Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 349 AM EDT Tue Mar 13 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 13/1200 UTC thru Mar 16/1200 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. Days 2/3... ...The West... A deep long wave trough axis extends from AK and down the length of the western US coast Wednesday morning. Upper level low pressure will close off west of BC through Wednesday, then shift south to the OR coast by Thursday night. This main core of forcing will focus on the northern half of CA. Only half inch PW wrapping in from the temperate Pacific will be available and thus keep precip rates generally limited to moderate intensity. Snow levels will decrease as the upper low approaches Thursday. Therefore no excessive rain risk is expected at this time. That said, instability under the upper low may allow locally enhanced rainfall in the valleys of northern CA. A shortwave trough rounds the long wave trough through this time with that axis coming ashore to southern CA Wednesday night. PW will decrease in Day 2 off southern CA as the tropical plume that had been coming ashore is shunted south. Light to locally moderate rain will occur over portions of southern CA Wednesday night into Thursday morning. WPC QPF was based on a blend of the 00Z GFS/ECWMF with considerations given to the prior forecast. Further information on snow is in the QPFHSD. ...Central Great Plains... A shortwave trough over Kodiak Island Wednesday morning will ride the 110 kt jet around the long wave trough over the West Coast, come ashore over southern CA Thursday, then cross southern CO Thursday night. Surface low pressure ahead of this shortwave will develop over KS Thursday night. Western gulf moisture, pushed up the plains by return flow as surface high pressure shifts east along the Gulf Coast Wednesday through Thursday. Progression of the CO shortwave/low slowed slightly in both the 00Z GFS and ECMWF compared prior runs with the GFS still more progressive/farther east with an axis of QPF along a developing warm/stationary front northeast of the surface low center. Clustering of the 00Z UKMET and EMCWF warranted a heavier weight in the blend to the ECMWF than the GFS. An easterly component north of the low looks to make a max precip area Thursday night across southeast WY where upslope and upper level forcing combine to enhance precip. Further information on snow is in the QPFHSD. ...Northeast... The upper low that is currently centered over the central Great Lakes will be ejecting northeast from PA to New England Wednesday before running into the blocking ridge over the Labrador coast and stalling over the Canadian Maritimes. This will keep an upper trough over the northeastern CONUS through the rest of the week. Lake enhanced precip will be locally heavy along with mainly moderate intensity in the precip shield from the low lingering just north of Maine Wednesday. The low occludes into Thursday with decreasing precip rates expected. WPC QPF is based on a blend of the 00Z GFS/ECMWF and NAM. Further information on snow is in the QPFHSD. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml