Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Mar 13 2025 Valid 12Z Sun Mar 16 2025 - 12Z Thu Mar 20 2025 ...Heavy rain and severe weather possible for the East Coast Sunday... ...Atmospheric river for Oregon and northern California Sunday... ...Overview... As the medium range period begins Sunday, a cold front with reinforcing fronts behind it will sweep across the East. Widespread thunderstorms are likely ahead of the primary front, which could be severe and have embedded heavy rain causing flooding concerns. Warmer than normal temperatures in the East ahead of the front will cool toward normal behind it, but temperatures are forecast to warm back up in the central and then eastern U.S. as the week progresses. Meanwhile in the West, an atmospheric river could cause locally heavy rain and higher elevation snow on Sunday ahead of an upper trough. As this upper trough moves moves across the western and central U.S., rain and snow is forecast to spread through the Interior West Monday-Tuesday and into the central and east-central U.S. Tuesday-Wednesday. ...Guidance/Predictability Assessment... Model guidance remains in good agreement early next week with the synoptic scale pattern that consists of an upper trough axis moving eastward from the Mississippi Valley into the East, another trough tracking from the eastern Pacific into the Interior West, and upper ridging in between. A multi-model blend including the 12/18Z GFS and 12Z ECMWF and CMC could be used to depict this pattern. By Monday-Tuesday and beyond, both the western and eastern troughs show some spread regarding how quickly they progress eastward. Near the East Coast, GFS runs have been persistently fast in lifting the trough east compared to the slower ECMWF. Even around Monday this led to model differences in how quickly precipitation moves off the East Coast, and then positioning differences in the placement of a resulting upper low in the Atlantic toward midweek. An intermediate solution like the ensemble means and even the CMC seemed more reasonable. Meanwhile as the other trough moves into the western and then central U.S., the 12Z ECMWF along with many of its ensemble members have been slow outliers with its progression eastward. GFS and CMC runs and their ensemble members tended to be faster in sweeping the trough toward the central U.S. as well as the associated surface low. The EC-based AI/ML models were uniformly faster than the control 12Z ECMWF, which gave confidence to lean away from the slow ECMWF/EC mean solutions. For the forecast blend, leaned more toward the 18Z GFS and CMC and their ensemble means by the late period. The newer 00Z ECMWF is now faster and thus in better alignment with this larger guidance cluster. However, the 00Z GFS is shallower with the trough and progresses it east so quickly that the surface low and precipitation are farther northeast than its previous favored run. ...Weather/Hazards Highlights... The Eastern Seaboard is forecast to see widespread convection on Sunday as moisture streams in ahead of the cold fronts. Severe thunderstorms are possible from the Mid-Atlantic to Carolinas and Southeast per the Storm Prediction Center. Locally heavy rainfall is also possible. The Day 4/Sunday ERO maintains a Marginal Risk stretching across much of the Eastern Seaboard. There is some uncertainty in how quickly the cold front moves eastward into the Atlantic off the Southeast coast, but heavy rain with high rates is likely ahead of it. Meanwhile farther north in the northern Mid- Atlantic and Northeast, moisture anomalies are generally over the 95th percentile and some modest instability is in place, so rain rates for the region could be locally heavy. All this provides continued support for a Marginal Risk but future adjustments certainly could be necessary. The quick movement of the heavy rain rates eastward ahead of the front could limit flooding potential to stay below Slight Risk levels. Rain and storms may continue into Monday for coastal areas before moving away. In the West, an atmospheric river of moderate strength is forecast to impact Oregon and northern California on Sunday and produce heavy precipitation (lower elevation rain and higher elevation snow). Some localized rain rates of over an inch per hour are likely, and the AR may stall briefly near the CA/OR border. Continue to delineate a Marginal Risk in the Day 4/Sunday ERO, but held off on any Slight Risk as the area likely to receive the heaviest rain is not very sensitive to flooding. The AR looks to weaken as it pushes into central and southern California later Sunday and into Monday, which should preclude too many flooding issues over recent burn scars. Heavy snow may pile up over higher elevations of the Cascades, Sierra Nevada, and the Interior West and Rockies into early next week as precipitation chances gradually shift east. While the details are still coming into focus, a low pressure system is forecast to strengthen in the central Plains by Tuesday (though weaker than the short range low) and move east or northeast. This low and its frontal boundaries will allow for some precipitation to spread generally across the central/northern Plains into the Great Lakes, with some mixed precipitation and/or snow possible northwest of the system track. Rain is forecast to develop in the east-central U.S. by midweek. Another threat as the upper trough pushes east through the western and central U.S. will be high winds. Winds in the southern Rockies and southern Plains could once again cause fire danger Monday-Tuesday. Above normal temperatures are likely across the eastern the U.S. on Sunday, with greatest anomalies of 20-30 degrees warmer than average for highs likely in the Mid-Atlantic and Northeast. Expect morning lows to be even more above average, well into the 50s and 60s, which could set records for warm mins--if the locations do not have those temperatures fall lower before the calendar day after the cold frontal passage. While temperatures cool in the East for Monday, milder than average temperatures are forecast to rebuild in the central U.S., spreading into the East by Tuesday-Wednesday underneath upper ridging. Temperatures will once again cool from west to east behind another cold front Wednesday-Thursday. Meanwhile, periods of troughing across the West will produce below normal temperatures on average, likely most below average on Tuesday-Wednesday. Highs should generally be a bit more below normal compared to low temperatures given clouds and precipitation. Tate 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