Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 24 2023 23:28:34 FOUS30 KWBC 242328 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 727 PM EDT Mon Apr 24 2023 Day 1 Valid 01Z Tue Apr 25 2023 - 12Z Tue Apr 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEASTERN FLORIDA... FL Peninsula... Convection initially was active across east-central FL and South FL near and south of a baroclinic trough/weak front. A large outflow boundary from South Florida intersected the West Coast Sea Breeze Front, which has led to active convection near and south of Lake Okeechobee which has been stationary as of late. While much of the activity in east-central FL has weakened, there is a patch not far from the Treasure Coast that remains active. Farther northwest, a small rotating thunderstorm near an apparent mesoscale low closer to the upper level shortwave is approaching the coast near Yankeetown, and a couple activity cells remain to its east. The expected evolution is this: activity approaching northwest FL and across central FL should be in decline shortly as CIN is setting in and CAPE continues to decline. While there is a non-zero chance for isolated heavy rain/flash flooding concerns, do not feel it rises to the level of a Marginal Risk. Across the southeastern Peninsula, as a cold pool tries to form with the convection inland of the Treasure Coast and across the Everglades, convection should show increasing progression to the east and southeast. One way or another (increasing CIN or the movement of an upstream shortwave across FL), heavy rain concerns should diminish around 03z, which is normally when convection within a 1.75" precipitable water air mass with sufficient instability would be in decline. Local 5" amounts are possible, along with hourly totals up to 3", before convection either moves out of the Florida Peninsula or weakens due to developing CIN.=20 Roth Day 2 Valid 12Z Tue Apr 25 2023 - 12Z Wed Apr 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF SOUTHERN KANSAS INTO MUCH OF OKLAHOMA, THE RED RIVER VALLEY, AND THE ARKANSAS RIVER VALLEY... 2030Z Update... Overall, the model signal remains much the same, so no large-scale chances were made to the previous outlook areas. The 12z guidance continued to shows a bi-model distribution in heavier QPF across the Plains, with one maxima centered over northeastern Oklahoma and the second closer to the Red River Valley. Probabilistic guidance from the HREF indicated that amounts of 2-3 inches are more likely in the northern maxima. While confidence in the details remains limited, the HREF guidance showed continued support for keeping heavier QPF and the associated outlook areas a little farther west compared to previous forecasts -- which is reflected in the 2030Z update. Previous Discussion... ....Central-Southern Plains to the Ozarks, Arkansas River Valley, and the ArkLaTex... A vigorous shortwave pivoting across the 4-Corners and southern Rockies on Day 2 will allow for a more difluent upper flow pattern downstream across parts of the Southern Plains east toward the Mid MS Valley. Deep-layer forcing ahead of this shortwave will be enhanced by lingering right-entrance region forcing from a 90-100+ kt upper level jet streak across the Mid MS Valley-OH Valley. The models (including some of the high-res guidance) continue to show a signal for rainfall totals in the 2-3" range with embedded higher totals, however there remains quite a bit of areal spread in terms of where convection will organize and propagate. Areal-average rainfall totals over the Slight Risk area thus may be a bit too high in some locations, while underestimating the potential elsewhere, leading to a continued rather broad risk area. One aspect where forecast confidence has increased is with respect to a western shift in the Slight Risk and Marginal Risk areas compared to yesterday's Day 3 ERO. However, the guidance continues to hint at a bi-modal distribution in the QPF, with one maxima located farther north (northeast OK into south-central-SE KS), likely associated with the best low-level FGEN from the aforementioned upper jet streak to the north, with a secondary maxima closer to the Red River. Regardless, given the noted environment conducive to heavy rainfall and the QPF totals, the potential for heavy rainfall has remained consistent while future hi-res guidance should help to narrow down any more specific adjustments to the location. ....Colorado Front Range into the Rocky Mountain Piedmont... A mid-upper level low will drop southeastward over the region as surface low pressure organizes over the southern Rockies-southern High Plains. This will bring anomalously moist SE-E flow into eastern Colorado. This will lead to a significant ramp up in low-level upslope flow during the day Tuesday along the Front Range, while after 16-18Z the CAMs show the character of the pcpn becoming more convective per simulated reflectivity and hourly QPFs as MUCAPEs climb between 200-600 J/Kg. All CAMs show hourly rainfall rates peaking around 0.75-1.25", with isolated Day 2 QPF totals of 2-4+ inches. Given the isolated nature of these higher amounts, any resultant impacts should be localized and limited to minor street flooding. In addition, as the upper-level low approaches, snow levels will likely quickly lower starting Tuesday evening from around 8000 feet to 6000 feet, bringing a changeover to snow for locations in the foothills and Palmer Divide and limiting the areal coverage of any continued excessive rainfall threat. Pereira/Hurley Day 3 Valid 12Z Wed Apr 26 2023 - 12Z Thu Apr 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN PLAINS INTO THE ARKLATEX, ARKANSAS RIVER VALLEY, AND SOUTHERN OZARKS... 2030Z Update... The 12Z guidance did not present any reason to deviate significantly from the previous forecast, so made only minor updates based on the latest guidance. Previous Forecast... ....Central-Southern Plains to the Ozarks, Arkansas River Valley, ArkLaTex, and Lower Mississippi Valley... The aforementioned shortwave on Day 2 will track slowly along or north of the Red River Valley Wed-Wed night. The 00Z guidance shows somewhat better clustering with the heavier QPF compared to prior runs, however there remains a decent amount of spread, with again hints of a bi or multi-modal distribution of the maxima per the global guidance (especially the ECMWF, CMC, and UKMet). Once again it appears the models are attempting to resolve the likelihood that areas with the best deep-layer forcing (associated with the aforementioned right-entrance region upper jet forcing) will likely not coincide with the best deep-layer instability (farther south, i.e. south of the Red River Valley). For now, will maintain rather broad Slight and Marginal Risk areas, hoping to gain more clarity over the next few forecast cycles as the event gets within the high-res CAM windows. Pereira/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9Yy30nwNO-HGSLoJMbuGfsVFnOkUzYjslNvzskmuXiNk= xh39_AXj6NxXy127sEkLkQZRes-d-igQPj2BpgZh3ZmgC5U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9Yy30nwNO-HGSLoJMbuGfsVFnOkUzYjslNvzskmuXiNk= xh39_AXj6NxXy127sEkLkQZRes-d-igQPj2BpgZhDUfuRzE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9Yy30nwNO-HGSLoJMbuGfsVFnOkUzYjslNvzskmuXiNk= xh39_AXj6NxXy127sEkLkQZRes-d-igQPj2BpgZh5sAgN_o$=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) .