Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 16 2023 20:22:22 FOUS30 KWBC 162022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Wed Aug 16 2023 Day 1 Valid 16Z Wed Aug 16 2023 - 12Z Thu Aug 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE COASTAL SECTIONS OF THE CAROLINAS, SOUTHEAST GEORGIA INTO MUCH OF FLORIDA AND ACROSS PORTIONS OF THE SOUTHWEST, SOUTHERN GREAT BASIN, SOUTHERN CALIFORNIA AND THE NORTHERN SIERRA... 16z Update: Only minor changes made to the Marginal risk over the Southeast. Guidance suggests initial development this afternoon may be a bit further west than the original western Marginal border, so we made a minor westward expansion to the area. Activity still looks pretty disorganized and mainly pulse in nature...so flash flooding should stay pretty localized. We added a small Marginal risk over portions of upstate NY into VT. Clearing over this area into early this afternoon should allow for the development of scattered convection along a low level trough/convergence axis. These cells will tend to be slow moving and focused near the better convergence...which may result in a quick 1-2" on an isolated basis. Given the relatively low FFG over the area, can not rule out some localized flash flood issues. No changes to the inherited Marginal over the Southwest. There is some concern for a slightly higher threat over portions of eastern NV into southern and central UT...as this is where the highest PW anomalies overlap 1000+ J/KG of CAPE. There is also a weak wave evident on satellite moving towards this area...however guidance is not really all that organized with this convective threat...thus will stick with a Marginal risk for now. We do expect we will see some isolated flash flood issues...but confidence on the coverage and organization is too low for an upgrade at this time. Chenard ....Previous Discussion... ....Carolina, Georgia Coasts into Florida... No significant changes made to the broad marginal risk area along the Carolina coasts, southeast Georgia and across much of Florida. The surface frontal boundary currently stretching from the Central Gulf Coast, northeastward into the Southeast should remain stationary day 1 as the upper trof weakens and does not allow for further eastward push. Models continue to show PW values remaining above average along the immediate NC,SC and GA coasts and across FL, with values 2+ standard deviations above the mean.=20 While the upper trof is weakening , uvvs may still be enhanced by right entrance region jet dynamics that linger along the immediate coast. Model consensus is for heavy rainfall potential in the vicinity of this front and south across much of Florida. Isolated runoff issues possible, especially if heavy totals fall across urbanized areas. ....Southwest into the Southern Great Basin... Only some small changes to the previous broad marginal risk area across the Southwest, southern Great Basin and portions of California. Trimmed the marginal risk area over central Colorado to the southwest to align better with the latest HREF probabilities for 1"+ rainfall amounts day 1. Also extended the marginal risk into the Peninsular Range and eastern portions of the Transverse Range of southern California where showers popped up Tuesday and HREF neighborhood probabilities for 1"+ amounts show probabilities of 15-20% for the new day 1 period. The upper low off the central California coast is expected to remain stationary day 1. This will help to maintain a broadly upper flow pattern to its east. PW values will remain above average across the marginal risk area, supporting isolated heavy rainfall totals and isolated runoff issues from what may again be an afternoon of widely scattered convection.=20 Oravec Day 2 Valid 12Z Thu Aug 17 2023 - 12Z Fri Aug 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHWEST, SOUTHERN GREAT BASIN INTO THE NORTHERN SIERRA AND ACROSS MUCH OF FLORIDA, ALONG WITH SOUTHEAST L.P. OF MICHIGAN AND NORTHEAST PENNSYLVANIA INTO PORTIONS OF NEW ENGLAND... ....20z Update... The main change with this update was to expand the Marginal risk across portions of northeast PA into the Northeast. An interesting setup with the approaching trough from the west interacting with a more subtle wave moving north along the Atlantic coast. Guidance is trending higher with instability/moisture across the Mid-Atlantic during the day Thursday, and thus the potential is there for some locally heavy rainfall. Not as much forcing at this time however, and so confidence on convective coverage/organization is pretty low. Thus opted against a southward expansion of the Marginal into this area. The better risk appears to be from northeast PA into central/eastern PA and portions of New England on Thursday night. This could be a sneaky nocturnal convective event with the moisture/instability streaming northward and interacting with the approaching large scale trough. The potential is there for some warm rain processes and heavy rainfall rates. One limiting factor appears to be quick cell motions...and overall confidence is a bit below average given the interacting features in play. For these reasons think the risk is still at Marginal level...but will need to continue monitor trends. Chenard ....Previous Discussion... ....Southwest, Southern Great Basin, Northern Sierra... The closed low off the central California coast that becomes stationary during the day 1 period will begin to progress back toward the east southeast and toward the south central California coast day 2. A broadly upper difluent flow pattern will continue to the east of this closed low and to the west of the upper high centered from the Rockies into the Plains. PW values in this broadly difluent upper flow pattern will remain above average, ranging from 1 to 3+ standard deviations above the mean. This will support another day of widely scattered convection, isolated heavy totals and isolated runoff issues, especially across any burn scar areas or slot canyons. A broad marginal risk was drawn to encompass where models are showing some precip potential. ....Florida and coastal Georgia and South Carolina... An axis of tropical PW values, 1.75 to 2"+ will remain across much of Florida and along the immediate Southeast Coast from Georgia to North Carolina for the upcoming day 2 period. At the moment there does not appear to be any strong surface to upper level features to focus convection, but a general model consensus for widespread scattered convection in the tropical PW axis, with slow cell motion possibly resulting in runoff issues, especially across urban areas. The marginal risk was extended north along the Georgia and South Carolina coasts to cover where the HREF neighborhood probabilities from 1200 UTC Thu to 0000 UTC Fri show high probabilities for 1"+ amounts and low for 2"+ amounts.=20 ....Southeast L.P. of Michigan, Northern to Central New York State into far northeast Pennsylvania... Increasingly negatively tilted height falls will be pushing through the Great Lakes and into the Northern Mid Atlantic day 2.=20 While there is likely to be an organized band of frontal/pre-frontal precip moving through the lakes, it should be very progressive, keeping amounts from being very heavy. Concerns for any isolated runoff issues are for far southeast L.P. of Michigan where recent heavy rains have produced above average stream flow and lower ffg values. A small slight risk was introduced here and farther to the east over far northeast Pennsylvania into central to northern New York State where rainfall has also been above average over the past few weeks and stream flows are above average and ffg values lower. In both areas, isolated runoff issues possible from short term precip values up to an inch. Oravec Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_9vsHvZP7D1KKtkO02bvx9I0ZeN4tEaQynKojAwR7YQ1= SCFLYhZoieR-5s6T82Wk7D33GVoKGUBjyEhrWjUHCpdR_Yw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_9vsHvZP7D1KKtkO02bvx9I0ZeN4tEaQynKojAwR7YQ1= SCFLYhZoieR-5s6T82Wk7D33GVoKGUBjyEhrWjUH-jMybvE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_9vsHvZP7D1KKtkO02bvx9I0ZeN4tEaQynKojAwR7YQ1= SCFLYhZoieR-5s6T82Wk7D33GVoKGUBjyEhrWjUHkcanjmU$=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) .