Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 15 2023 00:48:33 FOUS30 KWBC 150048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 848 PM EDT Thu Sep 14 2023 Day 1 Valid 01Z Fri Sep 15 2023 - 12Z Fri Sep 15 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTH CENTRAL COLORADO INTO SOUTHWEST NEBRASKA AND OVER WEST TEXAS... Removed the Slight Risk from the central portion of Texas where convection from earlier in the day had diminished and re-positioned it over parts of West Texas where the HRRR has been suggesting upscale growth later this evening and overnight. The ARW2 had a similar idea but with somewhat smaller areal coverage.=20 With the expected low level jet formation and some upper level support combined with some overlap with storms from previous days...think there is enough of a signal for a Slight Risk though confidence. Farther north...there has been a signal for heavy rainfall along and east of a cold front extending from southwest Nebraska into northeast Colorado and westward towards the Rockies foothills. The axis of the Marginal risk area captures the axis... so simple realigned the Slight Risk. The big question was how far eastward to extend because of an increasingly stable atmosphere farther east. Convection over the Western U.S. convection should be tapering off quickly with the loss of daytime heating...except along and ahead of a shortwave trough progressing across the area.=20 Bann ....16Z Update... The main adjustment to the Slight Risk areas in place was the one in North Texas where CAMs show continued surface theta-e advection into the southern periphery of the Dallas/Fort Worth metro area into the early afternoon hours. Expanded the Slight Risk a little farther east where the ongoing cluster of storms may support a flash flood threat into the mid-afternoon hours. New 12Z HREF shows as high as 60-70% probabilities for >3" of rainfall today south of the Dallas/Fort Worth metroplex. A similar situation is anticipated over southeast Texas where surface-850mb theta-e advection will intersect a stalled frontal boundary through this afternoon and early evening. Given how dry the region has been in recent weeks, however, opted to maintain a Marginal Risk since most rainfall will be beneficial. Adjusted the Marginal Risk area in the central High Plains to account for latest CAMs guidance, but no significant adjustments to the Slight Risk in central CO as the overnight shift's forecast rational and latest 12Z CAM guidance supports the need for the Slight Risk. To add on to the HREF probabilistic guidance from the previous shift, the 12Z HREF showed similar to slightly higher probabilities for not only the >1" and >2" totals, but also included a 20-30% area for >3" totals around the Denver metroplex. The only addition to the forecast cycle is a Marginal Risk in southern GA and far southern SC. The 12Z sounding out of Jacksonville, FL showed PWs near 2" with not much in the way of capping present. While RAP mesoanalysis shows a little stronger capping to the north, this should be eroded rather quickly this afternoon while at the same time, the region lies at the nose of a weak 250mb jet streak positioned over the Mid-South. With MLCAPE forecast to top 1,000 J/kg, PWs near 2", and 20-25 knots worth of effective bulk shear aloft, storms will have some marginally organized characteristics to go along with 2"/hr rainfall rates within the heaviest t-storm activity. MRMS FLASH soil moisture ranged between 50-70% across parts of southern GA and new 12Z HREF showed neighborhood probabilities of 40-60% for rainfall totals >3". Given these reasons, chose to add a Marginal Risk to southern GA and far southern SC this forecast cycle. Mullinax ---Previous Discussion--- ....Great Basin into the Central Rockies and Central High Plains... Height falls expected to move west to east day 1 from the Great Basin into the Central Rockies and Central High Plains region.=20 This will support a fairly broad region of scattered convection day 1 in what will be a region of above average PW values, 1 to 2+ standard deviations above the mean. A small slight risk area was added across north central Colorado from approximately Colorado Springs, northward through Denver, Boulder and into Ft Collins.=20 Convection may maximize in this region late Thursday afternoon into Thursday evening as front drops south into northeast Colorado and the above mentioned height falls push east across northern Colorado and enhance upper difluence. The slight risk area fits where HREF neighborhood probabilities are high, 70-90%+ for 1"+ amounts, high also for 2"+ amounts 40-60%, albeit over a smaller region closer to Denver. This region has seen above average precip over the past week, resulting in lower ffg values through the slight risk area. The previous marginal risk area that extended westward into western Colorado and southern Utah was expanded farther west into northern Arizona and southern Nevada where locally heavy rainfall amounts are possible in the above mentioned high PW axis/upper trof axis with embedded shortwaves.=20 HREF neighborhood probabilities, while very splotchy in appearance, do show fairly high probabilities in spots for 1"+ amounts for the marginal risk area from western Colorado into southern Utah, southern Nevada and northern Arizona. ....Southern Plains... A surface frontal boundary expected to remain stationary day 1 from Southwest Texas, eastward into south central Texas and along the Gulf Coast. Strengthening southerly inflow into this front should support increasing convection to the north of this boundary prior to 1200 UTC Thursday, with this potential organized convection then pushing downstream early day 1 from across portions of central to eastern Texas. Much of this region has been dry over the past few weeks, resulting into fairly high ffg values. The one exception is over portions of south central Texas in the vicinity of WFO SJT. A slight risk area was denoted over northern WFO SJT and southwest WFO FWD where heavy rains early day 1 may overlap the lower ffg values. In the slight risk area, HREF neighborhood probabilities are high for 1 and 2"+ amounts, 50-80 and 40-60% respectively, with hourly rainfall totals of 1-2" possible.=20 Oravec Day 2 Valid 12Z Fri Sep 15 2023 - 12Z Sat Sep 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF WEST CENTRAL TEXAS... ....2030Z Update... Little changes made this forecast cycle to the overnight forecast guidance based on the latest QPF and where the most flood prone areas are likely to take shape. Following today's heavy rainfall across parts of the region, the heart of Texas remains most at risk as southerly 850mb moisture transport directs added moisture flux into the stationary front located over the Edwards Plateau. Convection will likely be highly scattered, but 1-hr FFGs will likely struggle to get above 2" and between the instability (1,000-1,500 J/kg MLCAPE), and the PWs mentioned from overnight, thunderstorms will be capable of producing excessive rainfall rates in excess of 2"/hr. The Slight Risk remains in place there while the Marginal Risk remains on track across the central Rockies and central High Plains. The thinking for coastal New England also remains on track as the bulk of the heaviest rainfall looks to come ashore later Saturday morning. Mullinax ---Previous Discussion--- The overall large scale pattern is not expected to change appreciably during the upcoming day 2 period from the Central High Plains, southeastward across the Southern Plains. An axis of PW values 1.50-1.75" expected to persist along and to the north of the west to east oriented stationary front that will not move much from the day 1 period, continuing to stretch from Southwest Texas into south central Texas. Additional overrunning convection possible along and north of the stationary front with locally heavy rains from northwest Texas southeastward into southeastern Texas. There is a fairly large spread with qpf details, leading to overall low confidence with max areas. A small slight risk area was added to the previous Excessive Rainfall Outlook for portions of west central Texas that may see an overlap of day 1 and day 2 precip across regions of relatively low ffg values. Not a lot of changes made to the previous broad marginal risk area from the Central High Plains, southeastward into Texas given the above mentioned model qpf spread. ....Coastal New England... Rain bands on the west and northwest side of Lee will begin to impact coastal New England Friday night into early Saturday morning. At the moment, the heaviest Lee rains should remain offshore through the day 2 time period, with the amounts that do fall along coastal New England, not resulting in any runoff issues. Oravec Day 3 Valid 12Z Sat Sep 16 2023 - 12Z Sun Sep 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN MAINE... ....2030Z Update... The latest QPF came in lower across eastern New England, largely due to a combination in a slight increase in forward speed and tighter precip gradient on QPF shield's western flank. The setup is also lacking sufficient instability, which should put a cap on hourly rainfall rates from becoming prolific. That all being said, it is still highly unusual to have a tropical entity so close to the coast of Maine. NAEFS shows PWs above the 90th climatological percentile and IVT values above the 97.5 climatological percentile over Downeast Maine by Saturday afternoon. Much of central and eastern Maine also sport 50-75% soil saturation on FLASH MRMS at this time. Rainfall rates will not have to be too impressive to result in flash flooding for these reasons. In addition, the strong tropical storm force wind gusts will likely rip leaves off trees, potentially clogging gutters and increasing the potential for ponding in poor drainage areas and on roadways. Should QPF increase across far eastern Maine over the next 24-36 hours, an upgrade to a Moderate Risk may become necessary. At this time though, have maintained a Slight Risk which still shows there is enough rainfall expected on the western flank of Lee to support concern for flash flooding. With the tighter QPF gradient over central New England, did trim back the western flank of the Marginal Risk area. In Texas, the Edwards Plateau and Hill Country remain most at risk for potential flash flooding given the sufficient moisture (1.5-1.75" PWs), instability (MLCAPE >1,000 J/kg), and the recent multi-day stretch of stormy weather contributing to increasingly saturated soils. A Marginal Risk remains in place, but should there be increased confidence in heavier rainfall rates transpiring over the lower FFGs of the Edwards Plateau, a Slight Risk upgrade may be necessary in future forecast cycles. Mullinax ---Previous Discussion--- ....Eastern Maine... The latest NHC track for Lee takes the storm northward toward western Nova Scotia/far eastern portion of coastal Maine by Saturday evening. Heavy rains likely to continue on the west side of Lee, impacting coastal New England. Model consensus is for the max axis to be along the Maine/New Brunswick border region into western Nova Scotia, where rainfall totals of 1 to 4 inches are possible. No significant changes made to the previous marginal or slight risk areas, with only a small eastward adjustment given the latest model guidance. The storm is expected to remain fairly progressive, limiting the very heavy rainfall potential. However, much of eastern Maine has seen above average precip over the past few weeks, elevating stream flows and lowering ffg values. Heavy rains from Lee may produce localized urban or small stream flooding. ....West Central to Central Texas... The stationary frontal boundary across Southwest to south central Texas during day 1 and 2 will begin to push back to the south day 3 as height falls push southeast across northern to central Texas. Similar to the day 2 period, there continues to be a large amount of qpf detail spread in the models, with continued low confidence. A marginal risk was added from the previous day 4 Excessive Rainfall Outlook across portions of west central to central Texas where additional moderate to heavy rains may fall day 3 and overlap regions of locally heavy rains on days 1 and 2. FFG values may end up low enough after the days 1 and 2 rainfall totals to warrant a slight risk area across these regions eventually.=20 Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5v77SYVcikkyvaMvXBs2zSbkF--WHeIoBnQG_vgceTOm= q0f8GI2kWznafctx470ONyiHev6Uz57izz1ZKzlR0GUQ0kA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5v77SYVcikkyvaMvXBs2zSbkF--WHeIoBnQG_vgceTOm= q0f8GI2kWznafctx470ONyiHev6Uz57izz1ZKzlRhxh3O3A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5v77SYVcikkyvaMvXBs2zSbkF--WHeIoBnQG_vgceTOm= q0f8GI2kWznafctx470ONyiHev6Uz57izz1ZKzlR5aZ6LKo$=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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