Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 353 PM EST Thu Mar 08 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 09/0000 UTC thru Mar 12/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Eastern Great Lakes - Northeast... The occluding cyclone off the coast of Maine early this afternoon, north of the closed mid-upper level circulation, will pivot slowly westward across Maine during the day 1 period. After 00Z Friday, additional light-moderate liquid equivalent precipitation totals (0.25-0.50") will fall across northern Maine early in the period (i.e. through tonight) -- owing to the slowly-retrograding shortwave (lingering DPVA) and deep moist easterly flow/elongated Atlantic fetch. At this stage, the models are fairly well clustered in terms of the QPF axes, though the GFS is one of the wetter pieces of guidance across northern ME. The WPCQPF was essentially a consensus of both global and high-res models, most notably the 12Z GFS, ECMWF, local bias-corrected ensemble QPF, HREF mean, and recent NBM, which tended to temper the wetter output offered by the GFS. Elsewhere, lake and/or diurnally-enhanced snow showers will bring additional modest accumulations (with the orographic enhancement) into the northern Appalachians-Laurel Highlands as well as the Tug Hill Plateau-Adirondacks, along with another area of 0.25-0.50+ inch liquid equivalent totals across northeast OH, northwest PA, and western NY downwind of Lake Erie with the predominant WNW low level flow. ...Northern CA and Pacific Northwest eastward into the Northern Rockies... Satellite shows a compact area of low pressure along the PAC NW coast early this afternoon. This low will continue to move northeastward and weaken with time later this afternoon and tonight. Onshore westerly flow will be around the climatological 99th percentile through 00Z, thus should see good orographic enhancement to precipitation totals and see rates as high as 0.3"-0.4" in an hour. The lack of anomalous PWATs and a quick progression of the strongest forcing/onshore flow will limit the overall magnitude of rainfall. However anticipate a longer duration of showery conditions given continued mid level troughing overhead...thus looking at amounts within the 1-1.5"+ range both along the coastal ranges and the Cascades through 12z Friday. Should see decent QPF further inland across the northern Rockies as well...as the deep layer zonal west to east flow is favorable for orographic enhancement. Thus anticipate some 1-1.25"+ totals in the more favored locations. Model agreement was pretty good across this region. Thus WPC was able to utilize a multi model blend for QPF. Days 2/3... ...California into Great Basin... A rather wet start to the weekend is expected for CA with the marginal potential for excessive rains. Currently, a positive tilt upper trough is over the eastern Pacific with a batch of upper dynamics in the base of the amplified feature. This short wave consolidates and intensifies on Fri, reaching central/southern CA on Sat before shearing-out downstream into the Great Basin/Southwest on Sun. A slight plume of anomalous pacific moisture transport with some mid and high moisture content from the subtropics will surge into Central/Southern CA on Sat and this onshore component will linger into Sat, as the upper dynamics fade while advancing downstream. There is some slight spread with the guidance on progression of this feature, ecmwf continues to be more progressive than gfs. WPC followed a blend compromise of the data, with moisture spreading into Central/Southern CA on Sat and a marginal threat for excessive rain on Sun from central CA coast/coastal ranges into Transverse ranges of southern CA, while heavy snow impacts the central/southern Sierra. Overall expect 1 to 2 inch areal avg amounts with locally higher numbers over the weekend. ...Central/Southern Plains and Mid/Lower MS Valley into South/Southeast... Operational and ensemble data continue to converge toward some similar solution over the southern tier of the country but as the weekend progresses, large spread still exists involving multiple streams. The consensus at the moment suggests a possible heavy rain event and WPC kept but shifted a slight risk for excessive rainfall south or from nern LA/swrn AR through central MS into parts of AL. A progressive polar short wave trough, sliding through the Pac NW today, will gradually dig south and east Fri and through the weekend, eventually reaching the southern tier of the country. Continuous lowering upper heights ahead of this feature will allow constant moisture return to slide north from the Gulf of Mexico and focus/overrun along a stalled frontal zone, before a surface low will form and begin to proceed downstream toward the Southeast. The focus for heavier rain on Sat will likely focus from southern AR/much of LA to MS. However, a number of solutions capture rather deep convection focusing over the central Gulf of Mexico, which may prohibit the deepest tropical moisture content to make its way north to interact with the surface boundary and arriving short wave. Then on Sun, this Gulf convection will calm/fade and allow more widespread potential for heavy rain ahead of the surface low moving west to east. This presents heavy rain potential from MS/AL into GA/southern Appalachians and SC. WPC followed a major blend compromise of the 12z gfs and 00z/08 ecmwf both days for amounts and excessive rainfall forecast. This means each day the potential for 1 to 3 inch areal avg amounts over the region, especially in Sun. Over time, the ecmwf becomes awfully suppressed and less wet, which continued at 12z, and the gfs is a bit bolder/amplified and more aggressive with qpf potential. There remains large spread, especially Sun into early next week, this is not atypical with a multi-stream feature and likely means changes to the forecast over the next day or two. Hurley/Musher Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml