Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 821 AM EDT Mon Mar 26 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 26/1200 UTC thru Mar 29/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southern Plains northeast into the Mid Mississippi Valley and Upper Midwest... An amplified split flow upper trough will move slowly through the West over the next day, as the digging southern stream trough eventually leads to a closed mid-upper low across the Southwest while the leading edge of upper height falls and upper disturbances along the subtropical jet emerge into the central portion of the country. This will be the start of a multi-day active convection and heavy to excessive rainfall event, as the amplification of the upper trough west/ridge pattern retards the w-e progression of frontal features and convection ahead of the trough until at least mid-week before the next northern stream shortwave diving se across the northern-central Rockies can finally kick the longwave trough eastward. For the upcoming day 1 period (12Z Mon-12Z Tue), broadening upper divergence downstream of the amplifying trough will be accompanied by deepening sw flow and an uptick in warm/moisture/theta-e advection through the column -- with the spike in PW values (1.5-1.75") owing to the moisture transport aloft (via the subtropical connection off Baja) as well as within the lower-mid layers (trajectories off the western GOMEX). The latest ESRL atmospheric river detection tool (ARDT) in fact depicts a bona-fide AR toward Monday evening (by 00Z Tue) from the southern Plains into the mid MS Valley, with integrated water vapor transport (IVT) anomalies climbing to near 2 standard deviations above normal. At the same time, both the SREF and GEFS show PW and 850-700 mn moisture flux anomalies reaching +3-3.5 over the MO/KS/AR/OK four state region. In terms of the QPF, the spring-like nature of the system is exemplified by the latitudinal spread among the global and high-res guidance, at least in terms of axis of heaviest rainfall. The global QPFs remain on the northern edges of the guidance spread, while the high-res CAMs make up the majority of middle to southern solutions. WPC opted to lean toward the southern members, including the 00Z WRF-ARW, while also giving some weight to the HREF mean given the spread. The attempt was to align the max QPF axis where the rather broad, favorable sw-ne low-level frontogenesis would more likely intercept a more unstable thermodynamic profile, as both the GFS-ECMWF show the northern periphery of the instability axis (MUCAPES up to 500 j/kg) lifting into northeast OK, northwest AR, and southern-central MO overnight. In addition, southwesterly 850 mb flow increasing to ~50kts Monday night will veer and align nearly parallel to the mean 850-300 mb flow by 08-12Z Tue over the aforementioned area, favoring increased upwind propagation (weak Corfidi vectors) and thus an enhanced potential for back-building and cell training. ...Pacific Northwest... A rather impressive short wave trough moving rapidly across the nern Pacific towards British Columbia will yield zonal moist onshore flow into WA and extending south into OR over the next 24 hrs. This upper disturbance and lower heights will not dig much at all, due to the mammoth upper ridge over the eastern Pacific and extending into the northwest quad of the country. However, an anomalous long fetch of Pacific moisture flux over-top the upper ridge will persist through Monday night across the Pacific Northwest. Overall WPC followed a blend of the GFS/ECMWF along with the 00z high-res package, which yielded areal avg totals of 1 to 2+ inches with locally higher values in the coastal ranges, Olympics and Cascades. Days 2 & 3 ***Southern Plains to the Deep South and southern Appalachians*** There is a strong multi-model signal in the Tuesday morning to Thursday morning time period for widespread heavy rainfall across portions of the southern plains and extending slowly eastward across Missouri, Arkansas, and into the Deep south and Tennessee. Sustained deep moisture flux from the Gulf of Mexico will be well established by Tuesday morning across eastern Texas and Oklahoma ahead of the pronounced upper level trough and closed low over the Four Corners region. This moisture surge is also being aided by return flow around the eastern U.S. surface high that is slow to move eastward, advecting 65+ degree surface dewpoints northward ahead of the cold front. This will intercept a slow moving frontal boundary that will be crossing the south-central U.S. during the middle of the week, with multiple waves of low pressure along it. By Tuesday morning, a strong low level jet of roughly 40 knots develops from eastern Texas to Missouri and advects 1.5 to locally 2 inch PWs (about 2 to 3 standard deviations above normal) northward ahead of the boundary. This same general region will be under the favorable right entrance region of the 250mb jet through this forecast period and will enhance deep layer ascent. Multiple mesoscale convective systems are expected to develop and move in a general southwest to northeast direction, with a good potential for training convection from southwest to northeast that is parallel to the mean layer flow, and merging outflow boundaries capable of producing several inches of rainfall, with rainfall rates easily reaching the 1 to 2 inch per hour range. The QPF forecast from WPC was based primarily from the ECMWF, GFS, and the WPC biased corrected model. Flash flood guidance values are relatively high from eastern Texas to Mississippi. However, given the magnitude of the expected rainfall, on the order of 2 to 4 inches with locally higher amounts within the heaviest QPF axis each day, and convective training potential, a slight risk of excessive rainfall remains in effect for the Day 2 period from eastern Oklahoma to the St. Louis area, and a Day 3 moderate risk was introduced across portions of the Arklamiss region where 3 to 6 inches of rain is expected. ***Great Lakes region to the Northeast U.S.*** Height falls moving east northeastward in the northern stream flow from the Upper Midwest to southeast Canada will be more progressive than the slow moving southern stream closed low affecting the south-central U.S. This will result in a shorter duration of rainfall with light to moderate amounts expected from Michigan to the northern Appalachians with amounts generally in the 0.25 to 0.50 inch range, and higher across Ohio and western Pennsylvania. ***Colorado and New Mexico*** An upslope precipitation event is expected to be ongoing across the mountains of southern Colorado and northern New Mexico and adjacent foothills and plains, primarily for the first half of Day 2 Tuesday morning and into the early afternoon. Post frontal upslope northeast flow combined with enhanced upward motions to the northeast of the southwestern U.S. closed low will support moderate snow from central Colorado to north central New Mexico. The lift is expected to weaken by Tuesday night and the precipitation should decrease in coverage and intensity. The winter weather discussion (QPFHSD) has additional information regarding the snow expected. ***Pacific Northwest into the Northern Rockies*** The main culprit for precipitation across northwest Oregon and western Washington will be sustained moist onflow flow from the northeast Pacific interacting with the coastal ranges and the Cascades. The majority of the precipitation is forecast to fall during the day 2 period on Tuesday/Tuesday night when the onshore flow is more orthogonal to the topography and thus maximizes the ascent. Locally heavy precipitation, mainly in the form of snow, is likely for the higher elevations of the Cascades. By Wednesday, the mean layer flow veers to a more northwesterly direction and the PW axis is mainly offshore by that time, and drier conditions return by the end of this forecast period. Additional light to locally moderate precip is likely from the northern Rockies of northern Idaho and western Montana and expanding southeastward into the northern high plains and Rockies of Wyoming. Hurley/Hamrick Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml