Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 17 2023 00:52:26 FOUS30 KWBC 170052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Wed Aug 16 2023 Day 1 Valid 01Z Thu Aug 17 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... ....01Z Update... ....Southwest U.S. into the Southern Basin... Short term trends in radar and satellite imagery showed widely scattered convection continuing over the Southwest U.S. as of early evening. Locally intense rainfall rates in and near the terrain has resulted in isolated problems with run off late this afternoon and early evening. The potential for flooding or run-off problems should gradual decrease over the next few hours with the loss of daylight...so will maintain the Marginal Risk area with the expectation that the threat is primarily during the evening hours. South Carolina to Florida... Trimmed some territory out of the western part of the Marginal Risk area in Georgia and South Carolina due to a subtle eastward shift in the position of a quasi-stationary boundary since late morning. However...weak low pressure moving northeastward along the front will continue to support locally heavy rainfall rates on the warm side of the front from coastal Georgia southward into Florida this evening and tonight. The threat of excessive rainfall will continue to be aided by the right entrance region of an upper level jet off the immediate coast.=20 Bann 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 Valid 12Z Fri Aug 18 2023 - 12Z Sat Aug 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST ARIZONA INTO SOUTHWEST UTAH... ....20z Update... Minimal change to the Marginal over the Northeast. However, this event bears watching as things seem to be trending towards a slightly greater flash flood risk. Heavy rain may be ongoing at 12z Friday (a continuation of the day 2 event described in that discussion)...with another round possible during the day Friday as destabilization occurs ahead of the approaching trough/closed low. One of the two CSU ML tools is now showing Slight risk probabilities...but think there is enough uncertainty to hold off on any upgrade at this point. The timing of the trough and how it interacts with the moisture/instability streaming up the coast will play a role in the magnitude of the eventual flash flood risk. We did add a Slight risk with this update over portions of northwest AZ into southwest UT. Seeing a notable uptick in mid/upper level forcing by this time as the closed low off the CA coast shifts eastward and the upper jet west of the area intensifies. Not really seeing much in the way of moisture from Hilary quite yet, but PWs are still above average...and will probably end up with ~1000 J/KG of instability. Thus should be enough moisture and instability around to support heavy rainfall rates, and the uptick in forcing may be enough to increase convective coverage/organization. Thus it seems like there will be an increasing flash flood risk over the susceptible areas of northwest AZ into southwest UT...so went ahead with a Slight risk upgrade. We also added a Marginal risk over portions of central/southern FL. This is really just a continuation of the Marginal risk from days 1 and 2...with the front remaining in the area along with elevated PWs and sufficient instability. Not expecting the risk to be any higher or lower than days 1 and 2...with the threat primarily confined to any more susceptible urban areas. Chenard=20 ....Previous Discussion... ....Southwest, Southern Great Basin, Southeast California and the Northern Sierra... The closed low moving toward the south central California coast day 2, will become stationary and elongated along the central California coast day 3. Not a lot of changes to the large scale flow to the east of this closed low and to the west of the upper high pushing eastward day 3 into the Central to Southern Plains.=20 Broadly difluent upper flow between these features in an axis of continued above average PW values across these regions will support another day of widespread scattered convection and isolated runoff issues, especially across any burn scar areas or slot canyons. Higher PW values may begin to enter the Southwest by the end of day 3 ahead of the potential tropical system models are forecasting to move northwest off the southern portion of Baja California Sur. At the moment, the risk level was kept at marginal, with higher risks denoted on the WPC days 4 and 5 Excessive Rainfall Outlooks.=20 ....Northern New York State into Northern New England... Strong height falls will be moving east from the eastern Lakes/Northern Mid-Atlantic into the Northeast day 3. Higher PW values are expected to be drawn northward ahead of the associated cold front pushing eastward across eastern NY State into New England. This will support locally heavy rains from northern New York State into western to northern New England in a region of defined frontal/pre-frontal convergence. Contemplated not having any risk area for day 3, but the CSU first guess fields still are showing marginal risk of 24 hour qpf exceeding the 1-2 year ARI across this area. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4tlJTZY23G0Uu9HoeFoyk_I28EwIVC4DQSAg_2BWJBo2= rBboKDA40Swu9EEWxOseGm4gLP-SQuBQr2hqbYkgCeTNjtw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4tlJTZY23G0Uu9HoeFoyk_I28EwIVC4DQSAg_2BWJBo2= rBboKDA40Swu9EEWxOseGm4gLP-SQuBQr2hqbYkgUW180fg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4tlJTZY23G0Uu9HoeFoyk_I28EwIVC4DQSAg_2BWJBo2= rBboKDA40Swu9EEWxOseGm4gLP-SQuBQr2hqbYkgw2b3sj8$=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) .