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615 FOUS30 KWBC 101543 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1143 AM EDT Thu Jul 10 2025 Day 1 Valid 16Z Thu Jul 10 2025 - 12Z Fri Jul 11 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NEW ENGLAND, THE MID ATLANTIC, AND THE CENTRAL PLAINS INTO THE MIDWEST... ...New England... The earlier flash flooding over southern New England has largely waned over portions of Massachusetts as expected with the threat of intense rain rates and flash flooding subsiding. The Slight Risk was therefore dropped. Locally heavier rain will still be a threat for a few more hours, but any flash flooding will be isolated and likely confined to mainly urban areas. For Vermont, a weak shortwave trough combined with modest amounts of moisture and growing instability should support scattered to numerous showers and thunderstorms through the early evening across much of Vermont today. Storm motions will be slow, at least initially, and will be capable of intense rain rates between 1-2"/hr at times. The overnight CAMS and now the latest 12Z runs do show potential for pockets of 2-4" totals with enough coverage to warrant an introduction of a Slight Risk for scattered instances of flash flooding. ---previous discussion--- A Slight Risk was introduced overnight to portions of east-central MA and surrounding portions of RI/CT, as highly anomalous moisture (PWs near 2.0", near the 99th percentile per CFSR climatology) has been supporting very efficient (but so far quite localized) hourly rainfall totals of 1-2" (with as much as 1" in 15-min). This is occurring on the northern/western periphery of an MCS tracking that is tracking across southeast MA, supported by idealized placement in the right-entrance region of a ~100 kt jet streak (at 250 mb) centered over northern New England/southern Quebec. While the best low-level (925-850 mb) moisture transport with the LLJ is located along/offshore with the MCS itself, this jet should veer into the morning hours and allow for a short period of better moisture transport into eastern CT/MA and RI. This should continue the risk of scattered flash flooding at least a bit beyond 12z, as RAP isentropic analysis indicates some of the best moisture transport (isentropic lift/upglide) through the 925-700 mb layer (295-305K isentropic surfaces). This upglide and resultant slantwise instability may be sufficient to overcome a lack of vertical instability for a period (though RAP analysis is still indicating a bubble of 500-1000 J/kg of SBCAPE at 08z). The tail end of this upglide may be positioned near the Boston metro area between 12-15z, and some CAMs suggest the potential for localized 2-4" totals from these training showers (as indicated by both the 00z HREF and experimental RRFS PMM QPF fields). Given 3-hr FFGs in the vicinity of Boston near 2.0", there may be a localized instance of more significant flash flooding (primarily in more vulnerable low- lying areas of the metro). ...Mid-Atlantic and Appalachians into the Southeast... The Slight Risk was extended further across southeast Virginia where probabilities for impactful rainfall totals and intense rain rates have come up since the overnight cycle, otherwise the rest of the Slight Risk across the Carolinas and central VA looks in good shape given the expected coverage and intensity of thunderstorms this afternoon/evening relative to the antecedent soil conditions as well. ---previous discussion--- A shortwave trough situated over southern Quebec begins to lift eastward today, but is slow to do so as it slogs through larger scale ridging. Vorticity streamers trailing the upper trough may allow for some convective organization today, though the highest coverage and best rainfall rates are expected to be relegated to southern portions of the Mid-Atlantic and into the Southeast (where anomalous tropospheric moisture will also be on the downtrend, but PWs still near 2.0" are around the 90th percentile per CFSR climo). Ridging is expected to amplify in the wake of the departing SW, and this will eventually result in height rises aloft that will be less amenable to organized deep convection. As a result, expect a fair bit less convective coverage today overall (particularly in more northern portions of the Mid-Atlantic, perhaps with the exception of the far northern Appalachians in NY/VT/NH where the closer proximity of the lifting shortwave may provide better localized uplift via DPVA and height falls with peak daytime heating). The inherited Slight risk was generally maintained for much of VA/NC and into SC (though the southern eastern portions of the Slight have a greater risk of localized 5" exceedance, per 00z HREF neighborhood probs near 10%, while the northern/western portions are more sensitive with lower FFGs having had localized 3-5" totals over prior days). ...Northern and Central Plains into the Upper Midwest... Convective organization has been quite impressive overnight across SD/NE/MN/IA, as storms tapped into the favorable left-exit region of a subtropical jet streak (~90 kts at 250 mb) on the northern periphery of an upper-level ridge that has been situated over the Southwest U.S. and northwestern Mexico. The main shortwave responsible for this jet streak has yet to move into the Plains, but will do so today as it digs from the Intermountain West into the Central Rockies and High Plains. Convection should fire once again today, likely organizing and growing upscale over the Central Plains with peak daytime heating this afternoon and evening. CAMs suggest these storms may train favorably from west-to-east over portions of NE/IA, while farther east an MCV from the storms overnight will likely reinitiate convection with daytime heating with the potential for backbuilding storms resulting in training and repeating along a north/south axis from northwest IL through southwest WI (though confidence in this scenario is a bit lower, but has been impressively indicated for days by the ECMWF and its accompanying AIFS solutions). PWs are generally near or above the 90th percentile across this whole region, though rainfall is badly needed in some locations where moderate to severe drought is present. Unfortunately too much rainfall too quickly may result in scattered instances of flash flooding, as 1-2"/hr rates drive a localized threat for 4-6" totals where storms are able to train and repeat. Churchill/Taylor Day 2 Valid 12Z Fri Jul 11 2025 - 12Z Sat Jul 12 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE UPPER MIDWEST AND INTO PORTIONS OF THE MID-MISSISSIPPI VALLEY AND EASTERN CENTRAL PLAINS... ...Upper Midwest into Mid-Mississippi Valley... Another shortwave trough is progged to dig southward from south- central Canada into the Northern Plains, merging with the separate aforementioned shortwave that stalled out near the Central Plains and Mid-Mississippi Valley. This will result in a much larger area of organized convection on Friday and Friday night, and the best axis looks to be positioned from central/eastern IA into southwest WI (as this is the best model consensus and positioning of the ECMWF and AIFS in particular, but there is still considerable uncertainty with regard to the nuance of how these shortwaves evolve/interact and where the highest totals ultimately fall). Similar to Day 1, anomalous moisture (PWs of 90th percentile or higher) looks to be in place prior to the arrival of drier, much more stable air behind the accompanying cold front (which is expected to sweep through the Northern Plains and into the Central Plains and Upper Midwest late into the overnight). While much of the rainfall will be beneficial for the region overall, the inherited Slight Risk was expanded rather significantly. Future targeted upgrades to Moderate Risk are possible, depending on what transpires on Day 1 and how much overlap there is with additional localized totals of 3-5" on Day 2. ...Appalachians and Mid-Atlantic into the Southeast... As mid-level troughing becomes more established in the Northern Plains into Day 2, and ridging/height rises build in response downstream and may finally begin to suppress convection more significantly across the Mid-Atlantic into the Northeast. While overall intensity and coverage of convection should be drastically lower compared to prior days, convective initiation is still expected across the higher terrain of the Appalachians and along the sea breeze of the Carolinas and bay breeze of the Chesapeake. Some convection may propagate and/or initiate in areas inbetween, and maintained a Marginal Risk as a result (also including areas of the Southeast where slow moving, largely short-lived storms with efficient rainfall rates of 1-3"/hr may present a localized flash flood risk as well). ...Rockies into the Central and Southern High Plains... As the Southwest upper-level ridge erodes tonight into early Friday, there should be substantially more convective coverage across much of the Rockies and into the Central and Southern High Plains coincident with peak daytime heating. Organization looks most likely over eastern CO and adjacent portions of WY/NE/KS, but may propagate into the Southern High Plains as well (as northwest flow becomes established with shortwave impulses rippling south, getting tugged around the upper-level ridge retreating westward towards Southern California). Localized 1-2" totals (which have been continually indicated by both downscaled deterministic GFS/ECMWF solutions and ensemble 1" exceedance probabilities) in the sensitive terrain may lead to localized flash flooding concerns. Should more substantial convective organization occur (as the introduction of more CAMs seems to be indicating), then localized 2-4" totals are possible (and may necessitate a Slight Risk being introduced with subsequent outlooks). Churchill Day 3 Valid 12Z Sat Jul 12 2025 - 12Z Sun Jul 13 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE SOUTHERN PLAINS INTO THE OZARKS... ...Southern Plains into the Ozarks... Concerns are growing for more organized convection developing into Day 3, stemming from a subtle shortwave/upper-level low forming and stalling out over the Southern High Plains. The bulk of the global guidance is now indicating this scenario, and the ECMWF suite (including the ECENS and AIFS) are of particular concern with the most robust signal of the models (with the AIFS indicating a southward shift of best QPF signal, quite close to more sensitive areas of central/North TX). A large Slight Risk was introduced to encompass the area where organized convective activity is expected to occur, though many of the details will still need to be ironed out as we enter the hi-res CAM period. In the meantime, the combination of already saturated soils and terrain sensitivities across the broader region with highly anomalous tropospheric moisture (PWs of 2.0"+ expected, near 90th percentile) is justification for a broad Slight Risk that can be more fine tuned in subsequent cycles. ...Upper Midwest, Great Lakes into Ohio Valley... Convection should become less abundant into Day 3 in association with the consolidated shortwave from Day 2, as it lifts into Canada and forcing becomes less impressive overall (though still located within a favorable right-entrance region of an attendant jet streak). Daytime heating will likely drive the main threat with short-term localized totals of 1-3" possibly resulting in isolated flash flooding. ...Appalachians and Mid-Atlantic into the Northeast... A broad Marginal Risk area continues into Day 3, as localized downpours (1-2" hourly amounts) in association with daytime heating present an isolated risk of flash flooding similar to the prior day. Coverage and intensity may increase a bit, based on the consensus guidance, but remains questionable as ridging aloft may continue to tamp down on convection. Churchill 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