Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Sun Mar 19 2023 Valid 12Z Wed Mar 22 2023 - 12Z Sun Mar 26 2023 ...Heavy rainfall threat over parts of the east-central U.S. after midweek... ...Overview and Guidance/Predictability Assessment... Latest guidance is consistent with the large scale flow aloft, showing an energetic pattern with a series of features moving into and through a mean trough over the West Coast/western U.S. while an upper ridge meanders over parts of the Gulf of Mexico/northwestern Caribbean into the Bahamas. A vigorous system moving into the West as of Wednesday will initially bring rain and significant higher elevation snowfall to some areas and then the potential for heavy rainfall over the east-central U.S. as moist leading flow interacts with a front trailing from a northern stream system. The next system reaching the West Coast later in the week should amplify farther east and thus yield a farther north precipitation emphasis than the first one, followed by low pressure emerging over the Plains next weekend. Yet another system may approach the Northwest by next weekend as well. Guidance is still showing some spread and detail trends for the southern Canada/northern tier U.S. shortwave and associated surface low pressure tracking out of the Upper Midwest mid-late week. Latest GFS/ECMWF runs have become a little more stable and comparable to the ensemble means in principle, favoring more weight on those two models. The new 00Z UKMET has trended much closer to the GFS/ECMWF idea versus he prior run that was much stronger than consensus. The CMC continues to weaken this system in deference to the trailing West/Plains system that becomes faster than other guidance. As for the initial western system, guidance is consistent with a closed upper low reaching the Great Basin by early Wednesday and the depiction of the associated surface reflection. However as the system progresses eastward, there has been a general trend toward the overall upper trough becoming more diffuse. The 00Z/18 and 12Z/18 ECMWF runs are the exception, developing a rather strong shortwave from the southwest part of the initial trough and tracking deep low pressure into the Ohio Valley/Great Lakes/Northeast. The 12Z ECMWF mean was much weaker than the operational run for the surface reflection and a little more suppressed, while latest GEFS means at least suggest more low pressure hanging around over the Northeast by early Saturday than indicated by the operational GFS. An operational model blend emphasizing the 18Z GFS/12Z ECMWF followed by increasing weight of the GEFS/ECMWF means provided reasonably good continuity in light of the guidance spread. The shortwave forecast to reach the West Coast by Friday seems to be reverting back to some earlier GFS runs that had been on the stronger side of the spread a couple days ago. Through the 12Z/18Z cycles, the GFS was generally on its own with a closed upper low tracking over the Southwest/Four Corners on Saturday versus consensus that has been showing a more open and less amplified trough. However the slightly slower 00Z CMC now shows a closed low as well, just on a slightly farther north track than the GFS. On the positive side, the new 00Z GFS has weakened its day 7 Plains surface low pressure by about 15 mb compared to the 12Z run and is now much closer to consensus. The 12Z ECMWF details become a little suspect as well, using some of the originating shortwave energy to leave an upper low off Vancouver Island and the Pacific Northwest in contrast to the majority cluster that pushes away all of the late week energy and uses another shortwave dropping down from Alaska to form a separate upper low that approaches the Northwest on Sunday. Guidance comparisons based on the 12Z/18Z cycles led to starting the updated forecast with more 18Z GFS/12Z ECMWF relative to the 12Z UKMET/CMC, followed by a rapid transition toward 60 percent total weight of the 18Z GEFS/12Z ECMWF means by days 6-7 as detail differences increase. The remaining operational model component of the blend used only small weights of the 18Z GFS/12Z CMC while splitting ECMWF input among the 12Z and prior 00Z runs to downplay the 12Z run's questionable evolution near the West Coast late in the period. ...Weather/Hazards Highlights... A leading system tracking out of the Upper Midwest should spread some light to moderate precipitation across parts of the northeastern quadrant of the lower 48. Some snow will be possible over far northern areas. Meanwhile the initially strong system over the West will spread a broad area of rain and higher elevation snow across areas from California through the central two-thirds of the Rockies. Areas of heavy snow will be possible over favored mountain terrain on Wednesday. Some moisture will begin to spread east into the Plains while low level Gulf inflow becomes established as well. The combination of this moisture and the front trailing from the departing Upper Midwest system may promote some areas of enhanced rainfall over the Midwest around Wednesday night. However guidance signals for this continue to be as mixed as they have been over the past day, and soil conditions over this area are neutral to dry, so the new day 4 (12Z Wednesday-12Z Thursday) Excessive Rainfall Outlook continues not to depict any risk areas. Even though there is still a lot of model spread for surface specifics, there has been a persistent guidance signal for heavy rainfall over parts of the east-central U.S. in the day 5 (12Z Thursday-12Z Friday) period with that period's Excessive Rainfall Outlook introducing a Slight Risk area approximately from eastern Oklahoma to southwestern Ohio. This axis provides the best overlap of greatest moisture anomalies, instability, and guidance QPF. Changes in mass field specifics may lead to some adjustments in coming runs but there is higher confidence in the general idea of a heavy rainfall event. Consult latest products from the Storm Prediction Center for information on severe weather threats. Meanwhile some snow will be possible over some northern areas, again depending on uncertain specifics of frontal waves, while rainfall should continues spreading over the East late in the week. Best rain/mountain snow emphasis over the West from late week into the weekend will be from the Pacific Northwest/northern California into the Rockies. There is potential for some moisture to extend a little farther south if the upper trough over the region leans toward the amplified side of the envelope. Currently the daily amounts do not look exceptionally heavy but areas of localized enhancement area possible. The overall pattern will keep much of the West and parts of the northern Plains quite chilly through the period, with broad coverage of highs 10-20F below normal each day. Temperatures over the Northwest should be less extreme but may still be mostly below normal. In contrast, an area of well above normal temperatures will spread from the central-southern Plains through much of the East mid-late week. Thursday-Friday will be the warmest days for highs with plus 10-20F anomalies and some morning lows could be as much as 20-25F above normal Thursday into early Saturday. Frontal passage across these areas will then bring temperatures closer to normal. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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