Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 517 AM EDT Mon Mar 12 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 12/1200 UTC thru Mar 15/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Mid Atlantic into the Northeast... A strong compact shortwave is centered over TN early this morning. This wave will continue moving southeast through the day...with an area of rain/snow associated with it. During the morning hours the developing coastal low will become the dominant low...intensifying as it moves northeastward off the Mid Atlantic coast. However still expect a well defined area of lift to accompany the mid level wave as it moves into the Carolinas...with a QPF maximum to the north northeast of this feature. For this aspect of the system (QPF across the Mid Atlantic) favored a blend of the latest GFS/UKMET/HREF/NBM which seemed to offer a good median solution. This shrinks/decreases the area of QPF over portions of VA and northern NC...which should see the combination of downslope flow east of the Appalachians and the mid level wave tracking further southeast...both help cut down on amounts. QPF totals over central and eastern NC and SC remain pretty similar to our previous forecast...just a bit more added detail. Some model differences were noted with the strength and track of the developing surface low...which should really begin to intensify tonight as multiple mid level waves interact with it. Overall thought the 12z ECMWF was too weak and thus too suppressed with QPF through the day 1 period ending at 12z Tuesday. Meanwhile thought the 0z NAM was likely too strong and thus too far northwest and heavy with its QPF. For now preferred to stay in the middle of this model spread. A blend of the 0z GFS/UKMET/HREF mean and the 03z NBM seemed to represent a good middle ground solution. This blend also ends up pretty close to what the older 0z ECMWF from last night was depicting. This blend brings the 0.5" line up to around Boston...with 0.75"-1" over portions of far southeastern MA on to Cape Cod. Additional QPF will occur after 12z...thus see the day 2/3 portion of the discussion or our heavy snow discussion for more details regarding the remainder of the storm. ...California... Precipitation will move into northern CA late tonight as a cold front moves ashore. Given the well defined nature of the front and good mid/upper level forcing forecast...would expect to see a broad area of steady precipitation move ashore. Rainfall rates in the 0.3"-0.4" range in an hour appear likely across the favored terrain of northern CA into the Sierras. Model agreement was generally good...allowing for a multi model blend for QPF. This keeps good continuity from our previous forecast...just bumps amounts up a tad over the Sierras. ...South Florida... Scattered convection will remain possible over south FL into mid afternoon ahead of the southward dropping cold front. A decent amount of model spread was noted...but at this point not expecting anything too organized too occur. Thus the WPC QPF ends up pretty close to the 0z HREF mean...with a broad area of 0.25"-0.5" amounts. Given the anomalous PWATs in place...will probably see some locally heavier totals...but at this point anticipate those will be isolated in nature. Days 2/3... ...The West... A deep long wave trough axis extending south from AK with vorticity maxima centered off northern CA comes ashore Tuesday pushes inland through the midweek, with a closed embedded upper low approaching WA Wednesday night. Multiple shortwaves wrap around this low, digging the trough axis to southern CA and providing forcing to the plume of 0.75 inch PW moisture that streams inland ahead of the axis by Tuesday morning. Heavy precip will be centered on the CA Cascades and Sierra Nevada Tuesday into Wednesday with moderate intensity precip spreading south along the central and southern CA coast east Tuesday, and reaching the Rockies late Wednesday. This is a multi-faceted complex of activity moving in with understandable differences in guidance. WPF QPF was based on a blend of the 00Z GFS/ECMWF with a touch of 00Z UKMET and consideration to the previous forecast. The heaviest precip could exceed four inches across both days is expected at higher elevations of CA which will be in the form of snow in the crashing height falls. See QPFHSD for further snow information. ...Northeast... Better agreement is seen among the 00Z global guidance suite compared to recent days for the phased low already developed off the southern New England coast Tuesday morning. The 00Z ECMWF returned farther east and strengthened which is in line with the preferred guidance consensus. WPC QPF is based on the 00Z ECMWF/GFS with a bit of the 00Z UKMET and consideration of the previous forecast. As the surface low nears offshore New England, there is a clear model signal for a deep layered frontogenesis arcing from south-southwest to north-northeast over southeastern New England/MA through the day Tuesday. Strong banding with potential convective elements and ocean enhancement appear likely. A strengthening deformation axis should impact eastern and northern New England while light to moderate snow falls farther west into the interior Northeast. Orographic influences are also expected as the cyclonic flow passes offshore. The larger scale upper low will persist over the eastern Great Lakes Tuesday before ejecting east and arcing north across New England Wednesday night. This low will ensure plenty of cold air to support lake effect/enhanced snow showers downwind of the eastern Great Lakes with a focus southeast of Lakes Erie and Ontario and wrap around moisture into northwestern New England. Further info on snowfall can be found in the QPFHSD discussion. ...Big Bend of Texas... Anticyclonic flow around a surface high building into Louisiana Wednesday allows one inch PW (2 to 3 standard deviations above normal) to push up mainly the Mexican side of the Rio Grande. The combination of upslope flow and peak diurnal heating allows around a quarter inch areal QPF Wednesday afternoon which is favored more in the 00Z GFS than the 00Z ECMWF. Neither model features instability...so there is no risk for flash flooding here. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml