Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 454 AM EDT Sat Apr 07 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 07/1200 UTC thru Apr 10/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 For the 12z issuance: most changes were minor in the first six hour period, and the changes were based on (1) radar trends and (2) the most recent HRRR output. A few minor changes were made after 08/00z, mainly to drop QPF amounts a bit on the northern and western edge of the precipitation shield with the offshore surface low. The dry air in place may be tough to dislodge, even as the surface low makes its closest approach to the coast this evening over the mid Atlantic and far southeast New England. ...California... Deep Pacific moisture embedded in a weakening atmospheric river is transported across the Central CA early in the period, resulting in heavy to excessive rainfall during the first half of Day 1. The moisture plume weakens as it moves southward during the second half of the period, reducing the flood threat. For the most part, there was good model agreement with the dissipation of the atmospheric river, as well as the placement of the maximum qpf, so the WPC QPF was based on a blend of the 00z GFS/12z ECMWF. The most recent blended TPW product showed 1.75 inch precipitable water air (which is near record level values for the northern CA coast, with the 00z OAK sounding showing 1.67 inches) just off the coast. The moisture in the atmospheric river is being driven onshore by a 25 to 35 knot low level westerly flow, which becomes increasingly orthogonal to the terrain. The deep moisture has become focused ahead of a frontal boundary approaching the CA coast this morning, and model soundings indicated between 100/250 J/KG of MUCAPE ahead of the front, which comes ashore with the moisture plume. The convection could boost hourly rainfall rates to between 0.50 and 1.00 inches...mainly before 07/15z. Otherwise, the combination of near record moisture and modest instability will feed bands of rain with embedded elevated convection to produce between 2.00 and 3.00 inches of rainfall across the favored upslope areas of the Sierra Nevada Range. The front is expected to come ashore near 07/18z across central CA, and the best mid level lift in the fast mid level flow occurs before that time. The additional rainfall is expected to result in new flooding, or exacerbate existing flooding during the first part of the period. Rock slides, mud slides and flooding of roads, small river and stream can be expected through the cold frontal passage. After the front passes, the deepest moisture is shunted southward, as any remaining elevated instability is exhausted. Rainfall amounts nearing an inch are expected across the southern portion of the range, which could result in local runoff issues. ...Pacific Northwest.... A strong closed mid level low off the Pacific Northwest coast early on Day 1 weakens as it approaches Vancouver Island by the end of the period. Moisture in a strong upslope flow is expected to produce heavy to locally excessive rainfall over the higher terrain. There was good model agreement with the evolution of the mid level system, so the WPC QPF was based primarily on a blend of the 00z GFS/00z ECMWF, augmented with the 00z HREF mean over the higher terrain. Surface low pressure associated with the strong closed low off the Pacific Northwest coast early on Day 1 weakens as it approaches far western WA by the end of the period. Ahead of a weakening cold front associated with the surface low, a 30 to 50 knot low level south southwest flow transports 0.75 inch precipitable water air over western WA/OR. The best mid level lift ahead of the closed mid level system occurs between 08/06z and 08/12z, during the strongest upslope flow. There was a multi model signal for an axis of 2.00/3.00 inches of qpf stretched from the Klamath mountains in southwest OR across the OR Coastal range into the Olympics in WA. Slightly lower amounts (generally between 1.50 and 2.50 inches) were extended across the WA and northern OR Cascades. Snow levels are relatively high ahead of the surface front, so the majority of the qpf is expected to fall in the form of rain. This much rain could result in local runoff issues, especially across the coastal ranges late in the period. ...Rockies/Great Basin... Short wave energy in a fast mid level flow provides synoptic scale ascent over a frontal boundary extending from western MT into CO, and the ascent aids in spinning up surface low pressure on the front over WY and CO. The combination of moisture from a Pacific plume, upslope flow and synoptic scale lift is expected to produce locally heavy qpf amounts over the CO Rockies. There was good model agreement with the overall synoptic setup, so the WPC QPF was based primarily on a 00z GFS/00z ECMWF blend, augmented with the 00z HREF mean over the higher terrain. As short wave energy ejected from a strong closed mid level low off the Pacific Northwest coast, the short wave interact with deepening moisture from a strong Pacific plume. Precipitable water values over the the Great Basin and Central and Southern Rockies rise to between 0.50/0.75 inches, which is between two and three standard deviations above the mean. The moisture becomes involved in an upslope flow near a frontal boundary extending from western MT to the CO Front Range, and the combination of upslope flow and synoptic scale ascent is expected to produce local 1.50 inch qpf amounts over the CO Front Range, with lesser amounts (closer to an inch) further south across the range. Further west, behind surface low pressure forming over southeast CO, the upslope flow over the Wasatch Range in UT taps into the deep moisture to produce an axis of 0.75/1.00 inches of qpf along with range, with the highest amounts south of KSLC. Snow levels are fairly high ahead of the short wave energy across the Central and Southern Rockies, so a majority of the qpf is expected to fall in the form of rain. While there could be some issues with runoff and snow melt, flash flooding is not expected over the Rockies and Great Basin. ...Gulf Coast/Mid Atlantic/Northeast... Short wave energy tracking from the Southern Plains across the Gulf States spins up low pressure on a frontal boundary over SC during the second half of the period. Ahead of the front and surface low, moisture and instability will feed convection that produces locally heavy rainfall from the eastern Gulf States to the Mid Atlantic coast. There was good model agreement with the evolution of the surface low, so the WPC QPF was based primarily on a blend of the 00z GFS/00z ECMWF. Short wave energy approaching a frontal boundary draped across the Gulf Coast states early on Day 1 is expected to provide sufficient lift for cyclogenesis to occur over southern AL this morning. As the surface low develops, it drags a cold front across the the eastern Gulf States and Southeast through 08/00z. Ahead of the front, a 40 knot low level southwest flow focuses 1.50 inch precipitable water over FL/GA into SC and NC, mainly before 08/00z. Model soundings showed marginal to locally moderate instability, and divergence in the right rear quadrant of a 150 knot jet streak crossing New England will help organize the convection. There was a multi model signal for 0.75 to 1.50 inches of qpf ahead of the front in the aforementioned locations, with the highest amounts expected over far southeastern NC. The shear is expected to be strong enough to the convection moving, though local cell mergers along the line are possible, producing locally higher amounts. At this time, due to the expected progression of the convection and higher three hour flash flood guidance values, the threat for flash flooding is expected to be very low. Surface low pressure organizing over SC after 07/18z is expected to track northeast, passing southeast of the benchmark after 08/12z. Soundings from 00z north of the Carolinas and southeast VA showed dry air in place, which should suppress the northern and western extent of the precipitation shield with the surface low. Because of this, the 0.10 inch contour was kept south of central MD/central DE to the islands of southeast MA. Days 2/3 ...Pacific Northwest to the Midwest... A series of weak shortwaves will ride along a broad upper trough that shifts east from the Intermountain West through the weekend. A developing lee side surface low east of the Front Range will shift east to the Great Lakes by Monday. A strong baroclinic zone will develop in advance/to the north of the surface low with resultant swaths of locally heavy snow from the Dakotas to southwestern MN to IA/IL by Sunday night (along with moderate precip in the central and northern Rockies). The 00Z GFS and ECMWF diverged a bit with the snow axis over eastern SD though an equal blend of the two provided a good model consensus. The upper level energy weakens as it enters the Midwest Monday with precip rates also decreasing. See the QPFHSD for more information on this snow. PW around a half inch will push ashore as the second shortwave pushes into western WA Sunday with locally heavy rain areally averaged to 1 to 2 inches in the WA cascades and Olympics. Rapid height rises behind the weak trough will lead to a dry spell for Day 3 (12Z Mon-12Z Tue) in the PAC NW to northern Rockies. WPC QPF blend was mainly 00Z ECMWF/GFS with some inclusion of the 00Z UKMET. ...Southeast and Mid-Atlantic... A trailing cold front from a low lifting north off the eastern seaboard will stall over the eastern Gulf/Florida through the forecast period. Gulf moisture is essentially contained to the gulf coast and FL. Little forcing over this area will limit precip totals, though an increasing jet will allow more precip in the the Deep South. No excessive rain risks are anticipated, though north-central FL could see some heavy rains both Saturday and Sunday along the stalled front. Hayes/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml