Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 535 AM EDT Wed Mar 21 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 21/1200 UTC thru Mar 24/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern U.S... On the synoptic scale it is becoming less common in the modern age to reach Day 1 and have trouble finding a QPF consensus. Yet this early spring Nor'easter remains an enigma. The forecast leading into this storm has shown remarkable sensitivity to small scale interaction among shortwaves embedded in the unusually energetic yet low of amplitude - flow over the southern U.S. The overall trend had been toward more of a major snowstorm with 1 to 2 inch liquid equivalent amounts from Baltimore northeastward. The trends from one forecast cycle to the next, however, have been so inconsistent whether considering one model or all of them, that we are largely left with a manual forecast effort, relying on conceptual models more than any specific piece of guidance (especially where QPF is concerned). The model mass fields seem to have finally settled down, with a nicely depicted track of a low deepening off the Mid-Atlantic coast, down to about 990 mb as it then slowly lifts northeast. The 850-700 mb front / inverted trough structure north of the low track would appear to have set up as was anticipated from the MD/PA border region up through NYC / Long Island and beyond. This should become a focus for heavier, banded precipitation, creating a swath of maximum QPF in the vicinity of this region. Farther south the easterly onshore flow and deep layer ascent will be less long lived, but still substantial in the early hours of the day. Watching animations of the model fields, the focus and pivot in the 850-700 mb layer should occur near NYC / Long Island, acting to maximize snow and liquid equivalent amounts there. Most guidance actually does agree to this concept, all but the 00z ECMWF whose QPF axis was inexplicably offshore despite little change to the mass field solution. An additional concern, however, is that the evening soundings indicated very dry air in the mid levels, and despite height falls overnight, snowfall has been a bit lazy to fill in over the Mid-Atlantic. The strongest height falls also do not extend up into the baroclinic zone over southern New England until evening when the pivot and eastward progression should be kicking in, thus limiting the residence time of the *best* deep layer forcing. All this being said, the WPC QPF may end up being slightly heavy handed, but we are confident that it in the right place, hopefully an accurate depiction of the pattern. Working initially with the trends in NCEP guidance, and supported a fair amount by the UKMET, WPC favored a QPF blend of the HREF mean and a version of our in-house ensemble which gives significant weight to the hi-res model suite. This yielded a similar precip pattern to the previous NWS forecast, but with heavier amounts, up to a half inch heavier liquid amount in the vicinity of Long Island. The forecast does still support heavy snowfall along the east coast - see WPC Heavy Snow Discussion. ...Western U.S... On Day 1 into Day 2, the closed low well offshore of California phases with and is kicked by the northern stream flow, creating a full latitude trough with a pronounced southern stream component at its base. The pattern as a whole makes some progress into the Western U.S. on Wednesday, bringing widespread rain/snow as far inland as the northern Rockies down to the lower Great Basin / lower Colorado River basin. By far, however, this event will be known as a California storm, with heavy Sierra snows and heavy rainfall along the coast. The inherited WPC forecast was ratcheted up about as high was was necessary, especially as some of the heavy coastal rain into southern California will spread over into the Day 2 period. Therefore, we started with the previous forecast, and looked to just make minor adjustments to keep up with trends. For one, we trimmed back on the coverage of quarter inch amounts in the rain shadow areas of eastern WA/OR, based on hi-res model consensus, NDFD, and the GEFS Re-forecast data based on analog events. Secondly, there has been a continued trend for the heaviest Day 1 rainfall (through 12z Thursday) to back up northward through San Luis Obispo County and Monterey County. There will certainly be heavy rainfall reaching down into Ventura County and possibly the L.A. basin, but overall we nudged the Day 1 centroid farther north up the coast. This will mean that the risk of flash flooding and debris flows associated with some of the largest fire burn scars will straddle the Day 1 into Day 2 periods, with the full brunt of the moist plume and deep layer forcing not arriving until early Thursday. QPF was derived from a high percentage previous WPC, combined with influence from the 00z NAM CONUS Nest and our in-house ensemble. The WRF-ARW did not follow the trend, and was more aggressive with heavy rain extending into Los Angeles suburbs. This is its tendency nationwide (south and east of consensus), but it is sometimes right. The Excessive Rainfall Outlook will remain generous with its coverage given this possibility. Day 2 California The axis of much above average pw values---4 to 5+ standard deviations above the mean---in the ongoing atmospheric river event will persist into the southern California coast range---Transverse range and Peninsular range during the first half of the upcoming day 2 time period. This anomalous pw axis will be shifting eastward later during day 2 out of southern California and into the northern Baja and northeastward into the Great Basin---central to northern Rockies and eventually into the northern to central high plains. For California---there is good agreement for additional heavy precipitation through the southern California coast range/Transverse range and through the Sierra. Hourly rainfall rates of .25-.50" possible during Thursday morning---beginning to lessen in intensity by early afternoon as the axis of the anomalous moisture flux and PW axis pushes eastward. Runoff issues will remain a great concern into Thursday morning over the recent burn areas across the Transverse range including the Canyon--Sherpa---Whittier---Rey---Thomas---Sand---LaTuna---Bluecut and Pilot Burn area. The high risk area during the day 1 period was maintained over the western portions of the Transverse range---with the moderate region expanded eastward from the previous day 3 outlook to cover the above mentioned burn scars. Heavy snows likely through the Sierra. See the latest QPFHSD for additional winter weather information. Great Basin---Northern Rockies The inland moving height falls through the Great Basin---central to northern Rockies will spread the above mentioned axis of much above average pw values northeastward day 2. This will support widespread moderate to locally heavy precipitation through the Great Basin and into the Northern Rockies. Heaviest totals expected from the Wasatch/Uintas into the northern Rockies from far eastern ID/western WY into central to northern ID. Pacific Northwest The low level flow that is forecast to have a mostly terrain parallel trajectory day 1 across the Pacific northwest is expected to veer to a more onshore direction as height falls rotate inland on the south side of the south and southeast side of the strong closed low forecast off the B.C. coast day 2. Widespread moderate to locally heavy precip likely from the Washington-Oregon Cascades west to the northwest California coast range---Oregon coast range into the Olympic range. Northern plains Strong isentropic lift will be spreading eastward into the Northern Plains by the end of the day 2 time period associated with the above mentioned height falls pushing through the Great Basin. PW values will be rising to much above average levels at this time---supporting increasing precipitation coverage from northeast MT into western portions of ND and SD. Model consensus is for moderate totals across these areas during the day 2 period---with the WPC qpf not deviating from this. Eastern New England The comma head/deformation precip band with the developing storm off the Mid Atlantic Wednesday will continue to affect eastern New England at the beginning of the day 2 period before lifting northeastward into the Canadian Maritimes. At the moment---any significant precip for the day 2 time period likely to be along the immediate coastal sections from Cape Cod into coastal Maine. Day 3 Northern California into the Pacific Northwest The strong closed low off the B.C. coast will sink slowly southeastward to off the Pacific Northwest coast day 3. This will maintain deep layered cyclonic flow into the Pacific Northwest and northern California. PW values not expected to be anomalous in this deep layered cyclonic flow---with values below seasonal norms. Heaviest precip totals expected over the northern Sierra---northwest California coast range into the southwest Oregon coast range where PW values will be greatest and the upslope component the strongest. Aerial average 1-2"+ totals expected across this area--with aerial average amounts in the .25-50"+ range north of this into the Pacific Northwest. Southwest---Great Basin---Central Rockies Precipitation will be waning during the first half of the day 3 time period from the Southwest---Great Basin into the central Rockies as height falls will be pressing northeastward from these areas and into the Central to Northern Plains. For the day 3 period---additional light to moderate totals possible Northern-Central Plains---Lower MO Valley---Mid to Upper MS Valley The above mentioned height falls exiting the Great Basin/central Rockies region and pushing into the central to northern Plains will support an expanding area of precipitation across portions of the Northern to Central Plains into the lower MO---mid to upper MS valley region. Favored the GFS/EC solution of a slightly farther south qpf axis---especially across portion of the lower MO Valley into the mid to upper MS Valley region. Both the GFS and ECMWF are showing convection developing in an axis of instability Friday night/early Saturday over eastern portions of the Central Plains---Lower MO Valley-Mid MS Valley region. The NAM does indicate this area of instability but seems underdone with qpf potential here. Accumulating snows possible on the northern portion of this broad precipitation region from large portions of ND---into western to southern MN--northeast IA and northwest IL. See the latest QPFHSD for additional winter weather information. Burke/Oravec Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml