Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 31 2024 19:41:10 FOUS30 KWBC 311940 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 340 PM EDT Wed Jul 31 2024 Day 1 Valid 16Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE NORTHERN PLAINS TO THE MIDWEST AS WELL AS PORTIONS THE SOUTHWEST US AND NEW ENGLAND... ....16Z Update... The primary areas of interest have not changed much since the last update, but the SLGT risk across the Northern Plains through portions of the Midwest and Ohio Valley was adjusted to match the current convective coverage and expected impacts later in the period. Despite the models struggling with the overall setup, the footprint based off the ensemble based QPF from the HREF blended mean and associated probabilities continues to highlight the area over the Dakotas into MN/IA and eventually IL by the early morning hours at the back end of the forecast period. There was enough consensus in the initiation point over Southeast ND and points east to expand the SLGT a bit further north to reflect the locally enhanced rainfall expected late afternoon through the evening thanks to another MCS initiation and propagation forecast. Further to the southeast into the Southern Ohio Valley, there is a growing concern for a locally significant training scenario in-of Southern IN and Northern KY, potentially impacting areas further east near Southwest OH. Approach of the mid-level energy from this mornings MCS across IL will provide a targeted focus of large scale ascent within a very moist, unstable environment positioned over the aforementioned area. Latest HREF neighborhood probabilities are more aggressive with the signature for upwards of 3-6" of rainfall within the above corridor as the focused ascent in tandem with the mean flow orienting with the Upwind Corfidi Vectors would garner a better chance for training convection to focus within a narrow corridor for several hours before pivoting to the southeast. The depiction has been evident in both multiple CAM outputs and has merit within some of the ML output (GFS Graphcast and ECMWF AIFS) for a small area that could receive a period of heavier rainfall later this evening likely due to the propensity for train echoes under the primary axis of vorticity. There is still some uncertainty within the exact placement of the highlighted potential, thus there was not enough confidence to warrant an upgrade away from the SLGT risk in place. If the setup were to evolve, the impacts could be locally significant, especially if it were to take place over a metro corridor within the proxy of the IN/KY/OH border. Rest of the CONUS risks are relatively unchanged with an expected flare up of scattered to widespread Monsoonal convection across Southern AZ after 19/20z. The other area of interest remains across Northern New England where scattered convective cores capable of rainfall rates between 1-2"/hr could exacerbate areas in Northeastern VT and Northern NH that received significant flash flooding less than 24 hours prior and are still very sensitive to flash flooding prospects. Elevated neighborhood probabilities for at least 2" across Eastern NY through VT/NH would be sufficient for locally impactful rains and flash flood threats thanks to the complex topography and antecedent conditions. Kleebauer ....Previous Discussion... ....NORTHERN PLAINS THE MID-WEST... Convection should be on-going across portions of Iowa at the start of the Day 1 period at 12Z...with some risk of excessive rainfall lingering early on. Renewed convection is expected later in the day as stronger forcing moves across the Plains later today. WPC favored the more southerly deterministic QPF solution offered by the HREF...and that was reflected in expanding the previously- issued Slight Risk area a bit southward in portions of Minnesota and Iowa. Farther south...a large anomaly of 850 to 700 mb moisture flux remains in place. With the HREF depicting an axis of 1 to nearly 1.5 inches of rain possible falling over lowered flash flood guidance values in southeast Kentucky...felt confident enough to agree extending the Slight Risk south and east into a region where the deterministic QPF from GFS/ECMWF/UKMET were meager. There is still a lot of model qpf spread with some typical latitude differences with the max qpf axis. ....Northeast US... Height falls with a wave approaching from the west will be spreading over portions of New York into New England during the day...accompanied by a risk of moderate to locally heavy rainfall given the abundant moisture already in place. Maintained the Slight Risk without change given its placement over an area soaked by 4 to 8 inch amounts within the past 24 hours...making the area especially vulnerable to any additional rainfall and locally significant flash flooding remains a possibility. ....Southwest US... Maintained the Slight Risk across portions of Arizona that was introduced on Tuesday. Models continue to show an uptick in both moisture and instability compared with the past couple of days, with satellite supporting the model indications of a weak wave moving in from the southeast. This combination of increased forcing and better thermodynamic environment should support greater convective coverage with locally heavy rainfall rates. HREF probabilities are elevated as well...with 2"+ neighborhood probabilities of 40-60% and 1" EAS probabilities of 5-15" indicating at least some ensemble agreement on location/coverage of convection. Bann Day 2 Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDWEST AND OHIO VALLEY... ....20Z Update... The general premise for the D2 SLGT risk was relatively unchanged, however there was an extension to the northwest to include more of IL through Southern and Central WI for the latest update. The reasoning was a growing consensus on the development and eventual path of the closed upper feature moving overhead of the Upper Midwest by late Thursday morning. A tongue of modest instability rooted at the surface depicted by SBCAPE values between 1000-1500 J/kg will be present within the southern half of WI by mid- afternoon with the upper center tracking slowly to the southeast over the areal surface based instability. Scattered convection will form under the low circulation with some cells becoming nearly stationary under the central portion of the upper pattern as steering flow drops between 3-5kts according to forecast soundings under the mean low/trough. Steep lapse rates, sufficient buoyancy, and relatively deep moisture field will impose some scattered cases of heavy rainfall with the potential for local flash flooding, especially if storms are slow-moving and impact more urbanized areas. The setup will continue to evolve downstream with convection causing concerns for localized flooding as we mover southeast through the Midwest and eventually the Ohio Valley by the end of the forecast period. 12z HREF probabilities for >3" of rainfall were highest (50-60%) across WI, Northern IL, and the IN/OH region as that region will see a multi-round impact from thunderstorms=20 during the 24 hour period.=20 The SLGT was also expanded to the southeast a bit through Eastern KY into Northeastern TN as a multi-day episode of scattered heavy precip will only exacerbate the flooding potential that is ongoing today. The general priming of the soils from previous periods will lead to FFGs being climatologically very low aligning to heightened vulnerability to any additional rainfall. With the threat of heavy convection lingering into D2, a SLGT risk expansion was warranted.=20 There was not much change to the D2 MRGL risk out west as the ensemble QPF was relatively unchanged outside the movement of local maxima related to the scattered convective signal. There is a change for a targeted upgrade to the Northeast quadrant of NM if the signal ramps up a bit further within the terrain as the ongoing sensitivity within remnant burn scars in the Sangre de Cristos leads to a higher risk of flash flooding organically. The signal is currently highest across the Northeast NM plains where FFGs are higher, thus it was unnecessary to upgrade for the time being.=20 Kleebauer ....Previous Discussion... The suite of numerical guidance from 31/00Z continued to support a Slight risk area over portions of the Ohio Valley from Thursday into Thursday night as fairly impressive forcing approaches the region. Low level moisture should be place as the closed mid-level low drops in from the north...helping to focus moisture flux convergence along a quasi-stationary front in the area at the same time that divergence aloft increases in response to the presence of an upper level speed maximum. Given uncertainty of where the low will be based on the model spread and where the strongest forcing gets directed introduces plenty of uncertainty with regard to placement of heavier rainfall. As stated earlier...the ingredients still suggest a Slight risk area is warranted and the WPC placement remains south of global QPF consensus...in fairly close proximity to the UFVS machine learning ERO. ....Southwest into southern California... The upper ridge over the Southern Plains is forecast to build to the west and northwest across the Rockies, Great Basin and Southwest during day 2...helping push the axis of above average precipitable water values farther west into southern to central California and southern Nevada. This will foster an environment in which scattered diurnal convection will be expanding into portions of central and southern California/southern Nevada. Bann Day 3 Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO/TENNESSEE VALLEYS, INTERIOR MID ATLANTIC, SOUTHWEST US, THE SIERRA NEVADA AND ADJACENT VALLEYS... ....20Z Update... The previous MRGL risk east of the Mississippi was expanded a bit on all sides to account for some model spread in where more organized bouts of heavy rain could occur. The upper pattern is relatively unchanged with the globals and ensemble output maintaining basic continuity on the approaching trough and focused ascent over the Ohio Valley into the Mid Atlantic. A lot of the spread is more thermodynamically driven with scattered convective signals underneath the mean trough, so it is tough to guage with certainty where the heavier cores will develop. The environment is conducive for locally heavy rains anywhere from the Central Midwest through the Central and Northern Mid Atlantic with the primary likely falling under the center of the upper circulation and mean trough. This pinpoints areas like Ohio and Western PA down into WV/KY as the best chance for convective coverage capable of more flash flooding capabilities. A lot is contingent on the pattern evolution and with time leading into the setup still in favor, was reluctant to add any upgrades at this time to allow for further assessment.=20 Over the Western U.S, the pattern is more simplified, but there was more emphasis on the recent ensemble forecast to amplify a heavy rain threat further north through the interior valleys of CA and far western NV. This was also emphasized within the ML QPF footprint indicating a northward expansion of the convective potential that could prompt localized flood concerns over a climatologically favored area for flooding during convective impact. This allowed for an expansion of the MRGL risk within the Sierra's and adjacent valleys with maintenance of the MRGL elsewhere from the previous forecast.=20 Kleebauer ....Previous Discussion... ....Ohio and Tennessee Valleys... A closed low initially near the Great Lakes at the start of the Day 3 period will continue to sage south and eastward into the Great Lakes region. Given plentiful moisture in place and increasing cyclonic circulation aloft...localized downpours are in the realm of possibility. One camp of thought in the models is that higher amounts will be tied to the upper dynamics early on in the period while the NAM seems to champion the idea of higher amounts along the leading edge of the height falls closer to the better instability. With the spread shown...went with a broad and unfocused Marginal risk area for the time being. ....Southwest US... With the upper level ridge continuing to build across the Intermountain region and monsoonal moisture lingering near the international border...another round of late day/evening showers and thunderstorms is expected. Some northward/eastward expansion compared with the previous couple of days is expected. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6Me6_0Y3r6PsTfyyCFFostOVnFCvmAH9sz-_1ffg_Hya= Jt1RjWwXJva0A8FDzvENwq9i3RXBvA--rCmWu5VdzKMbWFs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Me6_0Y3r6PsTfyyCFFostOVnFCvmAH9sz-_1ffg_Hya= Jt1RjWwXJva0A8FDzvENwq9i3RXBvA--rCmWu5VdG3V1gXM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Me6_0Y3r6PsTfyyCFFostOVnFCvmAH9sz-_1ffg_Hya= Jt1RjWwXJva0A8FDzvENwq9i3RXBvA--rCmWu5Vd7OjG3G4$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .