Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 501 PM EDT Sat Apr 21 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 22/0000 UTC thru Apr 25/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern portions of the Southern Plains---Lower MS Valley---Lower TN Valley---central Gulf Coast... There is good model agreement on the overall heavy rainfall potential day 1 across eastern portions of the Southern Plains---Lower MS Valley...Lower TN Valley into the central Gulf coastal region ahead of the strong closed low moving through the Southern Plains into the Lower MS Valley. An axis of above average pw values...1 to 1.5+ standard deviation above the mean expected to move along and ahead of this closed low. This and favorable upper difluence---strong pre-frontal to frontal convergence will support organized heavy rains pushing eastward across these regions----with relatively high confidence overall on this heavy rain potential---albeit with lower confidence in some of the shorter term details. No significant changes made to the excessive rainfall potential outlook for the day 1 time period through 1200 utc Sunday April 22. With dry antecedent conditions and the expected overall progressive nature of the heavy rain areas---the threat of runoff issues will remain on the low side. A slight risk was maintained over the ARKLATEX region where model consensus is for the greatest hourly rainfall rates---1"+---during the 0000 to 0900 utc Sun time frame. ...Florida... There is a multi-hi res model signal for locally very heavy rainfall amounts across east central Florida tonight into early Sunday. The frontal boundary lying west to east across central FL this afternoon is expected to move little during the upcoming day 1 period. Low level southeasterly to southerly flow into this boundary----along with pw values rising to 1.5 to 2 standard deviations above the mean---will set the stage for locally very heavy amounts. Still a lot of uncertainties with some of the details in the hi res guidance---especially some of the very high totals such as the 10"+ in the arw...nssl wrf and nam conest and the 14" amounts in the hrrr experimental. While these models all have very heavy totals---the placement differs from between 60 and 80 nm. This and the very small scale of the maxima is leading to the low confidence. Still---with the multi-model signal for potentially heavy rain amounts---a marginal risk area was introduced in the update to the day 1 excessive rainfall potential outlook over the east central coast of FL for localized runoff issues especially over urbanized areas. Days 2/3... ...Tennessee Valley and Southeast to the Ohio Valley and Mid-Atlantic States... Widespread moderate, to locally heavy and potentially excessive rainfall, is expected with a dynamic upper low as it drifts eastward through the Tennessee Valley Sunday night and then slowly lifts northeastward through the Ohio Valley and Mid-Atlantic states Monday and Tuesday. The Day 2 Slight Risk of Excessive Rainfall was maintained over portions of the Southeast and Tennessee Valley, where models continued to show the potential for strong moist southerly low level inflow ahead of the low to combine with strong large-scale ascent and produce heavy, excessive rainfall. Also, models continued to show the potential for rainfall to exceed relatively high flash flood guidance values as the dynamic upper low slows while lifting through the Mid-Atlantic states Monday into Tuesday, so the Day 3 Marginal Risk was upgraded to a Slight Risk from coastal regions of South Carolina to the terrain of northern Georgia and western North Carolina. ...Northern Rockies to the northern/central Great Plains... Widespread light to moderate precipitation is expected with a well-defined shortwave trough progressing from the northern Rockies to the northern/central Great Plains early next week, with some locally heavy amounts expected along the favored terrain of Montana and Wyoming. Models continued to show quite a bit of spread with the timing and amplitude of the shortwave and resulting axis of precipitation across the region (especially across the Plains on Day 3), and the WPC QPF stayed close to continuity considering the uncertainty. For more details on model spread, please refer to WPC's model diagnostic discussion and please refer to WPC's Winter Weather suite for details concerning the heavy snow potential across the region. Oravec/Gerhardt Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml