Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 03 2024 19:58:26 FOUS30 KWBC 031958 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 258 PM EST Sun Nov 3 2024 Day 1 Valid 16Z Sun Nov 03 2024 - 12Z Mon Nov 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND THE OZARKS... ....16z Update... Little change to the inherited Slight Risk area, owing to observational trends and the new 12z HREF suite. The bulk of the expected rainfall going forward looks to occur to the south and east of areas that were hardest hit over the past 12 hours (mainly central to northeast OK). Despite this, the Slight was maintained over the wetter antecedent conditions, as elevated convection looks to still result in scattered 2-3" totals (well represented by the 12z HREF blended-mean, and particularly the 12z HRRR solution). Farther south and east (into southeast OK and northwest AR), scattered to numerous totals of 3-5" are more likely (with isolated/localized totals of 5" possible). Drier antecedent conditions across this region will likely limit instances of flash flooding to scattered coverage, but locally significant flash flooding will still be possible (considering that consecutive 3-hr periods of 3" exceedance are progged at 20-40% after 00z, per HREF 40-km neighborhood exceedance probabilities). Churchill ....Previous Discussion... ....Southern Plains and the Ozarks... In coordination with TSA/Tulsa, OK; LZK/Little Rock, AR; and SGF/Springfield, MO forecast offices, the inherited Moderate Risk area was downgraded to a Slight with this update. The surrounding Marginal was trimmed out of Wisconsin, Minnesota, and much of Iowa. There was a notable south and eastward shift in the guidance as to the axis of heaviest rainfall expected across Oklahoma and Arkansas today into tonight. A longwave trough over the Southwest will shift eastward today. Embedded shortwave disturbances as well as a developing 90 kt jet streak will move over the region through tonight. A well-established low level jet (LLJ) will continue to advect plentiful Gulf moisture northward into the area. The result will be multiple rounds of storms with embedded training cells in OK/AR and into MO. Much of the area has been very dry lately, though yesterday some rain impacted the area, especially in Oklahoma and into southern Missouri. Multiple "streaks" of much heavier rainfall are anticipated where cells of much heavier rain train over the same areas. This is likely to cause widely scattered to scattered instances of flash flooding. Antecedent conditions will be the primary limiting factor keeping the flooding impacts in check. Very dry conditions prior to this event has led to drought conditions developing with very low river and stream levels. Thus...much of the rain expected today will be beneficial and work to return those rivers and streams to much more normal levels. However, given the plentiful moisture, training storms, and some flood prone areas, the heavy rain could cause localized flash flooding. The eastward shift in the guidance has also introduced the Ozarks into the highest risk area. The topography of the area will help to focus the heavy rainfall into the valleys...which could result in rapid river, stream and creek rises. This also locally increases the flash flooding threat as compared to the same amount of rain falling over relatively flat terrain. While forecast rainfall amounts haven't changed appreciably as compared with previous forecasts, the shift of the heaviest rain eastward and into many areas that did not receive appreciable rain today has reduced the likely flood impact threat as well as coverage. When added to the dry antecedent conditions, this should keep flooding impacts from becoming widespread enough to reach Moderate Risk levels. Hence, while flash flooding remains likely as amounts could locally exceed 5 inches, the impact and coverage was determined to be in the Slight Risk category...hence the downgrade. ....Upper Midwest... In the upper Midwest, the eastward shift in the guidance and a more west to east movement of the expected convection should limit how much instability and moisture reach the upper Mississippi Valley. It's likely any remaining convection will be in a much weakened state by the time the rain reaches this area. Since similarly dry conditions and drought have impacted the upper Great Lakes in recent weeks, the largely stratiform nature of the rain should mean all of the rain will be beneficial with no flooding concerns noted. Thus, the Marginal was trimmed towards the south. Wegman Day 2 Valid 12Z Mon Nov 04 2024 - 12Z Tue Nov 05 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEAST TEXAS THROUGH PORTIONS OF WESTERN LOWER MICHIGAN... 20Z Update: Changes were few and far between for the D2 ERO with the primary deviation being a slight shift eastward with the risk areas given the latest trends within the forecast precip. A secondary area of development over the course the evening into tomorrow morning will generate another outflow dominant convective pattern with a general expansion of the precip further to the north and east leading to areas downstream in the Ozarks becoming the benefactor of the heaviest precip. There's a strong consensus for heavy rain totals exceeding 3" across the above area with a the 12z HREF neighborhood probabilities exceeding 70% for the >3" threshold extending from Southeast OK up through Northwest and North-Central AR into Southeastern MO. EAS probabilities follow suit with an axis of 50+% of >2" probs located over the same area with a 20-30% for >3" encompassing a good portion of the Ozarks. The antecedent conditions leading in are relatively dry in the grand scheme with the NASA SPoRT indicators leveling off the soil moisture percentiles below 40% for much of the area anticipating heavy rain. The more complex terrain does offer a slightly better risk than typical compared to areas in the plains, so the prospects are within the higher end of the SLGT risk margin, but just outside the MDT risk upgrade for the time being. Pending the evolution of the activity tonight, an upgrade may be necessary, targeting locations likely centered over Northwest AR into Southern MO. Will continue to monitor the situation closely as it evolves.=20 ...Previous Forecast.. Few changes were made or needed to the Slight Risk area across the middle of the country. A deep longwave trough will continue move eastward but begin to weaken and lift northward back into the jet stream on Monday. As it does so, associated jet streaks will intensify downstream of the trough, which will support its deamplification. The LLJ that has been supplying Gulf moisture will also elongate towards the north reaching as far north as lower Michigan by Monday. This will cause the resultant moisture to focus in a SSW to NNE corridor from Texas to Michigan. Unidirectional SSW flow will support training thunderstorms. The strongest storms are expected from Texas through Missouri, which will have access to the greatest Gulf instability and moisture. The storms will gradually weaken the further north you go, but are expected to maintain their integrity into Wisconsin and Michigan. The topography of the Ozarks will further locally enhance rainfall totals due to some upslope support. Due to overlap with the rainfall footprint Monday with that expected today, there is some potential for a Moderate Risk upgrade from western Arkansas into southern Missouri. However, this will be highly dependent on how much rain falls in that area today/Sunday. Total rainfall amounts will be somewhat lower on Monday as compared to Sunday, hence the importance of rain falling over the same areas as Sunday to enhance the flooding threat to Moderate Risk levels. Thus, the potential for an upgrade will continue to be evaluated. Wegman Day 3 Valid 12Z Tue Nov 05 2024 - 12Z Wed Nov 06 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MISSISSIPPI AND OHIO VALLEYS... 20Z Update: Very minor adjustments were made to the MRGL risk located across the Mississippi and Ohio Valleys as the overall pattern and subsequent rainfall forecast maintained continuity from run-to-run. Areal average QPF runs between 1-2" for the period with the heaviest rainfall likely relegated over the Mid Mississippi Valley Tuesday morning and afternoon before petering out as the disturbance shifts eastward. Main threats will be relegated to mainly urban and small stream flooding with more isolated flash flood concerns in terms of an areal extent. ...Previous Forecast.. By Tuesday, a progressive cold front pushing across the Mississippi Valley and associated upper level trough and shortwave will separate...with the upper shortwave ejecting northeastward into the Great Lakes, leaving the front behind. The combination of steady forward movement of the storms through the day followed by the rapid diminishing of any and all upper level support by Tuesday night will rapidly diminish the flooding threat as storm coverage and intensity also diminish. With some supply of Gulf moisture continuing on Tuesday, the storms that form through the day will still have enough punch to cause locally heavy rainfall. However, the eastward progression of the front should allow this rain to fall largely over areas that remain dry today/Sunday and Monday. Antecedent dry conditions will preclude much of the flooding threat by Tuesday, so only isolated flash flooding in urban and flood prone areas is anticipated. The Marginal Risk is largely unchanged from previous forecasts. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6LZ8i1DGnrnHmAHubfPiUy_Bha_2G_RP8Kgp0pOWxmpL= g8arBu_6BERdNtE-mwrkOLAqU5ufaxDBRD1p-sn1ObbN8pU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6LZ8i1DGnrnHmAHubfPiUy_Bha_2G_RP8Kgp0pOWxmpL= g8arBu_6BERdNtE-mwrkOLAqU5ufaxDBRD1p-sn1xCD5Xpw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6LZ8i1DGnrnHmAHubfPiUy_Bha_2G_RP8Kgp0pOWxmpL= g8arBu_6BERdNtE-mwrkOLAqU5ufaxDBRD1p-sn1DnMwU0Q$=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) .