Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 502 AM EDT Wed Apr 11 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 11/1200 UTC thru Apr 14/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...West Coast... A cold front crosses the West Coast from WA through central CA during the second half of Day 1. Ahead of the front (and its long wave trough), a strengthening low level flow focuses Pacific moisture over southwest OR/northwest CA, where locally heavy rainfall is expected. As the surface and mid level systems cross the Sierra Nevada Range, the threat for heavy rainfall shifts there late in the period. There was generally good model agreement with the timing of the surface and mid level systems, so the WPC QPF was primarily on a blend of the 00z GFS/12z ECMWF. Some 00z HREF mean/00z WRF ARW were used to better delineate higher terrain qpf maxima. The most recent blended TPW product showed an axis of 1.00/1.25 inch precipitable water air reaching from the central Pacific to northern CA coast, and the 11/00z OAK sounding showed a 1.15 inch precipitable water reading. The moisture was streaming ahead of a long wave trough near 40N 135 W early this morning. As the long wave trough approaches the West Coast during the first half of Day 1, a 30 to 40 knot low level southwest flow transports 0.75/1.00 inch precipitable water air to the southwest OR/northwest CA coast, mainly between 11/15z and 11/21z. During this time, model soundings showed MUCAPE values between 100/125 J/KG along the WA/OR coasts, which could add a convective characteristic to the QPF ahead of a cold front associated with the long wave trough. There was a multi high resolution model signal for 2.00/2.50 inches of qpf over the higher terrain of the Klamath mountains, and given the strength of the low level inflow, as well as the presence of marginal elevated instability, these amounts seem plausible. As the long wave trough crosses the West Coast after 12/00z, the focus for higher qpf amounts shifts to the Sierra Nevada range and the OR Cascades, where the low level flow is more orthogonal to the terrain. In these areas, qpf of 0.75/1.25 inches were placed, with locally higher amounts possible in the favored upslope locations (as advertised by the 00z HREF mean/00z WRF ARW). Locally heavy snowfall is possible across the higher terrain of the Sierra Nevada range during Day 1. Please refer to the most recent suite of WPC Winter Weather products for more information. ...Northern Rockies... Short wave energy ejected from a long wave trough crossing the West Coast during the second half of Day 1 spins up surface low pressure on a low level boundary along a frontal boundary extending from western MT into eastern CO. The combination of low level convergence along the front and upslope flow is expected to produce locally heavy qpf amounts across the higher terrain. There was fairly good model agreement with the timing of the short wave and frontal positions, so the WPC QPF was primarily on a blend of the 00z GFS/12z ECMWF. Some 00z HREF mean/00z WRF ARW were used to better delineate higher terrain qpf maxima. A boundary drops southward during Day 1, extending across western MT into eastern CO by the end of the period. Lift associated with short wave energy ejected from a long wave trough approaching the West Coast is sufficient to spin up surface low pressure on the boundary over central WY, which tracks along the front into eastern CO by 12/12z. The combination of low level convergence along the front and increasing upslope flow is expected to produce qpf maxima over the Blue Mountains in OR (with amounts nearing an inch), as well as the northern Bitterroot Mountains in ID, and the Grand Tetons and Big Horn Range in WY. In these locations, local 0.75 inch qpf amounts are possible. Locally heavy rainfall is possible in the lower elevations (where marginal, mostly elevated instability could increase rainfall rates), which could result in local runoff issues where snowmelt is occurring. At this point, this appears to be a flood versus flash flood threat, so no Marginal Risk area was assigned. Locally heavy snowfall is possible across the higher terrain of the Northern Rockies during Day 1. Please refer to the most recent suite of WPC Winter Weather products for more information. ...Northern Plains into the Upper Great Lakes... Short wave energy tracking from the Northern Rockies across the Northern Plains into the Upper Great Lakes on Day 1 pushes along a surface low across this area. Moisture and lift become focused along the track of the surface low, with the highest qpf amounts over the Upper Great Lakes. The 00z GFS has trended south with its qpf maximum over the Upper Great Lakes, so the model choice was skewed toward the more consistent 12z ECWMF. A short wave tracking from north central MT across ND into MN/WI during Day 1 strengthens with time, and spins up surface low pressure on a frontal boundary. Ahead of the short wave, a 20 knot low level south southeast flow transports 0.50/0.75 inch precipitable water air up along the 295K isentropic across ND into MN/WI/MI. The best lift occurs between 11/18z (over ND) through 12/06z (over WI and the LP of MI), during which timed banded precipitation is possible, mainly over central WI into the central portion of the LP of MI. As noted earlier, the 00z GFS was further south with the surface low and best lift, and it the further south with the axis of highest qpf. The WPC QPF was based more on a 00z NAM/12z ECWMF for the placement of the qpf axis, and amounts were closer to the 12z ECMWF/00z HREF mean. This blend produced an axis of 0.50/0.75 inches of qpf over central WI into the LP of MI. Days 2/3 ...Pacific Northwest... An persistent onshore flow in the wake of a departing upper trough Thursday with PW increasing from below 0.5 in to 0.75 in allows light to moderate precip over the coast ranges and Cascades for OR/WA. The next trough rotates around the Gulf of Alaska low Friday night and sends a narrow pre-trough plume of 0.75 inch PW into the Olympic Peninsula on a 40kt southwesterly jet. Enhanced precip with snow in the mountains occurs in this narrow stripe. A marginal excessive rainfall outlook was issued for Day 3 (12Z Fri-12Z Sat) for lower western portions of both the Olympics and the northern WA Cascades. ...Intermountain West to the Upper Midwest... Shortwave energy rounds a trough as it shifts east across the intermountain west allowing it to close at 500mb over WY Thursday night and allowing a trough to amplify south to the southern Rockies. This amplification maximizes gulf moisture input with bountiful moisture wrapping around the system with a deformation zone and topographic enhancement on an easterly flow for MT Thursday then east across SD Thursday night and the Upper Midwest/Great Lakes Thursday night. Slow progress of the low will allow repeating precip along this axis north of the developing surface low near the KS/NE border through the Day 3 time period. ...Great Lakes to New England... Zonal flow will push a shortwave trough east from the Great lakes Thursday morning to across Maine by Friday morning. 0.75 inch PW in a southwesterly jet with Gulf origins allows moderate intensity QPF in the quick moving system. The 00Z GFS was notably farther north with much of the QPF for Day 2 (12Z Thu-12Z Fri) which caused it to be reduced in the QPF blend that favored the 00Z ECMWF/NAM. ...Lower Mississippi Valley/ArkLaTex... An amplifying trough around a closed low over the central Rockies Friday will enhance a 50kt southerly jet off the western Gulf and across east TX/OK and the lower MS Valley before shifting farther north along the east side of the developing surface low over the KS/NE border. PW of 1.75 to 2 inches is 2.5 to 3 standard deviations above normal with a heavy rain threat. The trough begins to swing from a positive tilt to a neutral one Friday night which should allow for at least some progression. Timing is the greatest unknown with the 00Z ECWMF slowest and the 00Z GFS fastest. The 00Z NAM was a decent compromise and was used for the QPF axis with max magnitudes similar in the 4-6" range. Introduced a Slight Risk for excessive rain from LA to the bootheel of MO and Memphis. The surrounding Marginal Risk was drawn based on the plume extremes from the ECMWF (for west side) and GFS (for east side). Hayes/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml