Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Mon Apr 15 2024 Valid 12Z Thu Apr 18 2024 - 12Z Mon Apr 22 2024 ...Overview... Guidance generally shows a transition toward more progressive flow after the start of the period this Thursday when an Omega block pattern prevails over the northeastern Pacific through northwestern North America into the northern tier U.S. Ahead of a southern Canada upper low, the front trailing from surface low pressure exiting the Great Lakes will continue to bring some showers and storms across parts of the East. Then the combination of opening/progression of the upper low and trailing shortwave energy should bring some troughing into the East by the weekend and early next week. This evolution may cause a secondary cold front to decelerate or briefly stall across the Central Plains-MS Valley region, and aided by weak shortwave energy through the Southwest/Southern Plains, may help to enhance precipitation from the Middle-Lower Mississippi Valley into southern Plains late this week into the weekend. Meanwhile northeastern Pacific energy undercutting the ridge over western Canada should bring a front into the Northwest during the weekend and possibly the northern Plains thereafter. However a fair amount of spread develops for eastern Pacific into northern tier flow by late weekend/early next week. ...Guidance/Predictability Assessment... Latest guidance agrees fairly well for the overall evolution aloft across southern/eastern Canada into the lower 48, as the initial upper low over southern Canada opens up and progresses eastward while the combination of trailing cyclonic flow and a southern stream shortwave reach the central-eastern U.S. by Sunday- Monday. Shortwave differences are smaller in scale and thus have low predictability multiple days out in time. There are more notable differences in southward progression of the southern tier surface front and location of heaviest QPF. 00Z ECMWF-initialized machine learning models (MLs) offer support for at least greater frontal/QPF suppression than the 00Z ECMWF if not even toward the GFS, while latest operational dynamical model runs are clustering toward ECMWF ideas for a time over the southern Plains and vicinity. The new 12Z ECMWF has nudged the front southward over the Southeast especially around Saturday. By next Monday there is reasonable agreement and continuity regarding a frontal wave developing off the East Coast in response to the upper trough reaching the eastern U.S. Latest dynamical models have maintained recent trends toward Pacific trough energy (with possible embedded upper low) reaching southwestern Canada and the northwestern U.S. by late Saturday or early Sunday with continued progression near the Canadian border thereafter. The 00Z MLs on average suggest slower progression of this energy versus the dynamical runs, but the new 12Z GFS/CMC runs maintain similar timing from the prior cycle. The new 12Z ECMWF has trended a little slower though. Ensemble spaghetti plots become chaotic not only with this system but also upstream over the eastern Pacific, lowering confidence in details to the immediate west of the aforementioned feature as well. The guidance distribution where discrepancies exist generally favored an intermediate blended approach. Thus the updated forecast based on 00Z/06Z runs incorporated an operational model composite early-mid period, followed by modest incorporation of the 06Z GEFS/00Z ECens means by days 6-7 Sunday-Monday. ...Weather/Hazards Highlights... Some generally light precipitation is likely on Thursday over parts of the Northeast/Mid-Atlantic with a leading cold front. The next front right behind it will renew rain and convection across the East late this week, while lingering across the south-central U.S. as the frontal system slows down and possibly stalls briefly. Anomalous moisture and instability present in the vicinity of the wavy front favors a low-end Marginal Risk area from near the confluence of the Ohio/Mississippi Rivers southwest through parts of Texas on one or both days (Thursday/Friday) covered by the Days 4-5 Excessive Rainfall Outlooks. 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. Weak shortwave energy reaching into the Southern Plains and low level upslope flow north of the surface front should help to further enhance precipitation across Texas and vicinity on Saturday with increasing moderate to heavy rain potential. Then some rainfall should move into the Southeast as the Northeast dries out. The system currently expected to brush the Northwest during the weekend may bring some light to locally moderate precipitation. Expect a broad area of above normal temperatures by 5-15 degrees over the southern half of the country into Thursday, while the northeastern quadrant of the U.S. gradually cools behind a cold front. Meanwhile the upper trough across the northern tier states will support below normal temperatures (with greater anomalies for highs than lows) over the northern-central Rockies/Plains as of Thursday. Cooler temperatures will gradually expand farther south and east behind the secondary cold front late week into the weekend, with near to below average temperatures expected to reach everywhere east of the Rockies but Florida by next weekend. Coolest anomalies will be for highs up to 15-20 degrees below normal over parts of the High Plains. Meanwhile, 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. Rausch/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