Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 344 PM EDT Sun Apr 15 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 16/0000 UTC thru Apr 19/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southeast into the Mid Atlantic... A very dynamic, highly amplified spring system continues to migrate eastward, with a well-defined, elongated QLCS progressing through the southeastern U.S. this afternoon. Deep-layer instability, largely surface-based, continues to be on the uptick ahead of this line over the southeast and lower mid Atlantic region, while the increasing upper level divergence ahead of the trough (especially as the trough becomes more negatively tilted) should maintain the widespread coverage of moderate-heavy rainfall through the new day 1 period. In general the convective line will remain progressive in nature, likely limiting the potential for any extreme rainfall amounts. However, deep layer mean winds parallel to the line supports at least some south to north training of cells embedded within the line. Also, current convection is actually forming ahead of the true cold front, thus the potential exists for additional storms to fire on the true cold front as well, which would result in multiple rounds of locally heavy rainfall in any one location. Overall, this looks like a widespread 1-3" rainfall from the Southeast into the Mid Atlantic region...with the aforementioned training and repeat cell potential allowing for localized 3-5" amounts. Impressive 850 mb moisture transport and PWAT vales exceeding the climatological 90th percentile certainly support these totals and the potential for high rates. Thus at least some flash flood risk exists where any cell training or repeat convection is able to occur. The orientation of the line will likely be more favorable for short term training and repeat cells from northwest NC into portions of WV/VA/MD closer to the negatively tilting trough and better mid/upper level forcing. Strong southerly flow ahead of the low should help enhance rainfall totals some into the southern and eastern slopes of the Blue Ridge and Appalachians as well. Thus will carry carry a Slight risk from northwest NC into eastern WV, western and northern VA into MD. It is across these areas where the best chance for localized 3-5" totals exists to go along with a bit lower FFG. For this issuance opted to include the DC/Baltimore urban corridor as well. Dry conditions of late should help, but this corridor is still more susceptible to flash flooding, and note several HREF members and the 12z extended experimental HRRR showing localized 3-5" totals in the vicinity...and most of this would fall over a short period. Thus while not a sure bet, think the conditional threat for flash flooding is high enough to warrant the slight risk inclusion at this time. Instability will be greater across SC/NC...so certainly some chance the convective line really gets organized there this afternoon...which could block moisture transport and limit amounts a bit further northwest over southwest VA and far northwest NC. Tough to nail down these details though with much lead time, so will need to monitor trends. Given this potential, also opted to include the Charlotte metro in the slight risk, given their susceptibility, and to account for the possibility of a further south focus. WPC QPF leaned exclusively on the high res guidance for this warm sector convection. Most of these solutions continue to seem a bit slow compared to the current radar, thus leaned more towards the quicker high res solutions. Also incorporated some of the 0z HRRR experimental, which was more robust with amounts over SC/NC than the HREF component members were. As mentioned above, given the better instability here, there is certainly a possibility that the higher amounts do verify here. ...Upper MS Valley---Great Lakes into northern NY and northern New England... A surface low will continue to slowly track east across the OH Valley today. To the north and northwest of this low will see continued wintry precipitation across MN/WI/MI. Continued to lean toward the slower model solutions in terms of lifting the TROWAL east of the area -- which is close to a blend of the ECMWF and UKMET along with the WPC in-house bias corrected ensemble QPF. Significant, widespread winter weather hazards continue with this dynamic spring system -- please refer to the latest QPFHSD for further details. ...West... A closed mid level low will move into the West today...with a focused plume of moisture out ahead of it producing some 1-2" QPF amounts from far northwest CA into southwest OR, the OR Cascades, and the CA Sierras. WPC QPF ends up close to the HREF mean. Days 2/3 ...West Coast/Intermountain West... At the start of the Day 2 period (0-6z Tues), the long wave trof has come ashore generally neutrally tilted centered along 120W. The northern portion of the wave/vorticity has progressed into southern Canadian Rockies with a trailing trof to the base of the large scale trof...crossing the Absaroka Range to just west of the Tetons. The remaining limited mid-level moisture will slowly progress eastward though orientation to the terrain supports decent orographic enhancement particularly through the Wasatch/Unita/Wind River and Big Horns where broad .25 to .5" are expected over higher terrain with a few spots in the .6" range in most ideally orientated positions (such as S Big Horn Range and southern Wasatch). Mainly focused on a GFS/UKMET/ECMWF blend enhanced by the 12z NMMB mainly across UT to WY for 00-12z on Tues...though eventually shifting a bit toward slower solutions of the ECMWF/UKMET toward Wed as the base of the wave exits into the Plains. Strong onshore flow in the wake of the northern shortwave will support westerly flow of 30kts to 45ks at 85-7H at or near orthogonal to the Cascade range and though moisture is fairly limited WPC forecast QPF are still in the range of 1.0" through 00z Wed in Washington and .5" in Oregon. Heavy snowfall is possible across the higher terrain of the West Coast and Intermountain West during Days 2 and 3. Please refer to the latest suite of WPC Winter Weather products for more information. End of Day 3... A QPF respite is expected across much of the Northern Intermountain West as the base of the large scale trof ejects into the central Plains late Tuesday into early Wed. However, another compact wave is already approaching the West Coast aiming toward the OR/CA coast with broad troughing extending north into Western Washington toward midday Wednesday. Light orographic ascent will affect the Olympic range with up to .10" by the end of the forecast period; however the focus will be with southwesterly flow/warm conveyor ahead of the wave by 18z Wed leading to increased mid-level shield precipitation across SW Oregon before expanding north and eastward. The main forcing/QPF is expected to reach NW California toward 00z Thurs. The models have come into better agreement over the last 3-4 cycles though a recent slight southward/strong shift was noted in the 12z GFS. This reduced some of the higher confidence in a higher confidence non-UKMET blend, but still the agreement is tight enough to continue to support this blend. ...Northern and Central Plains/Upper MS Valley... The base of the long wave trof progressing through the West Monday into Tuesday, emerges early Wednesday into the Central Great Plains with a negative tilt orientation. This spurs fairly rapid surface cyclogenesis and rapid precipitation development across Nebraska to N Iowa midday Wed, eventually occluding late in the day and past 00z Thursday. The 12z GFS did trend slower but run to run spaghetti plots show it remains the fastest solution and was discounted from preference. The 12z NAM, like its predecessors was also quite cold in the lower thermal profiles and supports the fastest deepening, typical of a known negative bias at Day 3. The 12z ECWMF shifted a bit faster and matches the UKMET within the slow to center of guidance packing, but also trended a bit deeper/stronger. Strong MUCAPE to 1000 J/KG will support some elevated convective nature to TROWAL of the system enhancing QPF from 09-18z before reducing in intensity with the occlusion of the low by 00z, leading to an area of 1.0 to 1.5" QPF from NE Nebraska to NE Iowa. At this time, it appears a portion of the QPF will be wintry in nature (snow/ice/mix) and likely reduce any flooding threat so no excessive rainfall area is suggested. ...Great Lakes/Northeast... The large closed low will continue to progress through the eastern Great Lakes/northern Mid-Atlantic with the surface wave/triple point refocusing in a tricordal wobble in the NH/ME border vicinity. This allows for a slight slowing of eastward progression of the deepest southerly moisture flux into the higher terrain of western ME, where multiple model signal remains for QPF of 2.5". While much of the instability will be exhausted (less than 50 J/KG), the IVT values remain will above 2-3 StDevs of normal and has the potential for inundation flooding (vs. flash flooding)...still the magnitude is high enough for the area to remain within the Day 2 (12z to 12z Mon-Tues) Marginal Excessive Rainfall Outlook. WPC forecast was mainly driven by a blend of the GFS/ECWMF/UKMET and some Hi-Res CAM guidance such as the ARW/ARW2/Nam-Conest. The southerly warm conveyor belt will shift out of the CONUS late Tuesday though the vast nature of the upper low will support internal vorticity centers returning across the Great Lakes into northern NY/New England Tues into Wed increasing forcing/cooling aloft to activate some NW to SE oriented Lake Effect Belts as well as some weak comma head return moisture in N NH/ME by later in the Day Wed. Still this QPF is not substantial but please refer to WPC Winter Weather products for more information. Hurley/Gallina Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml