Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jun 13 2023 00:56:27 FOUS30 KWBC 130056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Mon Jun 12 2023 Day 1 Valid 01Z Tue Jun 13 2023 - 12Z Tue Jun 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE CENTRAL HIGH PLAINS AND FROM THE ARKLATEX INTO PORTIONS OF THE LOWER MISSISSIPPI VALLEY... ....01Z Update... Changes to the early evening D1 ERO update are mainly cosmetic to address the latest satellite and radar trends. Some localized trimming of the Marginal Risk was done across western portions of the northern Mid-Atlantic where rains have come to an end. Elsewhere, the Slight Risk area over the CO Front Range was removed given the level of convective overturning/stabilization from earlier heavy shower/thunderstorm activity, but remains in place farther east across eastern CO into western KS. Some minor southward adjustment was also made into far northeast NM and the OK Panhandle. Finally, some eastward extension/adjustment was made to the Slight Risk over the ArkLaTex region and extending east-southeast through southeast AR and portions of west-central MS. This is to account for the latest HRRR guidance and 18Z HREF which suggests convective redevelopment in the 06Z to 12Z time frame with a somewhat more progressive downstream propagation. Orrison ....Previous Discussion... Central High Plains into the CO and southern WY Rockies...=20 An upper low drifts northeast from the Desert Southwest today adding some upper level lift to increase coverage in what has been a rather wet period for the region. A plume of west Gulf moisture continues to push up the southern Plains through the CO Front range with PWs of 0.75-1" in these higher elevations (2 sigma above normal). Moderate instability will return this afternoon with 1000-1500 J/kg of MUCAPE. While there is modest low-level upslope flow, the flow aloft is westerly with effective bulk shear promoting organized convection capable of heavy rainfall, up to 2" an hour. Storm motion should be eastward per mean layer flow with southwesterly downwind propagating convective motion vectors suggesting a northward tendency. Clusters of 2-4" QPF are in the 00Z HREF consensus over the CO High Plains into western KS which is where storm mergers and short periods of training should be most prevalent. The Slight Risk area is maintained - merely expanded a bit farther west through the Continental Divide. =20=20 =20 Oklahoma through ArkLaTex to the Central Gulf Coast...=20 Southerly flow abundant with western Gulf moisture converges over the front already stationary over northeast TX through the lower MS Valley (this front persists through midweek in this blocked pattern) maintaining a pool of 1.5-2" PWs with ample instability, 2000-4000 J/kg of CAPE, just upwind. Heavy rainfall is expected at times along this frontal zone with more discrete activity in the warm sector south. Hourly rain totals to 2.5" with local amounts to 4" are possible. A Slight Risk remains for the area, just with a southward shift to along the Red River into southern AR per the 00Z HREF consensus. Ongoing activity over northeast NM and North Texas tonight will need to be monitored for affecting the local frontal position. Northwesterly mean flow becomes westerly this evening with a warranted expansion of the Marginal Risk over North Texas to the Central Gulf Coast. =20 =20 Intermountain West...=20 Enough moisture and instability will be available to aid diurnal convective development under and ahead of the upper low. Hourly rain totals of 1.5" are possible should cells merge and/or train, but coverage is not expected to be dense enough for more than isolated to widely scattered heavy rain-related issues. The Marginal Risk area was shifted just a bit south. =20 =20 Northern Mid-Atlantic States...=20 Frontogenesis in the low to mid-levels ahead of a re-strengthening mid/upper low will pivot over central/eastern PA/Upstate NY through this evening. Some instability will be present, producing locally heavy rainfall that repeats over the areas with a risk for hourly rainfall over 1" and potential for 3" to 4" in 6 hours where cells repeatedly merge/train and/or where mesocyclones can form. This would overcome dry preconditions and also be a risk in urban areas (including Philadelphia on the southern end of the pivot), so the Marginal Risk is maintained with a bit of a northward expansion around Lake Ontario. =20 Jackson Day 2 Valid 12Z Tue Jun 13 2023 - 12Z Wed Jun 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLAMISS INTO FAR WESTERN GEORGIA... ....1930Z Update... ....Southeast... The most notable change made to the Day 2/Tuesday ERO was to significantly shrink the Slight Risk area across the Southeast, and focus both it and the surrounding Marginal Risk area on the rainfall footprint points south, noting the significant but not unusual southward trend in the guidance as to where the heaviest rainfall totals will be seen from the storms. There were 3 reasons noted for the diminishing confidence in flash flooding for this area, as per the recent 12Z CAMs guidance. 1) Antecedent conditions have been very dry, and as such FFGs are very high across this area. Given some of the favorable atmospheric conditions noted in the previous discussion below, including high PWATs and plentiful instability, there may be some chance a few cells may overcome the high bar set by these dry conditions, but confidence on that is low. 2) The time in between these rounds of storms will be a lot. The rainfall in this area is likely to come from 2 separate MCS's, the first ongoing at the start of the period and the second starting up in the very early morning hours of Wednesday. In between there is good consensus among the CAMs guidance that almost the entire area will be dry. This over 12 hour period of dry weather in between rounds of storms should allow for effective draining of the small creeks and streams before the second round hits, so the 24-hour rainfall totals are not reflective of the overall threat. 3) While the storms are moving across the region, they will be RACING. Some of the guidance indicates they could be moving at 40 kts. Thus, even with 2 or 3 consecutive rounds of storms over a given area due to training or as part of an MCS, the time any one area will see heavy rain will be very short. Given these reasons, the Slight Risk area was shrunken to be largely tailored to the HREF neighborhood probabilities of 3 inches of rain or more, with some buffer added to the south, again given the southward trends in the guidance. Further shrinking of the region or elimination of the area altogether may be needed with future updates. ....Intermountain West to the Southern High Plains... No significant changes were made to the aforementioned area. The forcing should lesser on Tuesday as compared with today, so the Marginal remains for much of the area, though given the very wet antecedent conditions, any organized convection may cause local flash flooding impacts to be greater. Wegman ....Previous Discussion... ....ArkLaTex/Mid-South/Southeast... An upper low currently over the Desert Southwest opens into a shortwave trough and crosses the southern Plains Tuesday afternoon/night. This will track over a stationary frontal zone that is already set up over north Texas east over the interior Deep South. Precipitable water values of 1.5-2" will be maintained along the frontal zone with ample instability (CAPE of 3000-4000 J/kg) will persist just south in the warm sector. There should be enough low-level inflow and effective bulk shear to organize convection with cell training in WNW-ESE motion and occasional mesocyclone formation leading to hourly rain totals up to 2.5". 00Z guidance maintains a consistent signal for local 3-5" totals. As this will be the second day of the upcoming three where heavy rain is expected across the region, flash flood guidance should be compromised to some degree from rainfall through tonight. The Slight Risk was shifted south a bit per global guidance consensus (further investigation into CAMs are necessary - as the 00Z NAMNest is farther north) and expanded zonally along this stationary frontal zone with repeating rounds of heavy convection expected. ....Great Basin/Northern Intermountain West to the southern High Plains... A sharp upper level trough will dig southeast from BC, enhancing aloft divergence over the northern Great Basin into the northern Rockies. Pockets of 0.75-1.25" PWs combined with diurnal max of 1000+ J/kg of MLCAPE will allow for convection to develop in the afternoon and evening hours. Hourly rain totals to 2" are possible within such an air mass where cells merge and/or train. 00Z guidance QPF totals continues to indicate a risk for up to 2" in spots, so the rainfall in any one place could occur within an hour and pulse out/send out outflow boundaries to initiate new convection. As any heavy rain-related issues are expected to remain isolated to widely scattered, have maintained the Marginal Risk for an area now from the Sierra Nevada through the northern Intermountain West, over the CO Rockies through western KS. Jackson Day 3 Valid 12Z Wed Jun 14 2023 - 12Z Thu Jun 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... ....1930Z Update... ....Southeast... No big changes were made with this afternoon's update. The guidance trends have increased as compared with both inherited and the expected rainfall on Day 2/Tuesday. Thus, there is significantly higher confidence of more flash flooding impacts across this area. Expected rainfall in portions of the Slight Risk area both this afternoon and again on Tuesday will help to moisten up the soils in this region, then with widespread 2+ inches of rain likely across this area along with better forcing, especially by Wednesday night, there's a higher likelihood the rainfall will overcome the otherwise naturally high FFGs in this area, particularly in urban and poor drainage areas. The Slight Risk area was adjusted following latest model guidance to more of a WNW to ESE orientation, with some expectation of a continued southward trend in the guidance, having southern portions of both the Slight and surrounding Marginal risk areas with more of a buffer. Previous days' rainfall may make the difference on whether or not further upgrades may be needed for portions of the Slight risk area. ....Intermountain West... Once again no major changes needed as there's good agreement on the rainfall footprint across this area and as mentioned in the Day 2 discussion, any areas were more local organization can occur may result in more widely scattered coverage of flash flooding impacts. Wegman ....Previous Discussion... ....Lower Miss Valley through the Southeast... Another day of enhanced convective focus along a stalled boundary over the Southeast is expected as remnants of the upper low currently over the Desert Southwest is drawn east south of the occluded low drifting east from the Midwest. Ample Gulf moisture (PWs of 1.5 to 2") continues to pool along the frontal zone which should sag south into the instability pool and toward the Gulf Coast from Tuesday activity. However, there is a risk for this repeating heavy activity to focus over similar areas in MS/AL/GA to the FL Panhandle Tuesday and Wednesday which may warrant an eventual Moderate Risk upgrade. ....Intermountain West and Northern Rockies onto the Northern Plains... An upper trough pivoting over the Northern Rockies sends a cold front to the central portions of the Intermountain West and a renewed upper trough extending over southern CA allows an elevated surge of Pacific moisture up through this front. An axis of QPF from the Sierra Nevada across the Great Basin to the CO Rockies is forecast with a risk for local 1"+ precip in diurnally enhanced convection warrants a Marginal Risk. The cold front crosses the northern Rockies with lee-side convective activity over the already saturated central portions of Montana warrants a Marginal Risk into the eastern Dakotas. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7ckJU-9uYhtJsS5sNLSnuDI4xUAminVVK9gNup-Vun0K= Pkkk-Wmgp6PM-Tld-1c6dwNjbfhiQuD0khTOqlA7Ihq0CLM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ckJU-9uYhtJsS5sNLSnuDI4xUAminVVK9gNup-Vun0K= Pkkk-Wmgp6PM-Tld-1c6dwNjbfhiQuD0khTOqlA7APDjOBA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ckJU-9uYhtJsS5sNLSnuDI4xUAminVVK9gNup-Vun0K= Pkkk-Wmgp6PM-Tld-1c6dwNjbfhiQuD0khTOqlA7eLKoGXQ$=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) .