973 FOUS30 KWBC 010756 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 AM EDT Thu May 1 2025 Day 1 Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN SOUTHERN OKLAHOMA, FAR NORTH TEXAS, FAR WESTERN ARKANSAS, WESTERN PENNSYLVANIA AND THE CENTRAL APPALACHIANS... ...Southern Plains... Shortwave ejection out of the Southern Rockies will interact with a cold front pressing south through the Central and Southern Plains by the late-afternoon and evening hours Thursday through Friday morning. A relevant theta_E differential will be aligned within the confines of the boundary providing a suitable thermodynamic alignment such that area convection anchors and pushes along the west to east oriented instability gradient across Southern OK. Majority of CAMs are in consensus of a heavy rainfall footprint along and south of I-40, generally configured to the timing of the cold front, shortwave interaction, and LLJ convergence signal. This is primarily right around 00z when the convective threat enhances with the integration of the nocturnal LLJ, a timing historically prevalent for this time of year and this setup is no exception. 00z HREF neighborhood probs for >1" are relatively high (50-80%) within the corridor extending from Southwest OK across the Red River Basin towards the AR border. >2" is highest just north of the river from I-35 on east, a testament to the timing of the shortwave arrival and upscale growth of the convective threat as it maneuvers to the east-southeast. Rainfall rates between 1-2"/hr are most likely when assessing the hourly rate probs from the both the NBM and HREF output, a rate assessment that is more than suitable considering the blanket low FFG intervals due to the antecedent moist soils from the past 36-48 hrs of rainfall. The areal QPF footprint of >1" aligned within those lower FFG's pretty much perfectly, a scenario that lends to a greater credence of a touch higher risk category that what is normal the case for this setup. The previous SLGT risk was relatively maintained with minor adjustments on the edges pertaining to recent hi-res QPF and in conjunction with the ML First Guess Fields. ...Interior Mid Atlantic... Shortwave trough currently positioned over the Southern Plains will lift northeast through the Mississippi Valley with sights on the Great Lakes region by the second half of the D1. Downstream, an improved diffluent signature will align over the Ohio Valley and points northeast allowing for regional ascent to boost convective initiation in-of places within that diffluent core. The most notable area for convective development lies within an established theta_E tongue riding northward through the Appalachians Front with poleward expansion into the southeastern Great Lakes. This allowed for moisture rich air exuding PWAT anomalies between +2 and +3 within the corridor extending through much of WV up through Eastern OH and Western PA to the Lake Erie border. SBCAPE between 1500-2500 J/kg will be a common occurrence during peak diurnal destabilization creating a corridor of enhanced thermodynamic prospects to coincide with the arrival of the better mid-level forcing downstream of the approaching trough. Regional shear will be improving as well, especially within the confines of a shallow warm front that will be lifting northward through the course of the afternoon across the interior Mid Atlantic. Cell initiation will begin between 18-00z today with greater multi-cell cluster coverage likely to occur towards the back end of the above time frame due to anchoring along the warm front as it approaches. Areas across Western PA have been hit relatively hard recently with convective rainfall adding to the areal decoupling of the FFG indices meaning the prospects for flash flooding have increased with the arrival of this next disturbance. 00z HREF neighborhood probs for >2" are between 30-50% across a large section of Western PA extending south into the northern High Country of WV, rooted in the Appalachians Front, an area notorious for flashy rivers and smaller towns located within the valleys. There was enough consensus to warrant an upgrade considering the antecedent conditions, atmospheric environmental favorability, as well as the increased probs denoting a locally heavy rain prospect over the region. A SLGT risk has been added to much of Western PA down into portions of the Central Appalachians. Kleebauer Day 2 Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FROM THE SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY... Steady progression of a cold front through the Southern Plains will lead to textbook convergent pattern with an addition of a strong upper diffluence signature within the bounds of the southern jet proxy. High areal theta_E averages will denote the significant instability pattern positioned ahead of the approaching cold front leading to ample thermodynamic assistance for strong convective cores and expected heavy rainfall. Recent CAMs have finally come into view for the primary window of cell initiation with a widespread convective pattern likely to extend from the Stockton Plateau to points east within the I-10 corridor up through all of Hill Country, especially over the I-35 section of south-central TX. HREF blended mean QPF is over 2" now within a large area encompassing the terrain focused across the Central portion of the state. This had led to elevated neighborhood probs of >2" for the 12-00z time frame in the forecast running >70% over pretty much all of the above region extending into portions of East TX. The East TX corridor has become a newer development in terms of anticipated QPF magnitude as the the CAMs and in some