• MESO: Heavy rainfall

    From Weather Alert@21:1/175 to All on Sat Jan 2 20:54:00 2021
    AWUS01 KWNH 021810
    FFGMPD
    ORZ000-WAZ000-030600-

    Mesoscale Precipitation Discussion 0003
    NWS Weather Prediction Center College Park MD
    109 PM EST Sat Jan 02 2021

    Areas affected...Pacific Northwest

    Concerning...Heavy rainfall

    Valid 021800Z - 030600Z

    SUMMARY... A prolonged period of moderate-to-heavy rainfall will
    persist this afternoon and into the evening hours as deep moisture
    associated with an atmospheric river is aimed at the Pacific
    Northwest. Localized areas of flooding are possible.

    DISCUSSION... GOES-17 water vapor imagery shows an impressive
    fetch of subtropical moisture originating as far west as the
    north-central Pacific. An expansive North Pacific jet streak with
    wind speeds at the nose of the jet approaching ~150 kts is aimed
    at the region. Meanwhile, an impressive low 960s mb cyclone is
    co-located beneath the jet's left exit region. As of 17Z, the warm
    front associated with this system was not far from the west
    Washington coastline with RAP analysis indicating WSW flow of
    50-60 kts within the warm sector.

    Latest 12Z model guidance indicates a tongue of high 850mb theta-e
    values embedded within the strong low level flow begins to come
    ashore late morning. Along with rich theta-e advection, an IVT
    plume of ~700 kg/ms also supports the parade of rich atmospheric
    moisture directed at the Pacific Northwest this afternoon and
    evening. PWs up to 1" are embedded within the mean southwesterly
    flow aloft thus giving credence to an environment becoming more
    favorable for heavier rainfall rates. The highest content of the
    aforementioned 850mb theta-e tongue reaches western Washington
    between 21Z-03Z. As that tongue atmospheric moisture pushes
    through the region, hourly rainfall rates may climb up to ~0.5"
    per hour with locally excessive rates in the southwest facing
    slopes pf the Olympics due to more pronounced orographic lifting.

    The 12Z HREF shows high probabilities (over 70%) of 3-hour
    rainfall totals surpassing 1" in the highlighted area between
    21-00Z and 00-03Z. Antecedent conditions have been quite wet,
    especially along the windward side of the Olympics where 7-day
    percent of normal precipitation is over 200% in spots. Some
    locations in and around the Puget Sound, including along the I-5
    corridor have also witnessed abnormally wet conditions in recent
    weeks. Localized flooding, especially near rivers and streams, is
    possible this afternoon and into the evening hours. The cold front
    associated with the northeast Pacific storm system swings through
    the Olympics around 03Z and heads south and east, pushing the rich
    850mb theta-e axis towards southwestern Washington and northwest
    Oregon overnight. This may result in additional heavy rainfall in
    the southern periphery of the highlighted area beyond 06Z tonight.

    Mullinax


    ATTN...WFO...PQR...SEW...

    ATTN...RFC...NWRFC...NWC...

    LAT...LON 49042244 48492214 47752186 46822192 46082277
    45442312 45262386 45572418 46902429 48362479
    48292365 48412314 48632296 49022276
    #
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  • From Weather Alert@21:1/175 to All on Sun Jan 3 13:46:00 2021
    AWUS01 KWNH 030724
    FFGMPD
    CAZ000-ORZ000-031900-

    Mesoscale Precipitation Discussion 0005
    NWS Weather Prediction Center College Park MD
    223 AM EST Sun Jan 03 2021

    Areas affected...western Oregon into northwestern California

    Concerning...Heavy rainfall

    Valid 030719Z - 031900Z

    SUMMARY...Despite weakening moisture transport, heavy rain will
    affect western OR into northwestern CA through morning. Rainfall
    rates will briefly eclipse 0.5 in/hr and rainfall totals near the
    OR/CA border will likely peak in the 2-4 inch range through 19Z.
    The greatest impacts will be urban flooding and excess runoff near
    burn scar locations.

    DISCUSSION...07Z water vapor imagery showed a shortwave trough
    west of the Pacific Northwest coast with an inflection/vorticity
    max noted near 44N 128W. Precipitable water values were just shy
    of 1 inch along the Washington/Oregon coastline via 00Z RAOBs and
    GPS data. The associated cold front was analyzed from the northern
    WA Cascades to just south of Astoria, OR into the eastern Pacific
    at 07Z. There could be a weak wave along the front ahead of the
    inflection on water vapor imagery, just offshore of the OR coast,
    as a frontal band of heavy rain stalled near the mouth of the
    Columbia River with observed rainfall rates between 0.6 and 0.8
    in/hr ending 0530Z. 850 mb winds were 50-55 kt at KRTX (via VAD
    wind profile) and RAP analysis data indicated 70+ kt just
    offshore, reaching the coast within the next hour. Temporary
    stalling of a frontal rain band will continue to support the
    threat for heavy rainfall rates in excess of 0.6 in/hr in the
    short term for northwestern OR. The greatest concern for excess
    runoff will be near recent burn scar locations downstream into the
    Cascades.

    The shortwave just west of the Pacific Northwest coast is expected
    to move inland, sending the front progressively southward over the
    next 6 hours, before the fronts stalls near the OR/CA coast in
    advance of another shortwave observed between 155 and 160W. Strong
    low level moisture transport will continue the threat for
    occasional rainfall rates in excess of 0.5 in/hr from the OR
    Coastal Ranges into the Cascade Mountains but the threat should be
    short lived lasting an hour or two at most for any given location.

    Around 12Z and beyond, the front is forecast to stall near the
    OR/CA coast, and while moisture transport should weaken
    substantially given projected 850 mb winds only in the 30-40 kt
    range, precipitable water values are expected to remain near 1
    inch. Several hours of rainfall rates in the 0.3 to 0.6 in/hr
    range will be possible near the OR/CA border from about 12Z
    through 18Z. While the heaviest rates should focus closer to the
    coast and west of the Slater Burn Scar, the prolonged period of
    heavy rain may lead to 2-4 inches of additional rainfall through
    19Z with localized flooding along roadways and other impervious
    surfaces from Curry County to Del Norte County.

    Otto

    ATTN...WFO...EKA...MFR...PDT...PQR...

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

    LAT...LON 45942387 45912339 45482263 45332198 45032176
    44542175 43982192 43332218 43092224 42712219
    42492239 42482279 42962335 42502363 42012351
    41812340 41372355 41032385 41142429 42212456
    42972460 44002459 45102440 45762404
    #
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  • From Weather Alert@21:1/175 to All on Tue Jan 12 00:39:00 2021
    AWUS01 KWNH 112117
    FFGMPD
    ORZ000-WAZ000-120845-

    Mesoscale Precipitation Discussion 0006
    NWS Weather Prediction Center College Park MD
    416 PM EST Mon Jan 11 2021

    Areas affected...northwestern OR into western WA

    Concerning...Heavy rainfall

    Valid 112113Z - 120845Z

    SUMMARY...The likelihood of rainfall rates in excess of 0.5 in/hr
    will increase after 03Z across western WA into northwestern OR.
    Through 09Z, maximum rainfall totals in the Olympics are expected
    to be between 3 and 4 inches, while 2-3 inch max totals are
    anticipated from southwestern WA into northwestern OR. This will
    be the beginning of a prolonged atmospheric river event for the
    Pacific Northwest.

    DISCUSSION...A 72 hour loop of Blended TPW imagery showed a
    continuous axis of 1+ inch precipitable water values extending
    offshore of the West Coast, back into the tropical western
    Pacific. Embedded higher values of precipitable water existed
    within the plume with over 1.5 inches centered near 35N 150W at
    14Z, or about 1500 miles offshore of the OR coastline. The eastern
    end of this moisture axis is located south of a slow moving closed
    mid to upper-level low centered near Kodiak Island. At the
    surface, two fronts were analyzed at 20Z, the lead boundary
    extending southwestward from a surface low located near 44N 134W,
    and a second (stronger) surface low and associated cold front near
    47N 139W.

    As the shortwave tied to the western surface low near 139W
    advances toward the British Columbia coast tonight, a corridor of
    strong southwesterly to south-southwesterly 850 mb flow between
    60-70 kt (occassionally higher) is forecast to arrive along the WA
    and OR coasts near 03Z according to a consensus of the latest
    model guidance. The strengthening low level flow along the coast,
    combined with precipitable water values between 1.0 and 1.1
    inches, is expected to support IVT values over 750 kg/m/s by
    roughly 06Z over northwestern OR into far southwestern WA.

    The orientation of the low level flow will favor southwest facing
    slopes of the Olympic Peninsula and Coastal Ranges into
    northwestern OR with rainfall rates occasionally climbing above
    0.5 in/hr. While peak rainfall rates are expected to be below 0.75
    in/hr through 09Z, the transition from fairly light rainfall to
    heavy rainfall rates is expected to occur somewhat quickly, and
    high rainfall rates should persist for hours, continuing into the
    day on Tuesday.

    Given above average rainfall across the region over the past two
    weeks (150 to 300 percent of average from northwest OR into
    western WA), locations may be more susceptible to flooding
    concerns. Peak 12 hour rainfall totals of over 3 inches are
    expected into the favored terrain.

    Otto

    ATTN...WFO...PQR...SEW...

    ATTN...RFC...NWRFC...NWC...

    LAT...LON 48072474 48032425 47772392 47622322 47392318
    46972338 46752343 46422324 46182328 45602343
    44892365 44842416 45302434 46032441 46592449
    47632484 47982494
    #
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  • From Weather Alert@21:1/175 to All on Tue Jan 26 23:03:00 2021
    AWUS01 KWNH 261911
    FFGMPD
    CAZ000-270610-

    Mesoscale Precipitation Discussion 0014
    NWS Weather Prediction Center College Park MD
    210 PM EST Tue Jan 26 2021

    Areas affected...Central California Coast

    Concerning...Heavy rainfall

    Valid 261910Z - 270610Z

    Summary...A strong atmospheric river event will bring widespread
    rainfall to the central California coast later today into tonight.
    Hourly totals between 0.5-0.75 inches and amounts through 06Z of
    2-3" are likely. This heavy rainfall could cause flash flooding,
    especially over burn scars and other sensitive areas.

    Discussion...Water vapor imagery and the latest blended PW product
    shows an anomalously strong plume of moisture off the western U.S.
    coast. As the system approaches this evening, showers associated
    with the warm front are expected to move onshore the coast between
    00-03Z then quickly followed by the more intense rain associated
    with the cold front after 03Z as it drops south along the coast.
    Precipitable water values associated with the atmospheric river
    are forecast to be between 1-2 standard deviations above normal
    while IVT values by this evening are forecast to be near 4
    standard deviations above normal (approaching 5 std later on
    Wednesday). 850 mb flow increases to 50-60 kts by 04Z and is
    aligned nearly orthogonal to the terrain across the North Bay
    areas, which should enhance rain rates.

    The 12Z HREF supports hourly totals between 0.50 and 0.75" between
    03-06Z across the North Bay then advancing southward later in the
    night. The various hi-res models depict some higher-end potential
    for 1" hourly totals, though that risk is greater beyond the
    current MPD time frame. Through 06Z, 2-3" will be common across
    the North Bay areas, some locally higher amounts 3-4" will be
    possible.

    These rainfall rates and totals over the burn scars and other
    sensitive areas could lead to flash flooding, possible
    mudslides/debris flows. Even outside burn scar areas, rapid onset
    runoff and flooding will be possible.

    Taylor

    ATTN...WFO...EKA...MTR...STO...

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

    LAT...LON 39192309 39032255 38772217 38492199 38272203
    38192242 37972260 37942286 38252308 38552326
    38802353 39172384
    #
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  • From Weather Alert@21:1/175 to All on Mon Feb 1 21:52:00 2021
    AWUS01 KWNH 011727
    FFGMPD
    CAZ000-ORZ000-020000-

    Mesoscale Precipitation Discussion 0021
    NWS Weather Prediction Center College Park MD
    1227 PM EST Mon Feb 01 2021

    Areas affected...Northwest CA...Southwest OR

    Concerning...Heavy rainfall

    Valid 011720Z - 020000Z

    SUMMARY...Periods of moderate-to-heavy rain are expected late this
    morning and into the afternoon as Pacific moisture moves ashore
    ahead of an approaching frontal boundary.

    DISCUSSION...GOES-17 satellite imagery depicted a stream of
    Pacific moisture out ahead of a deep -2 to -2.5 STD longwave
    trough. 15Z surface analysis indicated an elongated frontal
    boundary was located just off the Pacific Northwest coastline. PVA
    and WAA surging out of the base of the upper trough is advancing
    the core of rich Pacific moisture east towards the Pacific
    Northwest coastline. PWs topping out as high as 1.00", just shy of
    ~2 STDs above normal, are being transported within a strong
    southwesterly mean 850-300mb flow. These atmospheric factors
    support the likelihood for ongoing periods of rain, potentially
    heavy at times, as the axis of Pacific moisture reaches the coast.

    Latest CAM guidance show the heaviest period of rainfall occurring
    between late morning and early afternoon. Hourly rainfall rates
    will be maximized along the more orthogonally favored upslope
    regions of northwest California and southwest Oregon. Rates in
    these areas could range between 0.25-0.50" with several hours
    worth of these rates possible. Total rainfall estimates during
    this MPD suggests up to 1.5" of precipitation is likely along the
    coastal range of northern California and southwest Oregon with
    locally higher amounts possible. While totals and rates are not
    overly impressive, antecedent conditions show these same areas
    have received between 150-300% of normal rainfall over the last 7
    days. FLASH guidance also highlights some areas with over 85% soil
    saturation. These factors along with some additional snow melt
    could result in rising streams and rivers with very localized
    flooding possible.

    Mullinax

    ATTN...WFO...EKA...MFR...

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

    LAT...LON 43162426 42762391 42002354 41142352 39962336
    39452331 39262397 39852460 40922481 41582470
    42042470 42482470 42882468 43122462
    #
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