the overnight forecast. ...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 for parts of the southern Plains and Mississippi Valley. This overall area may 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, so the Marginal is pretty borderline. Favorable flow into the Central Rockies should work to enhance precipitation there too, with some snow in the higher elevations at least. 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. Model guidance overall has trended toward a southwest shift with the heavy rain, and thus have shifted/expanded the Slight Risk toward this trend. The area covers west-central Texas eastward toward Texarkana with a broader surrounding Marginal. The front should progress and lay flatter across the Southeast on Sunday and bring some rain there before shifting mostly offshore for the workweek. Meanwhile some light to locally moderate precipitation could come into the Northwest on Saturday and shift across the northern tier early next week but with considerable uncertainty. 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. Tate/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