Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 11 2024 00:10:24 FOUS30 KWBC 110009 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 809 PM EDT Wed Jul 10 2024 Day 1 Valid 01Z Thu Jul 11 2024 - 12Z Thu Jul 11 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF UPSTATE NEW YORK THROUGH MUCH OF VERMONT AND NORTHERN NEW HAMPSHIRE... ....01Z Update... Main change to the on-going Excessive Rainfall Outlook was to trim off areas where rainfall has ended. Maintained the Moderate Risk area with only a minor eastward adjustment to the boundary based on latest radar imagery from the area. Within the moderate risk area...though...the signals have started to become stronger for 30 to 50 percent probabilities for 100 year ARI exceedance which is increasing the concern for occurrences of impactful flash flooding (especially through about 11/06Z). MRMS has been showing hourly=20 rates of 1.5 to 2.5 inches in an hour with the potential for=20 similar rates persisting through the evening and into the overnight hours...resulting in 3 to 5 inch amounts and isolated higher=20 totals. Refer to WPC Mesoscale Precipitation Discussion 604 for=20 more details on the mesoscale aspects. Overall...though...the risk=20 of excessive rainfall should taper off during the overnight hours as better dynamics shift away from the area. Across New Mexico and the adjacent southern High plains...kept the Slight Risk focused over the Sacramento mountains given their=20 hydrologic sensitivity...although the expectation is that the=20 areal coverage and rainfall intensity will diminish with the loss=20 of daytime heating. Bann ....16Z Update... Minor adjustments were made to the MDT risk with an expansion to cover the northeast coast off Lake Ontario near Watertown/Fort Drum, and took the risk area to the Canadian border. The 10 and 100 year ARI exceedance probabilities off the 12z HREF were very impressive across much of North Country through Northern VT with >50% 10 year ARI exceedance signatures and 30-50% centered over the Adirondacks. Totals between 2-4" are very likely for a large portions of Upstate NY through Northern VT/NH with maxima around 6" still expected. Further to the southwest over MI, a targeted heavy rain threat across Southeast MI will linger through 18z before moving out into the neighboring Ontario Province as the remnant circulation from Beryl continues to progress northeast. The risk areas over the Ohio Valley and MI were adjusted to conform to current radar trends and forecasted precip next 4 hrs. There were no changes necessary over the Southwest U.S, mainly in NM where scattered convection will exacerbate flash flood threats within the multitude of burn scars present from the Sacramentos up through the north-central part of the state. Rates will top between 1-1.5"/hr at peak intensity in the strongest cells, more than capable of causing issues within any burn scars and smaller towns within the complex terrain. Scattered heavy thunderstorms continue to impact the Central Gulf coast with rates reaching upwards of 2-3"/hr in stronger pulses. FFGs are sufficiently very high and the threat for flash flooding will be highly localized, mainly to the southern edge of New Orleans over to Biloxi. The threat will diminish across the area after 00z so have maintained the targeted MRGL for continuity purposes given the current radar depiction. ....Northeast... Remnant moisture and mid-level energy from Beryl will translate northeastward out the Ohio Valley and Great Lakes creating a heightened threat for widespread heavy rain and flash flooding across Southeast Michigan through New York and Central/Northern New England. At the surface, a warm front will bisect much of New York through Central New England, marked very well by a sharp theta-E gradient. This will be a focal point for convection prior to the main circulation moving overhead, as well as the axis of where the highest PWATs will be confined during the unfolding of the synoptic scale event. Low- level instability will be highest within the confines of the warm front and points south with the best upper level forcing likely along and north of the boundary creating a dynamic scenario with a zone of highest heavy rain potential where all three characteristics overlap. The highest heavy rain prospects lay within the Adirondacks and points east into north-central Vermont and New Hampshire, including the Green and White Mountains in the respective states. 12z HREF EAS probabilities were much more aggressive in the signals for at least 2" and 3" across the aforementioned areas with a 50-80% probability for at least 2" within the Adirondacks to just south of the Champlain Valley, a strong signal for higher totals given the necessary overlap of CAMs to exhibit such a larger probability. Historically, when an EAS signal is above 70%, the expectation for widespread coverage of that value of rain or more is very certain and regardless a higher risk consideration if the areal FFG indices allow. HREF EAS for 3" managed to come in more aggressive with a large coverage of 25-40% with the highest potential across the Adirondacks and northern Mohawk Valley characterized by a targeted 45-50% signature. This is right within the inflection of where the warm front is forecast to reside, creating a zone of higher confidence for heavy rainfall. Heavy rain will be possible all the way into Maine where the elevated PWAT anomalies between +2 to +3 standard deviations will be recognized leading to a higher end SLGT risk residing from Southeast Michigan all the way into western and central Maine. Totals of 2-4" are anticipated with local maxima to 6" plausible within the above areas in the Moderate Risk. 1-3" will be possible as far south as central and northeastern PA up through the NY Capital District due to convection developing along and ahead of the trailing cold front moving through the area tomorrow afternoon and evening. Kleebauer ....Previous Discussion... ....Central Rockies and portions of the Southwest... A Slight Risk for excessive rainfall remains in effect across the higher terrain of central New Mexico. During this period monsoonal moisture and diurnal heating will maintain convection across portions of the Central and Southern Rockies and southwest New Mexico. Some of the highest forecast QPF is expected to focus in the vicinity of the Sacramento Mountains where there have been a few recent wildfires. he burn scar complex (Blue-2, South Fork, Salt and McBride) are highly sensitive to rainfall and can easily lead to flooding and debris flows, especially with the 0.25 to 1 inch QPF that is forecast. ....Central Gulf Coast... Training convection expected to continue along the stalled frontal boundary. Very high FFGs within the area highlighted by a Marginal Risk which will limit flash flooding to very isolated and mainly within larger urban zones like New Orleans and towns based in the southeastern Parishes. Given the limited threat, should the convection shift further south offshore there may be the need to downgrade in future updates. Campbell Day 2 Valid 12Z Thu Jul 11 2024 - 12Z Fri Jul 12 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR NORTHERN MAINE, DELMARVA AND SOUTHERN VIRGINIA, EASTERN CAROLINAS, CENTRAL MIDWEST, AS WELL AS THE SOUTHWESTERN U.S... ....20z Update... Previous MRGL risk across the east coast was broken into two different areas; Northern ME and the coastal Mid-Atlantic down through the eastern Carolinas. An additional MRGL risk was issued across the Central Midwest (More on this threat in the "Midwest" sub-heading below). There were no necessary changes to the orientation of the MRGL risk across the Southwest as guidance remains steadfast on a highly isolated convective regime with the primary areas of interest being the burn scars in NM and small urban threat of towns within complex terrain across Southeast AZ through much of NM. ....Northern Maine... Remnants of Beryl will finally eject out into the Canadian Maritimes, but some lingering convective threat across Northern ME could induce some local flash flood concerns, mainly within the Caribou CWA. Ensemble QPF remains heaviest across Downeast ME where the highest PWAT anomalies remain leading to some deterministic output indicating 1-2" of rainfall within the first 3-6 hrs of the D2 period. This is subject to some variability as some guidance has the bulk of the precip out of the region by 12/13z tomorrow which would constitute a nil for the ERO proposal. Considering the environment and ensemble output, decided to maintain continuity in the outlined areas, but removed areas further south where dry air advection is more likely and flash flooding threat is next to zero. ....Coastal Mid Atlantic... Stalled frontal boundary will bisect portions of the eastern sea board with elevated PWATs mirroring the alignment of the front to points east. Local environment will be primed for convective potential, but will have some assistance in upper level support as a meandering wave off the Southeast coast lifts northwest on the western fringe of the ridge in the Atlantic, acting as a beneficial forcing mechanism to enhance regional rainfall potential. The best chance will lie from the Southern Delmarva through the Virginia Tidewater down into the Eastern Carolinas. Latest HREF probabilities are fairly aggressive in their signature for local totals between 2-5" as neighborhood probs for 2-3" totals are running between 60-90% with an area of 5" probabilities between 50-70% across Eastern NC. This area has been very dry as of late, and is well reflected within the FFG indices present for 1/3/6 hr markers. Sandier soils located within the area encompassed by the MRGL risk will deter a higher threat for flash flooding, however isolated urbanized flooding is still plausible considering the potential for 2-4"/hr rates as per the latest HREF mean hourly QPF output. The MRGL risk was maintained in this update for the aforementioned area, but cut out the extent of NNJ and NYC/LI due to drier air advecting overhead, limiting the threat of flash flooding to basically zero. ....Midwest... Trailing shortwave trough pivoting southeast around the backside of the mean trough over the Great Lakes and Ohio Valley will strengthen as it moves across IA into IL by tomorrow afternoon with large scale forcing increasing across the Central Midwest and adjacent Mid-Mississippi Valley. Scattered convection will develop over central and southern IL, MO, over into IN by late tomorrow afternoon through the evening with a few stronger cores possible given some modest buoyancy and favorable mid-level ascent focused in-of the region. 12z HREF signatures for heavy rainfall point to a more locally impactful setup with less organized convection, but rates of 1-2"/hr possible, heavy enough to breach the current FFG indices located over the area after getting impacted from Beryl. Antecedent moisture in the top layer of the soils is well-defined with NASA SPoRT moisture percentiles relaying the 0-10cm layer running between 75-90% leading to higher runoff capabilities. Typically a setup like this wouldn't cause too much concern in the area of focus, but the recent rains push this setup over into the low-end MRGL threshold thanks to the expected rates in convection and primed soils. A MRGL risk was introduced across portions of the Central Midwest into MO. Kleebauer ....Previous Discussion... ....Southwest... The persistent monsoonal pattern of diurnal convection will maintain an elevated threat for localized heavy rainfall and flash flooding concerns. Local totals of up to 1" signal a lower-end threat, however it remains within the Marginal Risk threshold. Campbell Day 3 Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC AND NORTH CAROLINA... ....20Z Update... A SLGT risk was maintained from previous forecast as a second round of convection along a stalled frontal boundary will lead to scattered instances of flash flooding, mainly within urbanized areas and areas that see repeat heavy rains from the period prior. Updated CSU First Guess fields maintain the alignment of a SLGT risk across the Eastern Carolinas with a MRGL up through the Delmarva into South Jersey. The favorable surface and mid-level ascent pattern will continue through the D3 time frame with the ridge flexing back further west from the Atlantic leading to elevated moisture and mid-level ascent being pushed further inland of the Mid Atlantic. It's possible the northern extent of the SLGT could be scaled back given some small trends in lower QPF focused north of the Virginia Tidewater. This will be the area to monitor for future adjustments. The Eastern Carolinas are still the primary target for heavier rainfall with some areas in the two day period potentially receiving over 5" from the evolving event. Wanted to also make mention of monitoring the progression and convective evolution within a migrating inverted trough axis through South TX into the Rio Grande. Current ensemble forecast is relatively tame within the realm of QPF, however the environment is more than favorable for higher convective impacts, including heavy rain within the Rio Grande and points just inland. First Guess fields do pinpoint a small MRGL located within the corridor from Eagle Pass down through Laredo, so perhaps if guidance ramps up the QPF signal in future updates, there could be a targeted MRGL risk area added to the central RGV. For now, decided against the addition as the threat remains non-zero, but outside the 5% threshold for issuance. Another area we'll be monitoring in future updates. Kleebauer ....Previous Discussion... The training of thunderstorms capable of producing several inches of rain will continue through this period. Rain from the day 2 period will have helped to raise soil saturation along the Mid- Atlantic and Carolina region thus lowering FFG. Most of the guidance is suggesting additional amounts of 1 to 3 inches from South Carolina to southern New Jersey with a couple showing local maximums up to 5 inches along this corridor. A Slight Risk was maintained for this part of the East Coast. A Marginal Risk spans from South Carolina northward to southern New hampshire. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-IbRHTeYDVOtvkhODy4oxpzXIWUkFecCl15DWQ7P0ABv= 8hwpBFRY2SVgZS5mmyPk4IsMZ87g4u568RueE4Y_RqEDm0M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-IbRHTeYDVOtvkhODy4oxpzXIWUkFecCl15DWQ7P0ABv= 8hwpBFRY2SVgZS5mmyPk4IsMZ87g4u568RueE4Y_cFWrZcE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-IbRHTeYDVOtvkhODy4oxpzXIWUkFecCl15DWQ7P0ABv= 8hwpBFRY2SVgZS5mmyPk4IsMZ87g4u568RueE4Y_nHJUnsw$=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. 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