Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 635 PM EDT Tue May 08 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 09/0000 UTC thru May 12/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern portions of the Northern Plains---Lower MO Valley into the Upper MS Valley... Convection is developing in southwest MN and northwest IA along an area identified by the recent RAP analyses and forecasts as frontogenetical. Coverage of showers/storms is expected to continue increasing this afternoon and evening as the region of well defined upper difluence in advance of the upper trough in an axis of pw values of 1.2-1.4 inches and lifted indices of -2 to -4 in Iowa this evening. Showers/embedded storms are expected to continue in the coupled upper divergence/lower convergence maxima across IA,southern MN, southwest WI, northwest IL and into northeast MO. Widespread moderate to heavy precip totals likely across these areas along with potential for isolated runoff issues where convection briefly trains. Shower and storms redevelop in the low level axis of inflow and instability Wed in WI and IL across Lake MI to lower MI. Manual QPF blended continuity with the global/Canadian regional GEM and high res ARW/ ARW2/ NMMB models. ...Pacific Northwest to the Northern Rockies... The closed low off the Pacific Northwest coast will be moving northeastward to the west of Vancouver Island by the end of the period. Organized precipitation along the associated frontal boundary will begin to affect the coastal Pacific Northwest Tuesday afternoon---moving farther inland through the Pacific Northwest Tuesday night-early Wednesday and toward the far northern Rockies by the end of the day 1 period. Model consensus is for widespread moderate precipitation totals from the Washington-Oregon Cascades--west to the coast ranges and over the Blues of northeast Oregon and far southeast Washington---with the day 1 WPC qpf close to the multi-model consensus. Days 2/3... ...Pacific Northwest... Light to moderate precipitation is expected over the favored terrain of the Pacific Northwest Wednesday night into Thursday as a series of shortwaves progress inland from the eastern Pacific. Activity should begin to wane on Friday while the shortwave starts to amplify downstream and increasing northerly flow shuts down the region's moisture supply. Model guidance was well clustered with keeping precipitation focused in the terrain and minimal adjustments were made to continuity. ...Northern/Central Rockies to the Great Plains and Upper Mississippi Valley... A large scale trough amplifying over the western U.S. will bring an increasing chance for precipitation from the Northern/Central Rockies to the Great Plains and Upper Mississippi Valley Thursday and Friday. The greatest threat for any excessive rainfall will be from north central Nebraska into southern Minnesota and northern Iowa, where the combination of a weak ripple of shortwave energy, strong southerly low level flow, and an axis of anomalous moisture will heighten the risk for a period of heavy rainfall Thursday night into early Friday. Also, some of these locations have received or are expected to receive moderate to heavy rain on Tuesday, which could add to run-off concerns. A Day 3 Marginal Risk of excessive rainfall was introduced across the region but could be upgraded to a Slight Risk once model guidance comes into better agreement with the axis of heaviest rainfall and details of developing precipitation. Favorable upper jet dynamics and upslope flow should lead to enhanced precipitation totals across the northern/central Rockies and adjacent High Plains, but current model guidance is in agreement with keeping amounts below flash flood guidance values. ...Great Lakes and Ohio Valley to the Central Appalachians and Northeast... A broad shortwave will continue to trigger shower and thunderstorm activity while it crosses the Great Lakes and Ohio Valley Wednesday night into Thursday. A plume of anomalous moisture ahead of the shortwave could support locally heavy rainfall, but the progressive nature of the system should minimize any widespread excessive rainfall concerns. The chance for excessive rain will especially be reduced Thursday night into Friday while the energy lifts towards the Northeast and gets absorbed into a large scale upper trough swinging across southeastern Canada. ...Southern Florida... The combination of a weakening frontal boundary and an axis of highly anomalous moisture could result in periods of heavy rainfall over the southern tip of Florida Thursday and Friday. There is some uncertainty with the exact amounts, but model guidance was in agreement with keeping the heaviest totals just offshore. Petersen/Gerhardt Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml