Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 633 PM EDT Thu Apr 12 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 13/0000 UTC thru Apr 16/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern to Central High Plains... There is overall good agreement in the latest model suite with the evolution of the multifaceted early spring storm expected to affect large portions of the nation over the next several days. The strong trof moving into the Great Basin this afternoon is expected to continue amplify into a strong closed low over the northern to central Rockies tonight---pushing into the central high plains during Friday. The developing comma head/deformation precip area over the Northern Rockies---northern high plains this afternoon will continue to expand eastward farther into the Northern Plains overnight and then begin to redevelop farther southward into portions of the Central High Plains as the closed low reforms across this area. This will result in a fairly large heavy precipitation region from southeast MT---southwest ND---much of SD into western to northern Nebraska. Heavy snow likely in this axis with max totals in the 12-18" range possible. See the latest QPFHSD for additional winter weather information. ...Upper MS Valley into the Great Lakes... To the east of the developing deep layered low over the Central Plains---a strong frontal boundary will extend west to east into the Upper MS Valley to lower Great Lakes region. Strengthening moist west southwest low level flow will overrun this front---with an expanding region of precip developing to the north of the front across the Upper MS Valley and into the Great Lakes. There is more spread among the latest operational and ensemble members with respect to amounts and the axis of the qpf in the vicinity of this boundary. A qpf solution closer to the in house ensemble mean and href mean was used to mitigate the spread in solutions---with areal average .25-50"+ a amounts depicted. Accumulating snow potential will be confined to the northern edge of this precip region from central MN into northern WI and the northern L.P. of MI. ...Eastern portions of the Southern Plains toward the Lower MS Valley... The low level south southwesterly flow will be strengthening considerably day 1 across the Southern Plains ahead of the above mentioned amplifying closed low---raising pw values to 1 to 2 standard deviations above the mean across portions of the Southern to Central Plains towards the Lower MS Valley. Strengthening upper difluence and boundary layer convergence along and ahead of the dry line/cold front to the east and southeast of the closed low will support increasing convective activity Friday afternoon across eastern portions of Southern Plains toward the Lower MS Valley. Moderate to heavy precip totals possible from northeast TX--northwest LA---eastern OK into central to western AR---southern MO. This active convection will continue to enhance past the end of the day 1 period---with a greater potential for more widespread heavy totals into the Lower MS Valley/Lower TN Valley during day 2. ...Pacific Northwest... Onshore west southwest low level flow will be re-strengthening into the Pacific Northwest tonight into Friday to the southeast of a developing closed low just to the south of the Gulf of AK. This will support moderate to heavy precip values from the Washington-Oregon Cascades---west to the northern Oregon coast range into the Olympic Range. Days 2/3 ...Pacific Northwest... A closed low is expected to drop south, west of the British Columbia coast on Saturday and near the Pacific Northwest coast Sunday evening. A series of smaller scale mid-level shortwaves are expected to crash into the Northwest coast south of the low center with PWATs near 0.75 inches reaching the coast and just inland, out ahead of a cold front. Some minor timing differences which were mitigated by staying near the middle of the spread given run to run changes. While PWATs and low level winds are not strongly anomalous, the long duration event into the Olympics and northern Cascades could produce some localized runoff issues which allowed maintaining the Marginal Risk for the Day 2 Excessive Rainfall Outlook (12Z Fri-12Z Sat). ...Northern and Central Rockies to the Great Lakes to the Northeast... A potent mid-level trough will continue advancing eastward into the central Plains on Saturday, with a closed low at 500 mb tracking along the NE/KS border Friday night and expanding north/south over IA/MO through Sunday morning as the mid-level trough axis acquires a negative tilt while swinging through the lower Mississippi valley. Moisture originating from the eastern tropical Pacific and western Caribbean then across the western Gulf will advect up through the Mississippi valley on a 50+ kt southerly jet ahead of the associated cold front, feeding into and across a strong stationary boundary extending eastward from the triple point low over northern IL to southern New England. A broad area of warm advection/frontogenetically enhanced precipitation will be ongoing at the start of the Day 2 period (00Z/14) over the upper Midwest while increasing upper divergence/diffluence enhances precipitation to the west within a developing deformation axis/coupled upper level jet signature. The northward extent of the precip will be determined by the presence of a strengthening surface high over northern Manitoba/Ontario. Dry air from this high will make for a sharp cutoff on the northern side which has continued to waver in recent runs and will continue to do so with hopefully lesser magnitude going forward. WPC stayed close to continuity despite some of these shifts, with this axis continuing to lie near the middle of the model spread. ...Lower Mississippi Valley/ArkLaTex... A north-northeast to south-southwest line of heavy rainfall is expected to be ongoing Friday evening across eastern Texas, northern Louisiana into Arkansas ahead of an advancing cold front. Early Saturday morning, enhancement to lift will be provided by an increasing upper level jet streak developing from the Ozarks, northward into the upper Midwest, combined with increasing upper level diffluent flow. A broad axis of 1.5 inch PWATS narrowing to a 1.75 to 2.0+ inch axis, or roughly 2 to 3 standard deviations above normal, is expected to be in place to support heavy rainfall with unidirectional flow promoting periods of training. Timing of the heavy rainfall progression toward the east remains uncertain with the GFS remaining fastest but the 12Z HRW NMMB, ARW and NSSL also showing a faster progression compared to continuity and the favored ECMWF/UKMET blend from 06Z-12Z Saturday. Given uncertainty in the hi-res guidance with a 48 hour forecast, decided to stay close to continuity and the preferred blend which allows for a slower progression toward the east. In addition, as the trough amplifies over the southern Plains, a stalling or slight backing of low-mid level flow is possible which could slow progression of the line and enhance already heavy precipitation. The average of the hi-res models and global guidance continues to support the heaviest rainfall through Saturday morning from northern Louisiana into southern Arkansas which is where the Moderate Risk was maintained for the Day 2 Excessive Rainfall Outlook, ending 12Z/14. As the mid-level trough axis swings across the lower Mississippi valley Saturday night, a somewhat faster progression of the cold front is anticipated along with a slight reduction in 3 hour rainfall totals ending Sunday morning. However, anomalous moisture and strongly divergent/diffluent flow aloft and embedded mesoscale boundary interactions will continue to support at least a Slight Risk for the Day 3 Excessive Rainfall outlook ending 12Z/15. Given timing uncertainty increases into the Day 3 time frame, confidence was not high enough to introduce anything higher than a Slight Risk, but an upgrade to Moderate could be possible in future forecast cycles as model convergence continues to occur. As of now, it appears 3-5 inches will be possible from eastern Louisiana into much of Mississippi, western Alabama into central Tennessee. Portions of Mississippi and Alabama have seen up to 200 percent of normal rainfall over the past 2 weeks which could help to support a higher risk category in future ERO cycles. Oravec/Otto Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml