Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 07 2023 20:49:27 FOUS30 KWBC 072049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 448 PM EDT Mon Aug 07 2023 Day 1 Valid 01Z Tue Aug 08 2023 - 12Z Tue Aug 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE NORTHEAST, THE MID-ATLANTIC REGION, EASTERN WASHINGTON, & NORTHERN IDAHO... ....Northeast... Based on the latest CAMs guidance, the consensus has been for the DC/MD/VA/DE region to likely see one very fast moving line of storms race across the area this afternoon, with very little to no rainfall activity occurring behind it. This presents a scenario where flash flooding is highly unlikely to occur, other than some nuisance flooding in poor drainage areas. The NAMnest is a notable outlier on this scenario, showing a much slower cell tracking across central MD and into the Baltimore region behind the line this evening, but this solution was discounted for lack of support among the other guidance. Thus, in coordination with LWX/Sterling, VA forecast office, have downgraded to a Marginal with this forecast update. The rest of the Northeast remains in steady state. Even here the primary threat by far will be the severe potential. While the flow becomes more parallel to the front the further north you go, and thus increasing the flash flooding threat broadly, the fast moving storms and the requirement for multiple rounds should keep the flooding threat relatively low in most areas. Particularly sensitive areas of northern NY and through the Philadelphia and NYC metros are at higher risk of flash flooding due to less rainfall needed. There was also a notable signature in much of the guidance across central and western WV with the storms now entering the area for increased rainfall totals as compared to the surrounding area. Training storms are beginning to develop with the event just getting underway. Either way the storms will be fast-moving, so it will take multiple rounds of storms to result in flooding. Thus, the area was kept in a Marginal. ....Pacific Northwest... The inherited Slight Risk area was expanded in coordination with PDT/Pendleton, OR forecast office. Stationary/very slow moving storms are ongoing just east of the Cascades, and a second line just east of the Tri-Cities, extending south into portions of northeast OR. The forcing will remain highest across WA today, so it's this region which is already seeing nearly stationary storms that are actively congealing into an MCS that is at the greatest risk of flash flooding in the now-term. Later this afternoon expect more widespread convection to break out across ID, with low FFGs contributing to flooding potential, especially in burn scars, canyons, any urbanized areas such as Spokane and Coeur d'Alene, and other poor drainage areas. ....Western High Plains... Scattered showers and thunderstorms will develop over parts of Colorado and Wyoming before spreading eastward. Storms will be moving into an airmass with deeper moisture which should allow for an uptick in rainfall rates with time. Locally heavy rainfall rates and isolated heavy rainfall amounts may lead to isolated instances of flooding or at least run-off problems in regions of poor drainage. ....Southwest... The combination of moisture rotating around the east side of now Post-Tropical Cyclone Eugene, a weak disturbance crossing northwest Mexico, and a shortwave across northwest AZ has led to a bit of convective expansion across portions of southeast AZ and central NM. Regionally, there is a broad area of 1000-4000 J/kg of ML CAPE and effective bulk shear in northern AZ and portions of NM is 25+ kts, enough for convective organization. The area with the most impressive moisture in southeast AZ, where convection has shown the coldest cloud tops. Hourly rain totals to 2" would not be a surprise where cells manage to backbuild, train, or merge. A mesoscale precipitation discussion is being considered for portions of the area, so a Marginal Risk area has been introduced in this special update. Wegman/Bann Day 2 Valid 12Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN NEW ENGLAND... ....19Z Update... ....Northeast... Guidance continues to come into agreement on a potentially significant heavy rainfall event across portions of New England on Tuesday. A slow moving but deepening low north of New York will track northeast to the west of Maine through the period. A 50 kt low level jet will advect unstable and unusually moist air from the Atlantic north into New England. This will support training storms as they move northward along with the flow. The storms will impact the terrain, particularly the White Mountains of New Hampshire into western Maine. Upslope support in these regions are likely to wring out additional rainfall to the south and east of the range, further enhancing totals. This area has been hard-hit in recent days and weeks, so streams and rivers are running high, with little space for additional rainfall on the order of 2 to 4 inches with locally higher amounts. The Moderate Risk area is focused in the region that has the most favorable antecedent conditions, with instability and persistent strong LLJ flow likely to result in training storms. Areas with the greatest persistence of training storms have the potential to locally exceed 6 inches. In coordination with BTV/Burlington, VT; GYX/Gray, ME; and CAR/Caribou, ME forecast offices, the Moderate Risk was introduced with this update. ....Central Plains... The Slight Risk area was expanded westward to include more of the Plains of northeast CO with this update. There has been increasing signal for more concentrated storms in this area, which will expand southeastward across far southern NE/KS through the period. ....Deep South.. No significant changes were made. Any flash flooding in this area will be very localized due to high FFGs and limited coverage of storms. The greatest threats are in urbanized and other poor drainage areas. Wegman ....Previous Discussion... ....Northeast... Convection capable of producing heavy rainfall rates and excessive rainfall amounts should be on-going mainly across northern New York and northern New England as Monday's system continues on its northward track. One area of surface low pressure is forecast to be located just north of New York while a second area of low pressure begins to organize south of Maine. Instability appears to be on the weak side but precipitable water values across the region should still be in excess of 1.5 inches...and on the order of 1.75 inches closer to the coast. That should still be sufficient to produce locally heavy rain rates early in the period...with the HREF neighborhood probabilities showing the potential for highest rainfall rates getting shunted eastward during the afternoon. The UKMET and the CMC both attempt to develop a decent band of deformation rainfall north of the border which stays north of the border...while the GFS was tended to rotate a second round of precipitation across northern New England later in the day. Kept a Slight Risk area in the area...trimming some area in deference to a model signal for a somewhat faster departure while keeping the risk across the north until it becomes more clear how energy north of the border will evolve. ....Southeast U.S.... Maintained the Marginal Risk area along and immediately south of an unusually well-defined front for this time of year where precipitate water values at or above 2 inches are expected to be in place. Exactly where a morning MCV will be located over Mississippi or Alabama...and how much interaction between any mesoscale boundaries...such as sea breezes or outflow from nearby convection...makes it difficult to pinpoint the most likely area to receive excessive rainfall.=20 ....Western Plains... Another day of late day and evening convection...with at least some threat of excessive rainfall...is possible as return flow draws moisture farther into the western Plains on the west side of a quasi-stationary front. Thinking is that the convection will initiate over the Western High Plains with the concern for excessive rainfall increasing as the convection moves east/southeastward towards deeper moisture as shown by precipitable water values of 1.5 inches. There was a fair amount of spread in the QPF placement over the region...tended to favor the more southern solutions like the ECMWF/CMC/UKMET (ignoring its 8+ inch convective-feedback bulls eye over northeast Nebraska) due to the better instability. Bann Day 3 Valid 12Z Wed Aug 09 2023 - 12Z Thu Aug 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY INTO THE SOUTHEAST U.S.... ....19Z Update... ....Mid-Mississippi, Lower Ohio, and Lower Tennessee Valleys... No major changes were made, but the Slight Risk was expanded in a number of directions to account for the development of widespread convection over an area that has been very hard hit in recent days with convection. Due to the uncertainty with some of the outlier models depicting heavy rain in areas that have been hard hit, such as far southern IA/western IL, the Slight Risk area was expanded a bit to account for that uncertainty. The area of highest risk within the Slight is around the Tri-Rivers area, which has both the most favorable antecedent conditions and the greatest likelihood in the guidance for persistent heavy rains. A Moderate Risk was considered in coordination with PAH/Paducah, KY office, but was decided to hold off until there was better certainty in the guidance on where specifically will be hardest hit. With that certainty, expect the Slight Risk area will be shrunk, but a future Moderate Risk remains probable. ....Southeast AZ... Monsoonal moisture will increase on Day 3/Wed above the potential from previous days. A Marginal Risk was introduced to account for the possibility heavy rains move over sensitive areas of AZ. There is potential the Marginal may need to be expanded to include portions of the Mogollon Rim in AZ as well with future updates. Wegman ....Previous Discussion... ....Illinois into the Tennessee Valley and portions of the Southeast... Convection is expected to develop during the late afternoon/evening and grow up-scale as shortwave energy initially over Kansas/Nebraska heads eastward...leading to height falls aloft overspreading the Mid-Mississippi Valley by 10/00Z. Gulf moisture gets pulled northward during the day with both the GFS and NAM showing precipitable water values exceeding 2 inches from the southeast corner of Missouri into parts of Arkansas, Mississippi and Alabama by 09/18Z while the flow aloft ahead of the strengthening shortwave becomes increasingly difluent. Models tend to develop convection during the first part of the Day 3 period...but the expectation is for the risk of highest rainfall rates and accumulations will not occur until the later part of the period due to the better thermodynamics and the arrival of 70+ kt speed maximum aloft. Given the time of year, the precipitable water values are above normal but not terribly so...only 2 to 2.5 standardized anomalies above climatology in Missouri/western Tennessee by Wednesday evening. However...the moisture flux is 3.5 to 4+ standard anomalies above climatology. The NBM and Ensemble Bias Corrected QPF focused 2 or 3 inch amounts along an axis from far southern Illinois into northern Tennessee...and the 07/00Z NAM and 06/12Z UKMET QPF axis was farther north while the GFS was farther south (supporting the GEFS idea of 3 inch contours into northern Georgia. Despite the spread...felt confident enough to begin realigning the axis a bit but left it fairly broad to deal with the spreads of QPF solutions and related excessive rainfall threat. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-LapBM_vBmEeV3lp56lQTc69S68841y5ZfKmXId72sVY= 9_Ujm8kmm4GP8NM5Oe0IBb1iwp33OdndwM3kd7SikzB4ivU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-LapBM_vBmEeV3lp56lQTc69S68841y5ZfKmXId72sVY= 9_Ujm8kmm4GP8NM5Oe0IBb1iwp33OdndwM3kd7SiegrPTjE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-LapBM_vBmEeV3lp56lQTc69S68841y5ZfKmXId72sVY= 9_Ujm8kmm4GP8NM5Oe0IBb1iwp33OdndwM3kd7SiRTJCBTM$=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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