Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 14 2025 18:52:23 FOUS30 KWBC 141852 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Wed May 14 2025 Day 1 Valid 16Z Wed May 14 2025 - 12Z Thu May 15 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC INTO THE CENTRAL APPALACHIANS... ....Mid-Atlantic... 16Z Update: Portions of the Mid Atlantic extending from the Tidewater of VA to the northwest across the Central and Northern Blue Ridge to the Appalachians Front are now in a SLGT risk after coordination with surrounding WFO's. The overall synoptic pattern remains unchanged with a slow progression of a negatively tilted 500mb trough currently centered over the eastern Ohio Valley when assessing WV satellite and recent UA analysis. Compared to yesterday, widespread elevated totals are not likely to be seen with the anticipated setup as the primary precip schema will lean to more scattered variety thunderstorms with potential for localized heavy maxima given the 90th percentile PWATs situated over the area (12z KIAD sounding of 1.31"). Convection is slowly beginning to initiate downstream of the mean trough across Southern VA into NC with a greater coverage anticipated between 18-00z during peak diurnal heating. MUCAPE between 500-1500 J/kg will be co-located within the terrain extending through the Blue Ridge and adjacent Appalachians with 1000-2000 J/kg bisecting the VA Tidewater up into Northern VA. This instability presence will be plenty to help maintain some stronger convective cores capable of localized rainfall rates between 1-2"/hr at peak intensity. Coupled with very low 1/3/6 hr FFG intervals after yesterdays heavy rain episode, this is sufficient for short term rates to exacerbate already compromised soils as NASA SPoRT maintains a signature of >90% soil moisture presence across pretty much the entire Central Mid Atlantic. 12z HREF neighborhood probs for >2" of precip indicates a broad footprint of 50 to 90% over the aforementioned area of focus with modest probs (20-40%) for >3" in the same zone. The most concerning depiction is within the 3 and 6-hr exceedance probability output showing a large chunk of the Blue Ridge, Central Appalachians, and Southern Laurels around 40-80% and 50-80% respectively. This is typically a good indicator for a locally more significant event potential in most areas of the CONUS, but especially over terrain with lower FFG indices starting out. This allowed for an upgrade to a SLGT risk in those locations extending from the VA Tidewater, northwest through parts of Northern VA, Blue Ridge and neighboring Shenandoah, into the Central Appalachians. A broad MRGL encompasses the metro corridor over MD along with Western and Central PA down towards NC as coverage and impacts are anticipated to be more isolated in these zones with greatest threat likely within urban areas due to higher runoff risk. Kleebauer ....Northern Plains... 16Z Update: After last nights shift of the QPF maxima further west, the guidance has come into agreement with little run to run variability when assessing the two main CAMs periods. The antecedent dry soils will be the "scale tipper" to the MRGL side of the threat as convective premise will likely be more of a welcomed sight, initially with some localized flood prospects in the Western High Plains most likely incurring due to hydrophobic soils if rainfall rates breach 2"/hr, or similar rates impacting a town with more runoff potential given urbanization factors. The nocturnal MCS development off the Front Range into SD/NE will be worth monitoring, but if the convection leans closer to the Sandhill domain along those borders into NE, the threat for flash flooding will likely be muted outside of direct impacts on any towns in the path of the complex. The MRGL risk was relatively unchanged given the above factors, but pending the output from the first round of convection along the lingering surface trough in conjunction with the secondary convective onslaught anticipated this evening, a targeted upgrade could be plausible. This is something we will monitor as the day progresses. Kleebauer ...Previous Discussion.. A deep upper level trough centered over the Intermountain West this morning will become negatively tilted as an impressive shortwave draws plentiful upper level energy into the northern Plains. Surface cyclogenesis is expected ahead of this trough over Kansas, which will track NNE into south-central North Dakota by Thursday morning. A line of convection associated with the leading/eastern edge of the trough is moving into far western South Dakota from Wyoming, and this line of storms will track NNE over the western Dakotas through the morning. The primary flooding threat will be from a new line of storms which will develop over western South Dakota with peak heating this afternoon, then track very slowly eastward, as embedded cells move north, parallel to the line. This will result in some training of convection, leading to an isolated flash flooding threat. In coordination with UNR/Rapid City, SD; FSD/Sioux Falls, SD; and ABR/Aberdeen, SD forecast offices, the inherited Slight Risk was removed with this update, and the surrounding Marginal shifted well to the west of inherited. The westward shift was due largely to a shift in the guidance slowing the forward motion of the trough. While this increased forecast rainfall amounts some, the area of the Dakotas now highlighted is in Moderate to in some localized cases Severe drought. NASA SPoRT soil moisture imagery shows the area has next to no moisture. Thus, despite the potential for heavy rainfall at times in some areas, the flooding threat was determined to be isolated. Wegman ....Ohio Valley... Scattered convective activity within an elevated moisture advection regime will occur this afternoon and evening with the greatest coverage anticipated in the evening thanks to added forcing from a mid-level perturbation moving up from the southwest. Sufficient low to mid-level buoyancy coupled with elevated PWATs will help with locally heavy rainfall potential across areas extending from Southern IN down into KY with totals potentially reaching between 1-3" in the strongest convective impacts. 12z neighborhood probs for >2" are modest (20-40%) across the above area with some elevated probs for >1" (50-70%) in the same locale. Considering the nature of the soil anomalies >80% within areas of Eastern KY into the Ohio River basin in conjunction with the heavier rainfall rates, a MRGL risk was expanded westward to encompass the threat. Kleebauer ....Michigan... Near record daily PWATs (12z KAPX sounding of 1.37") and anticipated lake breeze initiation thanks to strong differential heat flux off Lake Michigan will generate a period of slow-moving strong convective cores capable of rainfall rates of 1-2"/hr during anticipated impact. Modest 10-year ARI exceedance probs based on the latest hi-res ensemble suite (15-30%) across Northwest MI signal a modest threat for localized flash flood concerns over any urban zones within stronger convective cores. The key is the slower mean storm motions anticipated with much of the convective development likely anchored to the lake breeze and subsequent outflow generation. Ensemble mean areal average QPF around 0.5-1" with deterministic maxima between 2-3" was enough to warrant a targeted MRGL risk issuance within a zone situated west of I-75 and just north of I-96 in Lower MI. This is the primary area of concern with the greatest threat up closer to Traverse City and points just south. Kleebauer Day 2 Valid 12Z Thu May 15 2025 - 12Z Fri May 16 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS... ....Northern Plains... 20Z Update: Very little change necessary from the previous forecast as the expected synoptic and mesoscale evolution along with reputable precip forecast remains pretty consistent. Locally heavier cores will be embedded within the well-defined TROWAL located across ND through the period leading to some localized flash flood prospects within more urbanized zones. Considering the lack of a stronger convective output and rates likely more confined between 0.5-1"/hr at peak intensity within the higher QPF footprint, the threat for flash flooding will remain within the MRGL category. Kleebauer=20 ...Previous Discussion.. The low over the northern Plains will become vertically stacked as the upper trough catches up with the surface low. With the new vertically stacked low moving only very slowly towards the northeast, this will make for a prolonged period of rain on the cold/northwest side of the center, as it becomes the comma head region of the low. This area of the low is expected to be largely devoid of convection given the lack of instability making it quite that far north and west. Thus, expect the 2-3 inches of additional rainfall from today to have only a minimal impact given the still largely dry soils over much of North Dakota. This Marginal was shifted well to the west following the latest guidance trends, so the Marginal was removed from much of Minnesota due to lack of expected rainfall. Wegman ....Western Pennsylvania to Central Appalachians... Progression of a broad warm front will lead to convective development across the Ohio Valley, spreading northeast into Western PA and the adjacent highlands of MD/WV to the south. The progressive nature of the threat will curb the maximum potential, but the low FFG's situated across the higher terrain of the Mid Atlantic allows for a greater likelihood with any convective impact over the course of the forecast. Some of the CAMs are pretty bullish on the overall QPF footprint within the outlined zone with some localized 1-2+" totals sprinkled throughout the terrain up towards Northwest PA. Neighborhood probs of 60-80% for >1" and 25-40% for >2" are situated within that area from Erie, PA down to Tucker County WV. These probs are sufficient for some flash flood risk given the accompanying antecedent conditions. A MRGL risk was added to the above region to account for the setup.=20 Kleebauer Day 3 Valid 12Z Fri May 16 2025 - 12Z Sat May 17 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE OHIO AND TENNESSEE VALLEYS... A potent shortwave trough will round the periphery of the vertically stacked low over the northern Plains and upper Great Lakes on Friday. This shortwave will tap into a low level jet of Gulf moisture riding the prior low's occluded front. This will reinvigorate the front, resulting in rather fast northeastward moving storms across the risk area. Much of this area remains=20 saturated given elevated soil moisture anomalies on the latest NASA SPoRT due to prior storms, so any training thunderstorms will be=20 capable of producing flash flooding. The trend over the past 12 hrs is for a greater training threat within the confines of the west to east oriented front aligned from the Mid-Mississippi Valley through the Central Ohio Valley with a proxy close to the Ohio River down into KY. With such a well-defined theta_E gradient and mean flow becoming more parallel to the boundary orientation, the threat for waves of convection to funnel over a general area has risen with ensemble bias corrected output and ML QPF placement actually a bit further south overall. There is some merit to the front being located a bit further into KY as we move closer to the event, a trend that has occurred quite often inside 48 hrs with the best convection co-located near the better instability access. A SLGT risk is now embedded within the broad MRGL from previous forecast with the higher risk now running from the Mid-Mississippi River basin, east through the Central Ohio Valley to far western WV.=20 Wegman/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-oSDwGkaYWDUyksL-IMl-Tas-XXUI7JC3WTH4AFhMb7Q= 1N4zrgR_QVKBtZ1_cdJuR9WXz1oFgPFfedaf8yaSwjJDIQw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-oSDwGkaYWDUyksL-IMl-Tas-XXUI7JC3WTH4AFhMb7Q= 1N4zrgR_QVKBtZ1_cdJuR9WXz1oFgPFfedaf8yaSh1g0Anc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-oSDwGkaYWDUyksL-IMl-Tas-XXUI7JC3WTH4AFhMb7Q= 1N4zrgR_QVKBtZ1_cdJuR9WXz1oFgPFfedaf8yaShSdLJWM$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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