Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon Jul 10 2023 16:08:00 FOUS30 KWBC 101916 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 315 PM EDT Mon Jul 10 2023 Day 1 Valid 16Z Mon Jul 10 2023 - 12Z Tue Jul 11 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR THE CHAMPLAIN VALLEY AND MUCH OF VERMONT... ....WIDESPREAD SIGNIFICANT FLASH FLOODING LIKELY THROUGH TONIGHT... ....16Z Update... ....Northeast... Several changes to note as we progress through this very busy Monday. The High Risk area was extended southward to cover the rest of southern Vermont with the Special Update. No big changes were made to the High Risk area since that special update. Remarkably, the latest 12Z HREF guidance has increased 100-year ARI exceedance probabilities to over 80% now near the northwest corner of VT, near St. Albans, with over 70% chances from Burlington north to near the Canadian border along I-89 to near the spine of the Green Mtns. The surrounding Moderate Risk area was trimmed from the southwest to eliminate northwestern CT and the southwest corner of MA. Unfortunately the amount of trimming couldn't continue to follow the radar signature because the surface low driving this continues to strengthen and will pivot more Atlantic moisture into New England through tonight, so the back edge of the rain for western MA, VT, and NH is unlikely to continue to progress as daytime heating allows more widely scattered convection to begin redeveloping behind the plume of moisture. The heavy rain advancing to the west of Boston, including Worcester is likely to produce flash flooding, so the Moderate Risk area was expanded eastward for those storms moving through over the next couple hours. In similar fashion, for eastern CT (and eventually RI once the main band of storms moves through) while the rain has stopped here for now, additional convection is expected to form through the afternoon with daytime heating, and with very low FFGs, still could result in additional flash flooding, though nowhere to the extent to what has already occurred. By around sunset with diminishing solar heating so too will the flash flood threat from any afternoon storms. No changes were made to the Slight Risk area from MA north, with trims on the southwestern edge out of southern NY and western CT made. With a nearly stationary band of heavy rain set up in the eastern Lake Ontario region down through Syracuse due to the potent upper low pivoting southwest of that region, the Slight Risk area was expanded westward until that convection dissipates. Recent radar trends have been showing the band strengthening, so this may take several hours. For similar reasons, the possibility of widely scattered, but potentially strong afternoon storms this afternoon for southern NY and eastern Long Islands has prompted keeping the Marginal Risk largely the same. ....Southeast... Trimmed the Slight Risk out of central SC in coordination with CAE/Columbia, SC forecast office. The storms have been strong, locally producing rainfall rates to 1.5 inches per hour, but they are moving rapidly eastward. This rapid movement will greatly diminish the flash flooding potential, and the Slight along the SC coast is largely for the storms moving over potentially sensitive areas over the next 1-3 hours, with any following scattered cells behind the main line of convection. This is considered a low-end Slight for today. Along the Gulf Coast, largely as expected, most of the thunderstorm activity is off the coast, and those that are over north FL are moving quickly, so the Marginal was maintained with no changes. ....Southern Plains... Training showers and storms have developed across northeast TX and into northern LA, but fortunately they're moving quickly enough and are widely spaced apart from one another enough over areas that have high enough FFGs that the flash flooding threat remains low. The Slight for southern OK and far northern TX remains in place, and is for expected storms that will develop late this afternoon through the overnight, developing into an MCS. The MCS will be fast moving, but there are sensitive areas of west central OK and far northern TX that may get enough rain, especially with any cell mergers that occur ahead of the main line of storms that the flash flooding threat is largely elevated. The MCS has yet to form as of this update. Supporting the threat for flash flooding is the highly anomalous amounts of atmospheric moisture that will be advected into the region ahead of the storms, with PWATs likely exceeding 2 inches. This will allow the storms to be very efficient rain-makers compared to the typical hot and dry weather that is normal of mid-July. Thus, the sheer strength and abundance of moisture available to the storms may overcome their fast motion to result in more widely scattered flash flooding across the Slight Risk area. ....Washington State... A progressing but vigorous shortwave trough over southern WA will lift into the northeastern part of the state by late today. The showers and storms over central WA will develop into more of a comma-shaped appearance over the next several hours, with the Slight Risk area highlighting where the comma-head region is expected to develop. Thunderstorms are expected to develop within this region this afternoon, which could produce heavy rainfall rates. The flash flooding risk is heightened in those areas where the storms form or move over existing burn scars, canyons to the east of the Cascades, and any other flood-sensitive and more urbanized areas of north-central WA. In coordination with OTX/Spokane, WA forecast office, a Slight Risk targeting the afternoon and every evening hours today was introduced. Elsewhere into ID and MT few changes from inherited were made. These areas are ahead of the aforementioned shortwave, so while there is some potential for backbuilding, any individual shower and thunderstorm cells will be moving quickly, which will keep the flash flooding threat isolated. Wegman ....Previous Discussion... ....New England and Upstate New York... 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. (cont'd) --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .