Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 457 AM EDT Tue Mar 20 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 20/1200 UTC thru Mar 23/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...East Coast... All the ingredients were coming into place for the latest in a series of Nor'easters to produce a substantial swath of snowfall - enough to disrupt life/commerce in the big cities. Refer to the WPC Heavy Snowfall Discussion for winter weather details. The storm will also produce heavy rainfall over the coastal mid Atlantic. The event will be stretched over two days, through Wednesday. The latest WPC Day 1 QPF trended significantly upwards, now a full half inch heavier liquid equivalent heavier over the areas where MD meets PA, and extending to central New Jersey. More generally we are now a quarter inch heavier over the majority of the Mid-Atlantic states down through southeast Virginia, and nearing Long Island for QPF valid through 12z Wednesday. The 00z upper air analysis depicted a very energetic and fairly uncommon pattern over the southern states, with very short wave spacing between a lead wave which drove a severe weather outbreak over the South on Monday and an upstream wave that will amplify quickly over the same region today. Rather than move out and reinforce the offshore thermal gradients, the models now agree that the movement of the lead short will be dictated by the amplification of the trailing wave such that they are more in phase with one another and become part of a relatively consolidated upper air amplification over the Appalachians. This allows for cyclogenesis very near to the Mid-Atlantic coast, peaking Tuesday night and Wednesday. Additionally, with the lead wave now forecast to lift through the eastern Ohio Valley, it not only induces a broad scale warm-advection / frontogenetically forced precipitation event on Tuesday, but also acts to solidify an elevated baroclinic zone over northern MD / southern PA and across New Jersey. Strong lift during the initial setup phase, followed by continual easterly low level flow off the ocean, into the terrain, and then finally enhanced broad scale by the developing cyclone and broad deep layer height falls going into Wednesday, makes for a likely swath of hefty liquid equivalent precipitation in this zone. Other heavy amounts, although mainly in the form of rain, are expected over the Virginia tidewater region and DELMARVA peninsula, especially early Tuesday in association with the lead wave. The system makes an attempt at forming a TROWAL back through the Ohio Valley, and with nearly saturated air circulating around the gradually deepening upper low, expect at least quarter to half inch liquid amounts to occur back west. The NAM sometimes does a good job on Day 1, especially along the East Coast. Its QPF, at least through 12z Wed, was consistent with global model mass fields / synoptic thinking. Given a decent model consensus as well, we were able to use our in-house ensemble products which had trended upwards with amounts. Generally the GFS appears to have been too conservative leading into the event, and the ECMWF has a known dry bias even in strongly forced events, but its QPF did trend wetter. WPC used a 40/30/30 blend of our ensemble / 00z NAM / 00z HREF blended mean. ...California... A long advertised trough promises to bring widespread rain and some heavy rain into California. It still appears as though the bulk of the intense rain rates will occur on Day 2, beginning Wednesday morning - please see the Day 2/3 portion of the discussion, including an escalating risk of excessive rainfall / flash flooding. The ECMWF evolution of the system had been slower, but the 00z run started to catch up to the global consensus. With good large scale agreement, a consensus approach to the QPF, including bias-corrected tools, usually does well along the West Coast. For simplicity, we used the same QPF blend here as we employed elsewhere on Day 1, a 40/30/30 percent blend of our ensemble / 00z NAM / 00z HREF blended mean. This may have produced a result just slightly too heavy, given the WRF-ARW influence on the HREF mean. The WRF-ARW is doing something we keep seeing east of the Rockies, with a tendency to produce very heavy swaths of convective precipitation along the southern edge of synoptic forcing. This intensity, which would affect southern California on Day 1 if the WRF-ARW were correct, seems unlikely given the 850 mb flow really does not increase until Day 2. There is also a pronounced lead shortwave passing by Southern California Tuesday evening, with a brief period of subsidence in its wake. Therefore, we will maintain only Slight Risk probabilities for excessive rainfall on Day 1, leading into a more dangerous heavy rain event on Day 2. Day 2 California There is a strong model signal day 2 for widespread heavy precipitation across large portions of California from the transverse range/southern California coast range--through the Sierra and into the northwest California coast range. The atmospheric river event commencing during the day 1 period will continue into the day 2 period as a mid to upper level trof off the California coast continues to press eastward day 2. This will sustain anomalously strong and moist onshore flow into much of California with the emphasis for the strongest moisture flux focused into the southern California coast range/Transverse range and into large portions of the Sierra. Here pw anomalies of 4-5+ standard deviations above the mean and moisture flux anomalies of 5+ standard deviations above the mean are expected during day 2. Model consensus is for very heavy precipitation potential across these areas with a high likelihood of runoff issues over the southern California coast range/Transverse range and heavy snows into the central to southern Sierra. With respect to the runoff issues over the southern California coast range/Transverse range---a high probability was added from near Morro Pt---southeastward through the Transverse range to cover the Canyon--Sherpa---Whittier---Rey and Thomas burn areas. The expected magnitude of the rainfall across these burn areas---upwards to .25-50" per hour during the second half of day 2---will likely produce life-threatening debris flow/mudslide potential. Pacific Northwest Anomalous pw values will also be drawn northeastward into the Pacific Northwest day 2 ahead of the strong closed low amplifying and dropping southward to a position off the B.C. coast. While pw values will rise to 1 to 1.5 standard deviations above the mean into the Pacific Northwest---the low level flow will have a more terrain parallel trajectory---supporting much lower precip totals than areas farther to the south. .25-.50"+ areal average totals depicted from the Washington-Oregon Cascades to the Washington-Oregon coast ranges. Heavier totals in the 1-2"+ range likely from far southwest Oregon into coastal northwest California where the low level south southwesterly flow will have a greater upslope component. Inland into the Great Basin---widespread moderate precip totals likely day 2 as the upstream anomalous pw axis pushes inland. There is a lot of detail differences among the latest guidance leading to lower confidence than other areas in the anomalous pw plume. The day 2 qpf did not deviate from the model consensus of widespread moderate amounts. Mid Atlantic into New England The second and likely stronger of the two areas of low pressure associated with the early spring winter storm affecting the northeast will continue to deepen off the Mid Atlantic coast and push toward southeast New England day 2. There continues to be model spread with the details of this cyclogenesis event. The NAM is on the western edge of the guidance and farther to the northwest with heavy precip from northeast PA into southern NY state and into central New England. The NAM conest was farther to the southeast with its heavy axis---closer to the axes of the 1200 utc EC---in house bias corrected mean and sref mean. Heavy snows possible in what will likely be a well defined comma head/deformation precip band to the northwest of the deepening low from the Mid Atlantic into central to southern New England. Mesoscale snow bands likely which will support potential for enhanced snow totals depending upon where the bands set up. See the latest QPFHSD for additional winter weather information. Southern to central Appalachians Accumulating snows also likely from the central to southern Appalachians day 2 where persistent northwesterly to north northwesterly upslope flow is expected in the wake of the deepening coastal low. Day 3 California The axis of much above average pw and moisture flux values impacting southern California during day 2 will continue into the first half of day 3 before this axis sinks southeastward into the northern Baja. Model consensus is for additional heavy totals through the Transverse range--with moderate to heavy precip values extending southward through the Peninsular range day 3. The additional day 3 rains through the Transverse range will support the continued significant debris flow/mudslide issues across the numerous burn areas. A moderate risk is depicted into the Thomas burn area of the Transverse range---with a slight extended eastward through the remainder of the Transverse range. Precip totals not expected to be as heavy as day 2 given the lessening time of very anomalous pw values and moisture flux values into this area. Additional heavy precipitation and heavy snows likely through the central to southern Sierra day 3 as persistent west southwesterly upslope flow continues. Pacific Northwest The low level flow that had a significant terrain parallel component day 2 over the Pacific Northwest will veer to a more upslope direction day 3 as height falls from the strong closed low off the Pacific Northwest/southwest Canada coast swing inland. This will increase precipitation potential across the Pacific Northwest from the Cascades to the coast ranges. Northern Rockies into the Great Basin The inland push of height falls through the northern Rockies and Great Basin will raise pw values to much above average levels day 3. This will support additional widespread moderate to heavy precip totals across the Great Basin into the northern Rockies. Model consensus is for greater totals day 3 compared to day 2 with the heaviest amounts from the northern Wasatch into the far eastern ID/northwest WY and across the Rockies of central to northern ID. Northern plains The inland moving height falls across the northern Rockies will spread isentropic lift into the northern Plains early Friday morning. Strengthening southeasterly low level flow will support increasing precipitation potential from far northeast MT into western ND and western SD. At the moment---model consensus is for light to moderate areal average amounts in the .10-.25" range. Burke/Oravec Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml