Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 459 PM EDT Mon May 21 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 22/0000 UTC thru May 25/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...New Mexico into Texas... Confluent, moist southeasterly low level inflow along with broadly divergent flow aloft is expected to support developing convection this afternoon and evening across New Mexico into West Texas. Latest hi-res guidance continues to signal two areas of heavier accumulation. Storms developing initially along the high terrain may linger, producing locally heavy accumulations this afternoon and evening along the central New Mexico mountains into the high plains. Then the threat for heavy accumulations is expected to extend further south as intensifying southeasterly low level inflow supports the potential for training/repeating cell development across portions of West Texas during the overnight hours. WPC QPF generally favored a consensus of the 12z HREF members. ...Lower MS Valley into the TN/OH valleys and Great Lakes region... A mid-upper level shortwave and its associated surface wave are expected to move east from the upper Midwest across the Great Lakes this period. Pooling moisture along the preceding warm front, along with right-entrance region upper jet forcing, is expected to support a swath of moderate to heavy rains, with the 12z hi-res guidance continuing to suggest a period of back-building -- raising the threat for locally heavy accumulations across central and northern Ohio into western Pennsylvania during the evening and overnight hours. Further south, less organized convection is expected to develop along and ahead of a trailing boundary extending back into the lower Mississippi valley. Forecast confidence is less here, however a lingering weak shear axis may support some more organized heavier amounts back across the eastern Arkansas basin and lower Mississippi valley during the day on Tuesday. Here also, WPC QPF generally favored a consensus of the 12z HREF members. ...Mid Atlantic... A backdoor cold front stalled near the Virginia-North Carolina border is expected to remain a focus for developing convection during the afternoon and evening hours, with several of the 12z hi-res guidance members continuing to show locally heavy amounts along the border into southwest Virginia and western North Carolina. As convection begins to wane during the overnight hours, focus is expected to shift further north across western Virginia as the front begins to lift out. ...Florida and Southeast... Guidance continues to signal a stripe of moderate to heavy amounts from the western Florida panhandle into southeast Alabama as a weak low drifts north from the northeastern Gulf of Mexico this period. Guidance shows moderate to heavy rains drifting north along with the center across the Panhandle into southeast Alabama late Mon into Tue, with persistent inflow in the wake of center continuing the threat back to the coast. Early in the period, WPC QPF followed a blend of the HREF Mean and recent runs of the HRRR, with significant weight given to the HREF members through the end of the period. ...West... A deep upper low is expected to gradually fill and lift north from southern California and the lower Colorado basin this period. Deep northeasterly flow north of the low center along with favorable upper forcing is expected to support a widespread swath of light amounts from the northern Rockies back into the Sierra. Orographic forcing may support some heavier totals across the region. Days 2/3... ...Southeast/eastern Gulf Coast/FL... Deep moisture within confluent flow with corresponding lift provided by a weak 850 mb circulation drifting inland across a combination of AL/GA will support bands of showers over the eastern Gulf coast....including the FL panhandle inland to AL/GA Tuesday night through Thursday. Inflow off the gulf advects this tropical moisture into the Florida panhandle across into southern Alabama and Georgia. Heavy rain is expected from diurnal instability, sea breeze initiated lift, and convergence near the low. The models also depicted persistent, deep flow of moisture from the Bahamas into southeast FL with peak precipitable water values near 2 inches so showery conditions are expected along the southeast coast. The pattern favors inland penetration of the sea breeze from the east coast so peak afternoon convection should occur over interior portions of the FL peninsula to the west coast. QPF was based on the 12Z GFS/ECMWF with consideration of the NAM/GEFS ensemble mean and continuity. A Day 2 (12Z Tue-12Z Wed) marginal risk for excessive rainfall was maintained from the FL panhandle up AL/GA and expanded up to the southern Appalachians into NC. ...Great Basin/Northern Rockies/Northern Plains... An opening upper level low will lift north-northeast from NV to MT Tuesday night through Wednesday and into the Canadian Prairies Thursday. Instability under the trough will allow convective showers and 0.75 inch PW is above normal, but their sporadic nature does not yet warrant an excessive rainfall risk at this time. Greater moisture is east of the Rockies where Gulf influence is expected in continued southerly flow. A Day 2 (12Z Tue-12Z Wed) marginal risk for excessive rainfall was raised centered on the common border of MT/WY/SD near where a surface low is expected to form. Expect convective rain averaging one inch in this area where 6hr FFG is around two inches. ...Upper Midwest... A warm front associated with a low developing in the lee of the Rockies as a trough lifts over the western US will focus convection on a plume of Gulf moisture from the north-central plains to the upper Midwest of MN/WI Wednesday and Thursday. Raised a Day 2 (12Z Tue-12Z Wed) marginal risk for excessive rainfall centered on the common border of SD/MN/IA where the PW of 1.25 to 1.5 inches is anomaly is 2 standard deviations above normal. The marginal risk for Day 3 (12Z Wed-12Z Thu) was shifted northeast a bit on account of the 12Z consensus and an effort to include the MSP metro area. ...West Texas/eastern NM/TX-OK panhandles into west KS... A trough over the Desert SW Tuesday evening lifts north-northeast to MT through Wednesday. Another deepening low approaches CA Thursday. Gulf moisture streams north via the Rio Grande valley aided by surface high pressure centered east of FL. 1.25 inch PW over far west TX is 1.5 standard deviations above normal. The 12Z GFS continues convective feedback issues in this area, but heavy rain is possible over this area. Elected to not issue a marginal risk for excessive rainfall for Day 2 (12Z Tue-12Z Wed) as mean flow increases through the 2 period as the trough axis lifts out. Lighter QPF for Day 3 given lowering PW. Pereira/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml