Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Tue Apr 9 2024 Valid 12Z Fri Apr 12 2024 - 12Z Tue Apr 16 2024 ...Overview... The vigorous storm system affecting the East at the start of the forecast period on Friday will continue to track over eastern Canada and push its trailing front eastward as the supporting upper trough lifts northeastward. Some areas of rain and snow may linger over the Northeast into Saturday. A trailing Rockies/Plains upper ridge will bring initial western warmth into the central U.S. with parts of the Plains seeing highs 15-25F above normal during the weekend. Guidance is starting to stabilize for the deep upper low dropping into the eastern Pacific during Friday-Saturday, bringing a brief period of precipitation into California during the weekend. However after Saturday there is still timing spread for the upper low between California and the Plains, with some influence from differences in upper troughing expected to push gradually into the Northwest. This upper low will most likely reach the Plains by next Tuesday and support a fairly strong surface low at that time. ...Guidance/Predictability Assessment... On day 3, a general model blend consisting of the 00z EC/UKMET/CMC and 06z GFS was used, due to the general model agreement regarding the presence of an omega block pattern over the CONUS. By day 4 we shifted toward a Euro weighted GMB because it was more consistent than the GFS with respect to the digging West Coast trough. The EC also clustered reasonably well with the CMC, UKMET and ensemble means over the West Coast as well. There's much more spread on day 5 and that's reflected in a blend containing 6 different pieces of guidance. The 00z deterministic Euro is still favored but the 00z ECE and 06z GEFS were introduced to the GMB to account for spread in the Western mid-level low. By day 6, we've gone with an equal blend of the 00z EC/ECE/CMCE and 06z GEFS. The operational GFS still has the mid-level low center stationed over southern California while the favored blend positioned it near and around the Four Corners region. This same blend is continued through day 7. ...Weather/Hazards Highlights... The windy system tracking out of the Great Lakes as of early Friday will still support anomalous moisture over the Northeast in the strong southerly flow along/ahead of the cold front sweeping through the region during the day. A Slight Risk of Excessive Rainfall leading to Flash Flooding was introduced, in coordination with GYX, over portions of southern Maine and the White Mountains of ME/NH. Increased QPF trends, snow melt, and orographic enhancement were some of the supporting factors for the upgrade from the Marginal that remains over portions of Upstate New York, and central/northern New England. Colder air will filter in with the passage of the front which may result in areas of mixed precipitation type or changeover to all snow by Friday night and persisting into Saturday. The upper low dropping southward over the eastern Pacific late this week should approach/reach California during the weekend. While moisture anomalies do not appear too extreme with this system, the depth of the upper system offers potential for some localized moderate to heavy rain bands. Such activity could lead to some runoff issues given fairly high soil moisture anomalies currently existing along the coast. Thus the Day 5/Saturday ERO depicts a Marginal Risk area along approximately the central half of the California coast where the best average of guidance suggests potential for rainfall enhancement. Some precipitation in the Sierra Nevada may be in the form of snow. As the upper low progresses eastward, it should produce precipitation of varying intensity across the Great Basin and Four Corners states, followed by increasing coverage/intensity of rainfall over the Plains by next Tuesday as it develops potentially strong low pressure. The Northwest may see some light precipitation late this week and perhaps another period of rain/mountain snow early next week with the arrival of an upper trough. Farther east, a weak wave/frontal system may produce some rain over parts of the Ohio Valley/Great Lakes through Mid-Atlantic during at least a part of the weekend. The Northeast will see unseasonably warm lows early Friday ahead of the cold front crossing the region during the day. Behind the front, parts of the Ohio Valley/Great Lakes into central-northern Appalachians will see a brief period of cool daytime highs Friday- Saturday. Farther west, from Friday through the weekend an area of above normal temperatures will shift from the West/High Plains into the central U.S. Highs should reach up to 10-20F above normal on Friday while parts of the northern/central Plains may see plus 15-25F anomalies during the weekend. Some of this warmth with lesser anomalies (plus 5-15F) will likely spread into most of the East from Sunday onward. Persistence of a mean trough aloft may hold daytime highs closer to normal over the Northeast though. The upper low tracking in from the eastern Pacific will bring below normal highs into California and the Southwest during Sunday- Tuesday while farther north an upper trough and leading cold front should promote modestly below normal readings over the Northwest by early next week. Kebede/Rausch 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