Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 20 2023 09:02:32 FOUS30 KWBC 200902 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 501 AM EDT Thu Jul 20 2023 Day 1 Valid 12Z Thu Jul 20 2023 - 12Z Fri Jul 21 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN COLORADO INTO WESTERN KANSAS... ....Lee of the Central Rockies into the Central to Southern High Plains... A favorable setup for heavy rainfall is expected today/tonight across portions of eastern CO into western KS. In the mid levels, shortwave energy will rotate east southeastward on the northeast side of the Southwest upper high, pushing east southeast from the lee of the Central Rockies into the Central to Southern Plains. A jet streak in the upper levels will be favorably positioned for enhanced upper level divergence. In the lower levels a surface front will drop southward, with post frontal upslope flow and strong 850mb convergence helping initiate convection. This activity will likely enhance into organized convection that will push southeastward through the evening and into the overnight hours. Deep layer moisture will increase across the region as well, with mid/upper level moisture advecting in from the Great Basin, resulting in PWs over the climatological 90th percentile. Not looking at extreme instability, but HREF mean MUCAPE does get into the 1000-2000 j/kg range...plenty to support heavy rainfall rates given the forcing and moisture in place. The main question from a flash flood perspective will be the duration of these heavy rainfall rates. Most indications are that this activity will be pretty progressive off to the southeast, likely putting a cap on the upper rainfall potential. This is evident in the HREF neighborhood QPF probabilities, with a 40-70% chance of exceeding 3", but only a 5-15% of exceeding 5"...thus swaths of 3-5" seem most likely with this event. However given the degree of deep layer forcing present, tend to think this will initially be a messy convective evolution...with several convective clusters developing. Some merging of these clusters is likely, which should locally extend the duration of heavy rainfall rates and enhance the flash flood potential. And even though extreme rainfall totals are less likely, HREF probability of exceeding FFG is quite high, up to 50-80%...so seems pretty likely that we will end up with enough coverage of FFG exceedance to warrant a MDT risk upgrade. So even though the threat of 6"+ amounts is low, the coverage of 2-5" amounts should be enough to result in numerous instances of flash flooding later today into tonight, especially given the high rainfall rates expected. Portions of this area saw heavy rainfall and flash flooding Wednesday evening, further increasing confidence in the need for a MDT risk for today into tonight. The eastern extent of the MDT risk was bound close to the edge of the higher HREF probabilities. We did extend the MDT a bit further southwest than the HREF would support...to account for antecedent rainfall, a further southwest QPF footprint in some of the global models, and a common bias of models underdoing QPF over these further west areas. Scattered to numerous instances of flash flooding are anticipated within the MDT risk area, with localized significant impacts a possibility. With time convection should tend to become more linear and progressive in nature, so did not not extend the MDT into central KS...but some flash flood risk does likely still exist as this organized convection pushes east across KS and northern OK later tonight...with a Slight risk in effect for these areas. ....MO/AR into KY and TN... At 12z Thursday convection is likely to be ongoing across portions of southern MO. This activity should be quick moving, but enough moisture and instability is in place to suggest at least a low end flash flood risk. The bigger question is what will be ongoing across portion of KY and TN at 12z. Increasing 850mb moisture transport/convergence and an area of 1000-500 hPa thickness and height diffluence across the region implies the possibility of organized/training convection across the area this morning...with central TN having the highest chances. What may be an MCS by this time over southern MO is also expected to survive eastward into portions of KY and TN this morning/afternoon as well. There is some chance that this MCS merges in with the potential training convective complex over TN...and if this happens a more significant flash flood risk could evolve. Confidence is low on these details at this time as convection thus far early this morning has not been able to organize, so will not pull the trigger on a MDT risk upgrade. But this is considered a higher end Slight risk and the potential is there for significant flash flooding if things come together. By late Thursday night the aforementioned CO/KS convection should be moving southeast across OK, and we should have an axis of enhanced low level convergence stretching across AR and into northern MS/AL. There is some chance we get some nocturnal convective development along this corridor, but low confidence on these details. Given the potential convection today, and then again tonight...thought a broad Slight risk extending across the Plains and into the TN valley was warranted. ....Northern Mid-Atlantic and Northeast... The strong upper level trough will move from the Upper MS Valley through the Great Lakes, OH Valley and Southern to Central Appalachians Thursday and Thursday night. Impressive upper level divergence is also noted in the left exit region of an approaching jet. Instability of 1000-2000 j/kg will be plenty given the forcing to support an organized line of convection moving across portions of OH/PA and NY. PWs are not overly anomalous, but still high enough to support hourly rainfall of 1-1.5" or so. Most indications are that the convective line that does develop will be quick moving, limiting the rainfall total potential. However there are some signs that we could be looking a couple convective lines, which would allow for a bit more duration of high rainfall rates. Overall it seems like this is an event where max rainfall will peak in the 1-3" range, so not extremely high, but enough to support isolated to scattered FFG exceedance, especially over northeast PA end adjacent areas of NY where more saturated soil conditions and above average streamflows have lowered FFG. Chenard Day 2 Valid 12Z Fri Jul 21 2023 - 12Z Sat Jul 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR THE MID-SOUTH, NORTHEAST NEW MEXICO AND THE NORTHEAST... ....Mid-South/Southeast... An axis of above average PW values will stretch west to east from the Southern Plains into the Lower MS Valley/TN Valley and Southeast in the vicinity of the west to east oriented front forecast to be moving slowly south and southeast on Friday. An area of organized convection should be ongoing at 12z Friday in the vicinity of OK/AR. This activity may very well survive as it tracks east southeast along/near the aforementioned boundary. Exact convective mode/evolution becomes more uncertain as Friday progresses...but the overall environment will remain conducive for heavy rainfall with low level convergence and deep layered west northwesterly flow supporting some training/backbuilding potential. Some chance activity will have enough of a southward progression that the eventual flash flood threat ends up more muted, but given the ingredients and potential in place, think a broad Slight risk is warranted at this time and we can fine tune as we get closer. Models continue to nudge southward with the favored convective axis, so the Slight risk was expanded in this direction. ....NM... Low-level easterly upslope flow will strengthen Friday and Friday night over northeastern NM as the cold front continues to sink southward through the Southern High Plains. Meanwhile this area is clipped by the right entrance region of an upper jet to the north, resulting in a notable uptick in upper level divergence. This combo of low level upslope and convergent flow and divergent flow aloft should support scattered to numerous convective development Friday afternoon. Model guidance still supports the Slight risk across this region, with pockets of 2-4" of rain appearing likely. ....Northeast Mid-Atlantic into New England... Convection should be ongoing at 12z Friday across portions of the eastern Mid-Atlantic into eastern NY, but will probably be in a gradual dissipating state. The synoptic pattern becomes more stagnant Friday, with the trough slowing as it lifts north, and the upper jet remaining in a favorable position for enhanced upper level divergence. This pattern will result in a slowing of the eastward moving cold front as well. Thus as morning convection dissipates it is becoming increasingly likely that we see additional convective development by afternoon from eastern PA into the Northeast. With the slow movement of the front, these cells may tend to briefly anchor in place and/or only gradually move eastward with time. The 00z ECMWF and most of the HREF guidance is pretty aggressive with this convective development and rainfall totals. Through 00z Saturday the HREF is indicating the potential for swaths of 2-3" rainfall where these slower cells materialize. Given the wet pattern of late over this area, soil conditions and streamflows are more susceptible than normal for flash flooding. Thus it seems probable that at least an isolated to scattered flash flood risk will evolve across portions of eastern PA into eastern NY and into VT. There remains some question regarding the exact position of the front by Friday afternoon, and exactly how the transition from dissipating morning convection to developing afternoon storms evolves. Do think this event has higher end Slight risk potential, and can not rule out an embedded smaller MDT risk as the event nears. The lack of anomalous PWs is one limiting factor, but this should be overcome to some extent by the strong forcing and slow movement of cells. ....Southeast FL... Added a small Marginal risk across southeast Fl with this update. There is a notable uptick in HREF QPF output Friday afternoon as an approaching low level front helps enhance convergence and potentially help organize convection a bit more than a typical day. Plenty of instability and moisture to support heavy rainfall rates, so think at least a localized flash flood risk could evolve if convective clusters happen to merge over an urban area. Chenard Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_uo23bvy3XQMUOCEqz4IXQ57R2ardTVUXXzpjECtoIUV= 8GJXiVEQNn0QgrqZpLn-tjALb0pc3uaeJw4hXD3Fm43pXvM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_uo23bvy3XQMUOCEqz4IXQ57R2ardTVUXXzpjECtoIUV= 8GJXiVEQNn0QgrqZpLn-tjALb0pc3uaeJw4hXD3FWkmzZc8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_uo23bvy3XQMUOCEqz4IXQ57R2ardTVUXXzpjECtoIUV= 8GJXiVEQNn0QgrqZpLn-tjALb0pc3uaeJw4hXD3F-8yr1Qk$=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) .