Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 08 2023 00:52:15 FOUS30 KWBC 080052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Fri Jul 07 2023 Day 1 Valid 01Z Sat Jul 08 2023 - 12Z Sat Jul 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE SOUTH-CENTRAL PLAINS... ....South-Central Plains to Mid-South... Slight Risk is maintained for activity moving off the southern Rockies and the large, bowing MCS over the TX/OK Panhandles that will continue moving east overnight. This is a progressive system, but bookends to the bows will continue to prolong heavy rain and lead to isolated to scattered flash flooding overnight. Recent HRRRs seem on track for overnight precip of generally 1 to 2.5" along and south from the OK/KS border to the Red River from this potent MCS. Please see MPD 657 and subsequent MPDs for further information. Scattered activity over southern MO should congeal overnight as it tracks southeast through northeast AR and Memphis. 25kt mean layer flow should keep this activity fairly progressive, but recent HRRRs maintain a risk for 1.5 to 3" QPF overnight which could lead to instances of flash flooding. The Marginal Risk was maintained over this area and MPDs will be considered. ....Northeast... Ample moisture (+2 sigma PW) and lingering instability ahead of a frontal wave near the east end of Lake Ontario will allow localized heavy rain to continue this evening over interior portions of New England and the northern Mid-Atlantic. A Marginal Risk is maintained over the areas with a bit of shift east given an axis of 1000 J/kg MLCAPE over central/southern New England as of 00Z. Please see MPD 656 for further information. ....Central Mid-Atlantic through eastern Carolinas... The risk remains for isolated incidents of flash flooding in ample moisture with PWs exceeding 2" and instability, particularly over eastern NC this evening which warranted adjusting the Marginal Risk east. Please see MPD 658 for further information in NC. Over the central Mid-Atlantic, instability lingers overnight and continued low shear should allow slow movement of heavy thunderstorms that develop ahead of the approaching cold front which is currently just past the crest of the central Appalachians. ....Northern Plains... Fairly organized thunderstorms over eastern SD/western MN will continue moving south this evening where the Marginal Risk is maintained. PWs of 1 to 1.25" are not anomalous, but some localized repeating activity in this area of lower FFG warrants maintenance of the Marginal. ....North-central Rockies... Scattered moderate convection in central WY to northern CO where the PWs are 1 sigma above normal and topographical lift/localized flow may lead to instances of flash flooding this evening. ....Midwest... Was able to retract the Marginal South a bit over the Midwest to be south of the mid level low apparent in regional radar over central IA. This pushes the overnight heavy rain risk south of Chicago which was removed from the Marginal. Jackson Day 2 Valid 12Z Sat Jul 08 2023 - 12Z Sun Jul 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL/SOUTHERN PLAINS INTO THE MIDDLE MISSISSIPPI VALLEY... ....20Z Update... ....Plains to Mid-MS Valley... A few small changes were made for this afternoon's update. The Slight Risk area from the Plains to the Mid-MS River Valley was trimmed on the northeastern side in IN, which has somewhat drier antecedent conditions, such that the heavy rain associated with the thunderstorms should result in less widespread flash flooding. Meanwhile, in hard hit OK, there are very favorable antecedent conditions but less total rain expected. The heaviest rains are expected around the OK Panhandle and the southwest corner of KS, and also over northern MS and western TN. Guidance trends have been slowly drifting southward, so the northern end of the Slight was trimmed, where very little if any rainfall is forecast, whereas expanded a row of counties southward to include the northernmost LA counties. ....Mid-Atlantic to Northeast... The Marginal Risk area across the Northeast was expanded southward to cover the Piedmont of central NC and northeastern SC with this update. Locally heavier convection is expected across the Carolinas on Saturday as a subtle shortwave riding the southeastern periphery of a negatively-tilted longwave trough over the Midwest tracks across the Carolinas during peak heating. PWATs will be over 2 inches, or about 2 sigma above normal, which will provide plenty of fuel for any developing storms to be capable of heavy rains with local rainfall rates exceeding 2 inches/hour. High FFGs should prevent some flash flooding from occurring, but if storms move over cities such as Raleigh with locally lower FFGs, then the flash flooding threat from those storms will be considerable. ....Southeast FL... A small Marginal Risk area was introduced across the urban corridor of Southeast FL with another round of afternoon sea breeze convection with abundant moisture available to produce locally heavy rainfall resulting in an isolated flash flooding risk. Wegman ....Previous Discussion... ....Central High Plains to Mid-Mississippi Valley into Ohio Valley... Retrograding ridge in the southwest starts to amplify into the Four Corners concurrent with global scale trof dropping south with sharpening base across the Upper Midwest into the Great Lakes.=20 The shortwave at the base of the trof has been tapping some broad southwesterly flow to advect above average moisture back through the Ohio Valley increasing to 1.75". Otherwise, the latest in a parade of overnight MCS should be weakening at the start of the period across MO/AR bringing along with it, the highest concentration of Theta-E and deep moisture up to 2" into the Mid-Mississippi Valley area. Both waves will be generally weaker and more progressive, but will support redevelopment through Saturday evening into the overnight hours but with weaker/less focus allowing for numerous clusters of strong thunderstorms to progress across the Ohio, W TN and Lower MS Valleys supporting intense short-term rates over 2" but likely spotty clusters of 2-4". Upstream, another upslope day is likely to support return moisture through SE CO and with backing LLJ, new thunderstorms will develop in the lee of the CO Rockies. Moisture is going to be less robust and flux will be reduced compared to prior days, but scattered 1-3" streaks and short-term training in northwesterly flow is likely to once again induce localized flash flooding potential across SE CO, SW KS into N OK. As such, in combination across the OH, TN and MS Valleys, the inherited Slight Risk of Excessive Rainfall has been left unchanged though a small southward shift was made to the West while expanding the surrounding Marginal for the potential for a stronger cluster of thunderstorms across NE NM into the TX panhandle, as increasing members of guidance suggest more robust signal. Through the Tri-River Valleys, the Slight was expanding ever so slightly eastward to overlap slightly faster convective solutions into more rough/lower FFG terrain of SW OH, central KY.=20 .....Mid-Atlantic into New England... More active thunderstorms in the D1 period and slightly faster transition of shortwave energy northward into eastern Canada, has narrowed the moisture plume across New England toward the Mid-Atlantic as the shortwave ridging over the northwest Atlantic remains stubborn/amplified. The moisture axis has narrowed, but a narrow ribbon of 1.75-2" of moisture remains across N ME to the Megalopolis from NYC to DC. Southerly 850mb flow will be intersecting westerly down slope flow to support a narrow corridor of stronger ascent and scattered thunderstorms, but given the narrowing of the plume, the overall coverage should be reduced.=20 Very intense rainfall is likely to occur under these slow moving cells and spots of 2-3" are probable. However, there is limited consensus in placement of increased activity or higher magnitudes to support placing a Slight Risk at this time, though one may be warranted given the potential, magnitude and placement particularly relative to urban areas, lower FFG and areas affected in the D1 period. Gallina Day 3 Valid 12Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN OKLAHOMA TO THE WESTERN FLORIDA PANHANDLE AS WELL AS THE MID-ATLANTIC INTO NEW ENGLAND... ....20Z Update... Overall changes to the ERO for this afternoon's update were also relatively minor on D3/Sunday. From the Plains to the central Gulf Coast, expect an MCS ongoing Sunday morning in OK to track southeastward, clearing a line of storms that develops out ahead of it across the lower MS Valley on down to the central Gulf Coast. The storms out ahead of the MCS will themselves be slow-moving and capable of heavy rain before the heaviest rains associated with the MCS move through. A perhaps much more impactful round of rain is expected across the Northeast as a large negatively tilted shortwave trough rotates eastward into the Northeast from the Midwest. A corridor of heavy rain featuring training convection and above normal PWATs is likely to develop near the I-95 corridor from DC to NYC, then up the I-87 corridor through western New England. This area has already been hard hit from rains ongoing today and on previous days, but Sunday is shaping up to be considerably wetter on average and over a larger area than all the previous days, which mainly featured scattered thunderstorms. Sunday's rain looks to be frontal in nature, but the storms will be riding largely parallel to the front moving northward, which drastically increases the chances for training storms and a prolonged period of heavy rain. The area most likely to see those heavy rains will be from Philadelphia north through fear eastern NY and into western VT. This portion of the Slight risk is considered on the higher end of the risk category. While the heaviest rainfall for now appears to remain west of NYC, with an areal average of over 2 inches of rain expected in the city, likely to fall within a 6-12 hour period, the city is also included as part of the higher end of the Slight Risk category. Any additional eastward shift in the axis of heaviest rainfall would likely require an upgrade to a Moderate Risk from the city west through northern NJ, which remains a distinct possibility at this point. Meanwhile further south across the DelMarVa, antecedent conditions are also favorable for flash flooding across the DC and Baltimore metros, which have been hard hit in recent days, but less rain is expected here as compared with areas further north. Wegman ....Previous Discussion... ....Southern Plains to Deep South/Central Gulf Coast... As D3 period starts, the Southwest U.S. Ridge continues to strengthen and broaden centered along the New and Old Mexican boarder presenting broad strong northwesterly flow with 70-90kt 250mb upper-level jet streak from the Northern Rockies across becoming increasingly diffluent across the Southern Plains into the Lower MS River Valley before rounding the base of the larger scale troughing dominating the Great Lakes. The associated cold front will continue to drop south, while remaining deep layer moisture is pooled in 2"+ concentrations from the Red River across AR/LA into MS. Low level flow will increasingly confluent/convergent across the Lower MS Valley to develop into multiple clusters with some training elements along it, particularly across AR into S MS, given higher potential for upstream backbuilding. Further east, stronger thunderstorms will develop in areas of enhanced instability/convergence in proximity to the front; though are likely to be limited in duration without upstream recharging through advective processes from ongoing upstream complex(es) likely. Guidance continues a stronger amplification trend in the southwest and deeper trough eastward, this has resulted in the best QPF signal to continue a south and west trend and so have adjusted the broad Slight Risk south and west. Given multiple rounds over the prior 2-4 day period, a Slight Risk extends back into central OK, though adjustments are likely to be made depending on how the soil conditions worsen in the coming days.=20=20 ....Central Appalachians to Northeast/New England... The narrowed plume of moisture in the Day 2 period will see a recharge and broadening with the arrival of the next MCV/shortwave lifting northward through the Cumberland Plateau late in Day 2 into early D3 period. A tight gradient of moisture will exist across the Allegheny Plateau ranging from 1.25 to 2.0"+ from central PA to Long Island, this is due to strengthening low level flow, while weakly convergent, there is 30-45 degrees of fetch from the Carolinas narrowing toward the Delaware and Lower Hudson River Valleys. Increased moisture flux convergence will allow for numerous intense (up to 2-2.25"/hr rates) for thunderstorms across the region narrowing across northern New England with strong FGEN forcing given aforementioned flow. While the orientation of the QPF signature is dependent on timing of the approach and strength of the shortwave, there remains increasing signal for streaks of 2-4" totals across E PA to W MA, even with ensemble mean solutions reaching 2.5"+ values, driven by the GFS/UKMET and NAM, with the 00z ECMWF a tad slower but still near 3" in spots. At the fringes of the highest QPF streak, there remains solid clustering for risk across the Southern Mid-Atlantic into VA as well as northward across northern New England. At this point, a high-end Slight Risk is solidly in place, and given trends may even require a Moderate Risk in subsequent outlook particularly accounting for where grounds get further saturated in the D1 and D2 period. Further south across the Carolinas into the central Appalachians, forcing is a bit more scattered/isolated in the broader overall flow. Still ample moisture and unstable environment will exist, especially nearer E NC where highest moisture will be persistent.=20 As such, a broader Marginal Risk will remain in place even connecting over to the South across E TN/Upstate NC.=20 Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8k8-BPRFfnIR9vecdRH3UjpaWWF2JmLB-j57zZYEJs3j= Q13XKSAXap6A4NN_8nt-ZYj8OOWPYg5-aFdU4DdvZNQt5Sk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8k8-BPRFfnIR9vecdRH3UjpaWWF2JmLB-j57zZYEJs3j= Q13XKSAXap6A4NN_8nt-ZYj8OOWPYg5-aFdU4Ddv5eJ8F04$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8k8-BPRFfnIR9vecdRH3UjpaWWF2JmLB-j57zZYEJs3j= Q13XKSAXap6A4NN_8nt-ZYj8OOWPYg5-aFdU4Ddv8jPpSHE$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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