Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 18 2023 22:50:17 FOUS30 KWBC 182250 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 548 PM EST Sat Nov 18 2023 Day 1 Valid 01Z Sun Nov 19 2023 - 12Z Sun Nov 19 2023 ....Northern California... A cold front moving across the region is advecting in high enough moisture to lead to heavy rainfall across the region across area terrain. SPC mesoanalyses and NAM forecasts indicate pockets of 500+ J/kg of MU CAPE in the area and precipitable water values initially above 1" which decline with time after the front passes by, which would be most problematic in terrain like the Shasta/Siskiyou and northern Sierra Nevada. There is enough moisture and instability to lead to showers and possible thunderstorms -- which so far have been constrained to areas farther south in the Central Valley -- which given the degree of low-level inflow and effective bulk shear could try to organize should thunderstorms materialize. Beyond 02z based on RAP forecasts, precipitable water values broadly fall below 0.75" which should lowers the chance of 0.5"+ an hour rain totals based upon the available ingredients. The 18z HREF indicates the potential for such until 05z based on the aforementioned pockets of 500+ J/kg of MU CAPE, but at this point snow/freezing levels should be dropping. Overall totals are expected to remain modest, under 2". While the probability of excessive rainfall is deemed under 5 percent, because the window for such should be closing soon after this outlook's start time, it's nonzero for area burn scars. The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth Day 2 Valid 12Z Sun Nov 19 2023 - 12Z Mon Nov 20 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Southern Kansas/Northern Oklahoma... There remains a non-zero threat for excessive rainfall across south-central to southeast Kansas and north-central to northeast Oklahoma as strengthening mid to upper level dynamics are trying to draw favorable moisture/instability ingredients into the region ahead of a sharpening mid-level trough late Sunday into Monday.=20 This upper-level low will support strong cyclogenesis across the Southern Plains, while defining a sharpening dry line and strong southerly flow. Available moisture is lagging the increased wind flow, with total PWats only expected to reach 1.2-1.3" through late afternoon across central OK. A dryline bulge across NW Oklahoma is expected to support stronger moisture flux convergence for some scattered to widely scattered thunderstorms across the area of concern through late evening into the overnight hours of Monday. However, with the lack of moisture loading into the column combined with fairly quick steering flow/cell motion vectors, rain-rates and totals are more likely than not to reach even the naturally lower FFG values of the Flint Hills of SE Kansas into Osage county, OK (<2"/hr). 12z Hi-Res Convective Allowing Models do hint at spots of intense rates but generally run in the 1.5-1.75"/hr range with similar (or less) totals.=20 Orientation of the moisture flux convergence near the dry-line bulge may support upstream regeneration for possible training environment. However, longer term drought conditions are in place with soil moisture values in the top 40cm well below average. So only risk would be very intense short-term rates where soils are hydrophobic do to drought conditions or urban environment. So all in all, the risk will fall short of the 5% probability threshold for delineating a risk category, but an incident of flooding can not be fully ruled out, though most rain will be highly beneficial. Gallina Day 3 Valid 12Z Mon Nov 20 2023 - 12Z Tue Nov 21 2023 ....There is a Marginal Risk of Excessive Rainfall Over Portions of the Lower Mississippi Valley and Gulf Coast States... ....21z update... Very little change in the overall meteorological parameters discussed below with respect excessive rainfall and potential for localized flash flooding. Overnight and latest 12z guidance has denoted a slight eastward/faster trend to the axis of strongest forcing intersecting the return moisture flow off the western and central Gulf of Mexico. This flattens the overall flow with increased westerly steering, allowing for a tad increased axis of training/repeating for stronger, potentially rotating, updrafts along/ahead of the cold front in the Lower Mississippi Valley.=20 Very strong moisture flux should allow for very intense rainfall rates in the range of 2-2.5"/hr, but duration in a given place should be limited to those widely scattered areas of repeating cells. While the area of concern has been in a longer term drought and top 40cm soil conditions are at or below 30% moisture content (generally below 10th percentile historically), the sheer rates over dry soils may result in limited infiltration initially with increased run-off potential. As such, an incident or two of flash flooding remains possible, but nearly all parameters continue to fall below thresholds for an upgrade to a Slight Risk, but the broad Marginal Risk of Excessive Rainfall remains, if adjusted slightly, mainly eastward into portions of the Tennessee River Valley and central Gulf Coastal Plain.=20 Gallina ....Prior discussion... Flow aloft over the Lower Mississippi Valley and Gulf Coast region will back in response to the amplification of a long wave trough over the Intermountain region and the Rockies. There should be increasing divergence aloft...which sets the stage for cyclogenesis over Arkansas or far southern Missouri during the latter part of the day. Convection with some potential for producing some flash flooding due to downpours is expected to develop in an increasingly moist and unstable airmass along and ahead of a cold front tied to the surface low. The NCEP guidance and the ECMWF show precipitable water values increasing to between 1.8 and 2.0 inches from Louisiana to parts of Arkansas and Mississippi by 21/06Z and more than 2.5 standardize anomalies by 21/12Z. Those values approach 2 standardized anomalies above climatology for this time of year. The 18/00Z GFS and UKMET were most aggressive with maximum 24-hour rainfall amounts in the 2 inch to 3 inch range while the ECMWF was only showing isolated maximum amounts generally at or below 1.25 inches. Flash flood guidance values generally range from 2.5 to 3.75 inches at both the 1- and 3-hour time intervals...so it is possible that these values could be challenged in localized areas if the rates are sufficiently high. However...lack of any GEFS or SREF members showing 3 inch contours suggests not upgrading to a Slight risk at this point. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4R7mJaN4o7zIEfNMnl2z8TipLwXt60PTW5RVvDkIPGPv= ylzW_Vm_AwrAcoujvseTnu4AAFHjQ6lY_tVBvxrxut-fsgI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4R7mJaN4o7zIEfNMnl2z8TipLwXt60PTW5RVvDkIPGPv= ylzW_Vm_AwrAcoujvseTnu4AAFHjQ6lY_tVBvxrxvz1sJVU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4R7mJaN4o7zIEfNMnl2z8TipLwXt60PTW5RVvDkIPGPv= ylzW_Vm_AwrAcoujvseTnu4AAFHjQ6lY_tVBvxrxe0mnnV8$=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) .