Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 10 2023 08:27:59 FOUS30 KWBC 100827 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Mon Jul 10 2023 Day 1 Valid 12Z Mon Jul 10 2023 - 12Z Tue Jul 11 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR THE CHAMPLAIN VALLEY AND MUCH OF NORTHERN VERMONT... ....New England and Upstate New York... ....Widespread significant flash flooding likely today... A 500mb shortwave rotating within a large and broad longwave trough across the eastern CONUS will sharpen into a closed low over Upstate NY this morning and then pivot very slowly eastward through the day. This will drive intensifying downstream ascent through height falls and PVA, overlapped efficiently with the diffluent LFQ of a poleward arcing jet streak strengthening to 90-100 kts downstream of the primary trough axis. The increasing negative tilt to this feature will drive the 850mb low across central New England, with flow returning off the Atlantic and Gulf of Maine northwest back into New England at 20-25 kts, with the surface low tracking slowly northeast just inland of I-95. During this evolution, the large scale ascent will intensify across New England, resulting in an expansive area of heavy rainfall today. The 850mb inflow will drive impressive WAA northwestward, which will manifest as an impressive theta-e ridge lifting cyclonically into a TROWAL over New England. This theta-e ridge will then converge into a sharp mid-level deformation axis oriented N-S to produce locally even more intense ascent and provide a focus for heavy rainfall. This then produces a concerning setup for training and repeated rounds of heavy rainfall as mean 0-6km winds remain S to N through the day to impinge along this axis while inflow remains out of the E/SE. With PWs progged to be 1.75-2", around +2 sigma according to the NAEFS ensemble tables and above the 90th percentile from the SPC sounding climatology, modest 250-750 J/kg of MUCAPE through a deeply saturated column, and warm cloud depths above 11,000 ft, warm/efficient rainfall rates will likely peak above 2"/hr, and the HRRR suggest 15-min rainfall may exceed 1" at times (short term 4+"/hr rates). These rates in themselves could be enough to overwhelm soils to result in runoff and flash flooding, but what makes this event more alarming is that they will be occurring atop pre-saturated grounds. 14-day rainfall from AHPS has been 200-300% of normal across much of central and northern New England, leading to USGS streamflow anomalies that are almost uniformly above the 90th percentile, and in some places are well above all-time record flows. This has also resulted in FFG that is as low as 0.75"/1hr and 1-1.5"/3hrs, further indicating the sensitivity of the local soils. This suggests that even moderate rainfall will become runoff as the soils are fully hydrophobic. With rain rates of 2+"/hr likely, and training of echoes repeating through the day, many places will likely receive more than 3" of rain as reflected by all the ensemble probabilities, with local maxima of 5-8" probable noted by HREF probabilities reaching 60% for 5" and even 20-25% for 8". While there remains some uncertainty as to the exact placement of the heaviest rainfall axis, the HREF EAS probabilities for 3" are above 40% in western VT which is a notable value for this threshold. This indicates that guidance is coming into better agreement on the axis of heaviest rainfall which will focus beneath the deformation axis aligned near Lake Champlain. The inherited high risk and surrounding EROs still seem warranted for what could be a considerable flash flood event for the region. While rainfall on Sunday across parts of VT/NY was not quite as high as predicted, soils are still extremely vulnerable, and the high risk overlaps well the highest EAS probabilities and 24-hr/100yr RI exceedance probabilities which reach 40-50%. Repeated rounds of convection will likely plague much of Upstate NY and New England through the day, with WAA/TROWAL heavy rainfall transitioning by evening to a deformation band of additional moderate to heavy rain, and the high risk overlaps where the longest duration is expected. Minor cosmetic changes were made to the ERO, with the most significant adjustment actually spreading the MDT risk into the White Mountains of NH where impressive moist upslope flow could focus a secondary maxima in rainfall atop already saturated soils. ....Southeast... A broad but amplified longwave trough will continue to extend down into the Southeast today, with the base of the trough lingering along the Gulf Coast. This will result in more flattened, almost zonal, mid-level flow across the Gulf Coast/Southeast, within which weak vorticity impulses embedded within shortwaves will traverse eastward through the day. At the surface, a stationary front will ripple from Texas to South Carolina, interacting with the weak impulses aloft to produce waves of low pressure along it. Additionally, a zonally oriented jet streak across the lower TN VLY will pivot eastward and begin to arc to the north during the aftn, placing increasingly favorable diffluence within the RRQ aloft, to provide additional ascent. The result of this should be plentiful deep layer ascent through D1 to produce waves of convection. South of this front, 850mb winds will emerge out of the Gulf of Mexico at 20-30 kts to provide impressive inflow, which will then impinge into and along the front. This will surge PWs to as high as 2.25 inches, more than 2 standard deviations above the climo mean according to NAEFS, and produce IVT above 500 kg/m/s, highest in GA/SC. This moisture plume will overlap with MUCAPE that will approach 2000 J/kg to produce favorable thermodynamics for intense rain rates within convection, especially during peak heating. Rainfall rates as progged by the HREF will be 1-2"/hr, and with mean 0-6km winds aligned to the front and at least obliquely to the Corfidi vectors, it is likely these rates will train west to east through the day. Additionally, with higher instability lingering over the Gulf of Mexico, Corfidi vectors that are progged to collapse to just 5-10 kts indicates the potential for some backbuilding to prolong rainfall near the Gulf Coast. Anywhere training occurs will experience more than 3 inches of rain today, but there may be two local maxima as reflected by HREF probabilities of around 20% for 5". The first is along the Gulf Coast from near Plaquemines Parish, LA through the Big Bend area of the Florida Panhandle, where backbuilding into the Gulf of Mexico will likely train to the northeast to enhance the duration of heavy rainfall. The second area is along the coast of South Carolina where westerly flow along the front will allow for potentially significant training while also pinning the sea breeze near the coast to also prolong heavy rainfall potential. Consideration was made for two slight risk areas here, but recent rainfall, especially along the Gulf Coast has been light so souls are likely still hydrophillic there noted by FFG that is generally 3"/3hrs. However, after coordination with WFO CHS/ILM, a narrow SLGT risk was added for the coastal plain of SC where HREF exceedance probabilities are greater, and recent rainfall has been as high as 5" in isolated areas. ....Southern Plains... An anomalous mid-level trough encompassing much of the eastern third of the CONUS, and an amplifying ridge centered over the Desert Southwest will leave pronounced NW flow across the Southern Plains today. Within this flow regime, weak shortwave impulses and associated vorticity maxima will rotate down across the region to drive bouts of locally enhanced ascent. As the ridge to the west intensifies this evening, and a potent vorticity max shifts out of Colorado, low-level flow will back in response, driving a warm front northeastward and producing impressive WAA into an elevated boundary late. This WAA will surge PWs to around 1.75 inches, coincident with a ribbon of MUCAPE reaching 2000-3000 J/kg, providing robust thermodynamics for heavy rain producing convection, with thunderstorms likely organizing into an MCS through effective bulk shear of 40-50 kts. The high-res guidance is all in agreement that an MCS will develop, but as is typical with these subtle shortwaves in summer, the placement varies widely. This is noted in high-res simulated reflectivity, as well as HREF EAS probabilities that are minimal for 1 inch. However, the HREF neighborhood probabilities for 1"/hr and 2"/hr rates peak along and north of the Red River Valley of the South into Oklahoma, which is also an area that has received heavy rainfall the past 7-day noted by AHPS departures that are 300-600% of normal. Storm motions may generally be progressive within this MCS, but some increasingly right-angled Corfidi vectors to the mean flow suggest at least short-duration training is possible, and the HREF 3-hr maximum probability for 3" exceeds 30% tonight. Locally, this event could produce 3-5" of rainfall, which if it falls atop pre-conditioned soils from recent MCS events could lead to instances of flash flooding. Confidence in the position of the SLGT risk is admittedly modest due to uncertainty in the MCS track, but it was modified cosmetically to account for the newest guidance and most vulnerable antecedent soil conditions. ....Upper Midwest and Great Lakes... A cold front will dig southward from Canada today, but gradually become more elongated west to east across the northern tier of the CONUS. This evolution will be driven by a potent vorticity lobe rotating around the anomalous closed low near the Hudson Bay, with a secondary lobe swinging southward into MN late in the day help to stretch the front. By the end of D1 this front should finally eject southeast towards the Ohio Valley. This front will serve as a focus for showers and thunderstorms today as ascent maximizes through low-level convergence, modest height falls/PVA, and weak RRQ diffluence as a jet streak pivots southeast during the evening. This ascent will overlap favorable thermodynamics for heavy rain characterized by MUCAPE of 1000-2000 J/kg and impressive moisture flux as 30kt SW 850mb inflow surges PWs of 1.25 inches into the boundary. As the front becomes stretched west to east, this will allow the mean 0-6km flow to become aligned parallel to it, suggesting an increased training threat from Wisconsin through the U.P. of MI. At the same time, Corfidi vectors turning more into the intensifying inflow become nearly anti-parallel to the mean wind and collapse to just around 5 kts suggesting at least some backbuilding potential into the higher instability. With rain rates progged by the HREF to exceed 1"/hr at times, this training/backbuilding scenario could produce areas of 1-3" of rainfall. Despite modest recent rainfall leading to FFG that is around 2"/3hrs, there are some pockets of more sensitive soils, especially in the U.P., but HREF exceedance probabilities peak only around 10-20%. The MRGL risk was maintained and adjusted cosmetically, as some isolated flash flood responses are possible during the most prolonged training today. ....Northeastern Washington through Montana... A shortwave ejecting out of the Pacific Northwest this morning will amplify into a closed low as it moves into Alberta, Canada, leaving increasing downstream mid-level divergence, which will overlap with the favorable LFQ of an upper jet streak to provide pronounced ascent across the Northern Rockies and Northern High Plains. This will intersect a cold front digging southward out of Canada, with increased moist flow advecting from the southeast along the front. This will push PWs to 1-1.25", more than 1 standard deviation above the climo mean according to NAEFS. The atmosphere will marginally destabilize ahead of this front, with MUCAPE possibly exceeding 500 J/kg in places. The overlap of this deep layer ascent into this narrow ribbon of favorable thermodynamics should result in scattered thunderstorms, which will have rainfall rates that may reach 1"/hr at times as progged by the HREF neighborhood probabilities. Storm motions will generally be quick to the east, but repeated rounds could produce some areas of 1-2" of rainfall. While the FFG exceedance probabilities are modest, parts of MT have had excessive rainfall noted by AHPS departures that are 150-300% of normal leading to above normal USGS streamflows, while recent burn scars are also more vulnerable than depicted by FFG. Any repeating rounds of heavy rain could produce runoff and isolated instances of flash flooding. Weiss Day 2 Valid 12Z Tue Jul 11 2023 - 12Z Wed Jul 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-MISSOURI BASIN... ....Northern Plains into the Upper Midwest... Squeezed mid-level flow across the Northern Rockies will dive into NW flow over the Upper Midwest as a broad trough persists across the eastern CONUS. Shortwaves and accompanying vorticity maxima rotating through the flow will advect eastward and then drop southeast, joining the LFQ of an upper jet streaking from the Pacific to produce robust ascent across the area. This synoptic lift will interact with a wavering frontal boundary draped northwest to southeast from MT through MO, with increasing low-level S/SE flow impinging into this boundary. This low-level inflow will reach 15-25 kts, surging PWs to 1.25-1.75 inches, more than +1 standard deviation above the climo mean according to NAEFS, to provide substantial available moisture for heavy rainfall. Additionally, instability will climb to 1000-2000 J/kg within a ribbon lifting north along the front, with a steep CAPE gradient developing from SW to NE during this time. This overlap of forcing and moisture/instability should result in scattered to widespread thunderstorms along the front, with the greatest excessive rain threat likely developing through MCS development late in the forecast period. Thunderstorms that develop along the front from eastern MT through IA will likely have rainfall rates of 1"/hr or slightly higher within the favorable thermodynamics. The available high-res keeps activity generally scattered along the front, and storm motions using the 0-6km mean winds as proxy should be relatively quick, but along the front to support at least short-term training. This could result in isolated runoff responses, especially over more sensitive soils from 14-day rainfall that is 150-300% of normal in some areas. However, the greater flash flood risk exists from far eastern SD through northern MO where the guidance suggests an MCS will develop and potentially train along the instability gradient. There continues to be spread in the timing and placement of this MCS, but it is likely one will develop. At this time, the SLGT risk was tailored to best match the 12-hr HREF and 24-hr GEFS probabilities for 1 inch and 3 inches, but additional adjustments are likely. ....Northern New England... The excessive rain event from D1 will wane during D2, but guidance indicates the deformation will persist across northern VT/NH/ME during the first few hours of this forecast period. Additional scattered showers are possible through the day across central/northern New England as well before much drier air advects eastward into New England to shut off the rain. The ECENS/SREF/GEFS probabilities all suggest at least modest exceedance for 1" of rainfall, with briefly heavy rates likely within this pivoting deformation axis thanks to MUCAPE of 500-1000 J/kg. The rainfall in itself on D2 is not likely enough in any location to produce flash flooding. However, the soils will likely be extremely saturated from heavy rain and possibly significant flash flooding during D1, so any additional rainfall could result in further instances of flash flooding. The MRGL risk was trimmed to match updated guidance, but at least an isolated threat still exists before the system exits by late D2. ....Southern Plains through the Gulf Coast... A stalled cold front aligned along the Gulf Coast will weaken and dissipate into a trough Tuesday, but still provide a focus for ascent and thunderstorms during peak heating. A broad trough still encompassing much of the eastern CONUS will maintain an axis down to the Gulf of Mexico, producing subtle height falls and steepened lapse rates to help drive MUCAPE to 2000-3000 J/kg during the aftn/eve. This impressive instability combined with PWs still progged to reach around 2 inches will produce a sufficient environment for heavy rain rates exceeding 1"/hr. The overlap of highest PWs and greatest MUCAPE does not look ideal on D2 except near Louisiana and the northern Florida peninsula, so the excessive rain risk appears modest. However, rain rates of 1+"/hr within these favorable thermodynamics slowly training along the front through parallel 0-6km mean winds of just 10 kts, could produce locally 1-3" of rainfall as progged by HREF, SREF, and GEFS probabilities. The MRGL area is quite broad as determining a focus for greatest threat is challenging, but a subtle focus may exist along the sea breeze/collisions from eastern Louisiana through the western Florida Panhandle where isolated higher rainfall amounts are possible. Weiss 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!-t-qDd10pWf1WwTdlgbaNI8-IFAVHaS5S7KCDSnWrtM1= ocurQo47Oo6b_GVCFJb4w8_jVdl2HWWFAiAzruoUuDsBUs4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-t-qDd10pWf1WwTdlgbaNI8-IFAVHaS5S7KCDSnWrtM1= ocurQo47Oo6b_GVCFJb4w8_jVdl2HWWFAiAzruoUyZMDLVs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-t-qDd10pWf1WwTdlgbaNI8-IFAVHaS5S7KCDSnWrtM1= ocurQo47Oo6b_GVCFJb4w8_jVdl2HWWFAiAzruoUg3vwgRY$=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) .