Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 501 AM EDT Mon Jun 04 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Jun 04/1200 UTC thru Jun 07/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr ...Northeast... The elongated, negatively tilted trough nudging into southern Ontario and the eastern Great Lakes early this morning will become reinforced by an upper jet streak/additional shortwave energy entering the trough's base. The result will be a transformation toward a more positively tilted longwave trough as the low deepens through the mid-upper levels over southern Ontario-Quebec later Mon-Mon night. Favorable exit region forcing east of the trough (upper level divergence and strengthening lower tropospheric s-se flow and positive moisture/theta-e transport) will favor additional mod-heavy rainfall along the New England coast, along the apex of 1.5"+ PWs ahead of the surface occluded front. The consensus of the 00Z guidance depicted a a decided eastward shift in the heaviest rainfall during the day 1 period (12Z Mon-12Z Tue), i.e. mainly offshore Downeast ME while clipping eastern MA/Cape Cod. While relatively efficient considering the deep subtropical moisture plume (relatively high wet bulb zero levels and 500-300 mb layer PW values near 0.25"), the absence of even elevated instability will be a significant limitation to the short term rainfall rates, at least with respect to the excessive rainfall/flash flood potential given the antecedent soil conditions/current FFG values. As a result, the Marginal risk for excessive rainfall that was noted in yesterday's day 2 ERO has been removed. ...Southern Plains... A compact area of strengthened WAA and deepening elevated instability early in the period across the southern Plains, bolstered by the MCV ejecting east of the High Plains, has fostered an initial round of convection early, as the decaying MCS will bring moderate areal-average rainfall totals to central and north-central TX later this morning. Following round 1, lingering MCV energy and outflow boundaries along with the stationary synoptic front will favor additional convective development this afternoon-evening -- especially toward the mid-upper TX coast where robust deep-layer instability (model mixed layer CAPES 2500-3500+ j/kg) will be present. The model spread however is quite high with respect to the convection evolution (and QPF) later this afternoon and evening, though the solutions do begin to align somewhat more favorably late Mon evening and overnight (especially after 06Z) as increasing low level southwesterly flow (30-40 kts at 850 mb) supports another compact MCS from south-central OK, across the Red River and into North TX and western portions of the ARKLATEX. PW values are progged to spike aoa 1.75" Monday night along the path of this MCS, with the models (especially high-res CAMs) likely simulating the mesoscale environment to some degree. In terms of the QPF, given the disparity among the operational model runs (global and high-res), WPC once again leaned toward the means, particularly the HREF and NBM high-res means. Individual CAMs such as the WRF-ARW, ARW2. NMMB, NSSL-WRF, and NAM CONUS-Nest all show spotty areas of 24 hour rainfall totals of 3-5+ inches, the bulk of which would arrive late Monday night via the next MCS. Despite the relatively high FFG initially, the prospects of at least some reduction from rains early Mon, along with the potential for fairly prodigious short-term rainfall rates Mon night-early Tue with the MCS, would continue to warrant a Marginal risk area in the new day 1 ERO (similar to the day 2 outlook from Sun). Day 2 ...Northeast to Mid-Atlantic and Ohio Valley... Low pressure centered over Quebec opens as it shifts east across New England through Tuesday night. Light to moderate rainfall in sub one inch PW across New England. A cold front will progress south from the wave with a likely narrow swath of rainfall from the OH Valley to the Mid-Atlantic. One inch PW ahead of the front should allow for localized heavy rainfall in both the central Appalachians upslope areas and near the coast where bay/sea breezes could locally add moisture. ...Northern Plains... Gulf-sourced moisture will converge over ND where surface low pressure crosses as a shortwave trough lifts over the central CONUS ridge from MT into the Canadian Prairies. 1.25 inch PW is 2 standard deviations above normal. Ample instability with allow convective development though progressive/cold pool driven activity is expected in deep layer westerly flow. Therefore a marginal risk of excessive rainfall was maintained over eastern ND into northwestern MN. ...Gulf Coast... The cold front that has progressed south across the county will stall over the northern Gulf of Mexico through midweek. PW anomalies along the front are expected to be 1 to 1.5 standard deviations through Tuesday night. Mean layer flow along the orientation of the front and ample instability should allow repeating thunderstorm development, though most of this will be offshore. Will need to monitor southern LA and the FL peninsula for areas of repeat convective thunderstorms. Day 3 ...North-Central Plains to Upper MS Valley... Ongoing MCS activity is likely in the southerly low level flow streaming up the Plains from the Gulf. Heavy rain is a concern with PW around 1.5 standard deviations above normal over SD/NE and east into IA Wednesday. However, the mesoscale properties associated with this development make for a difficult location forecast. No excessive rain risk is raised as of now. ...Louisiana and Florida... The lingering cold front over the Gulf will continue to extend from LA to Central FL---focusing an axis of tropical moisture along and to its south through midweek. Heavy rainfall is likely though the PW anomaly diminishes to near normal Wednesday, so no excessive rain risk is raised at this time. Hurley/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml