Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 619 PM EDT Fri Apr 06 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 07/0000 UTC thru Apr 10/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...West Coast/Intermountain West... Ongoing atmospheric river event will continue to transport deep Pacific moisture across portions of northern and central CA into early Saturday morning. Low level flow focuses the deep moisture on the higher terrain and the resulting heavy rainfall over the higher terrain could produce flash flooding. WPC QPF was based mostly on the 12Z HREF mean with adjustments per the GFS/ECMWF. Trend in the 12Z hi-res guidance was just a bit farther south with a heavy rain max just north of San Francisco Bay and pulled the Moderate Risk area to the south as well. Larger Slight Risk area encompasses much of northern California. Morning sounding from OAK showed precipitable water value near 1.60" which was higher than any observed (1948-present) value in the months of November-May. PW values may peak near 1.80" at the start of the period tonight (00Z/07) which would be near the highest value observed during any month of the year -- an incredibly anomalous event. Warmth of the column is also of note with freezing levels above 10,000-12,000ft. Recent RAP/HRRR forecast soundings show an area of 100-400 J/kg of CAPE coming ashore at the start of the period this evening which should add a convective element to the precipitation character as both the moisture and instability drop slowly south ahead of the surface frontal boundary. Combination of upslope flow and modest inflow along with plenty of moisture supports several inches of rain for parts of the coastal ranges and also the northern and central Sierra Nevada range with lesser amounts (under 0.75") for the interior valley. This was supported by the 12Z hi-res guidance and the 12Z HREF mean offered a good starting point. After about 12Z Saturday, moisture axis sinks southward and diminishes as the cold front pushes through NorCal and then into the Great Basin later in the day. This will split the precip areas up the coast into Oregon and also eastward through Nevada into Utah. Favored a modest compromise among the hi-res guidance with the GFS as the ECMWF appeared too dry. ...Southern Plains across the Lower MS valley into the Mid Atlantic... Elevated convection north of a warm front over Arkansas into Mississippi will push its outflow southward into warm/unstable air surging northward out of the Gulf as evidenced by streamers into southern Louisiana this afternoon. Convergence should help develop additional convection south of the current area which is forecast to sink southward as the surface cold front over Oklahoma and north Texas dives southeastward. Models have had a good signal for locally heavy totals in excess of several inches and continued the small Moderate Risk area for northeastern Louisiana into west central Mississippi for this evening into tomorrow morning. Concern is for training ahead of the cold front with sufficient moisture in place (PW values around 1.50" which is about +1.5 standard deviations) and CAPE near 1000 J/kg. Model spread remained both N-S and E-W but the 12Z ARW and recent HRRR runs captured the ongoing convective evolution the best. Front will push diminishing activity eastward through the day on Saturday with less amounts overall due to the quick progression. Cold air to the northwest will help keep the system rather suppressed. Days 2/3 ...Pacific Northwest eastward through the Northern and Central Rockies... Heavy rain and snow will continue across the Pacific Northwest Saturday night into Sunday. PWAT values are not all that anomalous...however the position of the strong deep layer low off the WA coast will support persistent westerly flow into the terrain maximizing orographic effects. While not that anomalous, moisture will certainly be sufficient to support some heavy totals given the persistent forcing present. Some model differences were noted with the location of the low...with the 0z ECMWF and 12z GEM north of the NCEP models. The end result is for heavier amounts further south across western OR in the GFS/NAM and high res runs compared to the ECMWF. The UKMET is more similar to the NCEP guidance and trends seem to support a slightly further south solution as well. Thus while WPC followed a multi model blend for QPF...leaned more towards the axis depicted by the GFS/UKMET with details from the high res runs added. Looking at a broad 1-3" across the WA/OR Coastal Ranges and Cascades, with localized 3"+ amounts in the most favored terrain. This a slight southward shift from our previous forecast. As the forcing and westerly upslope flow moves into the northern Rockies, should see some 1" totals in the favored terrain there as well. Also looking at some hefty totals across the CO Rockies. The broad and anomalous atmospheric river moving into the west coast today (Friday) has its sights set on the central Rockies Saturday night into Sunday. Models are forecasting around record PWATs and integrated moisture transport over CO associated with this moisture plume. The large scale forcing is not all that impressive...however the anomalous moisture parameters combined with strong westerly flow supports some heavy totals in the favored terrain. The favorable setup is fairly short lived, but should see a shot of moderate to heavy precipitation Saturday night into early Sunday with totals between 0.75"-1.5" (locally higher) anticipated. Overall the new WPC QPF is a bit heavier and shifted south from our previous forecast over this area. ...Northern/Central Plains into the Upper and Mid Mississippi Valleys... As all this moisture entering from the Pacific overruns the arctic air over the Plains...will see snow spread east into the Plains and MS Valley Saturday night into Sunday. Although, at this point not looking like QPF totals will be all that significant. Despite the impressive atmospheric river moving into the west coast...moisture anomalies are not progged to be that high across the Plains. Looks like much of the moisture will struggle to make it over the Rockies and instead dissipate and sink southward over the Southwest with time. The lack of a strong wave and forcing in the mid and upper levels is likely playing a role in keeping moisture lower...along with the dry retreating arctic high pressure. Thus, if anything, models are trending a bit lighter with QPF magnitudes east of the Rockies. Models were in pretty good agreement...with WPC blending the 12z GFS/NAM Nest/ENS BC and the 0z ECMWF for QPF across this region. ...Southeast and Mid-Atlantic region into southern New England... An area of low pressure will form over the southeast on Saturday...passing off the Mid Atlantic coast Saturday night into Sunday. Models have shown some trend northwest with this low...although seemed to have stabilized some with recent runs. Model agreement is pretty good. And thus WPC utilized a multi model blend...incorporating the 12z GFS/UKMET/NAM Nest and 0z ECMWF. This shifts the QPF a tad northwest from our previous forecast across the eastern Mid Atlantic and into southern New England. Likely to have some convection across FL along the southward drifting front. The GFS is probably too dry with this activity, with the NAM too far north. The new WPC QPF is a bit higher than the previous forecast...with enough convergence and instability forecast to result in some locally heavier totals. On day 3 broad troughing is forecast to move into the east. A pretty messy pattern by this time with no real strong wave forecast...instead multiple weak ones. Thus no real good focus for QPF by this time...with moisture limited as well. The 12z suite of models have generally come into better agreement here...showing a continued downward trend in amounts. The 12z NAM now stands out as an outlier...thus a non NAM blend was used for QPF in the east on day 3. Also note a continued southward trend with the axis of convection over FL in most of the guidance, which was incorporated into the new WPC forecast. Fracasso/Chenard Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml