897 FOUS30 KWBC 180024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 824 PM EDT Wed Apr 17 2024 Day 1 Valid 01Z Thu Apr 18 2024 - 12Z Thu Apr 18 2024 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT LAKES, OHIO VALLEY AND WESTERN PARTS OF THE NORTHEAST... ...01Z Excessive Rainfall Outlook... Progression of the cold front has allowed the removal of Michigan and much of Ohio from the Marginal Risk area. The threat of excessive rainfall remained over far eastern Ohio into parts of adjoining states where the low/mid level flow continues to draw moisture ahead of the approaching cold front. In the immediate term...the concern is greatest where the southern part of a line is becoming aligned with the mean low-level flow setting up the possibility of cells training. Part of the convective line farther north was more progressive but poised to move over areas that were soaked with very heavy rain recently...so there is at least a low- end threat into the overnight hours. Some convection over northeast Mexico may make it across the international border into parts of the middle Rio Grande Valley in Texas during the evening...but the 18/00Z sounding from Del Rio showed an atmosphere that was largely capped atmosphere...an idea that the HRRR has been suggesting for a few runs and which should preclude much of an excessive rainfall threat. Bann ...16Z Update... Minor adjustments were made to the southern edge of the previous MRGL issuance across the Ohio Valley into the Mid Atlantic. This was in conjunction with the latest trends in hi-res deterministic and the QPF footprint within the NBM. Mid-level shortwave will rotate around the base of the closed 5H reflection over the Great Lakes and propagate eastward over northern IL/IN/OH before pivoting across PA by the end of the forecast cycle. Locally heavy rainfall from progressive convection will allow for flash flood concerns within that area bordering Lake Erie down to about I-70 where the best forcing will occur. This will cross over an area of lower FFGs due to recent rainfall events causing antecedent conditions to be primed for easier exceedance thresholds. The area of focus will be over northeast OH into western PA where even 1 and 3-hour FFG markers are hovering around that 1-1.5" range which is very low and a possibility within today's setup. 12z HREF EAS probability field is between 20-40% for exceeding 1" total rainfall between Cleveland to Erie, aligned well with the ML algorithms from both the GFS and ECMWF for the targeted location for best chance of impact. As a result, did not feel there was a need to adjust the northern extent of the risk area as it solidifies the forecast and maintains continuity. Across the Deep South, conditions have been fairly tame with regards to the previously highlighted area. 12z CAMs remained "hot" with the QPF interpretation, but radar and observational trends negate the former runs and the newer hourly guidance is more in- line with what is occurring which would be well below any chance of a MRGL risk issuance. Will continue to monitor, but the threat of flash flooding is trending closer to 0% than it is to the bottom of the Marginal Risk threshold (5%). Kleebauer ...Previous Discussion... Showers and thunderstorms tracking across the Ohio Valley may produce periods of heavy rainfall. Much of eastern Ohio and western Pennsylvania have Low FFG which does maintain an elevated threat for excessive rainfall and local flooding concerns. Model consensus has accumulations less than 2 inches across this area. Multiple rounds of convection will track through eastern portions of Pennsylvania and into New Jersey could reach 1 inch within areas of low FFG indices, especially in the 3 and 6-hr FFG intervals. The Marginal Risk area that was already in effect was maintained for this period. Campbell/Kleebauer Day 2 Valid 12Z Thu Apr 18 2024 - 12Z Fri Apr 19 2024 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND MISSISSIPPI VALLEY... ...20Z Update... Marginal Risk area was expanded to the northeast to include more of the Ohio Valley as latest guidance trends more bullish with the prospects of greater convective coverage in-of eastern MO, southern IL, and western KY. Latest probabilities for at least 1" and 2"/hr rates have ramped up to 40-60% and 10-20% respectively across the Mid-Mississippi Valley with the secondary pulse of heavier rates located within central TX. Regional instability is highest across the southern plains, but the combination of stronger mid-level forcing along with sufficient buoyancy is located over the Mid- Mississippi Valley where the best prospect for flooding is anticipated. Overall, the primary synoptic and mesoscale pattern are unchanged with only deviations anticipated in the general convective placement within the MRGL risk area. Relative progressive nature of the cold front will limit higher end potential over the Missouri Basin, but an upgrade to a Slight is not completely out of the question, pending evolution and trends upstream as the shortwave exits out of Nebraska. Kleebauer ...Previous Discussion... A surface low pressure system and associated front will develop across the Central Plains and advance toward the Mid-Mississippi Valley and convection will fire off as the moist return flow from the Gulf of Mexico drawls northward into the system. Locally heavy rain will setup over Missouri and points southwest to central Texas. The better concentration of the heavy rain will focus across Missouri and surrounding locations. Recent rains have lowered some of the FFG across this region and may have an increased sensitivity to additional rain/heavy rain. A Marginal Risk area covers part of northeast Texas to southern Missouri and western Kentucky. Campbell Day 3 Valid 12Z Fri Apr 19 2024 - 12Z Sat Apr 20 2024 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE TEXAS BIG BEND AND WEST TEXAS... ...20Z Update... The low pressure system will continue to have its associated frontal boundary draped across the Midwest and Plains during period while dryline sets up from the Oklahoma Panhandle to the Big Bend area. Convection is expected to be along and south of the front over the Southern/Central Plains and Lower Mississippi Valley with the higher QPF near/east of the dryline in west-central Texas. The consensus for the area over the Texas Big Bend up through the Edwards Plateau has grown within this suite of guidance with the environment primed for higher end convective development from the Lower Trans Pecos, eastward into the Rio Grande and adjacent Edwards Plateau. Ensemble probabilities remain on the lower end, however they are within the 10-20% range for totals exceeding 2" which would be sufficient for localized flood concerns within the terrain focused over the area. Further northeast into the Red River and southern OK, there's some differences in timing of the approach of the front into that area with the convergence pattern expected towards the back end of the forecast period with a higher potential for flash flooding in the successive D4 time frame. As a result, have removed the MRGL, for now and will monitor any trends over the next series of numerical guidance output. As we intake more hi-res deterministic data in the following 48 hours, a secondary MRGL is possible for the Red River basin with a gap in-between as probabilities for any appreciable convective coverage is low between the Concho Valley into north TX. A Marginal Risk area spans the Big Bend area northeast into the Edwards Plateau and Concho Valley. Campbell/Kleebauer 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