Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 22 2024 00:49:37 FOUS30 KWBC 220049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 748 PM EST Sun Jan 21 2024 Day 1 Valid 01Z Mon Jan 22 2024 - 12Z Mon Jan 22 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA AND SOUTHWESTERN OREGON... ....California and southwest Oregon... 0100 UTC Update -- Only change was to remove the Marginal Risk out of the southern portions of the Central CA Coast, based on both the latest observational as well as model trends. For Santa Barbara and areas farther down the coast, the latest HRRR QPF trends, along with the 18Z HREF and RRFS probabilities of receiving 0.50"/hr rates, would imply the steadier, heavier rainfall holding off until after 12Z Mon (current Day 2 ERO period). Previous discussion below... Overall, not much change from the previous forecast. The southern extent of ongoing precipitation across California will continue to spread further south and east across southern California, as a shortwave embedded within a broad upper trough amplifies the flow across the region later this morning. This may bring some orographically-focused moderate to heavy amounts into the coastal ranges before shortwave ridging brings drier conditions across central to southern California by the afternoon and continuing into the evening. Meanwhile, models have been consistent in showing another shortwave and accompanying plume of deeper moisture moving into western Oregon and northern California during the overnight into Monday morning. Increasing IVT along with favorable forcing will focus again across northern California into southwestern Oregon, supporting increasing rainfall rates across the same areas impacted by moderate to heavy rainfall on Saturday. HREF neighborhood probabilities indicated that locally heavy amounts of 3 inches or more likely within the Slight Risk area by 12Z Monday. Given the near-saturated to saturated soils across the region, these additional rains are are expected to contribute to further runoff concerns. .....Southern and central Texas... As a broad upper trough moves across the Southwest into the southern Rockies and High Plains, southerly low level inflow is expected to increase across southern to eastern Texas. PWs are forecast to increase to 1.25-1.5 inches (1-2 standard deviations above normal) along a 50 kt low level jet extending from the northwestern Gulf of Mexico through eastern Texas. This moisture, interacting with increasing ascent afforded in part by favorable upper jet forcing, will support the development of rainfall across southern to central Texas. Despite ample moisture and forcing, limited instability is expected to help keep rainfall rates and the threat for widespread excessive runoff concerns in check.=20=20 Hurley/Pereira Day 2 Valid 12Z Mon Jan 22 2024 - 12Z Tue Jan 23 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA AND FROM EASTERN TEXAS TO WESTERN LOUISIANA... ....2030Z Update... ....Eastern Texas and Southern Arkansas... The Slight Risk area was expanded northward to include the Arklatex region of southwestern Arkansas with this update, while the surrounding Marginal was expanded into northwestern Arkansas. Rain ongoing at the start of the period will continue through the 00Z-03Z Tuesday time frame. That rain will be heavy at times as copious Gulf moisture tracks northward on a 50 kt jet across eastern Texas and ascends the Ozarks of western Arkansas, where rates will be enhanced. On Monday morning, this warm, moisture-laden air will overrun cold air trapped from this latest cold snap, resulting in a significant icing event for a few areas, where winter headlines are out. However, with nothing to reinforce the cold air, the abundant warm air (and solar heating) will eventually overwhelm what cold air is left, resulting in a Ptype changeover from freezing rain to heavy plain rain. This is likely to occur in most areas by noon local (18Z). However, heavy rain will continue several hours beyond this, as mentioned. Thus, even areas that start out the day icing will eventually contribute to localized flooding concerns as heavy rain falls on melting ice. Soil conditions remain dry over the entire Slight and Marginal Risk area, but the combination of abundant warmth and Gulf moisture and a very slow-moving upper level system forcing the rainfall will lead to a multi-day event in portions of the area, particularly from Houston northward through the Arklatex. This should at least delay the onset of flash flooding in most of the ERO risk areas. However, the forecast of 2 to 4 inches of rain in the Slight Risk area should likely overwhelm flood-prone streams and creeks, especially since the recent cold weather will prevent much vegetative absorption of the rainfall resulting in most of the rain contributing to runoff. The greatest rainfall totals will be towards the Houston area and Texas Gulf Coast. ....California and Arizona... No significant changes were made to the inherited ERO risk areas. The Slight Risk area for the L.A. and San Diego metros continues to look good, as a solid 1 to 2 inches of rain are expected with an area of heavy rain that moves in ahead of a 160 kt jet streak just off the coast, which itself is slow-moving on the whole. (The winds within are extremely fast, but the jet streak itself is slow-moving). The enhanced lift associated with the jet streak and some modest instability will support the heavy rainfall into the L.A. and San Diego basins through the morning hours. By mid-afternoon the rain will be tapering off and relatively little rain is expected Monday night. Thus, the Slight Risk will likely be able to be canceled earlier than the 12Z end of the Day 2/Monday period. With the heaviest rain impacting southern California and the slopes of the Sierras...relatively speaking northern California may see a "break" in the rainfall with most areas picking up less than an inch of new rainfall for the period. A relative minimum is expected for the Bay area with less than a half inch expected in most areas. Despite recent rainfall from today, these meager amounts of rain are unlikely to contribute to flooding concerns, so the Marginal Risk has been trimmed out of the Bay area. A small portion of the coast near the CA/OR border remains in a Marginal Risk with locally heavier rain expected there due to the proximity of the coastal mountains and relatively higher amounts of rain seen in recent days and today. Wegman ....Previous Discussion... ....Western U.S.... Again, the forecast remains similar for much the much of the West, with unsettled weather expected to continue for another day as a series of shortwaves continue to impact the region. The shortwave approaching California Monday morning will continue to amplify, directing deeper moisture further south into southern California by late morning and continuing into the afternoon. Guidance shows PWs increasing to 2-3 standard deviations above normal along an axis of southwesterly flow ahead of the approaching wave. This moisture along with left-exit region upper jet dynamics is expected to help support some locally heavy amounts along the coast into the coastal ranges. These rains are expected to fall onto soils already moistened by this morning's rainfall, contributing to potential runoff concerns. As the trough continues to amplify and move inland, rain will spread across the lower Colorado Basin into southern Arizona.=20 While expected to be beneficial for most areas, this is likely to be the second of back-to-back days of widespread precipitation impacting the area, which may raise localized runoff concerns. Meanwhile, ongoing precipitation early in the period may contribute to some additional runoff concerns along the Sierra foothills and along the central coast and valleys before drier conditions spread across the region late Monday into Tuesday.=20 Further north, persistent onshore flow and energy aloft may contribute to additional locally heavy amounts along the far northern California and southwestern Oregon coasts into Tuesday. ....Eastern Texas to Louisiana and southern Arkansas... A mid-to-upper level shortwave moving east of the southern Rockies Monday morning, is expected to assume a negative-tilt as it moves east across the southern Plains on Monday. Strong upper forcing, interacting with deep moisture (PWs 1.25-1.5 inches) centered along a 50+ kt low level jet will support the development of heavy rainfall moving east across eastern Texas into portions of southern Arkansas and western Louisiana. In addition to the strong forcing and ample moisture, increasing instability is expected to raise the threat for heavy rainfall rates leading to localized excessive runoff. The 00Z model runs present a strong signal for amounts of 2-4 inches occurring over the region, but differ significantly on placement. While the models spread remains significant, the overall trend in the models has been to keep the axis of heavier amounts further west, which in reflected in the updated outlook areas. The Slight Risk area centers around the areas where the EC, GEFS, and CMC ensembles show their higher probabilities for accumulations of 2 inches or more. Pereira Day 3 Valid 12Z Tue Jan 23 2024 - 12Z Wed Jan 24 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS INTO THE MID-SOUTH... ....2030Z Update... ....Eastern Texas to the Mid-South... The Slight Risk area was broadly expanded in all directions with this update. While QPF is little changed from inherited, there remains significant uncertainty in the guidance as to where the heaviest rain will fall, with the GFS making the northwestern outlier, and the CMC/UKMET holding the heaviest rain over central LA. Most of the guidance is in between, and that's where the Slight remains in place today. However, given that all of these models suggest the axis of heaviest rainfall will support as much as 4 inches of rain, either into the Arklatex, central Louisiana, or anywhere in between, it was prudent that the Slight Risk area was expanded to cover any of these possibilities until there's better consensus in the guidance. Hopefully with this period moving into the Day 2 time frame in future forecasts, the consensus will become much clearer, and the Slight Risk area can shrink with time accordingly. It remains possible that should there be significant overlap between the Day 2/Monday and Day 3/Tuesday axes of heaviest rainfall, that a Moderate Risk may be needed for some of the current Slight Risk area, but at the moment given the wide variability in the guidance could not be justified. This concern extends into the Day 4 period as well. The area will continue to be monitored. ....Arizona... No significant changes were needed, but the Marginal was modestly tweaked a little further north to match where much of the guidance suggests where the elevated rain-snow line will be along the Mogollon Rim. Wegman ....Previous Discussion... ....Eastern Texas to the Mid-South... A brief window of relatively drier conditions is expected early in the period as the shortwave impacting the region on Monday lifts to the northeast through the mid Mississippi Valley Tuesday morning. However, precipitation is expected quickly redevelop back to the west as upstream energy continues to amplify an upper trough moving across the Southwest. Amplifying southerly flow will support a resurgence of deeper moisture across eastern Texas and the lower Mississippi Valley. PWs over 1.5 inches (2.5 standard deviations above normal) will begin to extend north across eastern Texas and the lower Mississippi Valley late Tuesday into early Wednesday. This moisture along with ample instability and large-scale ascent will support the development of heavy rainfall across the region. Some models also present a noteworthy signal for southwest to northeast training beginning to develop along a slow-moving frontal boundary, supporting an axis of heavy accumulations over the region. Unfortunately, the guidance is widely varied in its placement of heavy amounts. The UKMET and CMC are well south and east of the GFS and the NAM, while the ECMWF is slower -- keeping the bulk of its heavier amounts farther west. There is also uncertainty regarding how much convection will develop along the immediate coast and limit moisture advection and the development of heavy rainfall farther north.=20 Similar to Day 2, the outlook areas were shifted farther west, with the Slight Risk centered near the midst of the spread and close to where the 00Z GEFS showed higher probabilities for accumulations of 2 inches or more. However, given the model spread, confidence is limited. Models show the trough to the west continuing to reload beyond this period, suggesting this will be part of a multi-day heavy rainfall event, with potentially widespread impacts. However, given the uncertainty, did not have the confidence to introduce anything beyond a Slight Risk on either Day 2 or 3. However, suspect upgrades may be forthcoming, especially if model agreement improves and starts to show subsequent days of heavy rainfall across the same areas. ....Southeastern Arizona... An amplified upper trough along with anomalous moisture will continue to support precipitation across the region early in the period. While widespread heavy amounts are not expected, some additional locally heavy amounts may produce some isolated runoff concerns early in the period. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7-TC-UrFlgaleDVijREVBTmPEU57yJGyRGX82XivjCP5= aJXxT5p9mVBApttXiw9mBu23pgF8L4n-1S3NlkX4EPB760o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7-TC-UrFlgaleDVijREVBTmPEU57yJGyRGX82XivjCP5= aJXxT5p9mVBApttXiw9mBu23pgF8L4n-1S3NlkX40pg7pt4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7-TC-UrFlgaleDVijREVBTmPEU57yJGyRGX82XivjCP5= aJXxT5p9mVBApttXiw9mBu23pgF8L4n-1S3NlkX46PI4rOM$=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) .