Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 459 PM EDT Tue May 22 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 23/0000 UTC thru May 26/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...SD/MN/IA border junction... Tonight should be a good setup for an axis of elevated convection with heavy rains across portions eastern SD into southwest MN and northwest IA where a broad region of low-level inflow intersects and overrides a warm front. Elevation over the surface boundary might not be by much as MUCAPE values are expected to start in the 1500-2000 J/kg range at 06z and the best 850 hPa wind convergence/confluence appears to be near the SD/IA border. The 850 mb moisture transport increases and persists through the night downwind of the instability pool. Storm motion vectors are at a right angle to the low-level inflow expected, suggesting that storms could regenerate and train for a period from west-northwest to east-southeast. Still a good amount of uncertainty with model placement, though there is a strong enough signal for 3-5" totals. Think a solution somewhat south of the 12z ARW is in the ballpark of what could materialize. ...Northern Rockies/Divide... Upslope flow northeast of a deep layer cyclone combined with precipitable water values of 0.75-1" are expected to focus heavy precipitation near the western WY/MT border into eastern ID. For the most part, continuity resembled recent guidance, so let it ride. Increased amounts somewhat in the Wind River Range per the most recent guidance. ...East... A wet pattern is expected across the Southeast and Mid-Atlantic States. The main foci are the low-level circulation which moved inland from the Gulf yesterday into eastern AL and a frontal boundary shifting southward through the region. This afternoon, heavy rains are expected to start in north-central VA and southern MD and shift east and southeast this afternoon and evening before fading overnight near the VA/NC border. The guidance has trended in this direction so have followed suit. The portion of the pattern which most resembled continuity was with the eastern AL circulation which is expected to spread heavy rains into GA. The flow pattern in FL suggests a west coast sea breeze maximum, which resembled continuity which was strongly maintained. ...Southwest OR/northern CA... Diurnal convection each afternoon and evening is expected near and downwind of the terrain within the western periphery of a deep layer low. Increased amounts somewhat across this area to edge continuity towards the latest model guidance. ...AR/LA/TX.... An upper low near the southern LA/TX border should act to focus some convection over portions of AR/LA/MS through tomorrow within a region of relatively weak flow. Diurnal convection with brief heavy rainfall is possible here before the storms pulse/become outflow dominant. While very localized heavy totals are possible, areal averaged rainfall should not be all that high. ...NM/TX border... Followed the guidance trend to shift activity farther southward within a regime with shortwaves rotating east to northeast around the southern periphery of a deep layer cyclone across the Great Basin. Days 2/3... ...Southeast/eastern to central Gulf Coast/FL... Deep tropical moisture within confluent flow encounters a cold front moving across the southeast from the north, particularly Thursday. Diurnal heating produces an areal average QPF max of only around one inch on Wednesday, but given recent and ongoing activity in the area, as well as slow storm motion per 850 to 300 mb mean wind of about 5 kt, heavy rain can be expected. A marginal risk for excessive rainfall was raised for Day 2 (12Z Wed-12Z Thu) along the southern AL/GA border. Elsewhere expect locally heavy showers in areas of diurnal instability and sea/lake breeze initiated lift and convergence near the front. 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 primarily on the 12Z GFS and some 12Z ECMWF which came into better alignment over the central Gulf coast compared to the 00Z ECMWF. The 12Z NAM continues to be an outlier bringing low pressure closer to the central Gulf coast much sooner that other solutions so the NAM was given no weight. The cold front weakens for Day 3 (12Z Thu-12Z Fri) and the average QPF max shifts to south-central GA where less rainfall has occurred in the past week than along the AL/GA border, so no excessive rainfall risk was raised. ...Pacific Northwest/Great Basin/Northern Rockies/Northern High Plains... An amplified trough off the entire west coast Wednesday evening closes into an upper low and reaches the northern CA coast Friday. PW increased to around 1 inch ahead of the low over OR/WA which is 2 standard deviations above normal, so an excessive rainfall risk is possible by Day 4. In advance, an upper level trough and corresponding 700 mb low over MT Wednesday evening will lift northeast into the Canadian Prairies Thursday. Instability under the trough will allow convective showers and 1 inch PW is 2 to 2.5 standard deviations above normal, so amounts are higher than you typically see with a late May system. Showers occur in the area of low-mid level convergence and ascent near the low, which combined with terrain near Yellowstone and snow melt warrants a marginal risk for excessive rainfall for Day 2 (12Z Wed-12Z Thu). The focus for heavy rain shifts to the Canadian Prairies with the low on Thursday. QPF was mainly based on the 12Z GFS/ECMWF. ...Eastern Northern Plains/Upper Midwest... A warm front will focus convection on a plume of moisture with the ECMWF forecasting near 1.5 inches precipitable water at its peak as it lifts north across MN Wednesday night and Thursday. Activity will focus ahead of a lee trough in the high Plains, with another plume of enhanced moisture forecast to form ahead of the trough and approaching cold front Thu evening in MN. Clusters of showers and storms are expected again with convergence on the nose of the low level jet progressing across MN, areas near the SD/IA border, and eastern Nebraska. PW is 2 to 3 standard deviations above normal, but the main convergence is expected to lift into Canada with a limited threat for excessive rainfall over the northern plains and upper Midwest. QPF based on the 12Z ECMWF/GFS. ...West Texas/eastern NM/TX-OK panhandles into west KS... Another deepening low approaches CA Thursday. The trough over NM lifts out is replaced by a building mid-upper level ridge. Isolated to scattered showers/storms are possible but with less coverage due to weakened low level convergence and lift. The 12Z GFS continues to demonstrate convective feedback, but locally heavy showers and thunderstorms can again be expected into Wednesday before the forcing lifts well north. Roth/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml