Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 546 AM EDT Thu Mar 22 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 22/1200 UTC thru Mar 25/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Western U.S... On Day 1, the models target California for the heaviest precipitation with several inches of liquid equivalent precipitation in the coastal ranges of southern CA and along the spine of the CA Sierra Nevada range. One distinct area is within the plume of deepest moisture streaming onshore into southern CA into the southern CA Sierra Nevada ranges. The integrated water vapor imagery is well above normal and the slow movement of the max precipitable water values in the streaming streaming onshore in the counties north of Los Angeles combines with 850 mb convergence and 300 mb divergence to produce areas of heavy rain, with ascent aided by windward terrain. The continuation of the high moisture and water vapor transport into interior southern Ca leads to maxima along the foothills and mountains of interior CA with heavy Sierra Nevada snows, with several feet likely over the next day and a half. For further details regarding the excessive rainfall potential, please refer to the latest QPFERD. A second area covers northern CA to the OR border as the northeast Pacific upper low and trough drift towards the coast... with a jet max streaming onshore and inland into the ranges of ID and adjacent western MT. The upper jet moving into NV Thu afternoon supports upper divergence maxima streaming northeast from the CA Sierra across NV into UT with a maxima near the northern Wasatch/Uintas of UT, driving precipitation in windward terrain. QPF was derived from a combination of the latest high resolution guidance -- particularly the HREF means and the 04Z NBM. See the winter weather forecast amounts and snow probabilities for heavy snow potential in the mountains of CA. Days 2/3 ...Pacific Northwest/Northern Rockies... As a strong closed mid level low weakens and comes ashore over the Pacific Northwest during Days 2 and 3, short wave energy ahead of the closed low taps Pacific moisture to produce locally heavy qpf amounts across portions of the Pacific Northwest into CA. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based mainly on a blend of the 00z ECMWF/GFS. Day 2... A strong closed mid level low off the Vancouver Island at the start of Day 2 drops south, reaching a position off the OR coast by the end of the period. Short wave energy ejected from the closed low early crosses the Northern Rockies. There is some Pacific moisture in the flow as it reaches the Northern Rockies, and upslope flow is expected to focus the moisture on the higher terrain. Areas of 0.25/0.50 inches of qpf were placed over the Blue Mountains in OR, as well as the northern Bitterroot Mountains in ID. A strong short wave ejected from the closed mid level low comes ashore over southwest OR during the second half of Day 2. Ahead of the short wave (and its attendant surface low), a low level southwest flow focuses 0.50/0.75 inch precipitable water air over the Klamath Mountains of southwest OR and the Shasta and Siskiyou ranges in northern CA. There was a multi model signal for 1.25 to 2.50 inches of qpf over this area, with the highest amounts over the northern Klamath Mountains. Snow levels drop sufficiently with the short wave passage as to preclude a flash flood threat here on Day 2. Day 3... The closed mid level low off the OR coast at the start of Day 3 weakens into a short wave as it crosses northern CA and northern NV before reaching the Northern Rockies by the end of the period. The low level southwest flow becomes focused on the northern portion of the CA Coastal Range, as well as the Sierra Nevada range in northern CA. There should be enough moisture in the low level flow to produce axes of 1.00 inches of qpf across the Shasta/Siskiyou Ranges. There was a multi model signal for an axis of 1.50 inches of qpf across the favored upslope areas of the Sierra Nevada Range. Snow levels should be low enough in all areas to prelude the threat for flash flooding on Day 3. Snow is expected over the higher terrain of the West Coast and the intermountain West on Days 2 and 3. Please refer to the most recent suite of WPC Winter Weather products for more information. ...Central and Southern Rockies/Great Basin/Southwest... Short wave energy tracking from Southern CA crosses the Central and Southern Rockies during Day 2, bringing with it some of the Pacific moisture for locally heavy QPF amounts. For the most part, there was good model agreement with the the track and timing of the short wave energy. With this in mind, the WPC QPF was based on a blend of the 00z ECMWF/GFS. As short wave energy tracks from Southern CA across the Central and Southern Rockies during Day 2, it brings with it some of the deep moisture from the Pacific plume. Precipitable water values ahead of the short wave briefly peak at 0.50 inches across portions of the Four Corners area, and model soundings showed marginal instability (due mainly to steepening lapse rates). Low level flow on the back side of surface low pressure forming over eastern CO focuses the moisture ion the CO Rockies, where there was a multi model signal for 0.25/0.50 inches of qpf. Further west across UT and AZ, local upslope flow is expected to produce generally 0.10 to 0.25 inches of qpf over the Wasatch Mountains in UT, as well as the Mogollon Rim in AZ. ...Northern Plains/Upper MS Valley/OH Valley/Mid Atlantic... Short wave energy tracking from central CA reaches the Northern and Central Plains during Day 2. Lift with the short wave spins up surface low pressure on a frontal boundary extending the Central Plains into the Mid MS Valley. The low tracks across the lower OH Valley during Day 3, before weakening. There was good model agreement with the synoptic scale features, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. Day 2... Ahead of short wave energy tracking from central CA to the Northern Plains, a 35 to 45 knot low level southeast flow focuses Pacific based moisture on the 295 K isentropic surface across the Northern Plains into the Mid and Upper MS Valley. The best isentropic lift occurs between 23/18z and 24/00z (west) and between 24/00z and 24/06z (east), and model soundings showed some elevated instability across portions of southern IA into northeast MO and west central IL. The combination of moisture and lift is expected to produce an elongated axis of 0.75/1.25 inches of stretching from western ND across eastern SD/southwest MN over much of IA into northernmost MO and much of central and southern IL. The 00z GFS showed maximum amounts closer to 2.00 inches across southwest IA, but this could be overdone given the limited amount of instability available in the moisture plume. Day 3... The short wave energy tracks from the Upper MS Valley across the lower OH valley into the Mid Atlantic states during Day 3. Its weakening surface low tracks along a similar path, and the track of the surface low is expected to keep instability south of the track across portions of KY into eastern TN. Model soundings over this area showed mainly elevated instability (with MUCAPE values below 250 J/KG), with little of this instability surviving the trip over the frontal boundary. South of the surface low and its associated frontal boundary, a 40 to 45 knot low level southwest flow transports 1.00 inch precipitable water air into the lower OH Valley, mainly after 25/00z. There was a multi model signal for an axis of 0.75 to 1.50 inches of qpf extending from central IL into southeast VA and northeast NC, with the highest amounts over northern and eastern KY (where the best instability is expected). At this point, with little in the way of instability, no excessive area was issued for northern and eastern KY. Snow and ice are expected along the main qpf axis extending from the Northern Plains into the Mid Atlantic states during Days 2 and 3. Please refer to the most recent suite of WPC Winter Weather products for more information. Hurley/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml