174 FOUS30 KWBC 250052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 852 PM EDT Wed Apr 24 2024 Day 1 Valid 01Z Thu Apr 25 2024 - 12Z Thu Apr 25 2024 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR EASTERN OKLAHOMA AND WESTERN ARKANSAS... 01Z UTC update... Realigned the orientation of the Marginal Risk area to better match the low-level thetae gradient...with higher values advecting northward across the western High Plains ahead of an approaching shortwave trough and cooler/drier air feeding southward from strong surface high pressure over the Great Lakes region into the Mississippi Valley region. While there should be increasing isentropic upglide overnight that leads to the development of showers and thunderstorms...and a sufficient combination of CAPE on the warm side of the low level front and increasing moisture over the outlook area...localized rainfall amounts of 1 to 3 inch amounts are possible but the latest HREF continued to be unenthusiastic about the chances for 3 inch totals. Confidence about the potential for excessive rainfall is lowest in the Kansas portion of the outlook area given the dewpoint depressions...suggesting that the threat may not develop until after the end of the Day 1 period at 12Z. Bann Previous discussion issued at 0700 UTC... ...OK/AR... OK/AR will remain an area of diffluent 1000-500 hPa thickness just east of a migrating mid- level ridge axis translating east from the Four Corners region. At the surface, a front will lift north Wednesday night into early Thursday in advance of the approaching upper level trough from the Southwest. Mid-level capping should be minimal based on the forecasted 700 hPa temperatures, under 9C. Precipitable water values 1-2 sigmas above average for late April -- 1.5-1.75" -- are forecast by the model consensus to be in place near and just north of the surface front with elevated CAPE of 1000 to 2000 J/kg (higher to the south) may allow for stronger cells capable of high rainfall rates. Effective bulk shear/low- level inflow should increase to 35-40 kts. Non- traditional cell training within a WNW steering flow near and poleward of the front could allow for hourly totals to 2" and local 4" amounts. While the 00Z HREF isn't sold on 3"+ amounts in the area, precipitable water values in the area have trended upwards each day, and now the 00z Canadian Regional, though at a smaller scale, supports the wetter ECMWF solution. Went ahead and introduced a Marginal Risk in this update. Roth Day 2 Valid 12Z Thu Apr 25 2024 - 12Z Fri Apr 26 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS... A mid-level trough over the southwestern U.S. is forecast to eject out across the central and southern High Plains by Friday morning, spurring development of a surface cyclone across the High Plains. Winds at 850 mb will be south to south-southwest across the Great Plains but with increased magnitude -- up to 50-60 kt -- and moisture transport will allow precipitable water values to exceed 1.5" across portions of the Plains and Midwest. A broad area of instability east of a forming central High Plains surface low and attendant cold front/dryline, with values of 1000 to 3000 J/kg from northern TX/southern OK into KS and NE. Initially, some convection may be ongoing at the start of the period over OK/AR along a warm front lifting northward, but additional storm development is anticipated to occur during Thursday afternoon ahead of the cold front/dryline in KS/OK with thunderstorm development expanding through the evening and early overnight across TX and northern locations including NE and IA. Strong to severe thunderstorms are likely, ramping up the precipitation potential, despite decently fast cell motions. Deep layer south- southwest flow supports training of cells with rains up to 2.5" in an hour and local amounts up to 6". Models continue to show good agreement for the slight risk across portions of KS/OK/AR/MO which portions of could overlap with some heavy rainfall potential tonight into Thursday morning. The surrounding marginal risk was extended slightly from the overnight issuance southward along the cold front into central TX, eastward along the warm front into the Tennessee Valley, and westward more into Nebraska/South Dakota to the north and west of the surface low. Santorelli/Roth Day 3 Valid 12Z Fri Apr 26 2024 - 12Z Sat Apr 27 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS NEAR AND NORTH OF THE ARKLATEX AND NEAR THE ILLINOIS/IOWA BORDER... As a surface cyclone pulls northeast across the Plains and Midwest, moisture and instability will lift northward through the Midwest. Inflow at 850 hPa will be near or above 50 kts with precipitable water values as much as 2 standard deviations above normal and more than sufficient instability to help organize convection. Within a broad marginal risk from the northern Plains to Upper Midwest and southward along the cold front into the lower-mid MS Valley and eastern Plains, continued two slight risk areas - one near the IA/IL border and another across northeast TX, southeast OK, and northwest MO. Both of these slight risks remain due to heavy rainfall expected Wednesday into Thursday, and some recent rainfall during the past week. Cell training is possible, with some strong to severe storms as per the latest SPC Outlooks. Hourly rainfall totals to 2" with local amounts to 4" are possible. Santorelli/Roth Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt