Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 03 2024 20:01:07 FOUS30 KWBC 032000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Sat Aug 3 2024 Day 1 Valid 16Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHWESTERN FLORIDA GULF COAST, WESTERN FLORIDA KEYS, AND PORTIONS OF THE SOUTHEAST AND MID ATLANTIC... ....16Z Update... The latest advisory from the NHC on Tropical Depression 4 (TD4) has shifted the track of the center of the system bit to the west. As such, the associated precipitation fields have also shifted west, resulting in a significantly lower threat for flash flooding today into tonight for southern Florida. As such in the 12Z HREF guidance, only a very small sliver of coast including Naples and Marco Island remains in a 60-70% chance of seeing 5 inches of rain over the current 24 hour period ending 12Z Sunday, dropping to 10-15% for 8 inches of rain in the same time frame. In coordination with MFL/Miami and TBW/Tampa forecast offices, the Slight risk area was trimmed to just the immediate southwest coast, and with KEY/Key West for just the western Keys remaining in the Slight. Meanwhile across southeast Florida, any showers and storms so far today have been disorganized and struggle to even reach 1 inch per hour rates. While there remains some potential this afternoon and overnight for a more consolidated band...not only are none of the guidance suggesting such, but should a band form it's likely to be quite localized, favoring a Marginal threat for flooding there. ....Southeast... As regards the stalling out front across the Southeast, 3-hr FFG exceedance probabilities only peak to the 15-20% category this afternoon in the Florida Panhandle and coastal Alabama. Think it would take persistent heavy rainfall in Pensacola or other flood- sensitive areas to get flooding, so trimmed the Slight northeast away from the coast into Georgia. No big changes for Georgia or the Carolinas for potential training convection this afternoon with the slow-moving front. However, there's a notable minimum in expected rainfall across much of Virginia. In coordination with AKQ/Wakefield and RNK/Blacksburg offices, trimmed out the Slight across most of Virginia. ....Mid-Atlantic... The only area where the Slight has been expanded was in the Mid- Atlantic. Strong storms ongoing across Ohio and western Pennsylvania will push eastward as additional convection in the lee of the Blue Ridge develops this afternoon from northern Virginia northeast across eastern PA. The 12Z HRRR suggests the storms will evolve into separate segments as they move across the I-95 corridor from DC through NYC. Ample moisture will be in place with PWATs approaching 2 inches, so the strongest storms alone may cause localized flash flooding in any one of those metros, plus some of the CAMs suggest post-linear showers and storms may train into the evening over the same areas. The inherited Slight was expanded west to cover the ongoing through midday convection in western PA, and nudged east over most of Long Island for urbanization concerns as the strong storms move through. Once the storms cross the lower to middle Hudson Valley the instability should largely be confined to the coast, so inland areas appear to have a lesser threat for flooding. The Slight was trimmed out of the middle and upper Hudson Valley. ....Minnesota... The Marginal risk was expanded to include all of northern Minnesota for the ongoing severe convection occurring in that area. Local rates exceeding 1 inch per hour have been noted on radar, which may cause isolated instances of flash flooding. ....West... Rainfall across the West remains largely the same for forecast footprint today. Nudged the Marginal a hair further west in OR and northern CA for ongoing showers with some convective elements, but no significant changes were made. Wegman ....Previous Discussion... ....South Florida... Tropical Depression #4 over western Cuba will become better organized over the eastern Gulf west of the FL Peninsula over the next 24 hours, with the system expected to reach Tropical Storm status by 00Z 8/4 per the latest advisory from the National Hurricane Center. The guidance (both the CAM and global QPFs) indicate the heaviest rain just off the coast in SW FL. However, given the better destabilization east of the center, even if just a little, with tall/skinny CAPEs of 1000-2000 J/Kg, rainfall rates will be quite prolific given the ample deep-layer theta-e/moisture profiles (TPWs 2.4 to 2.6+ inches). Given the current and projected forward speed of the TD through Saturday, it appears as the Slight Risk (higher-end Slight) will suffice. Many locations across SW FL (including FMY) will see between 3-5" per the guidance, with lesser areal-average totals to the east. However, given the rich thermodynamic environment, localized totals of 6-8+ inches are anticipated within the Slight Risk area. The latest (00Z) HREF in fact shows 60-80% probabilities of 24hr rainfall exceeding 5" over a portion of SW FL along the coast, with 30-35% probs of exceeding 8". ....Southeast and Mid Atlantic... Farther north across the Southeast and Mid Atlantic, ample low- level moisture transport/flux and anomalous PWs ahead of a slow- moving mid/upper level trough and surface front will allow for numerous showers and thunderstorms across much of the Eastern Seaboard up through the northern Mid-Atlantic Region and southern NY-southern New England. MLCAPEs of 1000-2000+ J/Kg during peak heating along with PWs of 1.75-2.0+ inches will make for hourly rainfall rates of 2+ inches underneath the strongest cells, with localized totals of 3-5" during the period per the CAMs and latest HREF probabilities. Urban areas will be at the highest risk for flash flooding within the broad Slight Risk area. ....Southwest... Monsoonal convection over the Southwest (precipitable water values up to 1.75" near the International Border) continues to support a Marginal Risk area. Locally heavy rainfall would be most problematic in any area burn scars and dry washes/arroyos. ....Sierra Nevada into Oregon Cascades, Blue Mountains, and parts of the Northern Rockies to western portions of the Great Basin... Anomalous moisture (TPW anomalies topping 3-3.5 standard deviations above normal per the GEFS) will advect northward through the West on the backside of the upper ridge and ahead of the shortwave trough off the PAC NW coast. The guidance gets TPW values up to 1-1.25", which would be most anomalous in terrain. Given these moisture profiles, along with the diurnal heating (MLCAPEs peaking at least ~500 J/Km), spotty hourly or sub-hourly rainfall rates aoa 1" will make for localized runoff issues, especially over burn scares. ....Upper Midwest... The guidance is generally showing a modest signal for heavy rainfall in and near Minnesota this period. Precipitable water values rise about 1.25" with a warm advection pattern. This led to an introduction of a Marginal Risk here, as flash flood guidance values are modest after recent heavy rainfall. Hurley Day 2 Valid 12Z Sun Aug 04 2024 - 12Z Mon Aug 05 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA BIG BEND... ....20Z Update... ....Southeast... In coordination with TAE/Tallahassee, FL; JAX/Jacksonville, FL; and TBW/Tampa, FL forecast offices, a Moderate Risk upgrade was introduced with this update for the Florida Big Bend. The latest 15Z advisory from the NHC has Tropical Depression 4 (TD4) making landfall along the Big Bend at the end of the Day 2 period around 12Z Monday. Thus, all of the rainfall in this area is likely to be the portion of the tropical cyclone ahead of the center, with generally parallel to the coast to offshore flow. Forecast guidance has had a notable trend slowing the forward speed of TD4, which in turn notably increases the forecasted rainfall along and east of the track. The increased rainfall is for 2 reasons during this period: 1. A slower forward speed means extra time the storm has over the very warm (near 90F) waters of the northeastern Gulf, resulting in additional time the storm has to intensify before landfall, given very favorable meteorological conditions for intensification. 2. The slower forward speed increases the time the storm has to drop prodigious amounts of rain in the area. Despite much of this area being swampy with high flash flood guidance (FFG), the increased strength and longer duration of time for heavy rain prior to landfall should result in more widespread instances of flash flooding...hence the upgrade to a Moderate.=20 Further, wind and storm surge (storm surge just east of the center), should also contribute to more and widespread flooding impacts at the coast, with the wind moving inland resulting in more debris, both natural and artificial, potentially clogging drainage ditches. Thus, even if there weren't also an increase in expected rainfall, the higher wind alone may increase flooding impacts locally, especially where the eyewall comes ashore. On the other side of things, the slower forward speed has significantly delayed the heavy rainfall component from TD4 into the Carolinas. Thus, the Slight was trimmed well south essentially entirely out of the Carolinas. The surrounding Marginal remains in place due to another expected round of afternoon convection Sunday afternoon across the Carolinas due to the front that will eventually capture the circulation of TD4. Thus other than=20 plentiful Gulf moisture in place across both the eastern=20 Carolinas and necessarily around TD4, there will be much less=20 contribution from TD4 to rainfall in the Carolinas than previous forecasts have indicated. This of course increases the preparation time South Carolina has to prepare for this tropical cyclone. A small westward shift in the short-range guidance resulted in a very small westward nudge to the Slight and Marginal risks on the back side of the storm across southern and eastern Florida due to diminishing expected impacts in this area. ....Marginal Risks... Northeast: Little change was made other than trimming the Marginal across eastern Virginia and the Carolinas. There will be a renewed round of convection along the I-95 corridor from DC to Boston again with higher confidence from NYC to Boston. The 12Z HRRR suggests the storms should be rather fast moving segments, which should keep any flooding to a minimum and in urban and flood-sensitive areas. Southwest/Four Corners: No significant changes were made for the monsoonal moisture moving into the Southwest, as there has been little change in the guidance. Northern Plains...A small narrowing of the Marginal Risk area was=20 made from the Dakotas to Minnesota with slightly increased=20 confidence on the corridor of where a few storms may track along a=20 stalled front. Wegman ....Previous Discussion... ....Florida through the Southeast Coastal Plain.... TD #4 will eventually evolve into a defined tropical cyclone with the latest NHC forecast developing the system into our next tropical storm later this (Saturday) evening, carrying through the D2 time frame. The uncertainty of exact location of the low given the timing and interaction with the approaching trough/front to the north has limited the capabilities for discerning specifics for the Sunday into Monday period. There is some general consensus on heavy rainfall potential within the Western FL coastal plain, including areas like Sarasota up through Tampa and Clearwater, as the expected tropical storm lifts west of the FL Peninsula before expected to make landfall just east of the FL Big Bend. Many of the models keep the heaviest rain just offshore (closer to the center), while for now solid model consensus is somewhat lacking with the QPF during this period (Sun-Sun night). For this reason, per collaboration with the FL offices, have maintained an enhanced Slight Risk in the Day 2 ERO, opting for now to hold off on a Moderate. Meanwhile, some of the guidance continues to develop a Predecessor Rain Event (PRE) across the Southeast coast thanks to the timing of the system's approach with interaction of the coastline. This something to monitor in future updates as these phenomena are tough to gauge at range, but can be focused significant rain events where they occur. Regardless if a PRE forms or not, the highly anomalous tropical moisture flux and persistent heavy rain banding around the storm circulation will cause locally enhanced flash flood concerns across much of FL into the GA/SC coast. The SLGT risk was maintained given the ensemble QPF continuity and expected NHC track with the opportunity for future upgrades pending increased confidence on the storms forward motion. ....Montana into the Northern Plains and Upper Mississippi Valley... Aforementioned axis of anomalous TPW out West during Day 1 will ultimately get absorbed in the upper level jet streak and mid- level vort lobe that will cross MT and the Dakotas on Day 2. Dynamical forcing along with the thermodynamical profile will favor more widespread showers and thunderstorms, though the shear profiles will certainly favor more progressive/forward-propagating cell motions and thus make for more of an isolated flash flood threat. Areas across northwest MT, including Glacier NP, would be the most susceptible given the lower FFG (1" or less within 3 hours for most areas). An upgrade to Slight may be needed over this region in later outlooks, especially if we start seeing a consensus of heavier QPF from the models. ....Southwest.. Monsoonal moisture advecting on the underside and backside of the mid-upper level ridge will once again generate scattered showers and thunderstorms during the afternoon into evening peak heating hours. MLCAPEs of 500-1000 J/Kg will favor pockets of 1"/hr rainfall rates, which will likely lead to localized areas of flash flooding, especially over burn scars and arroyos. Hurley Day 3 Valid 12Z Mon Aug 05 2024 - 12Z Tue Aug 06 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20 OF THE SOUTHEAST... ....20Z Update... ....Southeast... Due to the expected slowing of the forward speed of Tropical Depression 4 (TD4), the forecast rainfall totals have increased and have been shifted south and west closer to the center of a likely somewhat stronger tropical cyclone as compared to inherited. The Moderate Risk was shifted south and west accordingly, in coordination with JAX/Jacksonville, FL; TBW/Tampa, FL; TAE/Tallahassee, FL; FFC/Atlanta, GA; and CHS/Charleston, SC forecast offices. 24-hour rainfall totals have increased to close to 9 inches just on the Georgia side of the border northwest of Jacksonville, which would fall in addition to the 1 and change inches in that same area expected prior to the arrival of the circulation and more associated with a stalled front in the area which will eventually capture TD4. With increased confidence therefore, much of southeastern Georgia down to the Big Bend is now considered in a higher-end Moderate Risk for flash flooding with this update.=20 A High Risk upgrade was discussed with the offices, but for now have opted to hold off. This was due to several factors, including some forward speed of TD4 at this time (albeit slowing), high FFGs due to much of this area being forested and flood-resistant, and the notable change in the forecast largely driven by changes in the guidance of the forward speed of TD4. The expected full stoppage of the storm followed by chaotic meandering of the center into Days 4 and beyond pose a larger flash flooding threat, despite the storm starting off the period stronger. Ultimately flash flooding is driven more by the forward speed of the storm as opposed to its intensity. Further, since this is still in the Day 3 period, there is still time to assess the storm in future guidance to hopefully achieve a better degree of consistency before the probable upgrade to a High Risk for portions of the South Carolina and Georgia coasts. Stay tuned. Very few changes were made to the surrounding Slight and Marginal risk areas, but any flooding across much of North Carolina will be primarily driven by the front corralling moisture from TD4 north along the front. ....Midwest... A low over the central Plains, high pressure to the north over Canada, a passing shortwave, and somewhat higher than normal moisture from Minnesota to Michigan will cause training thunderstorms across this region, tracking ESE along a stalled front. Guidance has shifted a good bit northward...an unusual direction...so the Slight was shifted accordingly out of Chicagoland and covering more of the Twin Cities, much of Wisconsin and west-central Michigan.=20 ....New England... The same front draped over the Midwest will focus convection across northern New England, though faster flow should mean the storms move along at a very fast pace, reducing the threat for flash flooding. The Marginal was adjusted northward accordingly with this update. ....Southwest... The typical monsoon flow will continue in this region. Very few changes were made due to sparse storm coverage expected in this area Monday afternoon. Wegman ....Previous Discussion... ....Southeast... Per collaboration with the WFOs, have hoisted a Moderate Risk during Day 3 (Mon-Mon night) across NE FL through the eastern coastal plain of GA-SC. This based on the latest NHC track and speed of TD #4, along with the anticipated rainfall from the day prior (Sun-Sun night). Expect further refinements in the outlook area over the next several forecast cycles, as there are still some location and timing differences with the models. UKMet and ECMWF in fact are on the lower edges of the guidance spread (UKMet farther west/inland with the QPF max), CMC much farther inland with the heaviest rain (central-northern GA), while the 00Z GFS QPF appears to align best with the NHC forecast track of TD #4. With the TC's forward speed expected to slow to 6-8 kts over SE GA by Day 3, that would put TD #4 in the 20th percentile of storm forward speed per climatology. As a result, areas of 5-10", perhaps with higher max totals, will be possible within the Moderate Risk area. Again, later shifts will refine the outlook, while also assessing the potential need for a High Risk. The slowing forward speed by Day 3 would be a big concern as such would elevated the excessive rainfall threat considerably. ....Upper Midwest... Transient 100-120kt upper level jet streak just north of the U.S.-Canadian border, along with a series of fast-moving shortwaves within the deep westerly mid-upper level flow, will provide ample dynamical forcing for more numerous showers and thunderstorms during Day 3. Some of the activity will be anafrontal, as the surface front is expected to drop into IA-IL-IN later Monday. However, sufficient deep layer instability will remain (i.e. even elevated), with the 00Z ECMWF and GFS indicating MUCAPE values of 1500-2500 J/Kg within the new Day 3 Slight Risk area during the day Monday. The Slight Risk was pulled a little farther south from yesterday's Day 4 outlook, aligning with the more favorable deep- layer instability, which is getting support from the 00Z GEM- Regional and higher resolution ECMWF runs. Both of these models show localized totals between 3-4", which will likely fall within a 3-6 hour period per the 3-hourly QPF output. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8LZBZUPLt3Bo4HUMlLB5C_bS4saEnCTekPauMgeRbBaf= XBMngdArRWeG8g2Jh5keLMqHa7HYl2mrwiDF_H2hfiZZAwA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8LZBZUPLt3Bo4HUMlLB5C_bS4saEnCTekPauMgeRbBaf= XBMngdArRWeG8g2Jh5keLMqHa7HYl2mrwiDF_H2hpsWJ1sg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8LZBZUPLt3Bo4HUMlLB5C_bS4saEnCTekPauMgeRbBaf= XBMngdArRWeG8g2Jh5keLMqHa7HYl2mrwiDF_H2ha0OeWus$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .