Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 445 AM EDT Tue Mar 27 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 27/1200 UTC thru Mar 30/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central and Eastern U.S... A highly-meridional upper flow pattern is evident via the early morning satellite WV/IR loops, with a closed mid-level circulation over the Southwest-far nw Mexico, another closed upper low northeast of Bermuda, with a broad ridge in between across the central-eastern CONUS. 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 eastern portions of the Southern Plains into the lower MS Valley. 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 closer to the surface front from eastern OK into northern AR and southern MO. WPC attempted to reflect this by incorporating some of the 00z ECMWF, WRF-NMMB, 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 1.5-3.0 inch areal average amounts across northeast TX-eastern OK into much of AR and southern MO. This idea resulted in an increase in QPF farther north compared to continuity (previous forecast), supported by the multi-model consensus (including the WRF-ARW) of a bi-modal distribution of 2 max QPF axes (one n and another s). Wherever small scale training occurs, there certainly may be some 5 inch totals. The nocturnal uptick in elevated low-level southwesterly flow (while also aligning more parallel to the mean 850-300 mb flow) will maintain the highest risk of convective back-building and cell training Tue morning across northeast TX into eastern OK and northern AR, then again Tue night into Wed morning a bit farther south to include portions of central TX into the ARKLATEX, central-southern AR, and northwest LA. The day 1 excessive rainfall risk incorporated a MODERATE risk inherited from yesterday's day 2 ERO, with the area expanded a bit to encompass both rounds of more enhanced/organized and potentially repetitive convection (and likely reduction in 1/3 hourly FFG values with time), while maintaining the fairly broad SLIGHT and MARGINAL risk areas where the short-term rainfall rates are not expected to be as robust (FFGs a bit more challenging to overcome), while also capturing the spread among the guidance (especially CAMs). ...Pacific Northwest... QPF trends from the previous forecast were minimal, however the new day 1 totals remain 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. Days 2 & 3 Southern Plains/Lower MS Valley/Gulf Coast/TN & OH Valleys/Central Appalachians/Northeastern U.S... A well anticipated heavy rainfall threat will continue to migrate south and east from the Day 1 QPF/excessive rainfall risk area. This squarely places the best threat for hydrologic impacts across sections of the Arklatex into the lower MS Valley and mid-South region. Multiple boundaries will likely be in play, primarily convectively-driven which complicates the Day 2 forecast a bit. Model surface winds depict a main baroclinic zone which would be an impetus for organized convection while additional convergence zones will unfold in response to the thunderstorm activity. Through Wednesday, there should be a decent chance for training convection to extend southwest/northeast across the moderate risk area covering far northeast TX across northern LA/southern AR and into northwestern MS. General unidirectional flow will be likely ahead of the upstream kicker shortwave which may lead to instances of flash flooding within this high precipitable water environment. By the overnight hours, some sort of linear convective complex would likely unfold along or in advance of the cold front sweeping through TX. While this poses a severe weather threat, it would also decrease the threat for flash flooding given the forward propagation. Unfortunately, the operational models are not aligned with one another, particularly the GFS/ECMWF. The past several runs of the GFS favor a more southern axis of heavy rainfall extending from northern LA/southern AR across northern MS and into central TN. Meanwhile, the 00Z ECMWF as well as previous cycles show a broad swath of somewhat lighter amounts (relative to other models) across a more expansive area. Considering other global models, the 00Z CMC/UKMET trended markedly higher with QPF over southeast TX/west-central LA. With a range of solutions in hand, it proved to be a challenging forecast. Much of the precipitation was driven by a combination of the 00Z GFS/18Z GEFS mean which was a bit more consistent with continuity and the preference to stay on the southern end of the spread. By Day 3, the axis of heavy rainfall will finally exit the center of the country and bring unsettled conditions to the south-central U.S. up toward the Great Lakes. At this point, there will be two drivers of precipitation across the region. First, the shearing shortwave lifting out of the Ozarks should spread a broad axis of moderate to locally heavy rainfall to the lower OH Valley. This would pose a threat for hydrologic issues given rather low flash flood guidance values in place. The parent shortwave and attendant cyclone should continue lifting northeastward into New England while additional lowering of heights is likely back across the Central Plains. However, with the region post-frontal and the main moisture axis displaced toward the Gulf Coast, only scattered showers will be possible. Elsewhere, some active convection lingering from Day 2 will march eastward into the south-central U.S. up across the TN Valley. Given unknowns in the timing/state of the overnight convective complex, the Day 3 forecast will prove to be a challenge. It is safe to say a flash flood risk would extend into this period as well with a slight risk advertised from central MS up into central TN. Compared to Day 2, the QPF amounts are decidedly lighter given the eastward progression of the trough. A blend of the 00Z GFS/ECMWF/in-house pseudo-ensemble bias-corrected model were utilized here. Western WA/Upper Intermountain West/Northern to Central Rockies into the adjacent High Plains... Along the western flank of a longwave trough extending across much of central Canada down into the northern tier states, a multitude of shortwaves will spark light/locally moderate precipitation over the Upper Intermountain West into the northern/central Rockies. Areas of enhanced upslope flow will lead to locally higher amounts across sections of the Rockies. As these perturbations in the mid-latitude flow shift toward the Great Plains, a more pronounced system arrives out of British Columbia. The 00Z GFS/ECMWF diverge with the intensity and timing of this feature. The quicker 00Z GFS was recommended from the WPC model diagnostics discussion which favors heavier amounts across the WA Cascades eastward into the Upper Intermountain West. Hurley/Rubin-Oster Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml