Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 549 PM EDT Mon Mar 26 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 27/0000 UTC thru Mar 30/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...21Z Update... Made some short term modifications primiarily in the first 6 to 12 hour period of the day 1 forecast based on trends seen in satellite imagery and radar imagery across the Southern Plains. With convection developing along and ahead of a cold front and dry line, thought that there was at least some potential for some enhanced amounts early in the 00Z to 06Z period across northern TX into the southeast corner of KS before the front moves through. This was an extension of the day-shift reasoning across the area. At this point, the dry air seen in the forecast soundings is enough to generate cold pools strong enough to keep cells propagating forward despite increasing amounts of low level moisture. Noted that the ARW and NMMB cores still generate some higher rainfall rates/accumulations later this evening which was already accounted for in the forecast. Also modified the areal coverage of some of the lighter QPF in the upper midwest based on the short-term evolution and the higher resolution guidance. ...Central and Eastern U.S... A multi-day heavy rain event will initiate this evening in the southern Plains and lower Missouri Valley. A low was closing off in the lower portion of a deep, large scale trough. The pattern will remain only slowly progressive, with periods of repetitive / training convective systems anticipated from OK/TX eastward and northeastward. The conveyor belt for this weather system, marked by PW anomalies of 2 to 3 standard deviations above climatology, will extend up into the Ohio Valley, resulting in a very lengthy and broad swath of heavy rain, but with the most intense storm totals and hourly rates occurring upstream in the more unstable airmass over the Plains and lower MO/MS valleys. For the Day 1 period, the WRF ARW continues to emphasize warm sector convection to such a degree that it greatly limits the expanse of heavier rain northward. There is, meanwhile, plenty of evidence from a multi-model approach, to expect that synoptic forcing will yield a secondary swath of higher amounts overnight from central and northeast Oklahoma into southern Missouri. WPC attempted to reflect this by incorporating some of the 12z GFS, WRF-NMM, and HREF Mean into the forecast. The result ends up being a bit washed out in the 24-hour totals, but has the effect of broadening the 2-4 inch areal average amounts while eliminating some higher amounts (around 5 inches in the previous forecast). The convection will likely try to fight through the diurnal minimum in the morning over OK/AR, and then come back with enhanced intensity in the afternoon. Wherever small scale training occurs, there certainly may be some 5 inch totals. Excessive rainfall risk is maintained in the Slight Category as this is the initial onset of heavier rain, and there is some uncertainty as to exact placement, but the Slight category will need to be expanded northward based on these changes to QPF. ...Pacific Northwest... QPF trended downward somewhat, but remains heavy in the favored mountain terrain, with strong onshore flow on the southern periphery of an incoming 125 knot jet streak. A rather impressive short wave trough moving rapidly across the nern Pacific towards British Columbia will yield zonal moist onshore flow into Washington and extending south into Oregon. This upper disturbance and lower heights will not dig much at all, due to the mammoth upper ridge over the eastern Pacific and extending into the northwest quad of the country. However, an anomalous long fetch of Pacific moisture flux over-top the upper ridge will persist through Monday night across the Pacific Northwest. Overall WPC followed a blend of the GFS/ECMWF along with the 00z high-res package, which yielded areal avg totals of 1 to 2+ inches with locally higher values in the coastal ranges, Olympics and Cascades. QPF was based on a 50/30/20 blend of the 12z GFS / WRF-NMM / WPC Previous. Days 2 & 3 Southern Plains---Lower AR...Lower MS Valley into the TN Valley There has been no significant changes in the upcoming day 2 and day 3 time period with the strong multi-model signal for widespread heavy to excessive rainfall amounts from eastern portions of the Southern Plains into the Lower AR---Lower MS and TN Valley regions. Latest guidance is showing similar evolution to the well defined southern stream closed low moving eastward from the Southwest into the southern high plains day 2 and then eastward day 3 across the remainder of the Southern Plains into the Lower MS Valley. With pw values expected to remain much above average---2+ standard deviations above the mean ahead of this trof...and very favorable large scale forcing with favorable right entrance region jet dynamics--strong moist southwest low level flow ahead of the slow southeast moving frontal boundary---strong frontal convergence---confidence remains high for the widespread heavy to locally excessive rainfall amounts. There will likely be a period of training of precip areas in a southwest to northeasterly direction parallel to the deep layered mean flow day 2 and 3 in the vicinity of this front---supporting the excessive rainfall threat. While precip has been low over the past few weeks across these areas and stream flows below average---the training potential continues to be a significant threat for runoff issues. For the day 2 excessive rainfall potential outlook period 12z Mar 27 to 12Z Mar 28th---the overall axis of the slight risk area was not changed. However--- a moderate risk area was added from northeast TX into far southeast OK and southwest AR to cover the regions of max training potential in the 0000 to 1200 UTC Mar 27th period. This moderate risk area then matches up with the previous day 3 moderate risk area depicted from southeast AR--northwest MS into far northeast LA. The day 3 moderate area was extended 50-80 nm farther to the south and southwest into northwest LA/northeast TX border area. The slight risk was also extended farther to the south into the mid to upper TX coastal region. For both the day 2 and day 3 time periods---areal average precip totals of 2-3"+ depicted across these areas---with potential for daily totals in excess of 5" where training maximizes. OH Valley into the eastern Great Lakes and Northeast U.S. Similar to the past several runs---a more progressive northern stream flow will push eastward days 2 and 3 from the OH Valley/eastern Great Lakes and into the northeast. The more progressive nature of these height falls and the associated precip will result in a shorter duration of rainfall with light to moderate amounts expected across these areas. Northern Plains into the Upper MS Valley Northeast Pacific height falls that support heavy precip over the Pacific Northwest and Northern Rockies day 1 will be diving east southeastward day 2 into the Northern Plains. This will push a fast moving area of low pressure east along and just north of the U.S./Canadian border region from MT to MN. Most of the associated precip will remain north of the surface low track across southern MB and western Ontario---with only some light totals extending southward into the Northern Plains/Upper MS Valley region. Northern Rockies into the northern to central high plains Additional upstream height falls dropping southeast through the northern Rockies will support light to locally moderate precip totals trough the northern Rockies and northern high plains day 2. These height falls will spread farther southeast into the Central High Plains day 3. The 0000 UTC ECMWF was the most emphatic with a broad region of .25"-50"+ precip totals from southeast WY/northeast CO into southwest NE and west central KS during day 3. The 1200 UTC ECMWF---however---has backed off appreciably across these regions--more in the .10-.25" range---and now more similar to the remaining guidance. The day 3 final qpf trended down across these regions from the prelim day 3 to account for this better model agreement. Pacific Northwest In the wake of the inland moving height falls day 1 over the Pacific Northwest---onshore flow is expected to weaken, This will reduce precip totals day 2 to moderate levels through the Washington Cascades Burke/Bann/Oravec Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml