Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
244 FOUS30 KWBC 301550 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Tue Apr 30 2024 Day 1 Valid 16Z Tue Apr 30 2024 - 12Z Wed May 01 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST KANSAS AND NORTHERN AND NORTHEASTERN OKLAHOMA... ...16Z Update... Generally only minor tweaks were made to the inherited forecast. The large scale pattern described in the previous discussion below remains valid. ...Kansas and Oklahoma... The primary change made in this area was to expand the inherited Slight Risk area south to include much more of northern and northeastern Oklahoma with this update. CAMs guidance is in good agreement that convection that forms in Kansas and Oklahoma this afternoon will congeal into an east-west oriented line of storms largely on the Oklahoma side of the border this evening, persisting well into the overnight hours. The line itself may be nearly stationary for a few hours before slowly weakening and dissipating in the predawn hours Wednesday. There remains considerable uncertainty, as is typical, with exactly where the line of storms forms and how quickly it moves to the south, so the inherited Slight in Kansas was left largely intact, though the flash flooding threat there has diminished somewhat. Northern and northeastern Oklahoma was hit hard in recent days with multiple inches of rain from strong storms, so this area also has elevated river levels and reduced FFGs, making it more vulnerable to flash flooding. The storms are likely to focus in a relatively small area that in a north-south sense should only cover a row or 2 of counties. Both the expected small size of the highest risk area and inherent uncertainty with the ultimate behavior of the expected convection precluded any consideration of a further upgrade. However, should the storms impact the hardest hit areas, locally significant and considerable flooding could occur. ...South Florida... A Marginal risk area was introduced for portions of south Florida. Afternoon convection is expected to form in southwest Florida this afternoon as PWATs spike above 1.5 inches in weak steering flow. A frontal system is approaching the state`s Gulf Coast now, and when added to the moisture-rich atmosphere in place across the state, slow-moving storms are likely to develop, then gradually shift east towards the I-95 corridor. Conditions have been drier than normal across much of the Florida Peninsula lately, which should help preclude any widespread flash flooding, but slow moving storms in a moisture-abundant atmosphere have the potential to produce rates that may locally exceed FFGs. For this reason, a low-end Marginal was needed. ...New York and Pennsylvania... The Marginal Risk area in the Northeast was trimmed out of the Hudson Valley with this update. Afternoon convection is likely to form on the west side of the Catskills. As the storms grow upscale, they will encounter a faster steering flow in the upper levels, which will help accelerate them eastward. So while rates may exceed FFGs, the time at which those rates will be realized will decrease with time. Thus, the Marginal remains for the portions of NY and PA that are most likely to see the storms initially forming, before they move too fast to be much of a flash flooding threat. ...Elsewhere... For South Texas and the upper Midwest, no significant changes were made, see the previous discussion below for more details in these areas. Wegman ...Previous Discussion... ...Kansas and Oklahoma... A surface cyclone will develop over the northern plains with a cold front extending along the southern periphery of the surface reflection. A strong southerly component downstream of the surface low will allow for a pool of elevated theta-E`s to advect poleward into eastern KS and Oklahoma during the late afternoon time frame with an associated increase in regional instability within the axis of higher theta-E`s. There is a growing consensus for convective initiation across east-central KS towards the 23-01z period where the LLJ kicks and the aforementioned cold front converges on the region with rapid CI likely somewhere north of Wichita. A moisture rich environment thanks to low-level moisture advection coupled with primed instability in-of the area will lead to locally heavy rain within any convective development, especially under more prominent supercellular modes thanks to increasing shear within the area of positive buoyancy. 00z HREF blended mean QPF has really become bullish with totals approaching 2.5-3" now across that area in and north of Wichita with some guidance expanding further as cold pool propagation to the south would expand some pretty hefty totals to areas that were just recently impacted. Rates are likely to approaching 2-3"/hr based on the latest HREF probability fields (30-40%) which would easily eclipse the lower FFG indices in place after the recent deluge. The area near the KS/OK border is conditional to the cold pool propagation and the cold front expecting to struggle to gain latitude leading to an elongated front where convection can situate through the end of the period. Some models are aggressive with totals exceeding 3" if that were to occur, so the prospects for higher end impact lead to the addition of a SLGT risk over the area where the threat is maximized. ...Northeast... A stalled frontal boundary is analyzed across northern NJ, extending west through northern PA, southern NY, into Ontario. A cold front is currently moving through the Great Lakes in response to surface low moving through the northern Midwest. The stationary front, cold front, and attendant mid-level energy will all aid in convective development this afternoon with initiation over western NY and northern PA, moving eastward through peak diurnal instability. Consensus for scattered thunderstorms has grown across all guidance leading into the period with the latest CAMs persistent on an axis of locally heavy rainfall impacting areas from the Finger Lakes, eastward into the Hudson Valley. 00z HREF blended mean QPF footprint depicts a large swath of 1-1.5" precip totals in-of NY state with the northern fringes of PA out near Bradford and Susquehanna also within the mean QPF swath formed via the convective regime expected. Hourly rates will be the primary driver for localized flooding across the area in question with HREF probabilities for 1"/hr now topping out at 50-60% from a line extending between Utica down to northeastern PA with the time frame between 18-00z as the period of interest for convective initiation and impact. 2"/hr rates are non-zero, but limited in spatial coverage with probabilities peaking closer to 10-15% over portions of southern NY state. The area of interest will lie along the stalled front where the best low-level convergence axis will reside, as well as where the favored mid-level vorticity maxima advects overhead. Where these two align will likely lead to totals approaching 2" within a course of 1.5-2 hrs before convection steadily moves eastward. Areas with more urbanization will be subject to the best flash flood potential due to runoff, as well as areas that can pick up a quick 1.5-2" when impacted. The MRGL risk was maintained from the previous forecast with some minor adjustments on the fringes. ...Midwest to Upper Mississippi Valley... A quick-moving shortwave will eject out of the southern periphery of a negatively tilted longwave trough over the northern Plains and southern Canada leading to convective genesis over the northern and central plains. The latest HREF and associated CAMs highlight the potential within the mean QPF forecast with a swath of 1-1.5" possible over portions of MN/IA/Northwest WI with a signal for local 2-3" amounts given the potpourri of deterministic. There is less consensus on exactly where the stronger cells will reside, but the initiation point across southeastern SD and eastern NE is fairly consistent on guidance. The threat is low to mid-tier for the MRGL risk, but probabilistic signals for at least 1"/hr and up to a quick 2-3" are enough for some local impacts to areas who have substantially lower FFG indices thanks to the previous series of storms. The MRGL risk was maintained with little deviation from the previous forecast. ...Southern Plains... A dryline will focus over west TX into OK during the afternoon today with scattered convection likely to fire after 20z within the Stockton Plateau and portions of north TX towards the Red River. Precip rates will be capped between 1-1.5"/hr which is enough for some localized flood concerns in small towns with urbanization factors that could enhance runoff potential. The threat is on the lower end of MRGL, but resides within the scope of the threshold of 5%. Kleebauer Day 2 Valid 12Z Wed May 01 2024 - 12Z Thu May 02 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL IS FORECAST OVER PORTIONS OF THE MIDWEST & CENTRAL/SOUTHERN PLAINS... ...Midwest/Central Plains... A broad, closed upper level cyclone will persist over eastern MT into the adjacent northern plains and southern Canadian Provinces by Wednesday with a potent, mid-level shortwave rounding the base of the mean trough and exiting into the northern high plains by the second half of the period. A vigorous upper diffluent pattern will transpire by Wednesday afternoon and evening with a corridor of convection initiating towards nightfall. A stout LLJ will enhance areal shear in-of the central and northern plains within the Quad state area between NE/KS/IA/MO. Any convective development will congeal and grow upscale into a large cluster of heavy thunderstorms within the above area leading to widespread heavy rains capable of flash flooding as rates likely reach towards that 1-2"/hr marker with locally higher possible. The one factor that may limit the higher end potential will be the progressive nature of the convection as pattern yields faster storm motions limiting the prospects for training. This does not mean there will not be some areas that can overachieve and maximize the potential based on the prime mid-level energy involved, along with a strong meridional push of some very moist air approaching 2-2.5 deviations above normal for PWATs based on the latest GEFS and NAEFS output. The threat remains solidly within the SLGT with a chance at any upgrade if future CAMs indicate a better potential for totals to exceed 3" in a larger footprint than what is currently depicted. ...Southern Plains... The location on a secondary rainfall maximum across TX is becoming more clear as a quick moving shortwave embedded within the subtropical jet ejects out of MX and moves to the northeast by Wednesday evening. Guidance suggests that somewhere across eastern TX would be the primary target for convective development with the heaviest rainfall located within the tongue of higher theta-E`s positioned east of the line from ABI to DRT. A cluster of thunderstorms will develop over west TX with a steady propagation to the east during the afternoon and early evening hours. By nightfall, a prominent LLJ aligned across central and east TX will provide necessary maintenance to any convective clusters that will migrate over the state after peak diurnal instability. Surface dew points running into the upper-60s and low-70s relay the moisture rich environment that will provide the foundation for significant rainfall potential as the mid-level vorticity maxima continues to move over the region providing the additional large scale forcing that will keep the convection rolling through the evening. Current deterministic output is consistent with local totals breaching 4" across portions of central and north TX with the latest ensemble bias corrected QPF sufficiently in the range of 2-3" over an area extending from southwest OK down through the DFW metroplex and adjacent surrounds. This is quickly gaining steam for a higher impact event, especially when you compound the idea that the northern portion of the I-35 corridor in TX becomes a focal point. There is some spread on the exact positioning of the heaviest precip expected, but ML outputs have been persistent in the area over north TX for the past succession of runs. Should deterministic guidance converge further with similar amounts, or if ensemble support becomes more aggressive with totals and probabilities for exceeding higher end QPF markers, a Moderate Risk cannot be ruled out in later issuance`s. Moderate Risk impacts are certainly possible wherever the heavy rainfall materializes with the highest prospects within the major urban centers along I-35. Kleebauer Day 3 Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL IS FORECAST OVER MUCH OF THE MISSISSIPPI VALLEY EXTENDING FROM THE MIDWEST DOWN INTO EAST TEXAS, AND LOUISIANA... ...Summary: Two distinct areas of interest will arise for heavy rainfall on D3 as a strong mid-level shortwave continues to march eastward out of TX into the Lower Mississippi Valley by the middle of the period. Further north, a powerful, negatively tilted trough will maintain foothold across the northern plains with a strong vorticity maxima pivoting around the southern periphery of the closed upper reflection in place across southern Saskatchewan. Each upper forcing will play a role in locally heavy rainfall impacts spreading eastward from the continuation of the previous period. The primary threat is currently front loaded with the best flood prospects leaning towards Thursday morning and afternoon before the threat wanes heading into the second half of the forecast period. ...Midwest... Overnight convection from Wednesday will continue to march eastward as the congealed cold pools from the initial cell mergers will advance through the central Midwest with sights on eastern IA into WI and MN on Thursday morning. The complex will eventually peter out as it moves northward into a less favorable environment which will put an end to the flooding threat as we move into the afternoon. Later in afternoon and evening, a cold front will propagate to the east out of the central plains with a redevelopment of convection over eastern KS, IA, and northwest MO. This will allow for the secondary peak of flash flooding potential as thunderstorms migrate through the mid and upper Mississippi Valley, impacting some areas that were just affected in the last 24 hrs. With the lower FFGs expected, the threshold for flooding will be much lower than climatological norms for some areas within the expected secondary convective regime. Totals are currently within the 1-3" range on guidance which is significant enough to warrant at least the current SLGT risk in place with some chance at further upgrades pending prior period(s) evolution and precip outcomes. ...Southern Plains and Lower Mississippi Valley... A fairly significant complex of thunderstorms will likely be ongoing in the beginning of the period with the cold pool propagation forecast to impact areas of east TX into Louisiana, similar areas that were just hit recently with significant rainfall and flash flooding. The prospects for flooding are now well above climatological norm as areal FFG indices are solidly 50% of what they normally are during the time of year (2-3" compared to 4-5"). Forecast QPF is hovering between 2-4" with locally higher with the ensemble bias corrected forecast showing a wide swath of 2+" and some deterministic already ramping up over 5" across east TX into north-central LA. Considering the repeated nature of these heavy rain threats, the prospects for flooding is growing with a SLGT risk in place over the Arklatex down into parts of east TX and the Lower Mississippi Valley. Pending the forecast from the previous period with CAM guidance assistance, an upgrade to a Moderate Risk may very well be entertained. Further north across KS and OK, the cold front to the north will move south with a push from high pressure nosing in through the plains in wake of the low pressure moving to the north. An area of convection will likely form over the central plains and move south with the boundary leading to more locally heavy rainfall in association with the area of convection. The threat is less aggressive compared to what is expected further south, but the chances are still prevalent. The SLGT risk was maintained from the previous D4 connecting the two primary areas of interest to the north and south. 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