instances the global deterministic are now catching on to the deep moisture advection regime poking north out of the Western Gulf ahead of the front arrival. Forecast soundings across the area between Houston up to the latitude of Austin are now indicating MUCAPE or the order of 3500-5000 J/kg prior to any front arrival leading to significant heavy cores to transpire due to cap breach from diurnal destabilization initially, followed by more organized multi-cell clusters as we move into the evening as the LLJ re-intensifies and the cold front motions into the region. Soundings also note a period of mean flow and upshear components that would lead to potential backbuilding within the confines of the front. Very strong ascent between 850-300mb is also noted within multiple CAMs outputs, a testament to the favorability for the setup to become more organized and providing a vast areal expanse of heavy convection once the setup matures. PWATs between 1.6-1.9" with locally >2" are being forecast along and ahead of the front as it moves into Central TX, a forecast that traditionally is favored for greater flash flood prospects, especially over the favored I-35 corridor between Austin to San Antonio where high runoff capabilities are found due to soil type and limestone foundations. Local totals between 3-6" are likely over the entire stretch of Hill Country through East TX with the eastern most extent even shifting into Western LA as the convective pattern evolves and grows upscale entering into the adjacent Lower Mississippi Valley. Houston metro is now in play for convective impacts, especially as thunderstorms to the north provide outflow generation that could send cold pools south towards the Gulf coast with additional convective development along the edge of any approaching cold pool. The previous SLGT risk was relatively unchanged across Central TX. Some expansion to the west was made into the Stockton Plateau where initial cell formations will likely occur as the front arrives and interacts with a formidable instability axis aligned over the plateau itself. The SLGT was expanded southeast to cover the Houston metro and points north where there is growing consensus on convective impacts over and near the urban center. This is a high- end SLGT over much of Central TX, especially Hill Country to the I-35 corridor and points east. A risk upgrade is not out of the question in subsequent updates given the environmental favorability and flashy history of places expected to see the greatest thunderstorm coverage/rates. Kleebauer Day 3 Valid 12Z Sat May 03 2025 - 12Z Sun May 04 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE APPALACHIAN FRONT AS WELL AS PORTIONS OF THE GREAT BASIN... ...Appalachians... An anomalous upper low center evolving over the Ohio Valley will generate a strong diffluent signature downstream across the Appalachians and points east with a significant moisture fetch poleward from the Gulf, and eventual Atlantic Basin by the end of the period. Organized convective activity will brew within the RER of a strengthening upper jet core located over Ontario/Quebec as the upper pattern becomes closed off to the west after 00z/Sun. NAEFS PWAT anomalies between +1 and +2 deviations will offer a suitable moisture rich environment conducive for stronger heavy convective cores capable of locally enhanced rainfall prospects within the confines of the great diffluent axis. As of this time, this places the Appalachians Front from Northern GA up through PA as the primary region of interest, however some elevated instability notions are present within some of the recent deterministic output. As of this time, a general 1-2" is forecast within the above corridor with some heavier cells even possible as far south as the Central Gulf Coast, but those are more aligned within the decaying surface front that is progged to breakdown before exiting the coastal plain. The area of focus will reside within that broad ascent pattern enhanced by the evolving amplified pattern to the east of the Mississippi with the complex terrain providing a secondary element that would favor isolated flash flood prospects. As a result, a broad MRGL risk extends from the Southern Appalachians up through Northeast PA where the current ensemble mean QPF is highest while also residing within the lower FFG indices available in the Eastern CONUS. ...Great Basin... Omega block pattern initiating over the CONUS will lead to yet another amplified upper low generation across the Western CONUS through D3. The threat for elevated convection in-of interior NV up into Southeast OR have been forecast for several days with the deterministic guidance basically "locked in" to the expected evolution. Despite rainfall rates rather low grade compared to what you normally find for flash flooding, this area has a notorious low FFG bias that tends to accentuate the flash flood prospects in this area of the CONUS. Recent ensemble means have been hovering between 0.75-1.25" of total QPF within the corridor extending from Reno to points northeast with local maxima hovering up near 2" due to the added effect of greater upper forcing thanks to the maturing closed ULL reflection. This setup has been steady within the First Guess Fields for the threat and really hasn't wavered much in the MRGL risk output from the ML. Considering the factors above and a pattern more favorable for these convective schemes across the Great Basin, the previous MRGL risk inherited was maintained with a small expansion northward into Southeast OR to highlight QPF trends favoring better totals within those zones. 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