Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Tue Mar 13 2018 Prelim Day 1 QPF Discussion Valid Mar 13/1200 UTC thru Mar 14/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Northeast... The heavy snow event across New England will continue through the day Tuesday as an intensifying area of low pressure moves northward offshore. The setup continues to favor a broad area of moderate snowfall with the likelihood of a heavy snow band given the impressive frontogenesis forecast from eastern MA into portions of NH and ME. Some model differences were still noted. The 0z NAM and the 0z HREF were stronger and further west with the low/mid level low...resulting in heavier QPF totals both across eastern and western New England. Meanwhile the majority of the global models (GFS/ECMWF/UKMET) and the GEM regional were slightly weaker/east...resulting in a further east QPF distribution. At this point the feeling was that both camps had some aspects of the forecast more correct. Thus WPC QPF was close to a 50/50 compromise of the latest global model mean and the HREF mean. This does result in an increase in amounts across much of New England...but not as high as most of the 0z high res runs would suggest. Though the day 1 period (06z Tue- 12z Wed) this brings the 1.5" QPF line into coastal MA/NH/ME with some 2"+ amounts possible over far southeast MA and Cape Cod...while also expanding the 1" line further west into central New England. Another well defined shortwave will dive southeast on the southern periphery of the closed northeast mid/upper level low. This should result in increased snow shower coverage across portions of the Great lakes into the OH valley and the Appalachians today into tonight. Overall kept pretty good continuity here form our previous forecast...with the high res guidance more leaned on than the global solutions. ...West... Strong mid/upper level forcing will overspread the west coast today. Shortwave energy within the longwave trough...and strong upper level divergence in the left exit region of an approaching upper jet...will provide the synoptic ascent for precipitation through the period. PWATs will generally be running around the climatological 90th percentile and integrated moisture transport is closer to the 97th percentile. Also given the falling heights...would anticipate the development of weak instability through the day. The combination of the strong forcing, moderately anomalous moisture/IVT and weak instability supports the idea of pretty good rainfall rates today over CA...primarily over the Sierras and the western slopes of the terrain where orographic impacts will be maximized. Rainfall rates of 0.25"-0.5" within an hour are expected. Given the weak instability...the high res guidance supports the idea of convective elements embedded within the broader rain shield...with localized rainfall rates potentially exceeding 1" in an hour in the eastern valley and western slopes of the Sierras. These enhanced rates should not last all that long...as the enhanced moisture transport axis is rather progressive. Behind it the cooling temperature aloft and persistent upslope flow continues to support showers into the terrain...however dropping PWATs and forcing by this time should result in decreasing rates. Thus overall appears like a longer duration of steady showers through the day 1 period...with a window for a shorter period of more intense rates. QPF totals were increased some from our previous forecast with 1-3" expected where orographic enhancement is maximized west of the Sierras...with 0.25"-1" across the Valleys and around 1" across the favored coastal terrain. WPC QPF used a multi model blend...weighted towards the 0z HREF mean. Chenard