Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 06 2022 20:05:19 FOUS30 KWBC 062005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 304 PM EST Sun Nov 06 2022 Day 1 Valid 16Z Sun Nov 06 2022 - 12Z Mon Nov 07 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS COASTAL OREGON AND NORTHWESTERN CALIFORNIA... ....Coastal OR and Northwest CA... A large and energetic vertically-stacked low centered along the coast of British Columbia this morning will begin digging south to Washington State through Sunday night. Plenty of upper level energy associated with a 160 kt northerly jet streak will bring several shortwaves around the base of the low, which will direct a fetch of Pacific moisture into coastal southern OR and northwestern CA today. Soil moisture percentiles from NASA SPoRT-LIS are generally around the 50% mark, with local terrain as high as 90%, so somewhat more susceptible to rapid runoff compared to if the area was in an extended dry spell. Nonetheless, guidance is solid on a swath of 1-3 inches of rainfall along coastal OR and far northwest CA through early Monday and maximum rainfall rates near 0.5"/hr. Low and falling snow levels will diminish the impact of this precipitation since higher elevation precipitation will be in the form of snow, hence the Marginal Risk area remains confined to the coast and the adjacent ranges. Rain occurring over burn scars, particularly in northwest California, will pose the greatest chance of isolated flash flooding. Another limiting factor is the weak IVT values expected while also occurring from a more westerly direction (unimpressive moisture source) and the relatively fast progression from north to south of the higher rainfall rates. The previous Marginal Risk is unchanged for this forecast update. ....Lower Mississippi Valley and Eastern United States... An anomalously warm and moist airmass located across the East poses a non-zero threat for excessive rainfall through early Monday. Strong southerly flow on the western periphery of a large high pressure system in the western Atlantic and a building ridge over the Deep South are contributing to the record warmth and near record PW values into the Northeast compared to early November climatology. A lifting warm front is expected to reach into northern LA and southern AR later this afternoon and increase dew points into the upper 60s and low 70s. Slow-moving showers and thunderstorms are anticipated to develop along the aforementioned boundary (mainly after 03z Monday), but limited MUCAPE should keep convection short-lived and reduce the flash flood risk. For the western Carolinas and Southeast, fast-moving and small scale showers/thunderstorms may contain efficient rainfall rates as PWs range between 1.3-1.7". Combined with a surface trough swinging from the Southeast coast to eastern Georgia tonight, 12z HREF neighborhood probabilities do show some small percentages for exceeding 3 inches by this evening in eastern GA and Upstate SC. For the Northeast, a slow-moving NE-SW oriented cold front should pick up forward speed overnight, but until then there will be a threat for linear showers/storms to linger over parts of Upstate NY and northern VT. Most guidance keeps rainfall totals under 2 inches, while also occurring over areas with relatively dry soils. Snell Day 2 Valid 12Z Mon Nov 07 2022 - 12Z Tue Nov 08 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE TRANSVERSE RANGES OF SOUTHERN CALIFORNIA... ....Southern California... The deepening trough and vertically stacked low will continue propagating south down the coast from Washington State to just west of northern California on Monday. This will cause the longwave trough to amplify in a positively-tilted direction. The result will effectively be a shifting on the main onshore moisture flow southward down the California coast into central and southern California. The jet streak rounding the base of the trough will provide the best forcing as a lobe of vorticity remains well offshore. Since the main forcings won't align, rainfall totals across California will not be as robust on Monday as compared with areas further north on Sunday nor these same areas on Tuesday. Meanwhile, cold air associated with the trough will also continue advancing southward through northern CA. Thus, while 2 to 4 inches of liquid-equivalent precipitation are expected in the Sierras of central CA, the bulk of that moisture will fall as snow, which will prevent any flooding there. However, further south in the Marginal Risk area of the Transverse Ranges through the L.A. Basin, the cooler air will be much slower to arrive. The predominant southwesterly onshore flow will help the mountains upslope additional rainfall from the moisture plume driving into the coast. Now while soils have dried out a bit (they are around normal for this time of year), the 1-2" of expected rainfall in this area should help to saturate the soils. In areas with sensitive burn scars, this could be just enough for an isolated flash flooding threat, especially into late Monday night as rainfall rates exceeding 0.5"/hr are likely after 03z Tuesday per the 12z HREF. A Slight Risk was considered over the San Gabriel Mountains, but it would likely be too small to depict within a probabilistic product. ....Southern Arkansas... There is a nonzero chance for lingering heavy rainfall extending along a lifting and weakening warm front extending from the southern Plains to the Tennessee Valley. Convection is likely to be ongoing at 12z during the switch from the current day 1 period (ending 12z Monday) into day 2. Certain hires CAM guidance (especially the normally overzealous FV3) depict chances for 3"+ totals during the morning hours across southeast AR. PWATs are likely to be greater than 1.5" and above the 90th percentile compared to climatology, so the threat is worth monitoring. Instability will be lacking and the current thought is that any intense rainfall that does occur will be highly localized, thus the a Marginal Risk was omitted. Wegman/Snell Day 3 Valid 12Z Tue Nov 08 2022 - 12Z Wed Nov 09 2022 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS THE TRANSVERSE AND PENINSULAR RANGES OF SOUTHERN CALIFORNIA... ....Southern California... On Tuesday, the vertically-stacked low starting the day centered off the coast of northwest CA will begin driving southeastward into central CA by early Wednesday morning. A hyper-energetic jet with strong shortwaves rounding the base of the low will drive into SoCal on Tuesday. This will strengthen the southwesterly jet streak extending from SoCal into the northern Plains. The SoCal mountains will be in the right entrance region of this jet. The onshore flow will move a very moist airmass of up to 1" PWATs, which is +2 standard deviations above the climatological norm (according to the 12z GEFS) into SoCal. The combination of anomalous moisture and plenty of upper level support will result in heavy rainfall rates in the mountains especially, but also into the L.A. Basin down to San Diego. Upslope will be maximized in the Transverse Ranges as IVT values approach 600 kg/m/s within a strong SW fetch. These IVT values are not overly impressive and are not expected to linger for an extended amount of time, thus limiting the risk to no more than a Slight at the moment. Expect 1-2" of rain into L.A. and San Diego with totals of 3-5" in the adjacent mountains. Thus, a Slight Risk of Excessive Rainfall was maintained for this forecast cycle. The Slight is focused mostly in the mountains, but isolated flash flooding, rapid runoff, and urban flooding is also possible for areas along the coast as well. As always, areas near recent burn scars will be most at risk to debris flows and other impacts associated with heavy rain. While much higher totals can be expected in the Sierras of central CA, low snow levels will result in most of that falling as snow. The only minor updates to the outlook areas were to extend the Marginal Risk northward along the central CA coast to account for the latest QPF and to smooth/expand the Slight Risk area to account for heavier rates impacting more urbanized inland locations. ....Portions of the High Desert and Great Basin... Moisture hitting the West Coast will have a much easier than normal time getting past the mountains into interior sections of southern NV, northwest AZ, and southwest UT on Tuesday. This is thanks in part to the driving upper level low approaching this area, anomalous Pacific moisture, and a hyper energetic 140 kt southwesterly oriented jet moving over the area. The result of all of these ingredients coming together will be quite unusual amounts of rain with PWATs between 0.50 and 0.75", which is +1.5 standard deviations above the climatological normal. Expect 1-2" of rain broadly across the Marginal Risk area, with locally higher amounts in the mountains on the southwestward facing slopes. Even valley areas to include metro Las Vegas can expect around an inch of rain on Tuesday. Snow levels will waffle around 7000ft over most of the Marginal Risk as well. Wegman/Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!77DaYEMttvA72WQbMg_qpeLUMbnZEnQfQl3i9JtALkOW= Xc-Jz0Ez4a_LhNUUAiqcIKqxHbt3Erx4nyhJooodRxCQCAo$=20=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!77DaYEMttvA72WQbMg_qpeLUMbnZEnQfQl3i9JtALkOW= Xc-Jz0Ez4a_LhNUUAiqcIKqxHbt3Erx4nyhJooodCf_xWHw$=20=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!77DaYEMttvA72WQbMg_qpeLUMbnZEnQfQl3i9JtALkOW= Xc-Jz0Ez4a_LhNUUAiqcIKqxHbt3Erx4nyhJooodxBwQ62A$=20=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) .