Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Fri Apr 20 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 21/0000 UTC through Apr 24/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Lee of the Central Rockies---Central to Southern Plains... A welcome widespread moderate to heavy precipitation event is likely from the lee of the central Rockies southeast through large portions of the Central to Southern Plains. An incoming closed mid- to upper-level low will help draw increasingly anomalous moisture northward due to strengthening low level southerly to southeasterly flow into the area. Strong upper difluence ahead of this closed low in this increasingly anomalous precipitable water (PW) axis will support the likelihood of widespread moderate to heavy totals. There is some latitude differences with the max precip amounts. WPC QPF did favor the farther south axis of the CMC GEM and hi-res ARW across southern OK and north TX---closer to the west to east oriented frontal boundary across north TX, with the new rendition trending drier per recent radar trends and 12z ECMWF/12z GFS/18z in-house experimental bias corrected QPF runs. With recent dry conditions and the overall expected progressive nature of the organized heaviest precip---do not believe there will be any runoff issues. Areal average 1-2" totals expected over much of OK into north TX---with lighter totals in the .25-.50"+ range extending north and northwest of this into the Central Plains and lee of the Central Rockies where instability should be lower. ...Pacific Northwest into the Northern Rockies... A fast-moving shortwave will be moving inland along the Washington---B.C.---Idaho border region tonight/early Saturday and into southern Alberta later Saturday. An area of organized precip along and ahead of the associated frontal boundary will support moderate to heavy precip totals over the northern Washington Cascades---Olympic Range and into far southwest B.C. and Vancouver Island. Lighter precip totals expected south into the Southern Washington-Oregon Cascades---west to the coast ranges and into the Northern Rockies from far northern ID into northwest MT. ...Northern NY state into northern New England... The broad cold low will continue to progress across Newfoundland tonight and over the far northwest Atlantic during Saturday. The persistent cyclonic flow in its wake will support scattered snow showers from northern NY state into northern New England. Any precip/snow amounts are expected to be very light. Days 2/3... ...Central-Southern Plains to the Southeastern U.S.... While there are plenty of differences in the details, there remains a good model signal for widespread moderate to heavy and potentially excessive rains to develop and move east across the region as a compact mid-upper level low tracks east-southeast from the central Great Plains on Saturday to the lower Mississippi and Tennessee valleys by Monday morning. Ensemble trends of the 500 mb closed low have been somewhat slower over the past several runs as the system reaches the Mississippi valley, with a 12Z GFS/00Z ECMWF blend representing the greatest ensemble clustering and near the ensemble means. The 12Z NAM was considered too far north with its mass fields and was not used for the QPF generation. Convection is expected to be ongoing at the start of the Day 2 period (00Z/22) over the ArkLaTex, and perhaps heaviest just north of a returning warm front over the west-central Gulf Coast at the start of the period. The position of the upper low and jet placement support strong lift within the left exit region of a 110 kt jet core and strong upper level difluence over the lower Mississippi valley. However, 700-500 mb lapse rates remain rather weak...less than 7 C/km...which may only support elevated instability up to 1000 J/kg per a consensus of available model guidance. The closed nature of the low and PWATs in excess of 1.5 inches with 20-40 kt of 850 mb inflow should help to support heavy rainfall rates. However, the forecast shows the upper low to steadily progress toward the east which should mitigate rainfall totals to some degree. Another concern with some of the latest model guidance was the potential for precipitation to be displaced south of the global model QPF, given hi-res support farther south through 12Z/22. The 12Z ECMWF also supported higher QPF to the south of many of the deterministic guidance, which is often true and better co-located with forecast available instability. While continuity was adjusted toward a GFS/ECMWF blend, WPC QPF adjustments were made to move maxima south and to reduce apparent feedback bulls eyes noted in the GFS and ECMWF across the Southeast. Across the Southeast, a coastal convergence boundary is expected to setup on Monday with any significant instability along the coast or offshore. Overrunning of low level moist flow should support locally heavy rainfall in the vicinity of the GA/SC coast for Day 3 (00Z 23-24), with a secondary maxima just ahead of the mid-level low (greater mid-level lapse rates) and with a strong upslope component into the southern Appalachians. Future outlooks will likely be needed beyond the Day 3 Excessive Rainfall Outlook (12Z/24+), but marginal risks were drawn for the Day 2 and Day 3 Excessive Rainfall Outlook to cover the model spread from eastern TX/OK into the TN valley and Florida Peninsula, with a Slight Risk on Day 2 straddling the TX/LA border and from northern/eastern Alabama into western Georgia for Day 3. ...Florida... As the upper trough described in the above section advances east, a front is expected to initially lie east to west across the Peninsula, drifting north as a warm front Sunday night into Monday. A somewhat narrow axis of PW values in excess of 1.5 inches near and south of the front is expected to increase standardized PW anomalies to near +2 on Sunday. Southeasterly 925-850 mb flow of 20-30 kt early Sunday morning may focus localized cores of heavy rain with areas that are slow moving or regenerating over the same locations in the vicinity of low level convergence along the coast. Flow veers to a more southerly direction late Sunday, which is likely to shift any heavy rainfall focus offshore. The 12Z HRW-ARW, NSSL and NAM_NEST support the idea of 3-6 inch totals along the central and northern east coast of the Florida Peninsula for the 24 hour period ending 12Z/23. No additions were made on the Day 2 Excessive Rainfall Outlook at this time given low confidence in the details, but later high resolution model guidance may allow for a narrow marginal risk in future outlooks. ...Northern Rockies into the northern High Plains... After the departure of a strong shortwave into south-central Canada Saturday night, quasi-zonal flow in its wake will bring the passage of another shortwave, albeit weaker, across the West Coast during the day on Sunday. A 700 mb trough axis will likely amplify across the interior northwestern U.S. on Sunday, developing into a closed low near the ID/MT/WY border Monday morning. Model agreement on placement was generally agreeable, but the 12Z NAM was a deterministic outlier with taking energy from the base of the trough into the Southeast for Monday leaving less strength to the trough over the interior Northwest. WPC QPF was led largely by a 12Z GFS/00Z ECMWF blend. Forecast moisture anomalies are fairly minimal on Sunday across the Northwest, but as the low becomes better defined with eastward progression into Monday, 700 mb wind speeds will increase into the 30-40 kt range ahead of the closed low. Forecast guidance does show the low opening back up into a positively tilted trough axis during the day on Monday but weak moisture return from the Plains will join with the system, reaching the northern Plains and allowing for PWs to peak near 0.75 inches. Increasing overrunning north of a quasi-stationary front extending from northern Wyoming into the Dakotas on Monday along with low level upslope flow into the High Plains with some enhancement to lift in the right entrance region of a 70-90 kt 250 mb jet streak should all combine to aid in producing heavier liquid equivalents from the northern Rockies into the eastern MT/WY border. Roth/Oravec/Otto Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml