FOUS30 KWBC 081945 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 345 PM EDT Wed Jul 8 2026 Day 1 Valid 16Z Wed Jul 08 2026 - 12Z Thu Jul 09 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS THROUGH THE UPPER GREAT LAKES... ...Central Plains to the Upper Great Lakes... ...16Z Update... The inherited Slight Risk remains on track with only minor cosmetic adjustments after evaluation of the morning 12Z guidance. The overlap of a series of weak mid-level impulses with the diffluent RRQ of a departing jet streak will produce robust synoptic ascent atop a wavering stationary front. This lift acting upon elevated PWs in the presence of 30-40 kts of bulk shear will support clusters of convection moving slowly along the boundary, especially this evening as the LLJ ramps up to 25-35 kts. This is agreed upon by the majority of CAMs, and although mean winds of 20-25 kts will keep storms progressive, repeating rounds of 1-2"/hr rain rates will still support scattered flash flooding. ...Previous Discussion... Embedded upper-level shortwaves rounding an upper-high over the southwestern U.S. will lead to increasingly northwesterly flow over a quasi-stationary frontal boundary across the Upper Great Lakes/Midwest. An overnight MCS may still be ongoing through the start of the day 1 period across the Michigan UP/northern Wisconsin west-southwest through southern Minnesota, with the outflow from these storms likely helping to delineate this as the eventual position of the frontal boundary later this afternoon. This will favor renewed development of storms along and just to the south of the boundary from the UP/northern LP of Michigan southwest through central Wisconsin and into southern Minnesota/northern Iowa. Seasonably high PWAT values 1.5-2 standard deviations above the mean will support rain rates of 1-2"+ per hour. The latest hi-res guidance shows the potential for locally heavy rainfall totals of 3"+, backed by moderate (40-60%) HREF probabilities of totals exceeding 3", supporting the risk for scattered flash flooding. The Slight Risk has been extended further to southwest where heavy rainfall is also expected ahead of a developing surface wave along the cold front over Nebraska. This will help focus convergence along the front promoting more widespread storm development with storm clustering/cold pool mergers in the presence of northwesterly flow leading to the potential for upscale growth into an organized convective system. Seasonably high moisture flowing northward on the west side of surface high pressure to the east will pool ahead of the boundary with daytime surface heating leading to moderate to strong instability supporting heavy rainfall rates of 1-2"+. A developing low level jet over the central Plains will also help to maintain storms into the evening hours. Similar to the Upper Great Lakes/Midwest, the latest hi-res guidance supports the potential for locally heavy rainfall of 3"+ and scattered instances of flash flooding There may be a relative minima between these two regions that would not quite rise to the level of a Slight Risk. However, there's enough agreement amongst the guidance that this region would be limited if present at all so felt a full extension of the Slight Risk southwestward was warranted rather than introducing a separate risk area. ...Central/Northern High Plains... A cold front pushing into the central/northern High Plains will provide a focus for convergence as well as moist, post-frontal upslope flow to support scattered afternoon thunderstorms off the higher terrain. Locally higher PWAT values of 1"+ will support heavier downpours with an isolated risk for flash flooding. ...Gulf Coast, Tennessee Valley, into the Mid-Atlantic... The inherited marginal risk wasmaintained from the overnight issuance. A potent and slow moving shortwave will likely help spawn rapid convective development this aftn, with numerous thunderstorms likely, especially from KY/TN east towards VA. A lack of meaningful shear will keep storms of the pulse variety, but with PWs 1.75-2.00 inches and storm motions generally just around 10 kts, rainfall of 2-3" with locally higher amounts is possible thanks to rainfall rates which have a 15-25% chance of exceeding 2"/hr for short durations. The FFG exceedance does peak above 20% in some locations across this region due to compromised FFG from recent rainfall, but CAM simulated reflectivity keeps storms very scattered, supporting the MRGL risk. Farther south towards the Gulf Coast, airmass type thunderstorms will likely become scattered to numerous with 2+"/hr rainfall rates, but high FFG and a lack of organization should prevent more than isolated flash flood concerns. ...Arizona... A few isolated or scattered thunderstorms will develop within a modest surge of PW lifting into AZ this evening. The UA WRF, HREF, and REFS all indicate locally more than 1" of rainfall, but 1-hr FFG exceedance is only around 5%. Opted to not introduce a MRGL risk today due to the uncertainty in coverage and generally pulse environment, but should a vulnerable area experience a slow moving cell with heavy rain, some impacts could occur due to flash flooding. Putnam/Weiss Day 2 Valid 12Z Thu Jul 09 2026 - 12Z Fri Jul 10 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI AND OHIO VALLEYS, AS WELL AS THE MID- ATLANTIC STATES... ...Middle Mississippi and Lower Ohio Valleys... A slow moving cold front will drag southeast from the Upper Midwest into the Ohio Valley and Mid-Mississippi Valley by Friday morning. This evolution will occur in response to shortwaves embedded within generally near-zonal flow, but leading to subtle amplification of the mid-level trough. This will create increasing deep layer ascent, with RRQ diffluence of a weakening and departing jet streak also contributing. The overlap of this ascent on the baroclinic gradient/front will likely result in scattered to numerous thunderstorms tracking west to east across the area. Although confidence in the exact timing and placement of any convection is modest, it is probable (and reflected by available high-res CAMs) that rounds of thunderstorms will occur generally beneath the aforementioned weak impulses. These thunderstorms will blossom within favorable thermodynamics characterized by PWs potentially exceeding 2 inches (approaching daily records and above the 90th percentile of the SPC sounding climatology) overlapping MUCAPE of 2000-3000 J/kg. This will support heavy rain rates of 1-2"/hr, which through multiple rounds has a 40-60% chance of creating stripes of 3+" of rain, despite generally fast motion of 20-25 kts. Due to uncertainty in the exact timing in placement, but the likelihood of multiple rounds of intense rainfall, the broad SLGT risk was expanded, and also pulled north to include the Chicago metro area after coordination with WFO LOT. ...Mid-Atlantic... Farther east into the Mid-Atlantic states, another area of locally heavier rainfall has become more likely as suggested by many of the available CAMs and within the 24-hr neighborhood probabilities from both the HREF and the REFS, and a SLGT risk of excessive rainfall has been introduced for the eastern Mid-Atlantic. A shortwave positioned over KY in the morning will speed northeast bringing height falls and PVA across the area and interacting with a wavy stationary front across VA. This will lead to a weak surface wave moving along the boundary, and this low and its accompanying locally enhanced ascent (through WAA and accelerated moist inflow) will help spawnmore organized convection during the aftn and eve. While storms may move steadily ENE on 15-20 kts of 0-6km mean winds, some training along the warm front is possible as reflected by boundary parallel mean flow and propagation vectors. With hourly rainfall of 1-2" likely (50-70% chance), and short term training could overwhelm soils which have recently become more saturated due to recent rainfall. The threat for excessive rainfall is also supported by modest (but notable for a convective environment) EFI of 0.6 across northern VA and much of MD. The most likely areas for flash flooding will be where any of these rates train across urban/more vulnerable soils, but with HREF probabilities for 5" of rain up to 30%, scattered instances of flash flooding are possible anywhere within the SLGT risk region. ...Central High Plains... A familiar set-up will remain in place Thursday as a frontal boundary becomes quasi-stationary through the region. Moist/post- frontal upslope flow and convergence along the boundary will continue to support storms with locally heavy downpours and an isolated risk for flash flooding. ...Arizona... A plume of elevated PWs potentially exceeding 1.25" will squeeze into southern Arizona Thursday evening coincident with modest but increasing SBCAPE of 500-750 J/kg. This will result in scattered thunderstorm development which is supported by all available CAMs. These storms will move slowly and chaotically within 0-6km mean winds of just 5-10 kts, while the favorable thermodynamics produce short-term rainfall rates locally exceeding 0.75" (HREF and UA WRF support). If any of these rates linger across vulnerable soils or terrain, isolated instances of flash flooding could result, and a narrow MRGL risk was added. Weiss Day 3 Valid 12Z Fri Jul 10 2026 - 12Z Sat Jul 11 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI/OHIO/TENNESSEE VALLEYS AND CENTRAL APPALACHIANS... ...Middle Mississippi Valley to the Central Appalachians... A similar synoptic pattern/setup will exist Friday from the Mid- Mississippi Valley through the Mid-Atlantic states as a wavering stationary front stalls west to east across the region. This front will stall in response to generally zonal (weak troughing) mid- level flow, within which ripples of shortwave energy will traverse. Ahead of this front, moisture will continue to pool as persistent W/SW 850mb flow of 20-25 kts surges Gulf moisture northeastward, leading to a plume of PWs reaching 1.75 to 2.00+ inches. Moisture transport within this flow is reflected by impressively long transport vectors which converge strongly into the boundary, suggesting additionally enhanced ascent from Missouri through West Virginia, which will also be within a steep MUCAPE gradient of 1000-3000 J/kg. This pattern will support another day of scattered to numerous thunderstorms containing 1-2"/hr rain rates. The training risk on Friday appears a bit stronger than Thursday due to boundary-parallel flow along the steep CAPE gradient, with enhanced bulk shear of 20-25 kts supporting greater organization into clusters. The SLGT risk has been maintained and cosmetically adjusted for the latest guidance (generally to cover the ECENS/SREF probabilities for 3"/24 hrs). It is possible that an enhanced/slight risk area may be needed where rainfall D3 overlaps vulnerable soils from rainfall on D2, but confidence is not high enough in either coverage or location to introduce this yet. ...Central High Plains... Farther west, the tail of this lingering front will finally push out of the High Plains and into the Central Plains through Friday. Convergence along this front into favorable thermodynamics to support heavy rain will create at least an isolated risk for flash flooding due to training of 1-2"/hr rain rates on aligned Corfidi/Mean wind vectors. This continues to support the inherited MRGL risk with only modest cosmetic adjustments for recent guidance. Weiss Day 4 and Day 5 Valid 12Z Sat Jul 11 2026 - 12Z Mon Jul 13 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO/MID-MISSISSIPPI VALLEYS AND CENTRAL APPALACHIANS ON SATURDAY... There is a Slight Risk of excessive rainfall over portions of the Ohio and Mid-Mississippi Valleys eastward into the central portions of the Appalachians on Saturday as shortwave energy aloft drops into the Mid- Mississippi Valley on the east side of a building upper-level high over the Western US. A quasi-stationary front at the surface separating cooler/drier air to the north of the front from a moisture-rich atmosphere to the south will be acting to focus the convection. Locally intense rainfall rates and heavy to potentially excessive rainfall amounts are possible in areas where precipitable water values range from 1.8 to 2.2 inches as the shortwave aloft tracks eastward...reaching the central Appalachians by the end of the Day 4 period at 12/12Z. Conditions become somewhat less favorable for organized heavy to excessive rainfall on Sunday as the upper forcing pulls away but enough moisture/instability remains in the low levels...especially at time of maximum daytime heating...to support a broad and largely unfocused Marginal Risk in the area from parts of the Mid- Mississippi Valley across the Southeast US. Introduced a Marginal Risk area across parts of Arizona and New Mexico on Day 5 as moisture continues to get drawn northward. Precipitable water values on the order of 1.75 inches nose into southwest Arizona by Sunday. This Marginal risk area is an extension of the Marginal Risk area on the Day 4 time frame already in place. Bann 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 $$