Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 401 AM EST Thu Mar 08 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 08/1200 UTC thru Mar 11/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Northeast... The heavier snowfall from the east coast storm will shift into central and northern New England today. The area of low pressure will get tucked back west into Maine later today...with it filling out and weakening into tonight. Models remain in good agreement with the handling of this evolution and for QPF through the day 1 period. WPC thus utilized a multi model blend, which keeps pretty good continuity from our previous forecast. Highest totals should generally be within the 0.75"-1" range across portions of central and northern Maine. ...Northern CA/Northwest... Satellite shows a compact area of low pressure off the Oregon coast early this morning. This low will continue to move northeastward...moving ashore across northwest Washington later today. Precipitation intensity should pick up along the WA/OR coast by 15z and linger through the day. Onshore westerly flow will be around the climatological 99th percentile for a period today...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 1-3" 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" 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... ...Pacific Northwest to Northern Rockies and Adjacent Northern High Plains... One shortwave approaching from the northeast Pacific Ocean/Gulf of Alaska region will make its way onshore Friday morning with 30 kt to 40 kt low/mid level flow helping to push a plume of moisture inland ahead of the low shortwave. As the shortwave makes its way into the northern Great Basin, the fetch of moisture becomes disrupted by rising mid/upper level heights. As a result, areal coverage of precipitation will be tapering off as the system reaches the northern Rockies and the adjacent high plains WPC tended to follow the ECMWF/GFS here and their hi resolution counterparts to place precipitation in the complex terrain. ...Southern California... Flow aloft over the eastern North Pacific Ocean will back as shortwave energy rounds the base of a trough at 145W late Friday and approaches southern California late in the day on Saturday. Between strong upper level difluence/divergence and a plume of precipitable water values in excess of 1.25 inches approaching the coast...precipitation should reach the coast of southern California by Saturday evening and then spread inland overnight. Focused the heaviest amounts where the moisture plume interacted with the coastal ranges as well as the southern portion of the Sierra Nevada range. A general model compromise in terms of QPF worked out well. ...Gulf Coast to Southeastern States... Rainfall should become more widespread and more common beginning as early as early Saturday and continuing through Sunday night...with the axis of moderate to heavy rainfall amounts lining up with areas that were soaked a week or two ago. This raises the possibility of excessive rainfall...so have hoisted a slight risk of excessive rainfall over parts of eastern Arkansas into northern Mississippi and southwest Tennessee for the day 3 period /12Z Saturday to 12Z Sunday/. The GFS and UKMET were most aggressive with rainfall amounts in part because of a deeper surface low and more energetic shortwave approaching from the west while the ECMWF actually backed off rainfall amounts from its 07/12Z run. The NAM did not have widespread coverage of heavy rainfall but rather displayed a mottled look with heavy amounts scattered around the region. The uncertainty in the system's strength and track has gone back several runs now so confidence in the forecast is below average. Even so, the signal is present for at least a period of heavy rainfall. A fairly broad slight risk was issued in part because of the model spread in exactly where the most intense rainfall rates fall...the area will be refined in upcoming forecast cycles. But the axis of highest precipitable water values is close to this area in the GFS/ECMWF/NAM. The UKMET axis and associated precipitation was confined south of the outlook area but its idea was covered by a marginal risk that surrounds the slight risk. ...Northeast... Cyclonic flow around the departing storm that impacted the region on Wednesday/Thursday will continue to produce generally light precipitation over New England and portions of western and northern New York through much of the day 2/3 period. The heaviest precipitation on day 2 should be over far north/east Maine given the proximity to the low center and the amount of moisture within the circulation. Amounts taper off on day 3 and should be focused over the upslope regions of western Maine. To the south, some lake effect precipitation is expected to the lee of Lake Ontario as cold air is drawn across open water. Some lake enhanced precipitation is expected to the lee of Lake Erie, although amounts should be less than what falls downwind of Lake Ontario. Chenard/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml