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   FOUS30 KWBC 172006   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   306 PM EST Mon Nov 17 2025   
      
   Day 1   
   Valid 16Z Mon Nov 17 2025 - 12Z Tue Nov 18 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF   
   SOUTHERN CALIFORNIA...   
      
   16z update:   
   Current observational trends and 12z CAM guidance continue to   
   solidify the current thinking and placement of ERO areas and   
   magnitudes moving forward through 18.12z.   
      
   Core of the warm conveyor belt/atmospheric river is starting to   
   reach southwest California and rounding Point Conception. Weakly   
   confluent 850-700mb 35-45kt southerly flow along the front and 1 to   
   1.25" total Pwats (around .5" in the same layer) is resulting in   
   400-500 kg/m/s IVT values. Though the moisture plume is narrowing   
   and starting to advance with upstream height-falls, the onshore   
   flow will intersect the Transverse Range more ideally to maintain   
   solid rainfall rate potential over .5"/hr (HREF probs of 70-90% on   
   the Transverse and 30-40% on the Peninsular Ranges). The forward   
   progress is likely to limit overall totals along the spine of the=20   
   terrain to 1.5-2" with an isolated 2.5" not out of the realm of=20   
   possibility. Combine this with higher soil saturation in the 55-80%   
   range per NASA SPoRT 0-40cm relative soil moisture values (which=20   
   are well over the 95-98th percentiles), suggestive of enhanced=20   
   runoff capability and continued flooding potential. Even urban=20   
   locations of Southern California will remain with .5-.75"=20   
   potential total in short duration maintaining solid Slight Risk=20   
   level coverage for urban flooding concerns.   
      
   Downstream, early morning (18.06z-18.12z) south to north moving   
   thunderstorms given modest moisture and 500+ J/kg of MUCAPE through   
   the Lower Colorado Valley will have some potential for enhanced   
   rates capable of an isolated incident of flash flooding and so   
   little change in placement/thinking with the downstream Marginal   
   Risk into S NV and W AZ as well.   
      
   Gallina   
      
      
   ~~~~Prior Discussion~~~~   
   A vigorous mid-level wave (initially centered near 39.9N, 125.5W   
   at the start of the forecast period) will migrate south-   
   southeastward along the California coast today while cutting off   
   from westerlies farther north. As this occurs, strong ascent   
   (associated with areas of 0.3-0.6 inch/hr rain rates in northern   
   California this morning) will spread southeastward along a cold   
   front and also within a robust upslope regime on the western sides   
   of the Sierra. 1-1.2 inch PW values near the coast and aforementioned   
   synoptic factors will foster several areas of 1-2 inch rainfall   
   totals, with locally higher amounts possible across the Transverse   
   Ranges of southern California.   
      
   The aforementioned front will result in a focused axis of heavy   
   rain that will translate from north to south along the southern   
   California coast. Across the Slight Risk area, this heavy rainfall   
   will occur over both sensitive locales (from recent burn scars and   
   debris flows) and urban/hydrophobic surfaces. The greatest   
   concentration of flash flood risk will likely occur across the   
   Transverse Ranges and adjacent areas of Los Angeles Metropolitan   
   area through tonight.   
      
   Outside of this area, models indicate very isolated, but slow-   
   moving convective activity beneath the upper low center across   
   northern and central California, with heavy rainfall potentially   
   falling over soaked ground conditions and burn scars. Isolated   
   flash flood potential cannot be ruled out here.   
      
   Lastly, the cutting off upper low will spread strong mid/upper   
   difluence across the Lower Colorado River Valley especially in the   
   03-12Z timeframe. Models are consistent in developing at least one   
   or two bands of convective structures that drift northward across   
   the area fairly quickly, but also result in localized training. One   
   or two instances of flash flooding cannot be ruled out in this   
   regime.   
      
   Cook   
      
   Day 2   
   Valid 12Z Tue Nov 18 2025 - 12Z Wed Nov 19 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20   
   WEST CENTRAL ARIZONA...   
      
   20z Update:=20   
      
   Upgraded to a Slight risk across portions of west central AZ with   
   this update. Strong forcing just ahead of the upper low will   
   support repeated rounds of showers and embedded heavier convective   
   elements within the deep layered southerly flow. Not a lot of   
   instability forecast, but values upwards of 500-1000 j/kg should=20   
   be enough to support locally heavier rainfall rates...over 0.5"/hr   
   but generally under 1". Nonetheless the repeated rounds and some=20   
   upslope component should result in rainfall locally exceeding=20   
   2-3" over the day. HREF neighborhood probabilities of exceeding 2"   
   are 50-70% and 3" are 40-50%. This rainfall should be enough to=20   
   result in isolated to scattered flash flood impacts, especially=20   
   over any more sensitive areas.   
      
   We also added a small Marginal risk centered over KY. A low moving   
   across the OH valley will place KY in the warm sector, with   
   instability forecast to increase over 1000 j/kg by Tuesday   
   afternoon. An initial batch of showers should move across during=20   
   the morning with the warm front, with another more robust=20   
   convective round expected during the afternoon and evening as=20   
   instability increases ahead of the cold front. Both the HREF and=20   
   REFS indicate 1"/hr neighborhood probabilities of 20-40% and the=20   
   HREF neighborhood probabilities of exceeding 3" during the period=20   
   are locally over 15%. It's a pretty marginal situation, but given=20   
   the degree of moisture transport and forecast uptick in=20   
   instability it does seem like an event that could locally exceed=20   
   model QPF forecasts...thus can not rule out some isolated, mainly=20   
   urban, flash flood concerns.   
      
   Chenard   
      
   ...Previous Discussion...   
      
   Maintained the previous;y issued Marginal risk area over portions   
   of Arizona with little need for more than cosmetic changes in   
   deference to the model guidance. A closed low within a broader   
   trough digs southward along the the west coast of the US and then   
   starts to move inland late. The overall synoptic pattern seems to   
   be handled comparably by the ECMWF/GFS and UK models for much of   
   Day 2 but some of the subtle differences in the details result in   
   non-negligible differences in QPF and the attendant risk of   
   excessive rainfall.   
      
   Precipitable water values get near 2 standard deviations above normal   
   for this time of year in southern-western AZ and most-unstable   
   CAPE values climb above 500 J/Kg south of the Mogollon Rim and into   
   the deserts and Phoenix Metro. This will maintain the risk of   
   elevated (0.50"+) hourly rainfall rates Tue-Tue night as noted from   
   the RRFS exceedance probabilities (through 00Z Wed), while also   
   implied from the 16/12Z RRFS 3-hourly QPFs. Much like the models   
   from Sunday afternoon...the 24hr QPF guidance from the 17/00Z model   
   cycle has come up. At this point there is still not enough   
   confidence from a duration perspective to upgrade to a Slight Risk   
   over the lower terrain south of the Mogollon Rim, which would   
   conceivably include the Phoenix area, but the potential to do so   
   will exist over the next day or so. The machine-learning first   
   guess fields also highlight part of the Marginal risk area with   
   some spotty 5 to 10 percent of exceeding flash flood guidance. At   
   this point...the machine-learning covers only the first half of Day   
   2...so the upcoming day shift will have a complete look at the   
   objective guidance for the Day 2 period.   
      
   Bann   
      
      
   Day 3   
   Valid 12Z Wed Nov 19 2025 - 12Z Thu Nov 20 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF   
   SOUTH CENTRAL TEXAS TO A PORTION OF THE ARKANSAS RIVER VALLEY AND   
   OVER THE SOUTHWEST...   
      
   20z Update:   
      
   Added a Marginal risk across portions of NM and AZ with this   
   update. With the closed low trending slower, scattered showers and   
   embedded thunderstorms are likely to continue into Wednesday=20   
   across these areas. Overall it looks like less of a focus than day   
   2, but nonetheless still enough of a convective signal to suggest=20   
   localized flash flooding could continue to be a concern.   
      
   Chenard   
      
   ...Previous Discussion...   
      
   a mid- and upper-level trough will be making its way eastward   
   across portions of the southern Great Basin on Wednesday and into   
   early Thursday. East of that system...low level moisture will start   
   to be drawn northward with dewpoints in the 60s by Wednesday afternoon   
   which should help feed showers and thunderstorms from the afternoon   
   into Wednesday night/early Thursday morning. Most unstable CAPE   
   pushes 1000 J per kg and precipitable water values in excess of   
   1.5 inches becomes established from parts of south-centrsl Texas   
   into eastern Oklahoma and western Arkansas. Based on the   
   GEFS...the precipitable water axis is roughly between 2 and 2.5   
   standardized anomalies above climatology for this time of year.   
   Based on that combination...suspect locally heavy rainfall rates   
   are possible. There has been a tendency for the upper trough to   
   slow its forward speed with a similar delay in convection   
   initiation. With the event spanning into Thursday with broader   
   areal coverage and an increasing risk of excessive rainfall (see   
   WPC Medium Range graphics and discussions for further   
   details)...opted to maintain the Marginal risk on the uncertainty   
   of timing.   
      
   Bann   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!8PIjstew6NUUkIe4hAF9UGoWGVMuTGsrBbgYyR9BMtbs=   
   Na53Ucq8zQbJSOCpK5RkNYnNv0WkXNktLxzuXXVX7C6vVkU$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!8PIjstew6NUUkIe4hAF9UGoWGVMuTGsrBbgYyR9BMtbs=   
   Na53Ucq8zQbJSOCpK5RkNYnNv0WkXNktLxzuXXVX0V3aojY$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!8PIjstew6NUUkIe4hAF9UGoWGVMuTGsrBbgYyR9BMtbs=   
   Na53Ucq8zQbJSOCpK5RkNYnNv0WkXNktLxzuXXVXjVrSQy4$=20   
      
   =3D =3D =3D   
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