Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 457 AM EDT Sat Apr 21 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 21/1200 UTC thru Apr 24/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southern Plains into the Southeast... A closed and not quite stacked cyclone will make its way slowly and steadily out of the Rockies, through the southern Plains into the lower Mississippi Valley. Accumulating snow will come to an end this morning as the system pulls away from the high elevations in Colorado. Attention turns entirely to the rainfall pattern. At least some of the D3 to D4 drought areas in the southern High Plains received some rainfall to finally kick off the spring season. The parent weather system, itself, carries seasonably strong wind fields, but is marked by 500 mb height anomalies only one standard deviation below climo, and instability is somewhat difficult to find given persistent dry ridging along the Gulf Coast. Thus, initially moisture is taking the scenic route up through West Texas. The moist plume will broaden today, but ongoing rain/storms north of an advancing warm front will curtail the northward extent of surface-based instability while the compact nature of the height falls will mean only a narrow zone of steep lapse rates to the east of the cyclone. Combine all this with a jet streak cutting straight across central Texas during peak heating, and the heavy rain potential appears confined to areas just north of the jet streak, and then more briefly farther south and east where convection will be more linearly forced in the low levels. Given the limiting factors, WPC QPF is more conservative than the spotty 3 and 4 inch amounts from some of the hi-res models, but there will be a good soaking rain of areal average 1.00 to 2.25 inches from the ARKLATEX into easertern Oklahoma, with some potential for excessive rainfall on the local level, especially in the more unstable areas across east Texas. Overall, the guidance trend was to contain the heavier rates closer to the mid level cyclone, given the instability-based factors discussed above. This resulted in a westward shift of WPC QPF for Day 1, meaning lesser amounts creeping into eastern AR/LA and adjacent TN/MS. Still, warm advection should yield increasing coverage of rain to parts of the Southeast by Saturday night. ...Florida... Easterly onshore flow deepens today, and the moisture depth / magnitude increases. Models certainly signal a fair amount of coverage of convective showers. Some of the hi-res models indicate spotty heavy rainfall. We were less confident in the intensity given the looks of the observed and forecast soundings. Upper flow becomes more supportive and difluent by Saturday night, but mainly at high levels, perhaps not tapping into the rich low level moist layer. Convection may instead by governed by diurnal boundary layer influences and outflows. The mid level lapse rates do not seem too supportive of organized heavy rain, but we will have to watch out for small scale organization along Florida's east coast. WPC QPF leaned toward a global model consensus while using the WRF-ARW2 for some of the mesoscale details. Days 2/3... ...Lower MS and TN valleys to the Southeast and southern Mid-Atlantic... Widespread moderate, to locally heavy and potentially excessive rains, are expected to move east across the region as a upper low drifts east from the lower MS to the TN valley this period. Overall, models are in good agreement with the larger scale features - showing a compact mid-upper level low moving east from the southern Great Plains into the lower MS valley on Sun. Strong moist southerly low level inflow ahead of the low, interacting with strong large-scale ascent afforded in part by favorable upper jet forcing, is expected to support widespread moderate to locally heavy rains from lower MS into portions of the TN valley and Southeast Sun to early Mon. While models continue to show typical differences with respect to the finer details, there remains a general model signal for 1 to 3 inch totals, with locally heavier amounts, extending from western TN to western GA during the Day 2 period (ending Mon 12 UTC). With these amounts causing potential runoff concerns, a 'slight' risk for excessive was maintained across this area in the Day 2 Excessive Rainfall Outlook. By early Mon, models show a northern stream trough moving off of the Northeast/Mid-Atlantic coast, allowing an upper ridge to build along the East Coast and causing the upper low to slow over the TN valley. Strong southerly-southeasterly low level winds sandwiched between the low and high pressure anchored off of the New England coast will channel deep moisture from the Southeast coast back into the southern Appalachians. As on Day 2, there is plenty of uncertainty with respect to the finer details of forecast. However, there is pretty good model signal for heavy amounts on the order of 2-4 inches along portions of the SC coast during the Day 3 period. It is uncertain to what degree convection developing offshore and along the coast may hinder moisture advection further west, however most of the overnight guidance show a secondary max along the favored terrain of western Carolinas and northern GA. Given the forecast uncertainty and the antecedent dry conditions across portions of the region, only a 'marginal' risk for excessive was introduced into the Day 3 Excessive Outlook (ending 12 UTC Tue) at this time. However, an upgrade to a 'slight' for portions of the region may need to be considered in future updates. WPC QPF generally followed a blend that gave significant weight to 00 UTC GFS and recent runs of the ECMWF through the period. ...Northern Rockies to the Northern Great Plains... A well-defined shortwave trough is forecast to move across the northwestern U.S. into the northern Great Plains this period. This is expected to produce widespread light to moderate precipitation from the northern Rockies to the plains, with some locally heavy mountain snows across southwest MT and northern WY. WPC QPF gave more weight to recent runs of the ECMWF and NAM. The GFS is more progressive and amplified than the model consensus with a leading piece of energy, producing heavier precipitation further east across the plains on Day 3. Refer to the WPC Winter Weather suite for details concerning the heavy snow potential across the region. Burke/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml