Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 526 AM EDT Wed Apr 25 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 25/1200 UTC thru Apr 28/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast U.S... Some upper level phasing will occur over the Northeast today as the long-lived meandering low in the southern stream lifts and joins a more progressive northern stream trough. The mean resulting trough takes on a slightly negative tilt, with one lobe of stronger forcing over the eastern Great Lakes and the other pushing toward Long Island and southern New England. Certainly this will lead to widespread rainfall lifting out of the Mid-Atlantic states this morning. Heavy rainfall, however, will be a little difficult to find, given the vorticity center over upstate New York / eastern Great Lakes is farther removed from rich moisture and any instability, and along the coast a surface occlusion is forecast to remain just offshore. The warm conveyor belt will promote relatively heavier totals, likely 1 to 2 inch 24-hour amounts, from Connecticut to southern Maine, but expect this to mainly be a stratiform event. The phased upper trough should reach its peak efficiency around the time the conveyor is directed into Down East Maine, such that our 24-hour amounts are locally maximized there, using the the HREF Mean, WRF-NMM, and the 00z NAM as a guide. It will be interesting to see if any deeper convection can occur in southern New England, CT/RI/MA, this afternoon in the wake of the warm advection / conveyor precip, as a few hundred J/kg MUCAPE is forecast to form. If this does occur, the duration over any one location would be brief, but when added atop the morning rainfall in an urban corridor there could be some flashy surface runoff. The same models mentioned above were used to depict the few diurnally driven showers expected in the Mid-Atlantic and North Carolina today, as well as the northwesterly flow upslope into the Appalachians. ...Central/Southern U.S... A compact mid level low will drop from the central Plains down through Oklahoma and Arkansas today. At 06z the leading edge of height falls and associated frontal zone was crossing paths with a narrow instability axis in southwest Texas, giving rise to a few thunderstorms. Otherwise, precipitation was more greatly rooted in the mid and upper levels near the tight 700 mb frontogenetical zone on the periphery of the upper low - namely over Kansas and northwest Oklahoma. This zone of strong mid and upper level forcing is forecast to stay pretty much intact today, producing a swath of rainfall heading southeastward across parts of OK/TX/MO/AR. Lower level moisture return is somewhat inhibited by a recent frontal intrusion into the Gulf and residual surface ridging there. During peak heating, however, some quasi-surface based convection is possible over central and northeast Texas, eventually forming toward parts of Arkansas and Louisiana in the evening as well. A more robust MCS is possible within the Rio Grande Valley where, per the Storm Prediction Center Day 1 Outlook, the bulk of activity will be on the Mexican side of the border, but forecast MUCAPE is supportive of storms reaching or forming in the vicinity of Del Rio and Laredo, with activity working its way southward over time. There is a fair amount of agreement among the model QPFs over this region. WPC favored the HREF blended mean (average of its ensemble mean and probability matched mean). The WRF-NMMB was used for a few details as well, given that it was a bit more generous with areal coverage of QPF in the few pockets of surface based instability that are expected to develop over parts of TX, LA, AR. The flash flood risk appears very low today given that most activity will have roots in the mid levels and should display a progressive character. Slower cell motion and stronger rain rates could occur near the Rio Grande during the early stages of the afternoon convective event. Days 2/3... ...Tennessee Valley and Southeast to the Northeast... A compact mid-upper level low moving out of the lower Mississippi valley Thu morning is expected to support a swath of light to moderate amounts from the Tennessee valley eastward into Georgia and the Carolinas Thu into early Fri. Precipitation is then expected to shift northward to the northern Mid-Atlantic and along the Northeast coast Fri into early Sat as the system weakens and lifts north ahead of an amplifying trough over the Midwest. Marginal moisture and the progressive nature of this system are expected to hamper the potential for any widespread heavy amounts. WPC QPF relied heavily on a blend of the NAM and recent runs of the ECMWF. As the system begins to lift out of the Southeast early Fri, the GFS becomes a deep outlier, spreading precipitation much further west from the central Appalachians and into the Northeast Fri into early Sat. Therefore little weight was given to its solution beyond late Thu. ...Pacific Northwest/Northern California... An expanding area of precipitation, with increasing amounts, can be expected across Oregon into southern Washington and northern California as an upper ridge gives way to an upper low on Fri. While widespread heavy amounts are not expected, ample forcing and instability may be sufficient for some locally heavy accumulations across portions of the region. WPC amounts leaned closer to a blend of the NAM, GFS and UKMET, with less weight given to the ECMWF - which was a relatively dry outlier. ...South Florida... A shortwave trough digging into the north-central Gulf of Mexico may support heavy amounts developing across the Keys and the southern peninsula as southwesterly flow ahead of the wave begins to draw deep moisture across the region. As this moisture interacts with favorable mid-upper level forcing, some of the overnight guidance, including the NAM, UKMET and ECMWF show moderate to locally heavy amounts developing across the region Fri night into Sat morning. The GFS which is faster with the trough aloft, pushes the deeper moisture axis further south and develops heavy precipitation mainly out over the Florida Straits. Given its outlier position aloft, leaned away from the GFS and more toward the NAM, UKMET and ECMWF. Burke/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml