Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 606 PM EDT Wed Apr 25 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 26/0000 UTC thru Apr 29/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast U.S... It pretty much remains the case that 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. Heavy rainfall 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, lingering into tomorrow. 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 WPC continuity, the HREF Mean and the National Blend of Models as a guide along with a general composite of high-res model guidance. It remains 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 may be in play. 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 in or near an associated WPC marginal excessive risk area. The same models mentioned above were used to depict a few diurnally driven showers expected in the Mid-Atlantic and North Carolina today, as well as the northwesterly flow upslope into the Appalachians. ...S-Central and Southeast U.S... It is also still well depicted in guidance that a compact mid level low will drop from the central Plains down through Oklahoma and Arkansas today and into the Southeast into Thursday. At 17z the leading edge of height falls and associated frontal zone was crossing paths with a narrow instability axis in southwest Texas/nrn Mexico, giving rise to a few thunderstorms with some downpour promise. Otherwise, precipitation remains greatly rooted in the mid and upper levels near the tight 700 mb frontogenetical zone on the periphery of the upper low - now shifted toward the Red River and into Arkansas. This zone of strong mid and upper level forcing is forecast to stay pretty much intact today, producing a swath of rainfall that is ongoing with mainly modest to moderate amounts. Lower level moisture return is somewhat inhibited by a recent frontal intrusion into the Gulf and residual surface ridging there but blended tpw loops do show a modest moisture axis fed up into the system. During peak heating into the evening, however, some quasi-surface based convection is possible over central and northeast Texas, eventually forming toward parts of the lower MS valley then progress across the Southeast/srn Appalachians Thursday. There is a fair amount of agreement among the model QPFs over this region. The day 1 forecast was mainly derived from WPC continuity, the HREF Mean, and National Blend of Models as per with a general composite of high-res model guidance. Days 2/3... ...Carolinas...Mid-Atlantic and Northeast... Low pressure swings north-northeast from GA Thursday evening up the Eastern Seaboard through Friday night around an amplifying trough over the Great Lakes. One inch PW lifts from the Carolinas along the Eastern Seaboard which is only one standard deviation above normal. The 12Z GFS has the greatest signal for heavy rain (over the central Mid-Atlantic), but the progressive nature of the system is expected to limit excessive rain potential to 1 to 1.5 inches. No excessive rain outlook was issued. 12Z guidance is a little more agreeable than previous runs with the 12Z GFS still much farther west with rain. The 12Z UKMET made a notable shift west and was no longer a major east outlier. Final WPC QPF was based on a blend of the 12Z NAM/ECMWF/UKMET with some consideration of the 12Z GFS. Lesser rainfall is expected over the northern New England coast for Day 3 (00Z Sat-00Z Sun) than in Day 2 farther south. Half into three-quarters inch QPF is expected along the Maine coast with less inland per 12Z operational model consensus. ...Pacific Northwest/Northern California... A deep closed low shifts east to the OR/CA border Friday night and merely drifts inland through Saturday. 0.75 inch PW spreads north from OR into WA ahead of the low Thursday night and east into ID Saturday. Broad areas of moderate precip are expected to spread inland through both days with only very localized and short lived embedded heavy precip ahead of the low. A general model blend was used for WPC QPF. ...Southern and Central Rockies... The trough from low shifting inland along the OR/CA border Saturday allows a southerly flow with a tropical connection to reach the NM/CO Rockies Saturday. Precip breaks out along this axis late Saturday with generally light to moderate precip noted in 12Z operational model guidance. ...South Florida... A shortwave trough digging into the north-central Gulf of Mexico Friday night may support heavy showers/thunderstorms across the FL Keys and the southern peninsula as southwesterly flow ahead of the wave begins to draw deep moisture across the region with 1.5 inch PW approaches the Keys. As this moisture interacts with favorable mid-upper level forcing, 12Z guidance, particularly the NAM and ECMWF show moderate to locally heavy amounts developing just south of 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 for the Day 3 QPF. Schichtel/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml