• MESO: Heavy rain/flooding

    From Weather Alert@454:1/105 to All on Thu Dec 31 17:56:00 2020
    AWUS01 KWNH 311946
    FFGMPD
    LAZ000-ARZ000-TXZ000-010100-

    Mesoscale Precipitation Discussion 0884
    NWS Weather Prediction Center College Park MD
    246 PM EST Thu Dec 31 2020

    Areas affected...Eastern TX...Western LA...ext Southwest AR...

    Concerning...Heavy rainfall...Flash flooding possible

    Valid 312000Z - 010100Z

    SUMMARY...Multiple bands starting to merge along cold front with
    occasional rain-rates up to 1.75"+/hr though with increasingly
    progressive cell motions pose possible flash flooding

    DISCUSSION...GOES-E WV suite continues to show the inner core of
    the closed low shifting eastward with the strengthening eastern
    side shortwave. This continues to support solid surface
    cyclogenesis near the Houston Metro. This also drawn the
    stationary front across LA further south while steepening the
    isentropic packing along the front. As such, the very strong
    nearly orthogonal boundary layer southeasterly flow and
    strengthening 85H southerly to around 45-50kts. This leads to
    deeper moisture flux convergence and vertical development and
    therefore higher depth of rainfall generation. This also narrows
    the TROWAL angling back northwest to the motion of the surface
    low. The strong DPVA combined with diffluent 3H flow within the
    Left-Exit Region of the 125kt jet should maintain the deepening of
    the cyclone and isallobaric component for further rainfall
    efficiency as the 850mb front also aligns with the surface
    boundary around 22-23z. Given MUCAPES pushing 1000 J/kg and total
    PWs around 1.75" and strong flux convergence, rainfall efficiency
    of 2"/hr are possible with perhaps some occasional sub-hourly
    bursts of higher intensity with merging cells just northeast of
    the surface cyclone where streams converge greatest.

    Greater veering flow aloft and increasing steering flow as the mid
    to upper level jet streams northeast, the low level warm
    conveyor/TROWAL should allow slightly elevated remaining
    convective elements to stream well north into East-Central TX and
    Northwest LA. However, increased speeds will also reduce duration
    of the 2"+/hr rates with expectedtotals of 2-3" genearlly along a
    line from LFK to JAS to between IER/POE where cells may linger a
    bit longer along/just north of the front before upgliding
    producing scattered streaks extending northward. This is best
    represented by recent HRRR and HRRRv4 solutions. After 00z, the
    mid-level forcing wave may begin to overtake the lower-level
    surface boundary and start to reduce best deeper vertical ascent
    and reduce overall coverage as cells expand into central LA, and
    across areas of drier ground conditions, making flash flooding a
    bit less possible, though lower FFG values near SHV and into AR
    will need to monitored closely.

    Gallina


    ATTN...WFO...FWD...HGX...LCH...SHV...

    ATTN...RFC...LMRFC...WGRFC...NWC...

    LAT...LON 33429362 32869264 31129287 29749312 29479443
    29969478 30569513 31219526 31779547 32449556
    33039542 33389472
    #
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  • From Weather Alert@454:1/105 to All on Thu Dec 31 10:52:00 2020
    AWUS01 KWNH 311440
    FFGMPD
    LAZ000-TXZ000-312015-

    Mesoscale Precipitation Discussion 0883
    NWS Weather Prediction Center College Park MD
    940 AM EST Thu Dec 31 2020

    Areas affected...Southeast TX...

    Concerning...Heavy rainfall...Flash flooding possible

    Valid 311445Z - 312015Z

    SUMMARY....Intersection of greatest rain-rates/totals are
    decoupling from areas of lowest FFG/ground saturation; however,
    ingredients for isolated flash flooding rain-rates still remain
    possible particularly in urban settings and near the coast through
    early after-noon.

    DISCUSSION...GOES-E WV depicts a very strong winding up closed low
    across Northeast Mexico with very impressive baroclinic leaf
    across much of TX. The very sharp distinct mid-level drying at
    the nose of stronger height-falls/core of the approaching
    shortwave rotating around the base/SE side of the cyclone is
    advancing through Northern Nuevo Leon. Very strong DPVA
    associated with this shortwave, along with favorable placement of
    the 120kt 3H jet core just south of the TX coast provides ample
    support for low-level wind increases and surface-low level
    cyclogenesis. As such, low level wind response is further
    strengthening FGEN forcing along the slowly advancing cold front
    that extends from the surface wave near ARM due south, while very
    strong warm advection is pumping western Gulf moisture/instability
    across coastal Southeast TX (south of the stationary front from
    said surface low to s of LFK to AQV to ESF to HEZ. Solid MLCAPE
    of 500-750 along with 1.5" total PWATs will be increasing due to
    the surface/low level response and perhaps some weak solar
    insolation by midday reaching 1000-1250 J/kg and 1.75" and
    20-25kts of SSE flow converging with nearly due west flow at the
    cold front. Combined with frictional convergence at the coast,
    thunderstorms capable will start around 1-1.25"/hr rates possibly
    becoming 1.5"-1.7/hr through the early afternoon (18-20z).

    While the rainfall environment is deteriorating through the
    afternoon, the hydrologic situation may be a bit better.
    Currently, the cold front/axis of heavy rainfall is along the
    eastern edge of the heavy rainfall footprint from over night where
    greater than 5" was observed across Jackson to Waller counties
    where FFG is compromised to less than .5" through all time-steps
    (1/3/6hrs). This axis continues northeast through Trinity and
    Polk counties though lower totals were observed with blow-off
    showers from the deeper convection. Still, this is aligned with
    strong orthogonal intersection with the stationary front to keep
    some isolated convective development lingering and compounding
    longer duration totals. With the approach of the shortwave,
    there remains a hour or so of slow eastward motion before deeper
    upper-level steering flow and height-falls support eastward
    propagation vectors of the cold front itself. Cells may continue
    to train northward along the cold front but overall motions should
    reduce totals at a given location or over saturated areas.
    However, this also moves the axis through the urban corridor of
    the Houston Metro where naturally greater run-off will occur with
    hydrophobic ground conditions...so with 2-3" totals to be
    expected, flash flooding remains possible here before further
    moving west with the strengthening rates by early afternoon.
    Currently, the NMMB has been trending the best both in placement
    and magnitude likely due to the greater synoptic forcing over the
    thermodynamic forcing, though all Hi-Res CAMs are trending a tad
    faster on the cold front reducing totals slightly.

    A secondary elevated band of convection currently observed at the
    leading edge of the 85-7H front/wind shift associated with the
    shortwave aloft, lingering mid-level moisture will continue to
    support some moderate showers and perhaps an additional .5" totals
    which given that lower FFG values may once again yield some
    further flooding concerns. There will be a pivot to the TROWAL
    across SE TX that may allow for greater duration if at a lower
    rain-rate through upglide process rather than vertical/convective
    processes.

    Gallina

    ATTN...WFO...HGX...LCH...

    ATTN...RFC...WGRFC...NWC...

    LAT...LON 30949474 30609387 29619388 28989489 28429611
    29089631 29539633 30179620 30779549
    #
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  • From Weather Alert@454:1/105 to All on Sat Jan 2 11:16:00 2021
    AWUS01 KWNH 021453
    FFGMPD
    FLZ000-GAZ000-022000-

    Mesoscale Precipitation Discussion 0002
    NWS Weather Prediction Center College Park MD
    953 AM EST Sat Jan 02 2021

    Areas affected...FL Panhandle... Southern GA

    Concerning...Heavy rainfall...Flash flooding likely

    Valid 021445Z - 022000Z

    SUMMARY... Training band of heavy rain and embedded thunderstorms
    to support a growing threat of flash flooding this morning and
    into the afternoon hours.

    DISCUSSION... GOES 16 IR imagery shows cold cloud tops associated
    with deep convection over the Gulf of Mexico. This area of
    convection is focused along a quasi-stationary frontal boundary
    positioned from South Carolina and southern Georgia extending
    southwest through the the Florida Panhandle and into the Gulf of
    Mexico. The front is located between a deep upper trough over the
    south-central U.S. and a subtropical high over the northern
    Bahamas. 12Z sounding out of TLH sampled an environment favorable
    for heavy rainfall with a deep warm cloud layer that extended more
    than 10 kft up into the column. In addition, 850-300mb mean winds
    are out of the SW, oriented parallel to the frontal boundary.
    Ample atmospheric moisture is available with PWs of 1.78" and
    skinny MUCAPE was also present. This environment is also expected
    to translate farther north and east into south-central Georgia
    this morning.

    As the day progresses, the slow moving nature of the frontal
    boundary will remain a trigger for additional heavy rain and
    thunderstorms. Available MUCAPE may vary at times but generally
    should hover close to 500 J/kg into the early afternoon hours.
    Area averaged HRRR soundings early this afternoon suggest PWATs
    hover around the 1.8-1.9" range with some guidance indicating PWs
    up to 2" possible later in the day. Over the last several hours
    some locations have already reported over 2" of rainfall with
    hourly rates over 1" within heavier cells. The training of deep
    moist convection over these areas are likely to produce additional
    rainfall totals between 2-4" inches over the next five hours with
    locally higher amounts possible where the most intense
    thunderstorms take shape. There are already reports of flash
    flooding and with overly saturated antecedent conditions having
    already occurred, additional areas flash flooding are likely to
    transpire throughout the day.

    Mullinax

    ATTN...WFO...CHS...FFC...JAX...TAE...

    ATTN...RFC...SERFC...NWC...

    LAT...LON 32358213 32128172 31768162 31158234 30288360
    29708456 29438524 30038557 31308416 32028295
    #
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  • From Mike Powell@454:1/105 to All on Thu Aug 3 07:57:00 2023
    AWUS01 KWNH 031025
    FFGMPD
    TNZ000-KYZ000-INZ000-ILZ000-MOZ000-031500-

    Mesoscale Precipitation Discussion 0846
    NWS Weather Prediction Center College Park MD
    624 AM EDT Thu Aug 03 2023

    Areas affected...central and southeastern MO into central KY/TN

    Concerning...Heavy rainfall...Flash flooding likely

    Valid 031023Z - 031500Z

    Summary...Areas of heavy rain and flash flooding are likely to
    continue from portions of central and southeastern MO into western
    KY through 15Z. Farther east, possible flash flooding will exist
    for portions of central KY/TN. Additional totals of 2-4 inches are
    expected.

    Discussion...10Z regional radar imagery continued to show an axis
    of training heavy rain over central and southeastern MO, although
    the axis is no longer contiguous. Rainfall rates have ranged from
    1-3 in/hr across this region of MO since 06Z and portions of the
    region have seen 3 to 8 inches of rain over the past 24 hours
    (MRMS estimates). Farther east, a well-defined MCV was located in
    southern IN, tracking toward the southeast, associated with a
    small region of 1-2 in/hr rainfall rates ~30 miles south of
    Louisville, KY along I-65. VAD wind plots at 850 mb from KSGF and
    KPAH showed 35-40 kt from the west and tied with some of the
    coldest cloud tops on IR imagery. However, overrunning low level
    flow and areas of confluence were still producing thunderstorms
    upstream across central MO where some of the heaviest rain has
    fallen over the past 6-12 hours. Precipitable water values were
    estimated to be between 2 and 2.4 inches and instability ranged
    from a few hundred J/kg in central KY to 1000 to 2000+ J/kg near
    the confluence of the OH and MO rivers.

    The focus for the heaviest rain over the next 3-5 hours is
    expected to set up from southeastern MO into far southern IL and
    western KY, in line with the strongest isentropic lift atop a
    surface front draped across the central MO/AR border. Areas of NW
    to SE training with 1-2 in/hr rates (perhaps locally higher) are
    expected to continue areas of flash flooding. Farther upstream,
    ascent is expected to diminish over the next few hours, however, a
    localized flash flood threat will remain across central MO where
    ground conditions are hyper sensitive due to recent heavy rainfall.

    Locations farther east from KY into TN, ahead of the MCV, will see
    a localized flash flood threat where lift ahead of the MCV will
    continue areas of heavy rain with brief training just downstream
    of the MCV center. The greatest limiting factor for excessive
    rainfall across central KY/TN will be weak instability. However,
    peak rainfall rates of 1-2 in/hr may still be enough to support
    localized rainfall totals of 2-3 inches through 15Z.

    Otto

    ATTN...WFO...EAX...JKL...LMK...LSX...MEG...MRX...OHX...PAH...
    SGF...

    ATTN...RFC...LMRFC...MBRFC...NCRFC...OHRFC...NWC...

    LAT...LON 38809182 38199048 37758952 37478873 37348821
    37348770 37498725 38138612 38128551 37818483
    37348425 36908404 36388417 36108477 35828556
    35528656 35448764 35608834 36088934 36949067
    37859232 38349296 38789291

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  • From Mike Powell@454:1/105 to All on Sun Aug 6 08:11:00 2023
    AWUS01 KWNH 061300
    FFGMPD
    MOZ000-MNZ000-IAZ000-SDZ000-061900-

    Mesoscale Precipitation Discussion 0876
    NWS Weather Prediction Center College Park MD
    859 AM EDT Sun Aug 06 2023

    Areas affected...Iowa and southern Minnesota

    Concerning...Heavy rainfall...Flash flooding possible

    Valid 061300Z - 061900Z

    Summary...Showers and thunderstorms will increase in coverage this
    morning and then drift slowly to the northwest across Iowa into
    Minnesota. Rainfall rates of 2"/hr or greater at times are likely,
    which through slow motions could produce 2-4" of rain. Flash
    flooding is possible.

    Discussion...The regional radar mosaic this morning shows
    expanding coverage of showers and isolated thunderstorms across
    central Iowa. This convection is blossoming downstream of a
    stacked low pressure moving eastward out of SD, with downstream
    enhanced ascent driven by height falls, impressive PVA, and
    modestly coupled upper jet streaks. This ascent will continue to
    impinge into favorable thermodynamics characterized by PWs of 1.5
    to 1.7 inches measured by morning UA soundings, around the 90th
    percentile for the date, collocated with MUCAPE above 1000 J/kg
    analyzed by the SPC RAP. Within this environment, rainfall rates
    have been estimated via KDMX and KFSD to exceed 2"/hr, and while
    this may be a bit overdone, it is reflective of the possible
    intensity developing through this aftn.

    The stacked low will shift slowly eastward through the day,
    maintaining robust deep layer ascent across the area. At the same
    time, S/SE flow downstream of the wave should drive PWs to above
    1.75" this aftn, coincident with MUCAPE remaining 1000-1500 J/kg.
    Despite significant cloud cover which may temper surface-based
    instability except in clearings, impressive theta-e advection
    lifting cyclonically into a TROWAL should supply ample elevated
    instability to persist heavy rain rates. This is likely
    manifesting as the high MUCAPE in model output, and when combined
    with a well-collocated overlap of the highest PW pool and warm
    cloud depths surging above 11,000 ft, will drive extremely
    efficient warm rain processes. This will result in rainfall rates
    which the HREF indicates could exceed 2"/hr, with the HRRR
    suggesting brief 3+"/hr rates noted via the 15-min rainfall
    accumulation product. Storms that develop will generally move
    slowly NW on 10-15 kt 0-6km mean winds, but increasingly
    anti-parallel and collapsing Corfidi vectors suggest considerable
    backbuilding potential to result in training and at times storm
    motions of just 0-5 kts. Where the most persistent training can
    occur, rainfall of 2-4" is possible as shown by the HREF and
    experimental RRFS TL ensemble probabilities.

    Although 7-day rainfall across northern IA and southern MN has
    been well below normal according to AHPS, heavy rain did impact
    parts of this area in the past 24-hours. This has resulted in
    nearly saturated top-soils according to the HRRR, and locally
    compromised FFG as low as 0.75-1.5"/3hrs. While the slow motions
    of any intense rainfall rates could result in instances of flash
    flooding through the aftn, the greatest risk will be where
    training can occur atop these most vulnerable soils.


    Weiss

    ATTN...WFO...ARX...DMX...DVN...EAX...FSD...MPX...OAX...

    ATTN...RFC...MBRFC...NCRFC...NWC...

    LAT...LON 44479441 44219309 43479205 42409142 41139128
    40439170 40349241 40859342 41439461 41799562
    42379619 42889648 43299648 44069639 44379578


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  • From Mike Powell@454:1/105 to All on Sat Sep 30 08:11:00 2023
    AWUS01 KWNH 300932
    FFGMPD
    FLZ000-301530-

    Mesoscale Precipitation Discussion 1117
    NWS Weather Prediction Center College Park MD
    530 AM EDT Sat Sep 30 2023

    Areas affected...Portions of the FL Atlantic Coast

    Concerning...Heavy rainfall...Flash flooding possible

    Valid 300930Z - 301530Z

    SUMMARY...Slow-moving showers and thunderstorms this morning with
    locally extreme rainfall rates may produce some areas of flash
    flooding.

    DISCUSSION...The latest radar imagery has been showing an uptick
    in very heavy shower and thunderstorm activity around the Daytona
    Beach, FL area and extending offshore with some linear bands
    becoming aligned with a convergent low-level flow regime just
    north of a quasi-stationary frontal zone.

    A very moist and unstable airmass is pooled across the region with
    PWs that are running over 2 standard deviations above normal with
    values of 2.25 to 2.5 inches. The 00Z RA0B sounding from KXMR
    (Cape Canaveral) was reflective of a very deep and warm tropical
    column with WBZ heights well over 15,000 feet and a PW of 2.31
    inches. The latest RAP analysis shows MLCAPE values of 1500+ J/kg
    along the coast and already indicative of a moderately unstable
    airmass.

    The combination of convergent low-level easterly flow, westerly
    shear aloft, and this very moist and unstable airmass will support
    slow-moving and locally anchored convective cells focusing along
    and adjacent to the coast that will be capable of extreme rainfall
    rates.

    Some rainfall rates with the more organized convective cells this
    morning will be capable of reaching 2 to 4 inches/hour and this
    will also be connected to the fact there that is such a deep and
    moist warm cloud layer.

    The 06Z HREF guidance suggests a threat for localized storm totals
    reaching or exceeding 5 inches where the storms become more
    focused, and at least in the short-term, areas near Daytona Beach
    will be most at risk for seeing these heavier totals. Going
    through the morning hours, some additional expansion of convection
    especially to the north along the coast may be possible.

    Very high FFG values will certainly mitigate any flash flood
    threat to an extent, but with such high rainfall rate potential
    this morning, some flash flooding will be possible and especially
    if any urban areas can get into the stronger convective cores.

    Orrison

    ATTN...WFO...JAX...MLB...

    ATTN...RFC...SERFC...NWC...

    LAT...LON 29878130 29828118 29598108 29228087 28808077
    28598080 28678113 29008136 29428154 29758153


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  • From Mike Powell@454:1/105 to All on Thu Oct 26 08:43:00 2023
    AWUS01 KWNH 260941
    FFGMPD
    TXZ000-261540-

    Mesoscale Precipitation Discussion 1161
    NWS Weather Prediction Center College Park MD
    540 AM EDT Thu Oct 26 2023

    Areas affected...Much of Central and South-Central TX

    Concerning...Heavy rainfall...Flash flooding possible

    Valid 260940Z - 261540Z

    Summary...Rainfall totals of 1-2" may continue through morning,
    possibly leading to additional localized totals of 3-5"+.
    Scattered instances of flash flooding are possible, and some may
    be significant.

    Discussion...Showers and thunderstorms continue early this morning
    in the vicinity of a surface trough draped across North and
    Central TX. Rainfall rates have moderated a bit over the past
    several hours but have shown some recovery as of late, still as
    high as 1-2"/hr locally. While progression of the convection
    eastward has begun to limit the threat for additional flooding
    across northeast TX, there is less eastward progression farther
    south (into the TX Hill Country and South-Central TX) which is
    facilitating localized training and repeating of 1-2"/hr rainfall
    rates. This is in a mesoscale environment characterized by PWATs
    of 1.4-1.9" (between the 90th percentile and max moving average,
    per surrounding sounding climatology), appreciable instability (as
    measured by 500-1000 J/kg of ML CAPE), and effective bulk shear of
    20-40 kts.

    Hi-res guidance has come into far less agreement through 16z,
    despite much better agreement among 00z HREF members earlier
    overnight. Both the HREF suite and more recent hourly runs of the
    HRRR have struggled to depict the intensity of the southern end of
    the convection, but the most recent HRRR runs (since 06z) have
    begun to depict a more concerning scenario where a narrow band of
    training convection sets up near the I-35 corridor (including the
    Austin and San Antonio metro areas) and have shown some southerly
    trend. Six hourly totals are indicated to be on the order of 3-9"
    in these latest runs, which is certainly plausible given the
    highly efficient warm rain processes and potential for training.
    That said, the 00z HREF is far less aggressive, generally
    suggesting 20-40% probabilities for localized 3" exceedance (per
    the 40-km neighborhood method), and negligible (~5%) odds for
    localized 5" exceedance. The above introduces above average
    uncertainty. Given the concerning trends in the HRRR (with
    observational trends supporting these concerns), continued flash
    flooding is considered possible (as well as locally significant
    flooding, given the potential magnitude).

    Churchill/Roth

    ATTN...WFO...EWX...FWD...HGX...SJT...

    ATTN...RFC...WGRFC...NWC...

    LAT...LON 32729592 31519596 30249674 28889793 28869937
    29409997 29800000 30899903 31509830 32229726


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  • From Mike Powell@454:1/105 to All on Sat Dec 2 09:55:00 2023
    AWUS01 KWNH 021235
    FFGMPD
    FLZ000-GAZ000-ALZ000-MSZ000-LAZ000-021833-

    Mesoscale Precipitation Discussion 1186
    NWS Weather Prediction Center College Park MD
    733 AM EST Sat Dec 02 2023

    Areas affected...Central Gulf Coast into the Florida Panhandle

    Concerning...Heavy rainfall...Flash flooding possible

    Valid 021233Z - 021833Z

    SUMMARY...Showers and thunderstorms will generally be on the
    increase in coverage and intensity going through the midday hours.
    A combination of heavy rainfall rates and localized training cell
    activity may result in scattered instances of flash flooding.

    DISCUSSION...The latest surface analysis shows a west/east
    oriented low-level trough axis/mesoscale boundary lingering over
    the far northern Gulf of Mexico south of the FL Panhandle and
    extending westward into the coastal parishes of south-central to
    southeast LA. Strengthening deep layer warm air advection and
    moisture transport over this boundary in response to a weak
    low-amplitude shortwave perturbation advancing northeast across
    the central Gulf Coast has resulted in a general uptick in the
    coverage of heavy shower and thunderstorm activity.

    MUCAPE values of as much as 1000 to 1500 J/kg are noted across
    southeast LA and into the immediate offshore waters of southern MS
    and southern AL. The 3-hour MUCAPE differentials across this
    region are as much as +250 to +500 J/kg, and suggestive of the
    northeast transport of greater instability albeit a bit elevated.

    The PW values are gradually increasing with time with the NESDIS
    Blended TPW imagery showing a well-defined northward transport of
    tropical moisture with 2.0 to 2.25 inch PWs lifting north from the
    Gulf of Mexico toward the central Gulf Coast. The aforementioned
    low-amplitude shortwave perturbation shows up quite well in the
    850/700mb and 700/500mb CIRA-ALPW datasets approaching southwest
    to south-central LA, and there is evidence in surface observations
    of a low center near the coast.

    Already there are broken areas of convection with heavy rainfall
    rates of 1 to 2 inches/hour impacting parts of south-central to
    southeast LA and coastal areas of southern MS. Even stronger
    convection with heavier rates are noted offshore of the FL
    Panhandle to the south and southwest of Panama City as a more
    organized convective band with very cold cloud tops aligns itself
    in close proximity to the aforementioned mesoscale boundary/trough.

    Given the increasingly parallel orientation of the convection to
    the deeper layer mean flow, there will be concerns for some
    training convective bands/cell-activity that will favor excessive
    rainfall totals. The 06Z HREF and recent HRRR guidance may be a
    bit too far north with their QPF footprint this morning, and the
    latest radar/satellite trends suggest areas closer to the Gulf
    Coast itself will tend to be at risk for the heaviest amounts.
    Additional rainfall amounts of as much as 3 to 5 inches will be
    possible going through midday, and this may result in scattered
    instances of flash flooding and especially around the more
    sensitive urbanized locations.

    Orrison

    ATTN...WFO...JAN...LCH...LIX...MOB...TAE...

    ATTN...RFC...LMRFC...SERFC...NWC...

    LAT...LON 31398750 31188539 30868451 30328396 29738424
    29528504 29918577 30188672 30088841 29628958
    29369064 29309158 29809183 30399105 31138911


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  • From Mike Powell@454:1/105 to All on Thu Dec 21 19:24:00 2023
    AWUS01 KWNH 212131
    FFGMPD
    CAZ000-220330-

    Mesoscale Precipitation Discussion 1213
    NWS Weather Prediction Center College Park MD
    430 PM EST Thu Dec 21 2023

    Areas affected...Portions of the Central and Southern California
    Coast

    Concerning...Heavy rainfall...Flash flooding likely

    Valid 212130Z - 220330Z

    SUMMARY...The flash flooding threat continues along portions of
    the central and southern California coast. The heaviest rain will
    shift west, resulting in diminishing rainfall rates from east to
    west through this afternoon.

    DISCUSSION...Heavy rainfall continued this morning primarily
    through the Santa Barbara area, where considerable flash flooding
    is underway, including flooding of homes and businesses in the
    Santa Barbara area. The corridor of heaviest rain remains near
    Santa Barbara, and is likely to persist over the next several
    hours as it only slowly drifts westward. 6 hourly rainfall totals
    of 2 to 4 inches have been reported in and around Santa Barbara,
    with considerably lighter amounts both east and west, showing how
    narrow a corridor of heavy rain this has been so far.

    The upper level shortwave forcing supporting the heaviest rainfall
    and circulating around the broader upper low is moving offshore to
    the west. MUCAPE values have only diminished to between 250 and
    500 J/kg. CAMs guidance is in good agreement that on the whole,
    the heaviest rainfall within the corridor should gradually
    diminish with time. All of these factors are likely to result in
    more intermittent periods of heavy rain with rates up to an inch
    per hour, but the training and painfully slow broad movement of
    the heavy rain corridor will keep the flash flooding threat in the
    likely category through the afternoon, particularly around Santa
    Barbara.

    As the corridor of heaviest rain shifts west late this afternoon
    and evening, increasing easterly/offshore/downslope flow should
    result in a sharp cutoff on the eastern side of the precipitation
    shield. The upper level low driving all this rainfall is moving
    southeastward well off the coast of Mexico, which will result in a reorientation of the rain bands from the current SSE-NNW to SE-NW
    this evening, and eventually E to W by late tonight. This in turn
    will result in diminishing rainfall for areas further north into
    Monterey and SLO Counties, though coastal sections of those
    counties may continue to see heavy rain well into the late
    afternoon. However, this same trend may allow bands of heavy rain
    to impact the remainder of the CA coast down to the Mexico border
    late tonight.

    Wegman

    ATTN...WFO...HNX...LOX...MTR...

    ATTN...RFC...CNRFC...NWC...

    LAT...LON 36282129 36192061 36102037 36092022 35952002
    35571954 35151904 34831880 34461876 34021906
    34351958 34412052 35082071 35532122 36072166


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  • From Mike Powell@454:1/105 to All on Sat Feb 10 08:47:00 2024
    AWUS01 KWNH 101042
    FFGMPD
    LAZ000-TXZ000-101641-

    Mesoscale Precipitation Discussion 0055
    NWS Weather Prediction Center College Park MD
    542 AM EST Sat Feb 10 2024

    Areas affected...portions of southeast TX & western LA

    Concerning...Heavy rainfall...Flash flooding possible

    Valid 101041Z - 101641Z

    Summary...A low-level convergence zone is expected to gather an
    increasing amount of thunderstorm activity through the morning
    hours. Hourly rain totals to 2" with local amounts to 4" are
    possible.

    Discussion...A shortwave moving near El Paso has been enhancing
    outflow aloft across much of central and eastern TX and slowly
    eroding CIN. Precipitable water values near the TX coast have
    risen into the 1.5-1.75" range per GPS data. MU/ML CAPE has been
    more or less stable near 250 J/kg on SPC mesoanalyses. A
    low-level convergence zone, occasionally showing up in the surface
    pattern but more evident at 925 hPa and 850 hPa, has been moving
    little near the Middle and Upper TX coasts and across western LA
    within the warm sector of a low over TX, which helped focus a band
    of showers hours ahead of what was suggested by the 00z HREF.
    Around 3 a.m. CST, there was a spike in hourly rain totals up to
    1.5", also earlier/more than anticipated by the guidance, from the Galveston/Harris County border into southeast Liberty County.

    Various model fields suggest that instability will be stable or
    slowly increase initially, before going upward faster later this
    morning near the southern end of the low-level convergence zone
    either due to daytime heating or the approaching shortwave. A
    boundary itself may shift slightly northward. The past few HREF
    runs have been slowly trending to a wetter signal across this
    region, focusing more on this convergence zone and less on the
    front to the north. As the shortwave near westernmost TX
    approaches, the expectation is for convective coverage to increase
    near the convergence zone, particularly after 13z per the 00z/06z
    HREF guidance is to be believed. The 00z ARW appears best placed
    with what's expected, based on what's occurred thus far, though
    its amounts could be a little light. Hourly rain totals are
    expected to peak in the 2" range, with local amounts to 4" being
    possible, which would exceed the three hour flash flood guidance.
    Any heavy rain-related issues would be isolated to widely
    scattered, with urban areas most sensitive.

    Roth/Asherman

    ATTN...WFO...HGX...LCH...SHV...

    ATTN...RFC...LMRFC...WGRFC...NWC...

    LAT...LON 31799320 31209252 30029415 28999604 29509646
    30789509

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