Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 16 2023 22:37:57 FOUS30 KWBC 162237 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 636 PM EDT Wed Aug 16 2023 Day 1 Valid 2230Z 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... ....2230 UTC Update... Given latest radar and satellite trends in coverage of convection...opted to expand the western periphery of the Marginal Risk area northward and westward in southern California. Coverage still looks to be isolated to widely scattered...but some problems have started with run off amounts where convection has formed over or near complex terrain. The expectation is that rates should begin to settle back a few hours after loss of daytime heating. Bann 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 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!84r4jOxEbHsAIwY8iU9S-vSgiZhp6DDPF2q9vSkTAvu1= 91KJSNyOIhcslT1-saVYhzGcI8zrioOcAg5PXiOvfklt834$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!84r4jOxEbHsAIwY8iU9S-vSgiZhp6DDPF2q9vSkTAvu1= 91KJSNyOIhcslT1-saVYhzGcI8zrioOcAg5PXiOvGZB0K7M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!84r4jOxEbHsAIwY8iU9S-vSgiZhp6DDPF2q9vSkTAvu1= 91KJSNyOIhcslT1-saVYhzGcI8zrioOcAg5PXiOvVPzCMDo$=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) .