Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 09 2023 22:33:20 FOUS30 KWBC 092233 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 632 PM EDT Sun Jul 09 2023 Day 1 Valid 2232Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN MID-ATLANTIC THROUGH WESTERN NEW ENGLAND... ....22Z Update... The Northeast Slight Risk was expanded north over western Maine. The Moderate Risk was contracted for areas behind the main activity in interior northern Mid-Atlantic and the central Mid-Atlantic since the main event in these areas is over. The Deep South Slight Risk was contracted a bit based on expected activity ahead of the impulse over AR. ....Mid-Atlantic to the Northeast... Please see MPDs 0679 and 0680 for further information on the ongoing heavy rain threat over the Northeast. Only slow progress is anticipated with the moisture plume causing the heavy northern Mid-Atlantic into southern New England activity through the overnight. There is a risk for additional rounds of heavy rain where the Moderate Risk remains. ....PA/NJ/Downstate NY... Training storms have already broken out along the I-81 corridor through Scranton/Wilkes-Barre with less organized but quickly becoming numerous convection noted from the Philadelphia area northeast into northern NJ. As the upper level trough driving all of this rainfall across the Northeast pivots northeastward, the forcing lining up with the mean southwesterly flow will allow convection to continue growing along the aforementioned local scale fronts, boundaries, and instability axes. Since they will be training, portions of the area hardest hit should see a widespread 2-4 inches of rain, with localized totals as high as 8 inches in the hardest hit areas. Of course, it's very difficult to predict exactly where these boundaries set up and how able they are to sustained repeated storms moving over the area, but needless to say given the widespread urbanization of this area and inherent lower thresholds of rainfall to achieve flash flooding, that impacts are likely. 12Z HREF 100-year ARIs are up to 20-25% for the lower Hudson Valley and Fairfield County, CT. ....New England/Upstate NY... Given the very favorable antecedent conditions all across western New England, with widespread areas running 300% of normal based on 2-week AHPS output of the past 2 weeks of rainfall, even the first half of the event today is likely to cause numerous instances of flash flooding across New England and far eastern NY. Southerly flow of abundant Atlantic moisture will draw northward along with the developing convection, which will persist through the overnight and into the day on Monday. A surface low developing along the coast will help focus the convection as the flow gradually pivots southeastward. The surface low will be the dominant factor in the heavier rains on Monday, so more on that in the Day 2 discussion. 12Z HREF 100 year ARIs are around 30% through 12Z tonight along the VT/NH border in the southern portions of those states, as well as into central VT. ....Lower MS Valley... Please see MPD 0678 for further information. A weakening MCS in eastern AR is itself fortunately becoming less of a threat as time goes on, but with a very tropical air mass out ahead of it, expect daytime heating to result in scattered to numerous thunderstorms across the Slight Risk area into MS and AL. The northern boundary of the MCS over northern AR may become a focus for slow-moving thunderstorms as well this afternoon. For northeast TX, expect an area of training showers with maybe a few embedded thunderstorms to develop overnight at the nose of an LLJ, which will keep the flash flooding threat, albeit isolated, continuing in that area into tonight. ....Delmarva... Ongoing activity will continue to push east and offshore from the Delmarva this evening. ....Eastern NC... Heavy thunderstorms will continue to push east over the Outer Banks this evening. The Slight Risk was removed from NC and there is little threat overnight for additional activity in NC. Jackson Day 2 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... ....2030Z Update... ....New England... Continued increases in both the total amount of rain expected across Vermont and northeastern New York prompted much of the 12Z guidance, particularly the 12Z HREF to indicate even higher chances of exceeding some remarkable thresholds Monday through Monday night. 24-hour rainfall totals of 3-5 inches across essentially the entire High Risk area, with local amounts approaching 12 inches for storm total rainfall over areas that have already been particularly hard hit in recent days by heavy rains prompted the upgrade. 12Z HREF probabilities indicate a 70-80% chance of exceeding 5 inches of rain for east-central Vermont. This same area has a 45% chance of exceeding 3 hour FFGs and a 60-70% chance of exceeding 6 hour FFGs. However, it's important to note that these are based on current FFGs, which are already low, and will likely come down considerably further as the event gets going across VT tonight. As far as 100 year Annual Recurrence Intervals go, expect a 60-70% chance of exceeding those values of 5-7 inches on the Vermont side of Lake Champlain. With all of these thresholds in mind, expect widespread considerable flooding impacts across much of Vermont and nearby surrounding areas, except perhaps the northeastern corner of the state. As everything continues to play out largely as expected, there are likely to be scattered Flash Flood Emergencies declared, along with mudslides and widespread flash flooding of not only small creeks and streams, but also the larger rivers. The widespread flooding is expected to be analogous to the 2011 Irene remnants event for this area, though this time no tropical storms are associated with this rainfall. Little has changed regarding the overall setup. A surface low starting out the day near the Jersey shore will slowly track northward to near Boston by Tuesday morning. This will make for a prolonged southeasterly flow of deep Atlantic moisture that will advect northeastward into the Green and White Mountains. Upper level support in the form of a rapidly strengthening negatively tilted shortwave trough...a pattern far more common in the cold season than mid-July... will enhance synoptic scale lift considerably across New England as it lifts northeastward. Further, a southwesterly 90 kt upper level jet streak is expected to form off the coast, which will support the strengthening shortwave, strengthening surface low, and lift which will wring out additional moisture. It's never a good thing when nature combines cool season upper level dynamics that are completely out of place for mid-July with far more typical heat/instability and tropical moisture into an area that already has been hard hit with heavy rainfall in recent days. Thus, localized catastrophic flooding is possible. As the low lifts northward Monday night, the High Risk area will enter the comma-head region of the cyclone, which will limit the instability and heavy convection a bit, especially the further west you go, but rain will continue right through the overnight across this region as areas to the east dry out under the dry slot. By 12Z Tuesday morning, the start of the Day 3 period, all but some lingering rain will be over, but another 1/2 to 1 inch may fall through midday Tuesday in northern VT/northeast NY. For southern New England, proximity to that aforementioned surface low will allow the heavy rain which will be ongoing at the start of the Day 2/Monday period to end from southwest to northeast as the dry slot of the low slowly tracks northward. The slow movement of the low, typical of summertime systems, will of course prolong the period of heavy rain ahead of it into New England, but the abundant moisture and flood sensitive areas of Worcester and Springfield, MA along with Providence, RI and Hartford, CT will likely cause localized flooding problems in those respective cities, so the Moderate Risk area was expanded south through all of western MA and the northern halves of CT and RI. For the Tri-State area around NYC, a nominally faster motion of the surface low over the city should end the rainfall right as this Day 2/Monday period begins Monday morning, so the Marginal Risk area was trimmed towards the Northeast and much of NYC was removed for this update. ....OK/TX Border area... A mesoscale convective system (MCS) is expected to form over the TX Panhandle Monday evening and progress eastward down the Red River overnight Monday night. While much of the activity will be in the form of a steadily moving squall line, pre-line convection likely to form will increase the threat, particularly over southern OK, for the pre-frontal convection will merge with the line, resulting in locally prolonged heavy rains over this area. Synoptically a front over much of northern OK will clash with a low-level jet (LLJ) advecting some impressively moisture-laden air into the area. In fact, PWAT values over the area will be around record territory for mid-July, which is no small feat. Further, instability will be plentiful, over 3,000 J/kg, which may support hail development, but will also support the very heavy rainfall rates expected over the area. The factors working against flash flooding will be around average soil moisture over the area, particularly on the OK side of the Red River (it's a fair bit wetter on the TX side), and fast storm motions which will diminish general time of heavy rain in any one area. In coordination with OUN/Norman, OK and FWD/Fort Worth, TX forecast offices a Slight Risk area was introduced with this update. ....Northeastern WA through much of Montana... A front that will move across the Intermountain Pacific Northwest and the high Plains of MT will produce locally heavy rainfall of up to 1 inch per hour across this region. The heavy rains will be supported by a small but vigorous shortwave progressing eastward along the Canadian border. In coordination with OTX/Spokane, WA; MSO/Missoula, MT; and TFX/Great Falls, MT forecast offices, a Marginal Risk area was introduced with this update. ....U.P. of MI and northeast WI... Frontal precipitation increased a little bit with the 12Z suite of guidance, so the Marginal Risk area was expanded a bit to cover more of the U.P. and also into WI, but for now no upgrades were considered with this update. ....Gulf Coast... Frontal and sea breeze convection expected along the Gulf Coast from New Orleans to Apalachicola will pose an isolated flash flooding risk Monday afternoon. At the moment most of the rainfall is expected to be offshore, with immediate coastal areas having very high FFGs, the Marginal Risk was maintained with no significant changes with this update. Wegman ....Previous Discussion... ....Northeast... At the start of the day 2 period (Mon 10.12z), the shortwave will will reaching peak maturity with deep layer DPVA across the Northeast as it starts to close off across N PA/E NY with surface low in proximity to E Long Island with a well developed and focused TROWAL crossing southeast New England with 20-30kts of warm conveyor ascending across New England into a sharpening deformation zone from the Catskills across Northern New England.=20 There are multiple signals for a remaining axis of 500-800 J/kg of MUCAPE along and southeast of the convergence zone, which will remain critical for the elevated, slow moving convection along the TROWAL. Given prior day's heavy rainfall (3-6") and already elevated wet soil conditions (per AHPS 200-300% above normal precip anomalies) and potential for slow/stationary cells resulting in localized additional 2-4"+, the risk of flash flooding remains highly likely and may need to be further increased based on how much rain unfolds in the Day 1 period, particularly across the southeast facing Berkshires and Green Mt Ranges where upslope flow should enhance ascent/rainfall rates where FGEN signals overlap the greatest.=20 ....Gulf States into South Carolina... Deep Layer trough as pressed the cold front and pooled moisture/instability along the Central Gulf Coast with a stronger trend toward a further south solution. This places the best upper-level diffluence and moisture flux convergence overall along and just offshore of the MS Delta toward western FL Panhandle.=20 While synoptic guidance continues to hint a stronger moisture flux convergence within the downstream shear axis across S GA; this looks like a typical guidance bias, stronger thunderstorms closer to the AL/GA and FL line are more likely given proximity to better instability, but should be more scattered in nature and within the area of higher FFG. As such, kept the Marginal Risk to align with the best moisture/instability where isolated stripes of enhanced rainfall could result in isolated flooding concerns. ....Central to Southern Plains... Stronger closed ridge over the Southwest and reduced activity overnight late Monday across Central High Plains, should have allowed for weaker moisture return to build up higher values, increased instability for potential activity into Tuesday evening. 00z guidance along with prior cycles continues to hint at an organized cluster or small complex to form at the upwind side of the cold front in the vicinity of NE NM into the Texas Panhandle/W OK. Storm mode will likely determine localized very intense rainfall rates capable of inducing an isolated few instances of flash flooding, in proximity to saturated grounds across central Oklahoma, a Marginal Risk is extended from the area in the Lower MS Valley back across and north of the Red River.=20=20 Additionally, northern stream cold front as leading edge of larger scale height-falls will be pressing southward across the central Plains and intersecting some of the untapped moisture/instability that rolled back through eastern Colorado on Sunday and Monday.=20 Instability of 2000-3000 J/kg and 1.5"+ total PWats pooled along strengthening (20-25kt) LLJ is likely to converge in early Tuesday for thunderstorm development across Nebraska, eventually propagating into KS. As such an extension of the Marginal Risk is brought northward into southern Nebraska extending toward the Iowa boarder.=20 ....Upper Midwest... There is uncertainty to the level of moisture flux return along/ahead of the cold front dropping out of Canada crossing Lake Superior to N Nebraska in the day 2 period. Stronger storms are likely to dot the front, but cell motions should mitigate the highest totals across portions of Iowa into MN, particularly given recent dry soil conditions. However, timing of the front with peak heating and likelihood of Lake Breeze enhanced thunderstorms off Green Bay/NW Lake Michigan may result in thunderstorm collision for a few spots of intense short-term rainfall rates over 2" and spots of 2-3" overall. The area has seen above normal rainfall over the last few weeks per AHPS and lower FFG values (mainly along the northern U.P.) may overlap enough for an isolated potential for localized flooding. So in coordination with MQT local Forecast Office, a small Marginal Risk of Excessive Rainfall was added for the core of the U.P. of Michigan for this cycle. Gallina Day 3 Valid 12Z Tue Jul 11 2023 - 12Z Wed Jul 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST PORTIONS OF THE MID-MISSOURI BASIN... ....2030Z Update... Very few changes were needed to the Day 3/Tuesday period. The Slight risk area was expanded a bit, but the area of higher rainfall expected from southeastern SD through extreme northern MO looks in place, with a notable heavy rainfall threat for the Omaha/Council Bluffs area. In coordination with BTV/Burlington, VT and CAR/Caribou, ME forecast offices, the Marginal Risk for northern VT and northern ME was maintained with this update. As mentioned in the D2 discussion, the rain will be quickly ending from south to north across VT, so any lingering rainfall Tuesday morning will be light and highly unlikely to cause additional new flooding on top of what is likely to be ongoing by Tuesday morning. Rainfall totals are a bit higher across far northern ME, but due to some inherent uncertainty as to how fast the rain will end, opted to keep the Marginal risk area. A new Marginal Risk area was introduced for southeastern AZ and southwest NM for expected afternoon showers and thunderstorms that may cause isolated flash flooding if any storms develop over flood-sensitive areas such as local arroyos and burn scars. A stalled out front will remain along the Gulf Coast from northern FL to eastern LA, then extending northwest through northern LA, but rainfall forecasts remain modest, especially with high FFGs over the area, so no significant changes were made for this area neither. Wegman ....Previous Discussion... ....Northern Plains through Upper Great Lakes... The strong synoptic scale closed low over Canada is expected to drop south toward the Boundary Waters of Ontario/Minnesota with a sharpening shortwave digging along the western side across into the Northern Plains. The associated cold front will have already started slow slow from the Northern Great Lakes across South Dakota before angling northwest into northeast MT. Strong upslope moisture flux will increase across the Northern High Plains, with a narrow axis of enhanced moisture/instability through the western Dakotas. Mid-day thunderstorms will start to organize into a stronger cluster/complex intersecting with 35-40kt SSW LLJ through the central Plains. MCS with training thunderstorms along the frontal boundary will increase in coverage/rainfall efficiency across central SD into SE SD toward 12.03-06z with streaks of 2-4" becoming likely across the Mid-Missouri Basin. While the area has been dry, thunderstorms on Day 2 and intense rates continue to support a Slight Risk across SD into NW IA. A broad Marginal Risk extends along the track as well as up the frontal zone across N IL/S WI and Lower MI, where scattered clusters may result in more isolated heavy rainfall signature (given low level flow will be a bit more parallel reducing moisture flux convergence. ....Northern New England... At the start of the forecast period, the increasingly stacked cyclone will be exiting with the occluded/cold front likely to have shifted east/northeast of Maine. However, the lingering deformation zone/TROWAL is likely to have ongoing showers or embedded smaller thunderstorms across N VT/NH and maybe the top of the Rooftop of Maine. Additional 1-2" totals are possible but reducing given limited remaining instability. These totals and likely ongoing flooding rivers/streams and saturated soils will still be prone to any rates up to .5-1"/hr, but will be conservative in higher ERO categories until knowing the greatest affected areas are known after the prior two days. As such a Marginal Risk of Excessive Rainfall will be a placeholder until the ground conditions are better understood. ....Southern Red River through Central Gulf Coast... The lingering stationary boundary will continue to be the focus of pooled deeper layer moisture, though being at the tail end of the large scale flow (exiting deep layer trof), there will be limited translation of the axis. Anomalous deep layer moisture with values of 2-2.25" total PWat through the Lower MS Valley and full-sun should support strongly unstable environment with 2000-3000 J/kg CAPE by midday. Weak upper level diffluence and weak south-southwesterly flow should allow for localized clusters of stronger thunderstorms. Deep layer slow steering flow along/parallel to the boundary should allow for sufficient organization to maintain the clusters and slow motions may support very intense but locally focused enhanced rainfall totals to support isolated instances of flash flooding; enough to maintain the Marginal Risk of Excessive Rainfall from SE OK to NOLA as well as the FL Panhandle. Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5qgS_b_Jn_anbujJbIK3IwmyPc3SzAW-Hhq0UhY-7CTg= w_gKLeQFgIM8v5r5DBVQSwj3GzAgITH7pH1sWVxgW5HTZCQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5qgS_b_Jn_anbujJbIK3IwmyPc3SzAW-Hhq0UhY-7CTg= w_gKLeQFgIM8v5r5DBVQSwj3GzAgITH7pH1sWVxgyNMmKHs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5qgS_b_Jn_anbujJbIK3IwmyPc3SzAW-Hhq0UhY-7CTg= w_gKLeQFgIM8v5r5DBVQSwj3GzAgITH7pH1sWVxgdof_cG4$=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) .