Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 09 2023 07:37:06 FOUS30 KWBC 090737 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 336 AM EDT Sun Jul 09 2023 Day 1 Valid 12Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC INTO WESTERN NEW ENGLAND... ....Mid-Atlantic to New England... Signals for heightened risk for flash flooding with isolated significant localized impacts possible across the Mid-Atlantic into Northwestern New England are coming into focus. A potent shortwave is starting to round the base of the larger scale trough across the Ohio Valley, resulting in some increased negative tilt by late afternoon. Strengthening broad southerly flow off the very moist Northwest Atlantic will increase total moisture across the Delaware/Hudson Valley into New England with 1.75 increasing to 2"+ by midday. In response a surface low and associated cold front (and inverted trof across NY) should support strong moisture flux convergence along its length by mid-morning (if not already ongoing by 09.12z) across NY, central PA to Western VA/NC.=20 Efficient rainfall production should result in localized rain-rates to 2"/hr and possible instances of flash flooding, deep layer steering will support northward moving cells supporting short-term training as the front slowly progresses eastward.=20 As the afternoon evolves, upper-level divergence increases along with heating to increase instability across the eastern US while the surface low strengthens in N VA. This will back the local flow across the Delaware Valley into the Hudson Valley, increasing moisture flux and FGEN ascent through E PA, further slowing forward progress of the convective line while cells still train/repeat to the north. By evening instances of 3-6" locally will become increasingly plausible with 00z HREF probability fields greater than 60% from DC to N VT (with spots near 90-100% for 3" and 30-60% of 5" across NE PA/N NJ to S VT. 00z Hi-Res CAMs have trended slightly eastward to align more favorably to the greatest 14-day precipitation over 300% of climatological normal axis extending from near Baltimore to Philly to N NJ and W MA, suggesting increased runoff potential for these. As the surface wave advances toward the New York Bight by early morning, increased low level flow, will advect unstable across across S NY/New England and with continued isentropic ascent through warm conveyor belt along/north of the surface low, additional efficient thunderstorms a likely to linger overnight, in proximity to SE NY/SW New England as well as into south facing terrain making a more likely scenario for flash flooding. As such, the Moderate Risk of excessive rainfall was shifted eastward slightly to account for the guidance trends. ....Eastern North Carolina... At the tail end of the cold front, the Slight Risk was expanded south and west to account for lower FFG over recently affected areas of west-central VA. Some consideration was made toward including the NC Fall Line/Piedmont region including Charlotte as the southern end of thunderstorm activity may lay out outflow boundaries for late evening/early overnight redevelopment of thunderstorms on increased orthogonal ascent of LLJ across the boundaries. The signal in guidance has increased for this thunderstorm development, but remain generally too isolated/scattered with only isolated enhanced rainfall totals to expand the Slight Risk at this time. ....Southern Plains to Central Gulf Coast... Like prior days, there is likely to be an ongoing MCS across Oklahoma at the start of the forecast period from prior night's upslope flow. Current RADAR and satellite imagery trends denote some increased in elongation of the wave/convective line from west to east with weaker isentropic ascent, though the length should maintain duration for modest rainfall crossing saturated grounds from multiple prior days' complexes, there remains a slightly faster trend toward the east. By mid-day, insolation and convergence in proximity to the MCV across LA/S AR should spur an additional round of stronger, moderately organized thunderstorms.=20 00z Hi-res guidance trends depict a more loosely organized clusters with some weak southwesterly isentropic ascent with cells moving off/away from the boundary into weaker elevated cells across LA/MS into SW AL. This is a general reduction of repeating/training environment expanding the potential for clusters than one or two broader heavy rainfall signatures/footprints. As such, the broad Slight has reduced in confidence for number of potential flooding incidents but remains generally broad enough to maintain, with a reduction of the western extent to account for the faster cell motions of the initial MCS this morning. Gallina=20 Day 2 Valid 12Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF INTERIOR NEW ENGLAND... ....Northeast... At the start of the day 2 period (Mon 10.12z), the shortwave will will reaching peak maturity with deep layer DPVA across the Northeast as it starts to close off across N PA/E NY with surface low in proximity to E Long Island with a well developed and focused TROWAL crossing southeast New England with 20-30kts of warm conveyor ascending across New England into a sharpening deformation zone from the Catskills across Northern New England.=20 There are multiple signals for a remaining axis of 500-800 J/kg of MUCAPE along and southeast of the convergence zone, which will remain critical for the elevated, slow moving convection along the TROWAL. Given prior day's heavy rainfall (3-6") and already elevated wet soil conditions (per AHPS 200-300% above normal precip anomalies) and potential for slow/stationary cells resulting in localized additional 2-4"+, the risk of flash flooding remains highly likely and may need to be further increased based on how much rain unfolds in the Day 1 period, particularly across the southeast facing Berkshires and Green Mt Ranges where upslope flow should enhance ascent/rainfall rates where FGEN signals overlap the greatest.=20 ....Gulf States into South Carolina... Deep Layer trough as pressed the cold front and pooled moisture/instability along the Central Gulf Coast with a stronger trend toward a further south solution. This places the best upper-level diffluence and moisture flux convergence overall along and just offshore of the MS Delta toward western FL Panhandle.=20 While synoptic guidance continues to hint a stronger moisture flux convergence within the downstream shear axis across S GA; this looks like a typical guidance bias, stronger thunderstorms closer to the AL/GA and FL line are more likely given proximity to better instability, but should be more scattered in nature and within the area of higher FFG. As such, kept the Marginal Risk to align with the best moisture/instability where isolated stripes of enhanced rainfall could result in isolated flooding concerns. ....Central to Southern Plains... Stronger closed ridge over the Southwest and reduced activity overnight late Monday across Central High Plains, should have allowed for weaker moisture return to build up higher values, increased instability for potential activity into Tuesday evening. 00z guidance along with prior cycles continues to hint at an organized cluster or small complex to form at the upwind side of the cold front in the vicinity of NE NM into the Texas Panhandle/W OK. Storm mode will likely determine localized very intense rainfall rates capable of inducing an isolated few instances of flash flooding, in proximity to saturated grounds across central Oklahoma, a Marginal Risk is extended from the area in the Lower MS Valley back across and north of the Red River.=20=20 Additionally, northern stream cold front as leading edge of larger scale height-falls will be pressing southward across the central Plains and intersecting some of the untapped moisture/instability that rolled back through eastern Colorado on Sunday and Monday.=20 Instability of 2000-3000 J/kg and 1.5"+ total PWats pooled along strengthening (20-25kt) LLJ is likely to converge in early Tuesday for thunderstorm development across Nebraska, eventually propagating into KS. As such an extension of the Marginal Risk is brought northward into southern Nebraska extending toward the Iowa boarder.=20 ....Upper Midwest... There is uncertainty to the level of moisture flux return along/ahead of the cold front dropping out of Canada crossing Lake Superior to N Nebraska in the day 2 period. Stronger storms are likely to dot the front, but cell motions should mitigate the highest totals across portions of Iowa into MN, particularly given recent dry soil conditions. However, timing of the front with peak heating and likelihood of Lake Breeze enhanced thunderstorms off Green Bay/NW Lake Michigan may result in thunderstorm collision for a few spots of intense short-term rainfall rates over 2" and spots of 2-3" overall. The area has seen above normal rainfall over the last few weeks per AHPS and lower FFG values (mainly along the northern U.P.) may overlap enough for an isolated potential for localized flooding. So in coordination with MQT local Forecast Office, a small Marginal Risk of Excessive Rainfall was added for the core of the U.P. of Michigan for this cycle. Gallina Day 3 Valid 12Z Mon Jul 10 2023 - 12Z Tue Jul 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST PORTIONS OF THE MID-MISSOURI BASIN... ....Northern Plains through Upper Great Lakes... The strong synoptic scale closed low over Canada is expected to drop south toward the Boundary Waters of Ontario/Minnesota with a sharpening shortwave digging along the western side across into the Northern Plains. The associated cold front will have already started slow slow from the Northern Great Lakes across South Dakota before angling northwest into northeast MT. Strong upslope moisture flux will increase across the Northern High Plains, with a narrow axis of enhanced moisture/instability through the western Dakotas. Mid-day thunderstorms will start to organize into a stronger cluster/complex intersecting with 35-40kt SSW LLJ through the central Plains. MCS with training thunderstorms along the frontal boundary will increase in coverage/rainfall efficiency across central SD into SE SD toward 12.03-06z with streaks of 2-4" becoming likely across the Mid-Missouri Basin. While the area has been dry, thunderstorms on Day 2 and intense rates continue to support a Slight Risk across SD into NW IA. A broad Marginal Risk extends along the track as well as up the frontal zone across N IL/S WI and Lower MI, where scattered clusters may result in more isolated heavy rainfall signature (given low level flow will be a bit more parallel reducing moisture flux convergence. ....Northern New England... At the start of the forecast period, the increasingly stacked cyclone will be exiting with the occluded/cold front likely to have shifted east/northeast of Maine. However, the lingering deformation zone/TROWAL is likely to have ongoing showers or embedded smaller thunderstorms across N VT/NH and maybe the top of the Rooftop of Maine. Additional 1-2" totals are possible but reducing given limited remaining instability. These totals and likely ongoing flooding rivers/streams and saturated soils will still be prone to any rates up to .5-1"/hr, but will be conservative in higher ERO categories until knowing the greatest affected areas are known after the prior two days. As such a Marginal Risk of Excessive Rainfall will be a placeholder until the ground conditions are better understood. ....Southern Red River through Central Gulf Coast... The lingering stationary boundary will continue to be the focus of pooled deeper layer moisture, though being at the tail end of the large scale flow (exiting deep layer trof), there will be limited translation of the axis. Anomalous deep layer moisture with values of 2-2.25" total PWat through the Lower MS Valley and full-sun should support strongly unstable environment with 2000-3000 J/kg CAPE by midday. Weak upper level diffluence and weak south-southwesterly flow should allow for localized clusters of stronger thunderstorms. Deep layer slow steering flow along/parallel to the boundary should allow for sufficient organization to maintain the clusters and slow motions may support very intense but locally focused enhanced rainfall totals to support isolated instances of flash flooding; enough to maintain the Marginal Risk of Excessive Rainfall from SE OK to NOLA as well as the FL Panhandle. Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4pBtB-S7LjBkNoYvNe4o-f9-YvuO36rMGbQfd8Q6fCHh= d6O_BQENN8eUwV1uXDOAfs3Q-bri5RwMaQR8zOIx-byDkwA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4pBtB-S7LjBkNoYvNe4o-f9-YvuO36rMGbQfd8Q6fCHh= d6O_BQENN8eUwV1uXDOAfs3Q-bri5RwMaQR8zOIxH6JJUjM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4pBtB-S7LjBkNoYvNe4o-f9-YvuO36rMGbQfd8Q6fCHh= d6O_BQENN8eUwV1uXDOAfs3Q-bri5RwMaQR8zOIxRVL7itU$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .