688 FOUS30 KWBC 041549 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1149 AM EDT Sat Jul 4 2026 Day 1 Valid 16Z Sat Jul 04 2026 - 12Z Sun Jul 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN KANSAS AND OKLAHOMA AND PORTIONS OF NORTHERN ILLINOIS AND INDIANA... 16Z Update: Much of the central to eastern U.S. is in a warm, moist, and unstable air mass, all ahead of a slowly southward frontal boundary that is currently draped across portions of the Upper Midwest to Great Lakes. Several organized clusters of thunderstorms ongoing this morning will be the focus for pockets of heavy rainfall and instances of flash flooding. Ongoing convection across eastern Kansas this morning should eventually subside by early afternoon, but renewed convection later this evening just south of that along the residual boundary will be the focus for heavy rainfall later in the period. No significant adjustments were made to the Slight Risk area there, with probabilities remaining high for localized 3-5" totals through early Sunday morning. Another vort max moving into Illinois this morning will coincide with peak heating over northern Illinois and west-central Indiana to generate another round of thunderstorms capable of producing heavy rainfall. Pockets of 2-4" totals will be possible based off the latest HREF and 12Z hi-res models are in reasonable agreement for the placement of this next round of heavy rainfall. Through coordination with WFO LOT and surrounding offices, a Slight Risk was introduced with this update mainly for the afternoon to early evening hours where scattered instances of flash flooding are expected. Elsewhere across the eastern U.S., isolated to scattered thunderstorms are expected and will be a flash flood concern for mainly urban, low-lying, and other sensitive areas. Some concern for the I-95 corridor from DC to NYC today, particularly across northern NJ where guidance is a bit more bullish. For now, have kept the risk category at Marginal as storms may be more progressive. Taylor ---previous discussion--- Overnight convection over the High Plains should persist and possibly grow upscale as it moves eastward across Nebraska and Kansas. Increasing confidence in the generation of an MCS across the Central Plains, moving southeast into the Lower Mississippi Valley Saturday evening in conjunction with a cold front approach from the north and surface low positioned over the adjacent Southern Plains. Increasing low-level convergence ahead of the approaching front will help sustain/enhance convection. Low-level flow will become backed for a time across the area of southeast KS into northeastern OK ahead of the front according to forecast soundings from various deterministic CAMs. This leads to effective boundary layer shear approaching 35-40kts will be efficient in sustaining stronger updrafts and mesocyclone formations capable of strong to severe cells, even in the case of cell mergers. PWATs between 1.8-2.1" are expected in the area of interest allowing for a deep moist profiles suitable for heavy rain cores. HREF probs for >1"/hr rates are actually prolonged within the temporal aspect of the forecast over the region with a solid 30-50% prob from 00z-12z Saturday across the aforementioned area meaning some back-building behind any organized convective pattern is plausible. Modest 30-50% probs for >3" exist over the area from I-70 west of Kansas City down through Tulsa with the highest probs located over northeast Oklahoma. Isolated low-end probs for >5" also exist, but are generally noting the top-end of the potential for the threat as the progression of the MCS should limit the prospect for more appreciable impacts/coverage. A Slight Risk was maintained although modestly shifted southward covering southeast Kansas and northeast Oklahoma. ...Midwest into the Mid Atlantic... Pulse convection expected over a vast area spanning from the central/north-central states to the Mid-Atlantic/Northeast regions as shortwave energies move through the flow. Any thunderstorm will be capable of producing local rates of 1-2"/hr which could result in isolated cases of flash flooding over a broad area with the focus likely within any urban corridors or complex terrain (Appalachia). A broad Marginal Risk was maintained over the above regions to account for the threat. Kleebauer/Campbell Day 2 Valid 12Z Sun Jul 05 2026 - 12Z Mon Jul 06 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID-ATLANTIC... An east-west orientated front boundary will be sprawled across the Northeast/northern Mid-Atlantic region during this period. Moderate instability combined with PW values of 2+ inches are expected to overlap near the frontal boundary to produce periods of heavy rainfall. This area historically is more prone to flash flooding due to terrain influences and more sensitive hydrologic schemes thus maintaining the Slight Risk for excessive rainfall for this part of the region. Backbuilding and training of thunderstorms may be possible. Scattered flash flood concerns exist over the central Mid- Atlantic with an increasing risk of heavy convection over portions of Pennsylvania down through the Washington DC- Baltimore- Philadelphia corridor. A broad Marginal Risk area spans from eastern Oklahoma/Kansas eastward to the Mid- Atlantic/southern portions of the Northeast. Kleebauer/Campbell Day 3 Valid 12Z Mon Jul 06 2026 - 12Z Tue Jul 07 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID-ATLANTIC/NORTHEAST ON MONDAY... The threat for excessive rainfall and scattered areas of flash flooding will remain for portion of the Mid-Atlantic for this period. Rainfall occurring during the Day 1 and Day 2 periods may lead to increased soil saturation and lowered flash flood guidance for this region, thus making it more sensitive for additional heavy rainfall. Near and just south of this front instability of 2000 J/kg or greater and PWs over 2" are forecast, an environment supporting efficient rainfall and high hourly rates. Stronger low level convergence will be capable of producing a broader area of moderate to locally heavy rainfall across New York into southern New England.; however, with less available instability rainfall rates will likely not peak as high as those further south. Thus, this area is covered by a broad Marginal Risk from portions of Upstate New York to the Cape, south to the Carolinas and then southwest into Arkansas and northern Louisiana. Chenard/Campbell Day 4 and Day 5 Valid 12Z Tue Jul 07 2026 - 12Z Thu Jul 09 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS TO UPPER GREAT LAKES TUESDAY AND WEDNESDAY... Slight Risks of Excessive Rainfall have been introduced across portions of the Northern Plains into the Upper Great Lakes Tuesday-Wednesday. A seasonably moist, unstable airmass will be in place along the northern side of an upper-ridge building over the western to central U.S. early next week. An upper-level wave rounding the ridge will help trigger scattered to widespread thunderstorm development at the leading edge of the deeper moisture over the northern High Plains. Propagation ahead of a developing surface wave along a quasi-stationary frontal boundary and developing low-level jet will help maintain storms into the overnight hours through the northern Plains with the potential for at least organized clusters of storms if not a mesoscale convective system. A similar scenario will play out Wednesday as ongoing/renewed storm development will continue eastward into the Upper Great Lakes. Typical uncertainty on the exact location and coverage of convective development remains a concern particularly as the storm evolution Tuesday will likely impact storm development/evolution Wednesday. However, good consensus amongst the deterministic model guidance on areal average totals in the 1-2" range and locally higher amounts of 3-4" supports the threat for scattered flash flooding with potential areal adjustment of the risk in subsequent outlooks. Putnam 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 $$